XVII. CRETACEOUS GROUP
Divisions of the cretaceous series in North-Western Europe--Upper cretaceous strata--Maestricht beds--Chalk of Faxoe--White chalk--Characteristic fossils--Extinct cephalopoda--Sponges and corals of the chalk--Signs of open and deep sea--Wide area of white chalk--Its origin from corals and shells--Single pebbles in chalk--Siliceous sandstone in Germany contemporaneous with white chalk--Upper greensand and gault--Lower cretaceous strata--Atherfield section, Isle of Wight--Chalk of South of Europe--Hippurite limestone--Cretaceous Flora--Chalk of United States.
Having treated in the preceding chapters of the tertiary strata, we have next to speak of the uppermost of the secondary groups, called the Chalk or Cretaceous (No. 6. Table, p. 103.), because in those parts of Europe where it was first studied its upper members are formed of that remarkable white earthy limestone, termed chalk (creta). The inferior division consists, for the most part, of clays and sands, called Greensand, because some of the sands derive a bright green colour from intermixed grains of chloritic matter. The cretaceous strata in the north-west of Europe may be thus divided[209-A]:
Upper Cretaceous.
1. Maestricht beds and Faxoe limestone. 2. Upper white chalk, with flints. 3. Lower white chalk, without flints, passing downwards into chalk marl, which is slightly argillaceous. 4. Upper greensand. 5. Gault.
Lower Cretaceous.
6. Lower greensand--Ironsand, clay, and occasional beds of limestone (Kentish rag).
Maestricht Beds.--On the banks of the Meuse, at Maestricht, reposing on ordinary white chalk with flints, we find an upper calcareous formation about 100 feet thick, the fossils of which are, on the whole, very peculiar, and all distinct from tertiary species. Some few are of species common to the inferior white chalk, among which may be mentioned Belemnites mucronatus (see fig. 197.) and Pecten quadricostatus. Besides the Belemnite there are other genera, such as Ammonite, Baculite, and Hamite, never found in strata newer than the cretaceous, but frequently met with in these Maestricht beds. On the other hand, Volutes and other genera of univalve shells, usually met with only in tertiary strata, occur.
The upper part of the rock, about 20 feet thick, as seen in St. Peter's Mount, in the suburbs of Maestricht, abounds in corals, often detachable from the matrix; and these beds are succeeded by a soft yellowish limestone 50 feet thick, extensively quarried from time immemorial for building. The stone below is whiter, and contains occasional nodules of grey chert or chalcedony.
M. Bosquet, with whom I lately examined this formation (August, 1850), pointed out to me a layer of chalk from 2 to 4 inches thick, containing green earth and numerous encrinital stems, which forms the line of demarcation between the strata containing the fossils peculiar to Maestricht and the white chalk below. The latter is distinguished by regular layers of black flint in nodules, and by several shells, such as Terebratula carnea (see fig. 201.), wholly wanting in beds higher than the green band. Some of the organic remains, however, for which St. Peter's Mount is celebrated, occur both above and below that parting layer, and, among others, the great marine reptile, called Mosasaurus, a saurian supposed to have been 24 feet in length, of which the entire skull and a great part of the skeleton have been found. Such remains are chiefly met with in the soft freestone, the principal member of the Maestricht beds.
Chalk of Faxoe.--In the island of Seeland, in Denmark, the newest member of the chalk series, seen in the sea-cliffs at Stevens Klint resting on white chalk with flints, is a yellow limestone, a portion of which, at Faxoe, where it is used as a building-stone, is composed of corals, even more conspicuously than is usually observed in recent coral reefs. It has been quarried to the depth of more than 40 feet, but its thickness is unknown. The imbedded shells are chiefly casts, many of them of univalve mollusca, which, as they strictly belong to the Cretaceous era, are worthy of notice, since such forms, whether spiral or patelliform, are wanting in the white chalk of Europe generally. Thus, there are two species of Cypraea, one of Oliva, two of Mitra, four of the genus Cerithium, six of Fusus, two of Trochus, one Patella, one Emarginula, &c., on the whole, more than thirty univalves, spiral or patelliform, not one of which is common to the white chalk. At the same time, a large proportion of the accompanying bivalve shells, echinoderms, and zoophytes, are specifically identical with fossils of older parts of the Cretaceous series. Among the cephalopoda of Faxoe, may be mentioned Baculites Faujasii and Belemnites mucronatus, shells of the white chalk.
The claws and entire shell of a small crab, Brachyurus rugosus (Schlotheim), are scattered through the Faxoe stone, reminding us of similar crustaceans enclosed in the rocks of many modern coral reefs.[211-A] Some small portions of this coralline formation consist of white earthy chalk; it is, therefore, clear that this substance must have been produced simultaneously, a fact of some importance, as bearing on the theory of the origin of white chalk; for the decomposition of such corals as we see at Faxoe is capable, we know, of forming white mud, undistinguishable from chalk, and which we may suppose to have been dispersed far and wide through the ocean, in which such reefs as that of Faxoe grew.
White Chalk (2. and 3. Tab. p. 209.).--The highest beds of chalk in England and France consist of a pure, white, calcareous mass, usually too soft for a building stone, but sometimes passing into a more solid state. It consists, almost purely, of carbonate of lime; the stratification is often obscure, except where rendered distinct by interstratified layers of flint, a few inches thick, occasionally in continuous beds, but oftener in nodules, and recurring at intervals from 2 to 4 feet distant from each other.
This upper chalk is usually succeeded, in the descending order, by a great mass of white chalk without flints, below which comes the chalk marl, in which there is a slight admixture of argillaceous matter. The united thickness of the three divisions in the south of England equals, in some places, 1000 feet.[211-B]
The annexed section, fig. 193., will show the manner in which the white chalk extends from England into France, covered by the tertiary strata described in former chapters, and reposing on lower cretaceous beds.
Among the conspicuous forms of mollusca wholly foreign to the tertiary and recent periods, and which we meet with in the white chalk, are the Belemnite, Ammonite, Baculite, and Turrilite, all genera of Cephalopoda, a family to which the living cuttle-fish and nautilus belong.
Among the brachiopoda in the white chalk, the Terebratulae are very abundant. These shells are known to live at the bottom of the sea, where the water is tranquil and of some depth (see figs. 198, 199, 200, 201.). With these are associated some forms of oyster (see figs. 202. and 204.), and other bivalves (figs. 203, 205, 206, 207, 208.).
Among the rest, no form marks the cretaceous era in Europe, America, and India, in a more striking manner than the extinct genus Inoceramus (Catillus of Lamk.), the shells of which are distinguished by a fibrous texture, and are often met with in fragments, having, probably, been extremely friable.
[2 Illustrations: Fig. 210. Fig. 211. A branching sponge in a flint, from the white chalk. From the collection of Mr. Bowerbank.]
With these mollusca are many corals (figs. 209, 210, 211.) and sea urchins (fig. 212.), which are alike marine, and, for the most part, indicative of a deep sea. They are dispersed indifferently through the soft chalk, and hard flint, and some of the flinty nodules owe their irregular forms to inclosed zoophytes, as in the specimen represented in fig. 211., where the hollows in the exterior are caused by the branches of a sponge seen on breaking open the flint, fig. 210.
Of the singular family called Rudistes, by Lamarck, hereafter to be mentioned, as extremely characteristic of the chalk of Southern Europe, a single representative only (fig. 213.) has been discovered in the white chalk of England.
[4 Illustrations: Hippurites Mortoni, Mantell. Houghton, Sussex. White chalk. Diameter one seventh of nat. size.
Fig. 213. Two individuals deprived of their opercula, adhering together.
Fig. 214. Same seen from above.
Fig. 215. Transverse section of part of the wall of the shell, magnified to show the structure.
Fig. 216. Vertical section of the same.
On the side where the shell is thinnest, there is one external furrow and corresponding internal ridge, a, b. figs. 213, 214.; but they are usually less prominent than in these figures. This species has been referred to Hippurites, but does not, I believe, fully agree in character with that genus. I have never seen the opercular piece, or valve, as it is called by those conchologists who regard the Rudistes as bivalve mollusca. The specimen above figured was discovered by the late Mr. Dixon.]
The remains of fishes of the Upper Cretaceous formations consist chiefly of teeth of the shark family of genera, in part common to the tertiary, and partly distinct. But we meet with no bones of land animals, nor any terrestrial or fluviatile shells, nor any plants, except sea weeds, and here and there a piece of drift wood. All the appearances concur in leading us to conclude that the white chalk was the product of an open sea of considerable depth.
The existence of turtles and oviparous saurians, and of a Pterodactyl or winged-lizard, found in the white chalk of Maidstone, implies, no doubt, some neighbouring land; but a few small islets in mid-ocean, like Ascension, so much frequented by migratory droves of turtles, might perhaps have afforded the required retreat where these creatures might lay their eggs in the sand, or from which the flying species may have been blown out to sea. Of the vegetation of such islands we have scarcely any indication, but it consisted partly of cycadeous plants; for a fragment of one of these was found by Capt. Ibbetson in the chalk marl of the Isle of Wight, and is referred by A. Brongniart to Clathraria Lyellii, Mantell, a species common to the antecedent Wealden period.
Geographical extent and origin of the While Chalk.--The area over which the white chalk preserves a nearly homogeneous aspect is so vast, that the earlier geologists despaired of discovering any analogous deposits of recent date. Pure chalk, of nearly uniform aspect and composition, is met with in a north-west and south-east direction, from the north of Ireland to the Crimea, a distance of about 1140 geographical miles; and in an opposite direction it extends from the south of Sweden to the south of Bordeaux, a distance of about 840 geographical miles. In Southern Russia, according to Sir R. Murchison, it is sometimes 600 feet thick, and retains the same mineral character as in France and England, with the same fossils, including Inoceramus Cuvieri, Belemnites mucronatus, and Ostrea vesicularis.
But it would be an error to imagine, that the chalk was ever spread out continuously over the whole of the space comprised within these limits, although it prevailed in greater or less thickness over large portions of that area. On turning to those regions of the Pacific where coral reefs abound, we find some archipelagoes of lagoon islands, such as that of the Dangerous Archipelago, for instance, and that of Radack, with several adjoining groups, which are from 1100 to 1200 miles in length, and 300 or 400 miles broad; and the space to which Flinders proposed to give the name of the Corralline Sea is still larger; for it is bounded on the east by the Australian barrier--all formed of coral rock,--on the west by New Caledonia, and on the north by the reefs of Louisiade. Although the islands in these areas may be thinly sown, the mud of the decomposing zoophytes may be scattered far and wide by oceanic currents. That this mud would resemble chalk I have already hinted when speaking of the Faxoe limestone, p. 211.; and it was also remarked in an early part of this volume, that some even of that chalk which appears to an ordinary observer quite destitute of organic remains, is nevertheless, when seen under the microscope, full of fragments of corals and sponges; together with the valves of entomostraca, the shells of foraminifera, and still more minute infusoria.[215-A] (See p. 26.)
Now it had been often suspected, before these discoveries, that white chalk might be of animal origin, even where every trace of organic structure has vanished. This bold idea was partly founded on the fact, that the chalk consisted of pure carbonate of lime, such as would result from the decomposition of testacea, echini, and corals; and partly on the passage observable between these fossils when half decomposed and chalk. But this conjecture seemed to many naturalists quite vague and visionary, until its probability was strengthened by new evidence brought to light by modern geologists.
We learn from Lieutenant Nelson, that, in the Bermuda Islands, there are several basins or lagoons almost surrounded and enclosed by reefs of coral. At the bottom of these lagoons a soft white calcareous mud is formed by the decomposition of Eschara, Flustra, Cellepora, and other corallines. This mud, when dried, is undistinguishable from common white earthy chalk; and some portions of it, presented to the Museum of the Geological Society of London, might, after full examination, be mistaken for ancient chalk, but for the labels attached to them. About the same time Mr. C. Darwin observed similar facts in the coral islands of the Pacific; and came also to the opinion, that much of the soft white mud found at the bottom of the sea near coral reefs has passed through the bodies of worms, by which the stony masses of coral are everywhere bored; and other portions through the intestines of fishes; for certain gregarious fishes of the genus Sparus are visible through the clear water, browsing quietly, in great numbers, on living corals, like grazing herds of graminivorous quadrupeds. On opening their bodies, Mr. Darwin found their intestines filled with impure chalk. This circumstance is the more in point, when we recollect how the fossilist was formerly puzzled by meeting, in chalk, with certain bodies, called cones of the larch, which were afterwards recognized by Dr. Buckland to be the excrement of fish.[216-A] These spiral coprolites (see figures), like the scales and bones of fossil fish in the chalk, are composed chiefly of phosphate of lime.
[2 Illustrations: Fig. 217. Fig. 218. Coprolites of fish called Iulo-eido-copri, from the chalk.]
Mr. Dana, when describing the elevated coral reef of Oahu, in the Sandwich Islands, says, that some varieties of the rock consist of aggregated shells, imbedded in a compact calcareous base as firm in texture as any secondary limestone; while others are like chalk, having its colour, its earthy fracture, its soft homogeneous texture, and being an equally good writing material. The same author describes, in many growing coral reefs, a similar formation of modern chalk, undistinguishable from the ancient.[216-B] The extension over a wide submarine area of the calcareous matrix of the chalk, as well as of the imbedded fossils, would take place the more readily, in consequence of the low specific gravity of the shells of mollusca and zoophytes, when compared with ordinary sand and mineral matter. The mud also derived from their decomposition would be much lighter than argillaceous and other inorganic mud, and very easily transported by currents, especially in salt water.
Single pebbles in chalk.--The general absence of sand and pebbles in the white chalk has been already mentioned; but the occurrence here and there, in the south-east of England, of a few isolated pebbles of quartz and green schist, some of them 2 or 3 inches in diameter, has justly excited much wonder. If these had been carried to the spots where we now find them by waves or currents from the lands once bordering the cretaceous sea, how happened it that no sand or mud were transported thither at the same time? We cannot conceive such rounded stones to have been drifted like erratic blocks by ice[217-A], for that would imply a cold climate in the Cretaceous period; a supposition inconsistent with the luxuriant growth of large chambered univalves, numerous corals, and many fish, and other fossils of tropical forms.
Now in Keeling Island, one of those detached masses of coral which rise up in the wide Pacific, Captain Ross found a single fragment of greenstone, where every other particle of matter was calcareous; and Mr. Darwin concludes that it must have come there entangled in the roots of a large tree. He reminds us that Chamisso, the distinguished naturalist who accompanied Kotzebue, affirms, that the inhabitants of the Radack archipelago, a group of lagoon islands, in the midst of the Pacific, obtained stones for sharpening their instruments by searching the roots of trees which are cast up on the beach.[217-B]
It may perhaps be objected, that a similar mode of transport cannot have happened in the cretaceous sea, because fossil wood is very rare in the chalk. Nevertheless wood is sometimes met with, and in the same parts of the chalk where the pebbles are found, both in soft stone and in a silicified state in flints. In these cases it has often every appearance of having been floated from a distance, being usually perforated by boring-shells, such as the Teredo and Fistulana.[217-C]
The only other mode of transport which suggests itself is sea-weed. Dr. Beck informs me, that in the Lym-Fiord, in Jutland, the Fucus vesiculosus, often called kelp, sometimes grows to the height of 10 feet, and the branches rising from a single root form a cluster several feet in diameter. When the bladders are distended, the plant becomes so buoyant as to float up loose stones several inches in diameter, and these are often thrown by the waves high up on the beach. The Fucus giganteus of Solander, so common in Terra del Fuego, is said by Captain Cook to attain the length of 360 feet, although the stem is not much thicker than a man's thumb. It is often met with floating at sea, with shells attached, several hundred miles from the spots where it grew. Some of these plants, says Mr. Darwin, were found adhering to large loose stones in the inland channels of Terra del Fuego, during the voyage of the Beagle in 1834; and that so firmly, that the stones were drawn up from the bottom into the boat, although so heavy that they could scarcely be lifted in by one person. Some fossil sea-weeds have been found in the Cretaceous formation, but none, as yet, of large size.
But we must not imagine that because pebbles are so rare in the white chalk of England and France there are no proofs of sand, shingle, and clay having been accumulated contemporaneously even in the European seas. The siliceous sandstone, called "upper quader" by the Germans, overlies white argillaceous chalk, or "plaener-kalk," a deposit resembling in composition and organic remains the chalk marl of the English series. This sandstone contains as many fossil shells common to our white chalk as could be expected in a sea-bottom formed of such different materials. It sometimes attains a thickness of 600 feet, and by its jointed structure and vertical precipices, plays a conspicuous part in the picturesque scenery of Saxon Switzerland, near Dresden.
Upper greensand (4. Tab. p. 209.).--The lower chalk without flints passes gradually downwards, in the south of England, into an argillaceous limestone, "the chalk marl," already alluded to, in which ammonites and other cephalopoda, so rare in the higher parts of the series, appear. This marly deposit passes in its turn into beds containing green particles of a chloritic mineral, called the upper greensand. In parts of Surrey calcareous matter is largely intermixed, forming a stone called firestone. In the cliffs of the southern coast of the Isle of Wight, this upper greensand is 100 feet thick, and contains bands of siliceous limestone and calcareous sandstone with nodules of chert.
[2 Illustrations: Fossils of the Upper Greensand.
Fig. 219.
a. Terebratula lyra. } Upper greensand. b. Same, seen in profile. } France.
Fig. 220. Ammonites Rhotomagensis. Upper greensand.]
Gault.--The lowest member of the upper Cretaceous group, usually about 100 feet thick in the S.E. of England, is provincially termed Gault. It consists of a dark blue marl, sometimes intermixed with greensand. Many peculiar forms of cephalopoda, such as the Hamite (fig. 221.) and Scaphite, with other fossils, characterize this formation, which, small as is its thickness, can be traced by its organic remains to distant parts of Europe, as, for example, to the Alps.
The phosphate of lime, found lately near Farnham, in Surrey, in such abundance as to be used largely by the agriculturist for fertilizing soils, occurs exclusively, according to Mr. R. A. C. Austen, in the upper greensand and gault. It is doubtless of animal origin, and partly coprolitic, probably derived from the excrement of fish.
LOWER CRETACEOUS DIVISION. (No. 6. Tab. p. 209.)
That part of the Cretaceous series which is older than the Gault has been commonly called the Lower Greensand. The greater number of its fossils are specifically distinct from those of the upper cretaceous system. Dr. Fitton, to whom we are indebted for an excellent monograph on this formation as developed in England, gives the following as the succession of rocks seen in parts of Kent.
No. 1. Sand, white, yellowish, or ferruginous, with concretions of limestone and chert 70 feet. 2. Sand with green matter 70 to 100 feet. 3. Calcareous stone, called Kentish rag 60 to 80 feet.
In his detailed description of the fine section displayed at Atherfield, in the south of the Isle of Wight, we find the limestone wholly wanting; in fact, the variations in the mineral composition of this group, even in contiguous districts, is very great; and on comparing the Atherfield beds with corresponding strata at Hythe in Kent, distant 95 miles, the whole series has lost half its thickness, and presents a very dissimilar aspect.[219-A]
On the other hand, Professor E. Forbes has shown that when the sixty-three strata at Atherfield are severally examined, the total thickness of which he gives as 843 feet, there are some fossils which range through the whole series, others which are peculiar to particular divisions. As a proof that all belong chronologically to one system, he states that whenever similar conditions are repeated in overlying strata the same species reappear. Changes of depth, or of the mineral nature of the bottom, the presence or absence of lime or of peroxide of iron, the occurrence of a muddy, or a sandy, or a gravelly bottom, are marked by the banishment of certain species and the predominance of others. But these differences of conditions being mineral, chemical, and local in their nature, have nothing to do with the extinction, throughout a large area, of certain animals or plants. The rule laid down by this eminent naturalist for enabling us to test the arrival of a new state of things in the animate world, is the representation by new and different species of corresponding genera of mollusca or other beings. When the forms proper to loose sand or soft clay, or a stony or calcareous bottom, or a moderate or a great depth of water, recur with all the same species, the interval of time has been, geologically speaking, small, however dense the mass of matter accumulated. But if, the genera remaining the same, the species are changed, we have entered upon a new period; and no similarity of climate, or of geographical and local conditions, can then recall the old species which a long series of destructive causes in the animate and inanimate world has gradually annihilated. On passing from the lower greensand to the gault, we suddenly reach one of these new epochs, scarcely any of the fossil species being common to the lower and upper cretaceous systems, a break in the chain implying no doubt many missing links in the series of geological monuments which we may some day be able to supply.
One of the largest and most abundant shells in the lowest strata of the lower greensand, as displayed in the Atherfield section, is the large Perna mulleti of which a reduced figure is here given (fig. 222.).
In the south of England, during the accumulation of the lower greensand above described, the bed of the sea appears to have been continually sinking, from the commencement of the period, when the freshwater Wealden beds were submerged, to the deposition of those strata on which the gault immediately reposes.
Pebbles of quartzose sandstone, jasper, and flinty slate, together with grains of chlorite and mica, speak plainly of the nature of the pre-existing rocks, from the wearing down of which the greensand beds were derived. The land, consisting of such rocks, was doubtless submerged before the origin of the white chalk, as corals can only multiply in the clear waters of the sea in spaces to which no mud or sand are conveyed by currents.
HIPPURITE LIMESTONE.
Difference between the chalk of the north and south of Europe.--By the aid of the three tests of relative age, namely, superposition, mineral character, and fossils, the geologist has been enabled to refer to the same Cretaceous period certain rocks in the north and south of Europe, which differ greatly, both in their fossil contents and in their mineral composition and structure.
If we attempt to trace the cretaceous deposits from England and France to the countries bordering the Mediterranean, we perceive, in the first place, that the chalk and Greensand in the neighbourhood of London and Paris form one great continuous mass, the Straits of Dover being a trifling interruption, a mere valley with chalk cliffs on both sides. We then observe that the main body of the chalk which surrounds Paris stretches from Tours to near Poitiers (see the annexed map, fig. 223., in which the shaded part represents chalk).
Between Poitiers and La Rochelle, the space marked A on the map separates two regions of chalk. This space is occupied by the Oolite and certain other formations older than the Chalk, and has been supposed by M. E. de Beaumont to have formed an island in the cretaceous sea. South of this space we again meet with a formation which we at once recognize by its mineral character to be chalk, although there are some places where the rock becomes oolitic. The fossils are, upon the whole, very similar; especially certain species of the genera Spatangus, Ananchytes, Cidarites, Nucula, Ostrea, Gryphaea (Exogyra), Pecten, Plagiostoma (Lima), Trigonia, Catillus, (Inoceramus), and Terebratula.[221-A] But Ammonites, as M. d'Archiac observes, of which so many species are met with in the chalk of the north of France, are scarcely ever found in the southern region; while the genera Hamite, Turrilite, and Scaphite, and perhaps Belemnite, are entirely wanting.
On the other hand, certain forms are common in the south which are rare or wholly unknown in the north of France. Among these may be mentioned many Hippurites, Sphaerulites, and other members of that great family of mollusca called Rudistes by Lamarck, to which nothing analogous has been discovered in the living creation, but which is quite characteristic of rocks of the Cretaceous era in the south of France, Spain, Sicily, Greece, and other countries bordering the Mediterranean.
The species called Hippurites organisans (fig. 226.) is more abundant than any other in the south of Europe; and the geologist should make himself well acquainted with the cast d, which is far more common in many compact marbles of the upper cretaceous period than the shell itself, which has often wholly disappeared. The flutings, or smooth, rounded, longitudinal ribs, representing the form of the interior, are wholly unlike the hippurite itself, and in some individuals, which attain a great size and length, are very conspicuous.
Between the region of chalk last mentioned in which Perigueux is situated, and the Pyrenees, the space B intervenes. (See Map, p. 221.) Here the tertiary strata cover, and for the most part conceal, the cretaceous rocks, except in some spots where they have been laid open by the denudation of newer formations. In these places they are seen still preserving the form of a white chalky rock, which is charged in part with grains of green sand. Even as far south as Tercis, on the Adour, near Dax, where I examined them in 1828, the cretaceous rocks retain this character. In that region M. Grateloup has found in them Ananchytes ovata (fig. 212.), and other fossils of the English chalk, together with Hippurites.
FLORA OF THE CRETACEOUS PERIOD.
Although the fossil plants of the Cretaceous era at present known are few in number, the rocks being principally marine, they suffice, according to M. Ad. Brongniart, to show a transition character between the vegetation of the secondary and that of the tertiary formations. The tertiary strata, when compared to the older rocks, are marked by the predominance of Exogens, which now constitute three-fourths of the living plants of the globe.[223-A]
These exogens are wanting in the secondary strata generally, but in the Cretaceous period they equal in number the Gymnogens (Coniferae and Cycadeae) which abounded so much in the preceding Oolitic period, and disappeared before the Eocene rocks were formed.[223-B] The discovery of a tree-fern in the ferruginous sands of the Lower Cretaceous group of the department of Ardennes in France is one of many signs of the contrast of the flora, and doubtless of the climate, of this era with that of the Pliocene and Modern periods.
CRETACEOUS ROCKS IN THE UNITED STATES.
If we pass to the American continent, we find in the state of New Jersey a series of sandy and argillaceous beds wholly unlike our Upper Cretaceous system; which we can, nevertheless, recognize as referable, paleontologically, to the same division.
That they were about the same age generally as the European chalk and greensand, was the conclusion to which Dr. Morton and Mr. Conrad came after their investigation of the fossils in 1834. The strata consist chiefly of greensand and green marl, with an overlying coralline limestone of a pale yellow colour, and the fossils, on the whole, agree most nearly with those of the upper European series, from the Maestricht beds to the gault inclusive. I collected sixty shells from the New Jersey deposits in 1841; five of which were identical with European species--Ostrea larva, O. vesicularis, Gryphaea costata, Pecten quinque-costatus, Belemnites mucronatus. As some of these have the greatest vertical range in Europe, they might be expected more than any others to recur in distant parts of the globe. Even where the species are different, the generic forms, such as the Baculite and certain sections of Ammonites, as also the Inoceramus (see above, fig. 208.) and other bivalves, have a decidedly cretaceous aspect. Fifteen out of the sixty shells above alluded to, were regarded by Professor Forbes as good geographical representatives of well-known cretaceous fossils of Europe. The correspondence, therefore, is not small, when we reflect that the part of the United States where these strata occur is between 3000 and 4000 miles distant from the chalk of Central and Northern Europe, and that there is a difference of ten degrees in the latitude of the places compared on opposite sides of the Atlantic.[224-A]
Fish of the genera Lamna, Galeus, and Carcharias are common to New Jersey and the European cretaceous rocks. So also is the genus Mosasaurus among reptiles, and Pliosaurus (Owen), another saurian likewise obtained from the English chalk. From New Jersey the cretaceous formation extends southwards to North Carolina, Georgia, and Alabama, cropping out at intervals from beneath the tertiary strata, between the Appalachian Mountains and the Atlantic. They then sweep round the southern extremity of that chain, and stretch northwards again to Tennessee and Kentucky. They have also been traced far up the valley of the Missouri 275 English miles above its mouth, to the neighbourhood of Fort Leavenworth; and southwards to Texas, according to the observations of Ferdinand Roemer; so that already the area which they are ascertained to occupy in North America may perhaps equal their extent in Europe. So little do they resemble mineralogically the European white chalk, that limestone in North America is, upon the whole, an exception to the rule; and, even in Alabama, where I saw a calcareous member of this group, the marlstones are much more like the English and French Lias than any other secondary deposit of the Old World.
At the base of the system in Alabama I found dense masses of shingle, perfectly loose and unconsolidated, derived from the waste of paleozoic (or carboniferous) rocks, a mass in no way distinguishable, except by its position, from ordinary alluvium, but covered with marls abounding in Inocerami.
In Texas, according to F. Roemer, the chalk assumes a new lithological type, a large portion of it consisting of hard siliceous limestone, but the organic remains leaving no doubt in regard to its age.
In South America the cretaceous strata have been discovered in Columbia, as at Bogota and elsewhere, containing Ammonites, Hamites, Inocerami, and other characteristic shells.[225-A]
In the South of India, also, at Pondicherry, Verdachellum, and Trinconopoly, Messrs. Kaye and Egerton have collected fossils belonging to the cretaceous system. Taken in connection with those from the United States they prove, says Prof. E. Forbes, that those powerful causes which stamped a peculiar character on the forms of marine animal life at this period, exerted their full intensity through the Indian, European, and American seas.[225-B] Here, as in North and South America, the cretaceous character can be recognized even where there is no specific identity in the fossils; and the same may be said of the organic type of those rocks in Europe and India which succeed next in the ascending and descending order, the Eocene and the Oolitic.
FOOTNOTES:
[209-A] M. Alcide d'Orbigny, in his valuable work entitled Paleontologie Francaise, has adopted new terms for the French subdivisions of the Cretaceous Series, which, so far as they can be made to tally with English equivalents, seem explicable thus:
Danien. Maestricht beds. Senonien. Upper and lower white chalk, and chalk marl. Turonien. Part of the chalk marl and the upper greensand, the latter being in his last work (Cours Elementaire) termed Cenomanien. Albien. Gault. Aptien. Upper part of lower greensand. Neocomien. Lower part of same.
[211-A] See paper by the author, Trans. of Geol. Soc., vol. v. p. 246., 1840.
[211-B] Fitton, Geol. Trans., 2d series, vol. iv. p. 319.
[215-A] Proceedings of Geol. Soc., vol. iii. pp. 7, 8., 1842.
[216-A] Geol. Trans. Second Series, vol. iii. p. 232. plate 31. figs. 3. and 11.
[216-B] Geol. of U. S. Exploring Exped. p. 252. 1849.
[217-A] See Chapters X. and XI.
[217-B] Darwin, p. 549. Kotzebue's First Voyage, vol. iii. p. 155.
[217-C] Mantell, Geol. of S. E. of England, p. 96.
[219-A] Dr. Fitton, Quart. Geol. Journ., vol. i. p. 179., ii. p. 55., and iii. p. 289., where comparative sections and a valuable table showing the vertical range of the various fossils of the lower greensand at Atherfield is given.
[221-A] Archiac, sur la Form. Cretacee du S. O. de la France, Mem. de la Soc. Geol. de France, tom. ii.
[222-A] D'Orbigny's Paleontologie Francaise, pl. 533.
[223-A] In this and subsequent remarks on fossil plants I shall often use Dr. Lindley's terms, as most familiar in this country; but as those of M. A. Brongniart are much cited, it may be useful to geologists to give a table explaining the corresponding names of groups so much spoken of in palaeontology.
| Brongniart. | | |Lindley. | | | Examples. Cryptogamic. | |1. Cryptogamous amphigens, or cellular cryptogamic. | | |Thallogens. | | | |Lichens, sea-weeds, fungi. | |2. Cryptogamous acrogens. | | |Acrogens. | | | |Mosses, equisetums, ferns, | | | |lycopodiums--Lepidodendron. | | | | Phanerogamic. | |3. Dicotyledonous gymnosperms. | | |Gymnogens. | | | |Conifers and Cycads. | |4. Dicot. Angiosperms. | | |Exogens. | | | |Compositae, leguminosae, umbelliferae, | | | |cruciferae, heaths, &c. All native | | | |European trees except conifers. | |5. Monocotyledons. | | |Endogens. | | | |Palms, lilies, aloes, rushes, | | | |grasses, &c.
[223-B] A. Brongniart, Veget. Foss. Dict. Univ., p. 111., 1849.
[224-A] See a paper by the author, Quart. Journ. Geol. Soc., vol. i. p. 55.
[225-A] Proceed. Geol. Soc. iv. p. 391.
[225-B] See Forbes, Quart. Geol. Journ. vol. i. p. 79.
XVIII. WEALDEN GROUP
The Wealden divisible into Weald Clay, Hastings Sand, and Purbeck Beds--Intercalated between two marine formations--Weald clay and Cypris-bearing strata--Iguanodon--Hastings sands--Fossil fish--Strata formed in shallow water--Brackish water-beds--Upper, middle, and lower Purbeck--Alternations of brackish water, freshwater, and land--Dirt-bed, or ancient soil--Distinct species of fossils in each subdivision of the Wealden--Lapse of time implied--Plants and insects of Wealden--Geographical extent of Wealden--Its relation to the cretaceous and oolitic periods--Movements in the earth's crust to which it owed its origin and submergence.
Beneath the cretaceous rocks in the S.E. of England, a freshwater formation is found, called the Wealden (see Nos. 5. and 6. Map, p. 242.), which, although it occupies a small horizontal area in Europe, as compared to the chalk, is nevertheless of great geological interest, not only from its position, as being interpolated between two great marine formations (Nos. 7. and 9. Table, p. 103.), but also because the imbedded fossils indicate a grand succession of changes in organic life, effected during its accumulation. It is composed of three minor divisions, the Weald Clay, the Hastings, and the Purbeck Beds, of which the aggregate thickness in some districts may be 700 or 800 feet; but which would be much more considerable (perhaps 2000 feet), were we to add together the extreme thickness acquired by each of them in their fullest development.
The common name of Wealden was given to the whole, because it was first studied in parts of Kent, Surrey, and Sussex, called the Weald, (see Map, p. 242.), and we are indebted to Dr. Mantell for having shown in 1822, in his Geology of Sussex, that the whole group was of fluviatile origin. In proof of this he called attention to the entire absence of Ammonites, Belemnites, Terebratulae, Echinites, Corals, and other marine fossils, so characteristic of the cretaceous rocks above, and of the Oolitic strata below, and to the presence of Paludinae, Melaniae, and various fluviatile shells, as well as the bones of terrestrial reptiles and the trunks and leaves of land plants.
The evidence of so unexpected a fact as the infra-position of a dense mass of purely freshwater origin to a deep-sea deposit (a phenomenon with which we have since become familiar, in other chapters of the earth's autobiography), was received, at first, with no small doubt and incredulity. But the relative position of the beds is unequivocal; the Weald Clay being distinctly seen to pass beneath the Greensand in various parts of Surrey, Kent, and Sussex; and if we proceed from Sussex westward to the Vale of Wardour, we there again observe the same formation, or, at least, the lower division of it, the Purbeck, occupying the same relative position, and resting on the Oolite (see fig. 228.). Or if we pass from the base of the South Downs in Sussex, and cross to the Isle of Wight, we there again meet with the Wealden series reappearing beneath the Greensand, and cannot doubt that the beds are prolonged subterraneously, as indicated by the dotted lines in fig. 229.
The minor groups into which the Wealden has been commonly divided in England are, as before stated, three, and they succeed each other in the following descending order[227-A]:--
Thickness. 1st. Weald Clay, sometimes including thin beds of sand and shelly limestone 140 to 280 ft. 2d. Hastings Sand, in which occur some clays and calcareous grits 400 to 500 ft. 3d. Purbeck Beds, consisting of various kinds of limestones and marls 150 to 200 ft.
Weald Clay.
The first division, or Weald Clay, is of purely freshwater origin. The uppermost beds are not only conformable, as Dr. Fitton observes, to the inferior strata of the Lower Greensand, but of similar mineral composition. To explain this, we may suppose, that as the delta of a great river was tranquilly subsiding, so as to allow the sea to encroach upon the space previously occupied by freshwater, the river still continued to carry down the same sediment into the sea. In confirmation of this view it may be stated, that the remains of the Iguanodon Mantelli, a gigantic terrestrial reptile, very characteristic of the Wealden, has been discovered near Maidstone, in the overlying Kentish rag, or marine limestone of the Lower Greensand. Hence we may infer that some of the saurians which inhabited the country of the great river continued to live when part of the country had become submerged beneath the sea. Thus, in our own times, we may suppose the bones of large alligators to be frequently entombed in recent freshwater strata in the delta of the Ganges. But if part of that delta should sink down so as to be covered by the sea, marine formations might begin to accumulate in the same space where freshwater beds had previously been formed; and yet the Ganges might still pour down its turbid waters in the same direction, and carry seaward the carcasses of the same species of alligator, in which case their bones might be included in marine as well as in subjacent freshwater strata.
The Iguanodon, first discovered by Dr. Mantell, has left more of its remains in the Wealden strata of the south-eastern counties, and Isle of Wight, than any other genus of associated saurians. It was an herbivorous reptile, and regarded by Cuvier as more extraordinary than any with which he was acquainted; for the teeth, though bearing a great analogy to the modern Iguanas which now frequent the tropical woods of America and the West Indies, exhibit many striking and important differences (see fig. 230.). It appears that they have been worn by mastication; whereas the existing herbivorous reptiles clip and gnaw off the vegetable productions on which they feed, but do not chew them. Their teeth, when worn, present an appearance of having been chipped off, and never, like the fossil teeth of the Iguanodon, have a flat ground surface (see fig. 231.), resembling the grinders of herbivorous mammalia. Dr. Mantell computes that the teeth and bones of this animal which have passed under his examination during the last twenty years, must have belonged to no less than seventy-one distinct individuals; varying in age and magnitude from the reptile just burst from the egg, to one of which the femur measured 24 inches in circumference. Yet notwithstanding that the teeth were more numerous than any other bones, it is remarkable that it was not till the relics of all these individuals had been found, that a solitary example of part of a jaw-bone was obtained. More recently remains both of the upper and lower jaw have been met with in the Hastings Beds in Tilgate Forest. Their size was somewhat greater than had been anticipated, and even allowing that the tail was short, which Professor Owen infers from the short bodies of the caudal vertebrae, Dr. Mantell estimates the probable length of some of these saurians at between 30 and 40 feet. The largest femur yet found measures 4 feet 8 inches in length, the circumference of the shaft being 25 inches, and round the condyles 42 inches.
[2 Illustrations: Teeth of Iguanodon.
Fig. 230. Partially worn tooth of a young animal. (Mantell.)
Fig. 231. Crown of tooth in adult, worn down. (Mantell.)]
Occasionally bands of limestone, called Sussex Marble, occur in the Weald Clay, almost entirely composed of a species of Paludina, closely resembling the common P. vivipara of English rivers.
Shells of the Cypris, an animal belonging to the Crustacea, and before mentioned (p. 31.) as abounding in lakes and ponds, are also plentifully scattered through the clays of the Wealden, sometimes producing, like the plates of mica, a thin lamination (see fig. 235.). Similar cypriferous marls are found in the lacustrine tertiary beds of Auvergne (see above, p. 183.).
Hastings Sands.
This middle division of the Wealden consists of sand, calciferous grit, clay, and shale; the argillaceous strata, notwithstanding the name, being nearly in the same proportion as the arenaceous. The calcareous sandstone and grit of Tilgate Forest, near Cuckfield, in which the remains of the Iguanodon and Hyleosaurus were first found, constitute an upper member of this formation. The white "sand-rock" of the Hastings cliffs, about 100 feet thick, is one of the lower members of the same. The reptiles, which are very abundant in it, consist partly of saurians, already referred by Owen and Mantell to eight genera, among which, besides those already enumerated, we find the Megalosaurus and Plesiosaurus. The Pterodactyl, also a flying reptile, is met with in the same strata, and many remains of Testudinata of the genera Trionyx and Emys, now confined to tropical regions.
The fishes of the Wealden belong partly to the genera Pycnodus and Hybodus (see figure of genus in Chap. XXI.), forms common to the Wealden and Oolite; but the teeth and scales of a species of Lepidotus are most widely diffused (see fig. 236.). The general form of these fish was that of the carp tribe, although perfectly distinct in anatomical character, and more allied to the pike. The whole body was covered with large rhomboidal scales, very thick, and having the exposed part covered with enamel. Most of the species of this genus are supposed to have been either river fish, or inhabitants of the coasts, having not sufficient powers of swimming to advance into the deep sea.
The shells of the Hastings beds belong to the genera Melanopsis, Melania, Paludina, Cyrena, Cyclas, Unio, and others, which inhabit rivers or lakes; but one band has been found in Dorsetshire indicating a brackish state of the water, and, in some places, even a saltness, like that of the sea, where the genera Corbula (see fig. 237.), Mytilus, and Ostrea occur. At different heights in the Hastings Sand, in the middle of the Wealden, we find again and again slabs of sandstone with a strong ripple-mark, and between these slabs beds of clay many yards thick. In some places, as at Stammerham, near Horsham, there are indications of this clay having been exposed so as to dry and crack before the next layer was thrown down upon it. The open cracks in the clay have served as moulds, of which casts have been taken in relief, and which are, therefore, seen on the lower surface of the sandstone (see fig. 238.).
Near the same place a reddish sandstone occurs in which are innumerable traces of a fossil vegetable, apparently Sphenopteris, the stems and branches of which are disposed as if the plants were standing erect on the spot where they originally grew, the sand having been gently deposited upon and around them; and similar appearances have been remarked in other places in this formation.[230-A] In the same division also of the Wealden, at Cuckfield, is a bed of gravel or conglomerate, consisting of water-worn pebbles of quartz and jasper, with rolled bones of reptiles. These must have been drifted by a current, probably in water of no great depth.
From such facts we may infer that, notwithstanding the great thickness of this division of the Wealden (and the same observation applies to the Weald Clay and Purbeck Beds), the whole of it was a deposit in water of a moderate depth, and often extremely shallow. This idea may seem startling at first, yet such would be the natural consequence of a gradual and continuous sinking of the ground in an estuary or bay, into which a great river discharged its turbid waters. By each foot of subsidence, the fundamental rock, such as the Portland Oolite, would be depressed one foot farther from the surface; but the bay would not be deepened, if newly deposited mud and sand should raise the bottom one foot. On the contrary, such new strata of sand and mud might be frequently laid dry at low water, or overgrown for a season by a vegetation proper to marshes.
Purbeck Beds.
Immediately below the Hastings Sands we find a series of calcareous slates, marls, and limestones, called the Purbeck Beds, because well exposed to view in the sea-cliffs of the Peninsula of Purbeck, especially in Durlestone Bay, near Swanage. They may also be advantageously studied at Lulworth Cove and the neighbouring bays between Weymouth and Dorchester. At Meup's Bay in particular, Prof. E. Forbes has recently examined minutely the organic remains of the three members of the Purbeck group, displayed there in a vertical section 155 feet thick. To the information previously supplied in the works of Messrs. Webster, Fitton, De la Beche, Buckland, and Mantell, he has made most ample and important additions, so that it will be desirable to give them at some length, it appearing that the Upper, Middle, and Lower Purbecks are each marked by peculiar species of organic remains, these again being different, so far as a comparison has yet been instituted, from the fossils of the overlying Hastings Sands and Weald Clay. This result cannot fail to excite much wonder, and it leads us to suspect that the Wealden period, which many geologists have scarcely deigned to notice in their classification, may comprehend the history of a lapse of time as great as that of the Oolitic or Cretaceous eras respectively.[231-A]
Upper Purbeck.--The highest of the three divisions is purely freshwater, the strata, about 50 feet in thickness, containing shells of the genera Paludina, Physa, Lymnea, Planorbis, Valvata, Cyclas, and Unio, with cyprides, and fish.
Middle Purbeck.--To these succeed the Middle Purbeck, about 30 feet thick, the uppermost part of which consists of freshwater limestone, with cyprides, turtles, and fish of different species from those in the preceding strata. Below the limestone are brackish-water beds full of Cyrena, and traversed by bands abounding in Corvulae and Melaniae. These are based on a purely marine deposit, with Pecten, Modiola, Avicula, and Thracia, all undescribed shells. Below this, again, come limestones and shales, partly of brackish and partly of freshwater origin, in which many fish, especially species of Lepidotus and Microdon radiatus, are found, and a reptile named Macrorhyncus. Among the mollusks, a remarkable ribbed Melania, of the section Chilira, occurs.
Immediately below is the great and conspicuous stratum, 12 feet thick, long familiar to geologists under the local name of "Cinder-bed," formed of a vast accumulation of shells of Ostrea distorta (fig. 240.). In the uppermost part of this bed Mr. Forbes discovered the first echinoderm as yet known in the Purbeck series, a species of Hemicidaris, a genus characteristic of the Oolitic period. It was accompanied by a species of Perna. Below the Cinder-bed freshwater strata are again seen, filled in many places with species of Cypris, Valvata, Paludina, Planorbis, Lymnea, Physa, and Cyclas, all different from any we had previously seen above. Thick siliceous beds of chert, filled with these fossils, occur in a beautiful state of preservation, often converted into chalcedony. Among these Mr. Forbes met with gyrogonites (the spore vesicles of Charae), plants never before discovered in rocks older than the Eocene. Again, beneath these freshwater strata, a very thin band of greenish shales, with marine shells and impressions of leaves, like those of a large Zostera, succeeds, forming the base of the Middle Purbeck.
Lower Purbeck.--Beneath the thin marine band last mentioned, purely freshwater marls occur, containing species of Cypris, Valvata, and Lymnea, different from those of the Middle Purbeck. This is the beginning of the Inferior division, which is about 80 feet thick. Below the marls are seen more than 30 feet of brackish-water beds, at Meup's Bay, abounding in a species of Serpula, allied to, if not identical with, Serpula coacervites, found in the Wealden of Hanover. There are also shells of the genus Rissoa (of the subgenus Hydrobia), and a little Cardium of the subgenus Protocardium, in the same beds, together with Cypris. Some of the cypris-bearing shales are strangely contorted and broken up, at the west end of the Isle of Purbeck. The great dirt-bed or vegetable soil containing the roots and stools of Cycadeae, which I shall presently describe, underlies these marls, resting upon the lowest freshwater limestone, a rock about 8 feet thick, containing Cyclades, Valvata, and Lymnea, of the same species as those of the uppermost part of the Lower Purbeck. This rock rests upon the top beds of the Portland stone, which is purely marine, and between which and the Purbecks there is no passage.
The most remarkable of all the varied successions of beds enumerated in the above list, is that called by the quarrymen "the dirt," or "black dirt," which was evidently an ancient vegetable soil. It is from 12 to 18 inches thick, is of a dark brown or black colour, and contains a large proportion of earthy lignite. Through it are dispersed rounded fragments of stone, from 3 to 9 inches in diameter, in such numbers that it almost deserves the name of gravel. Many silicified trunks of coniferous trees, and the remains of plants allied to Zamia and Cycas, are buried in this dirt-bed (see figure of living Zamia, fig. 241.).
These plants must have become fossil on the spots where they grew. The stumps of the trees stand erect for a height of from 1 to 3 feet, and even in one instance to 6 feet, with their roots attached to the soil at about the same distances from one another as the trees in a modern forest.[233-A] The carbonaceous matter is most abundant immediately around the stumps, and round the remains of fossil Cycadeae.[233-B]
Besides the upright stumps above mentioned, the dirt-bed contains the stems of silicified trees laid prostrate. These are partly sunk into the black earth, and partly enveloped by a calcareous slate which covers the dirt-bed. The fragments of the prostrate trees are rarely more than 3 or 4 feet in length; but by joining many of them together, trunks have been restored, having a length from the root to the branches of from 20 to 23 feet, the stems being undivided for 17 or 20 feet, and then forked. The diameter of these near the roots is about 1 foot.[233-D] Root-shaped cavities were observed by Professor Henslow to descend from the bottom of the dirt-bed into the subjacent freshwater stone, which, though now solid, must have been in a soft and penetrable state when the trees grew.[233-E]
The thin layers of calcareous slate (fig. 242.) were evidently deposited tranquilly, and would have been horizontal but for the protrusion of the stumps of the trees, around the top of each of which they form hemispherical concretions.
The dirt-bed is by no means confined to the island of Portland, where it has been most carefully studied, but is seen in the same relative position in the cliffs east of Lulworth Cove, in Dorsetshire, where, as the strata have been disturbed, and are now inclined at an angle of 45 deg., the stumps of the trees are also inclined at the same angle in an opposite direction--a beautiful illustration of a change in the position of beds originally horizontal (see fig. 243.). Traces of the dirt-bed have also been observed by Dr. Buckland, about two miles north of Thame, in Oxfordshire; and by Dr. Fitton, in the cliffs of the Boulonnois, on the French coast; but, as might be expected, this freshwater deposit is of limited extent when compared to most marine formations.
From the facts above described, we may infer, first, that the superior beds of the Oolite, called "the Portland," which are full of marine shells, were overspread with fluviatile mud, which became dry land, and covered by a forest, throughout a portion of the space now occupied by the south of England, the climate being such as to admit the growth of the Zamia and Cycas. 2dly. This land at length sank down and was submerged with its forests beneath a body of fresh water, from which sediment was thrown down enveloping fluviatile shells. 3dly. The regular and uniform preservation of this thin bed of black earth over a distance of many miles, shows that the change from dry land to the state of a freshwater lake or estuary, was not accompanied by any violent denudation, or rush of water, since the loose black earth, together with the trees which lay prostrate on its surface, must inevitably have been swept away had any such violent catastrophe then taken place.
The dirt-bed has been described above in its most simple form, but in some sections the appearances are more complicated. The forest of the dirt-bed was not everywhere the first vegetation which grew in this region. Two other beds of carbonaceous clay, one of them containing Cycadeae, in an upright position, have been found below it, and one above it[234-A], which implies other oscillations in the level of the same ground, and its alternate occupation by land and water more than once.
Table showing the changes of medium in which the strata were formed, from the Lower Greensand to the Portland Stone inclusive, in the south-east of England.
1. Marine Lower greensand.
2. Freshwater Weald clay.
3. Freshwater } Brackish } Hastings sand. Freshwater }
4. Freshwater Upper Purbeck.
5. Freshwater } Brackish } Marine } Brackish } Middle Purbeck. Marine } Freshwater } Marine }
6. Freshwater } Brackish } Land } Freshwater } Land (dirt-bed) } Lower Purbeck. Freshwater } Land } Freshwater } Land } Freshwater }
7. Marine Portland stone.
The annexed tabular view will enable the reader to take in at a glance the successive changes from sea to river, and from river to sea, or from these again to a state of land, which have occurred in this part of England between the Cretaceous and Oolitic periods. That there have been at least four changes in the species of testacea during the deposition of the Wealden, seems to follow from the observations recently made by Professor E. Forbes, so that, should we hereafter find the signs of many more alternate occupations of the same area by different elements, it is no more than we might expect. Even during a small part of a zoological period, not sufficient to allow time for many species to die out, we find that the same area has been laid dry, and then submerged, and then again laid dry, as in the deltas of the Po and Ganges, the history of which has been brought to light by Artesian borings.[235-A] We also know that similar revolutions have occurred within the present century (1819) in the delta of the Indus in Cutch[235-B], where land has been laid permanently under the waters both of the river and sea, without its soil or shrubs having been swept away. Even, independently of any vertical movements of the ground, we see in the principal deltas, such as that of the Mississippi, that the sea extends its salt waters annually for many months over considerable spaces, which, at other seasons, are occupied by the river during its inundations.
It will be observed that the division of the Purbecks into upper, middle, and lower, has been made by Professor E. Forbes, strictly on the principle of the entire distinctness of the species of organic remains which they include. The lines of demarcation are not lines of disturbance, nor indicated by any striking physical characters or mineral changes. The features which attract the eye in the Purbecks, such as the dirt-beds, the dislocated strata at Lulworth, and the Cinder-bed, do not indicate any breaks in the distribution of organized beings. "The causes which led to a complete change of life three times during the deposition of the freshwater and brackish strata must," says this naturalist, "be sought for, not simply in either a rapid or a sudden change of their area into land or sea, but in the great lapse of time which intervened between the epochs of deposition at certain periods during their formation."
Each dirt-bed may, no doubt, be the memorial of many thousand years or centuries, because we find that 2 or 3 feet of vegetable soil is the only monument which many a tropical forest has left of its existence ever since the ground on which it now stands was first covered with its shade. Yet, even if we imagined the fossil soils of the Lower Purbeck to represent as many ages, we need not expect on that account to find them constituting the lines of separation between successive strata characterized by different zoological types. The preservation of a layer of vegetable soil, when in the act of being submerged, must be regarded as a rare exception to a general rule. It is of so perishable a nature, that it must usually be carried away by the denuding waves or currents of the sea or by a river; and many dirt-beds were probably formed in succession, and annihilated in the Wealden, besides those few which now remain.
The plants of the Wealden, so far as our knowledge extends at present, consist chiefly of Ferns, Coniferae (see fig. 244.), and Cycadeae, without any exogens; the whole more allied to the Oolitic than to the Cretaceous vegetation, although some of the species seem to be common to the chalk. But the vertebrate and invertebrate animals indicate, in like manner, a relationship to both these periods, though a nearer affinity to the Oolitic. Mr. Brodie has found the remains of beetles and several insects of the homopterous and trichopterous orders, some of which now live on plants, like those of the Wealden, while others hover over the surface of our present rivers. But no bones of mammalia have been met with among those of land-reptiles. Yet, as the reader will learn, in Chapter XX., that the relics of marsupial quadrupeds have been detected in still older beds, and, as it was so long before a single portion of the jaw of an iguanodon was met with in the Tilgate quarries (see p. 228.), we need by no means despair of discovering hereafter some evidence of the existence of warm-blooded quadrupeds at this era. It is, at least, too soon to infer, on mere negative evidence, that the mammalia were foreign to this fauna.
In regard to the geographical extent of the Wealden, it cannot be accurately laid down; because so much of it is concealed beneath the newer marine formations. It has been traced about 200 English miles from west to east, from Lulworth Cove to near Boulogne, in France; and about 220 miles from north-west to south-east, from Whitchurch, in Buckinghamshire, to Beauvais, in France. If the formation be continuous throughout this space, which is very doubtful, it does not follow that the whole was contemporaneous; because, in all likelihood, the physical geography of the region underwent frequent change throughout the whole period, and the estuary may have altered its form, and even shifted its place. Dr. Dunker, of Cassel, and H. Von Meyer, in an excellent monograph on the Wealdens of Hanover and Westphalia, have shown that they correspond so closely, not only in their fossils, but also in their mineral characters, with the English series, that we can scarcely hesitate to refer the whole to one great delta. Even then, the magnitude of the deposit may not exceed that of many modern rivers. Thus, the delta of the Quorra or Niger, in Africa, stretches into the interior for more than 170 miles, and occupies, it is supposed, a space of more than 300 miles along the coast, thus forming a surface of more than 25,000 square miles, or equal to about one half of England.[237-A] Besides, we know not, in such cases, how far the fluviatile sediment and organic remains of the river and the land may be carried out from the coast, and spread over the bed of the sea. I have shown, when treating of the Mississippi, that a more ancient delta, including species of shells, such as now inhabit Louisiana, has been upraised, and made to occupy a wide geographical area, while a newer delta is forming[237-B]; and the possibility of such movements, and their effects, must not be lost sight of when we speculate on the origin of the Wealden.
If it be asked where the continent was placed from the ruins of which the Wealden strata were derived, and by the drainage of which a great river was fed, we are half tempted to speculate on the former existence of the Atlantis of Plato. The story of the submergence of an ancient continent, however fabulous in history, must have been true again and again as a geological event.
The real difficulty consists in the persistence of a large hydrographical basin, from whence a great body of fresh water was poured into the sea, precisely at a period when the neighbouring area of the Wealden was gradually going downwards 1000 feet or more perpendicularly. If the adjoining land participated in the movement, how could it escape being submerged, or how could it retain its size and altitude so as to continue to be the source of such an inexhaustible supply of fresh water and sediment? In answer to this question, we are fairly entitled to suggest that the neighbouring land may have been stationary, or may even have undergone a contemporaneous slow upheaval. There may have been an ascending movement in one region, and a descending one in a contiguous parallel zone of country; just as the northern part of Scandinavia is now rising, while the middle portion (that south of Stockholm) is unmoved, and the southern extremity in Scania is sinking, or at least has sunk within the historical period.[237-C] We must, nevertheless, conclude, if we adopt the above hypothesis, that the depression of the land became general throughout a large part of Europe at the close of the Wealden period, a subsidence which brought in the cretaceous ocean.
FOOTNOTES:
[227-A] Dr. Fitton, Geol. Trans. vol. iv. p. 320. Second Series.
[230-A] Mantell, Geol. of S. E. of England, p. 244.
[231-A] "On the Dorsetshire Purbecks," by Prof. E. Forbes, Edinb. Brit. Assoc., Aug. 1850.
[233-A] Mr. Webster first noticed the erect position of the trees and described the Dirt-bed.
[233-B] Fitton, Geol. Trans., Second Series, vol. iv. pp. 220, 221.
[233-C] See Flinders' Voyage.
[233-D] Fitton, ibid.
[233-E] Buckland and De la Beche, Geol. Trans., Second Series, vol. iv. p. 16. Mr. Forbes has ascertained that the subjacent rock is a freshwater limestone, and not a portion of the Portland oolite, as was previously imagined.
[234-A] E. Forbes, ibid.
[235-A] See Principles of Geol., 8th ed. pp. 260-268.
[235-B] Ibid. p. 443.
[237-A] Fitton, Geol. of Hastings, p. 58.; who cites Lander's Travels.
[237-B] See above, p. 85.; and Second Visit to the U. S. vol. ii. chap. xxxiv.
[237-C] See the Author's Anniv. Address, Geol. Soc. 1850, Quart. Geol. Journ. vol. vi. p. 52.
XIX. DENUDATION OF THE CHALK AND WEALDEN
Physical geography of certain districts composed of Cretaceous and Wealden strata--Lines of inland chalk-cliffs on the Seine in Normandy--Outstanding pillars and needles of chalk--Denudation of the chalk and Wealden in Surrey, Kent, and Sussex--Chalk once continuous from the North to the South Downs--Anticlinal axis and parallel ridges--Longitudinal and transverse valleys--Chalk escarpments--Rise and denudation of the strata gradual--Ridges formed by harder, valleys by softer beds--Why no alluvium, or wreck of the chalk, in the central district of the Weald--At what periods the Weald valley was denuded--Land has most prevailed where denudation has been greatest--Elephant bed, Brighton.
All the fossiliferous formations may be studied by the geologist in two distinct points of view: first, in reference to their position in the series, their mineral character and fossils; and, secondly, in regard to their physical geography, or the manner in which they now enter, as mineral masses, into the external structure of the earth; forming the bed of lakes and seas, or the surface and foundation of hills and valleys, plains and table-lands. Some account has already been given on the first head of the Tertiary, the Cretaceous, and Wealden strata; and we may now proceed to consider certain features in the physical geography of these groups as they occur in parts of England and France.
The hills composed of white chalk in the S.E. of England have a smooth rounded outline, and being usually in the state of sheep pastures, are free from trees or hedgerows; so that we have an opportunity of observing how the valleys by which they are drained ramify in all directions, and become wider and deeper as they descend. Although these valleys are now for the most part dry, except during heavy rains and the melting of snow, they may have been due to aqueous denudation, as explained in the sixth chapter; having been excavated when the chalk emerged gradually from the sea. This opinion is confirmed by the occasional occurrence of long lines of inland cliffs, in which the strata are cut off abruptly in steep and often vertical precipices. The true nature of such escarpments is nowhere more obvious than in parts of Normandy, where the river Seine and its tributaries flow through deep winding valleys, hollowed out of chalk horizontally stratified. Thus, for example, if we follow the Seine for a distance of about 30 miles from Andelys to Elboeuf, we find the valley flanked on both sides by a deep slope of chalk, with numerous beds of flint, the formation being laid open for a thickness of about 250 and 300 feet. Above the chalk is an overlying mass of sand, gravel, and clay, from 30 to 100 feet thick. The two opposite slopes of the hills a and b, where the chalk appears at the surface, are from 2 to 4 miles apart, and they are often perfectly smooth and even, like the steepest of our downs in England; but at many points they are broken by one, two, or more ranges of vertical and even overhanging cliffs of bare white chalk with flints. At some points detached needles and pinnacles stand in the line of the cliffs, or in front of them, as at c, fig. 245. On the right bank of the Seine, at Andelys, one range, about 2 miles long, is seen varying from 50 to 100 feet in perpendicular height, and having its continuity broken by a number of dry valleys or coombs, in one of which occurs a detached rock or needle, called the Tete d'Homme (see figs. 246, 247.). The top of this rock presents a precipitous face towards every point of the compass; its vertical height being more than 20 feet on the side of the downs, and 40 towards the Seine, the average diameter of the pillar being 36 feet. Its composition is the same as that of the larger cliffs in its neighbourhood, namely, white chalk, having occasionally a crystalline texture like marble, with layers of flint in nodules and tabular masses. The flinty beds often project in relief 4 or 5 feet beyond the white chalk, which is generally in a state of slow decomposition, either exfoliating or being covered with white powder, like the chalk cliffs on the English coast; and, as in them, this superficial powder contains in some places common salt.
Other cliffs are situated on the right bank of the Seine, opposite Tournedos, between Andelys and Pont de l'Arche, where the precipices are from 50 to 80 feet high: several of their summits terminate in pinnacles; and one of them, in particular, is so completely detached as to present a perpendicular face 50 feet high towards the sloping down. On these cliffs several ledges are seen, which mark so many levels at which the waves of the sea may be supposed to have encroached for a long period. At a still greater height, immediately above the top of this range, are three much smaller cliffs, each about 4 feet high, with as many intervening terraces, which are continued so as to sweep in a semicircular form round an adjoining coomb, like those in Sicily before described (p. 76.).
If we then descend the river from Vatteville to a place called Senneville, we meet with a singular needle about 50 feet high, perfectly isolated on the escarpment of chalk on the right bank of the Seine (see fig. 248.). Another conspicuous range of inland cliffs is situated about 12 miles below on the left bank of the Seine, beginning at Elboeuf, and comprehending the Roches d'Orival (see fig. 249.). Like those before described, it has an irregular surface, often overhanging, and with beds of flint projecting several feet. Like them, also, it exhibits a white powdery surface, and consists entirely of horizontal chalk with flints. It is 40 miles inland, its height, in some parts, exceeding 200 feet, and its base only a few feet above the level of the Seine. It is broken, in one place, by a pyramidal mass or needle, 200 feet high, called the Roche de Pignon, which stands out about 25 feet in front of the upper portion of the main cliffs, with which it is united by a narrow ridge about 40 feet lower than its summit (see fig. 250.). Like the detached rocks before mentioned at Senneville, Vatteville, and Andelys, it may be compared to those needles of chalk which occur on the coast of Normandy, as well as in the Isle of Wight and in Purbeck[241-A] (see fig. 251.).
The foregoing description and drawings will show, that the evidence of certain escarpments of the chalk having been originally sea-cliffs, is far more full and satisfactory in France than in England. If it be asked why, in the interior of our own country, we meet with no ranges of precipices equally vertical and overhanging, and no isolated pillars or needles, we may reply that the greater hardness of the chalk in Normandy may, no doubt, be the chief cause of this difference. But the frequent absence of all signs of littoral denudation in the valley of the Seine itself is a negative fact of a far more striking and perplexing character. The cliffs, after being almost continuous for miles, are then wholly wanting for much greater distances, being replaced by a green sloping down, although the beds remain of the same composition, and are equally horizontal; and although we may feel assured that the manner of the upheaval of the land, whether intermittent or not, must have been the same at those intermediate points where no cliffs exist, as at others where they are so fully developed. But, in order to explain such apparent anomalies, the reader must refer again to the theory of denudation, as expounded in the 6th chapter; where it was shown, first, that the undermining force of the waves and marine currents varies greatly at different parts of every coast; secondly, that precipitous rocks have often decomposed and crumbled down; and thirdly, that many terraces and small cliffs may now lie concealed beneath a talus of detrital matter.
Denudation of the Weald Valley.--No district is better fitted to illustrate the manner in which a great series of strata may have been upheaved and gradually denuded than the country intervening between the North and South Downs. This region, of which a ground plan is given in the accompanying map (fig. 252.), comprises within it the whole of Sussex, and parts of the counties of Kent, Surrey, and Hampshire. The space in which the formations older than the White Chalk, or those from the Gault to the Hastings sand inclusive, crop out, is bounded everywhere by a great escarpment of chalk, which is continued on the opposite side of the channel in the Bas Boulonnais in France, where it forms the semicircular boundary of a tract in which older strata also appear at the surface. The whole of this district may therefore be considered geologically as one and the same.
The space here inclosed within the escarpment of the chalk affords an example of what has been sometimes called a "valley of elevation" (more properly "of denudation"); where the strata, partially removed by aqueous excavation, dip away on all sides from a central axis. Thus, it is supposed that the area now occupied by the Hastings sand (No. 6.) was once covered by the Weald clay (No. 5.), and this again by the Greensand (No. 4.), and this by the Gault (No. 3.); and, lastly, that the Chalk (No. 2.) extended originally over the whole space between the North and the South Downs. This theory will be better understood by consulting the annexed diagram (fig. 253.), where the dark lines represent what now remains, and the fainter ones those portions of rock which are believed to have been carried away.
At each end of the diagram the tertiary strata (No. 1.) are exhibited reposing on the chalk. In the middle are seen the Hastings sands (No. 6.) forming an anticlinal axis, on each side of which the other formations are arranged with an opposite dip. It has been necessary, however, in order to give a clear view of the different formations, to exaggerate the proportional height of each in comparison to its horizontal extent; and a true scale is therefore subjoined in another diagram (fig. 254.), in order to correct the erroneous impression which might otherwise be made on the reader's mind. In this section the distance between the North and South Downs is represented to exceed forty miles; for the Valley of the Weald is here intersected in its longest diameter, in the direction of a line between Lewes and Maidstone.
Through the central portion, then, of the district supposed to be denuded runs a great anticlinal line, having a direction nearly east and west, on both sides of which the beds 5, 4, 3, and 2, crop out in succession. But, although, for the sake of rendering the physical structure of this region more intelligible, the central line of elevation has alone been introduced, as in the diagrams of Smith, Mantell, Conybeare, and others, geologists have always been well aware that numerous minor lines of dislocation and flexure run parallel to the great central axis.
In the central area of the Hastings sand the strata have undergone the greatest displacement; one fault being known, where the vertical shift of a bed of calcareous grit is no less than 60 fathoms.[244-A] Much of the picturesque scenery of this district arises from the depth of the narrow valleys and ridges to which the sharp bends and fractures of the strata have given rise; but it is also in part to be attributed to the excavating power exerted by water, especially on the interstratified argillaceous beds.
Besides the series of longitudinal valleys and ridges in the Weald, there are valleys which run in a transverse direction, passing through the chalk to the basin of the Thames on the one side, and to the English Channel on the other. In this manner the chain of the North Downs is broken by the rivers Wey, Mole, Darent, Medway, and Stour; the South Downs by the Arun, Adur, Ouse, and Cuckmere.[244-B] If these transverse hollows could be filled up, all the rivers, observes Mr. Conybeare, would be forced to take an easterly course, and to empty themselves into the sea by Romney Marsh and Pevensey Levels.[245-A]
Mr. Martin has suggested that the great cross fractures of the chalk, which have become river channels, have a remarkable correspondence on each side of the valley of the Weald; in several instances the gorges in the North and South Downs appearing to be directly opposed to each other. Thus, for example, the defiles of the Wey in the North Downs, and of the Arun in the South, seemed to coincide in direction; and, in like manner, the Ouse corresponds to the Darent, and the Cuckmere to the Medway.[245-B]
Although these coincidences may, perhaps, be accidental, it is by no means improbable, as hinted by the author above mentioned, that great amount of elevation towards the centre of the Weald district gave rise to transverse fissures. And as the longitudinal valleys were connected with that linear movement which caused the anticlinal lines running east and west, so the cross fissures might have been occasioned by the intensity of the upheaving force towards the centre of the line.
But before treating of the manner in which the upheaving movement may have acted, I shall endeavour to make the reader more intimately acquainted with the leading geographical features of the district, so far as they are of geological interest.
In whatever direction we travel from the tertiary strata of the basins of London and Hampshire towards the valley of the Weald, we first ascend a slope of white chalk, with flints, and then find ourselves on the summit of a declivity consisting, for the most part, of different members of the chalk formation; below which the upper greensand, and sometimes, also, the gault, crop out. This steep declivity is the great escarpment of the chalk before mentioned, which overhangs a valley excavated chiefly out of the argillaceous or marly bed, termed Gault (No. 3.). The escarpment is continuous along the southern termination of the North Downs, and may be traced from the sea, at Folkestone, westward to Guildford and the neighbourhood of Petersfield, and from thence to the termination of the South Downs at Beachy Head. In this precipice or steep slope the strata are cut off abruptly, and it is evident that they must originally have extended farther. In the woodcut (fig. 255. p. 245.), part of the escarpment of the South Downs is faithfully represented, where the denudation at the base of the declivity has been somewhat more extensive than usual, in consequence of the upper and lower greensand being formed of very incoherent materials, the upper, indeed, being extremely thin and almost wanting.
The geologist cannot fail to recognize in this view the exact likeness of a sea cliff; and if he turns and looks in an opposite direction, or eastward, towards Beachy Head (see fig. 256.), he will see the same line of heights prolonged. Even those who are not accustomed to speculate on the former changes which the surface has undergone may fancy the broad and level plain to resemble the flat sands which were laid dry by the receding tide, and the different projecting masses of chalk to be the headlands of a coast which separated the different bays from each other.
In regard to the transverse valleys before mentioned, as intersecting the chalk hills, some idea of them may be derived from the subjoined sketch (fig. 257.), of the gorge of the river Adur, taken from the summit of the chalk downs, at a point in the bridle-way leading from the towns of Bramber and Steyning to Shoreham. If the reader will refer again to the view given in a former woodcut (fig. 255. p. 245.), he will there see the exact point where the gorge of which I am now speaking interrupts the chalk escarpment. A projecting hill, at the point a, hides the town of Steyning, near which the valley commences where the Adur passes directly to the sea at Old Shoreham. The river flows through a nearly level plain, as do most of the others which intersect the hills of Surrey, Kent, and Sussex; and it is evident that these openings, so far at least as they are due to aqueous erosion, have not been produced by the rivers, many of which, like the Ouse near Lewes, have filled up arms of the sea, instead of deepening the hollows which they traverse.
Now, in order to account for the manner in which the five groups of strata, 2, 3, 4, 5, 6, represented in the map, fig. 252. and in the section fig. 253., may have been brought into their present position, the following hypothesis has been very generally adopted:--Suppose the five formations to lie in horizontal stratification at the bottom of the sea; then let a movement from below press them upwards into the form of a flattened dome, and let the crown of this dome be afterwards cut off, so that the incision should penetrate to the lowest of the five groups. The different beds would then be exposed on the surface, in the manner exhibited in the map, fig. 252.[247-A]
The quantity of denudation or removal by water of stratified masses assumed to have once reached continuously from the North to the South Downs is so enormous, that the reader may at first be startled by the boldness of the hypothesis. But the difficulty vanishes when once sufficient time is allowed for the gradual and successive rise of the strata, during which the waves and currents of the ocean might slowly accomplish an operation, which no sudden diluvial rush of waters could possibly have effected.
Among other proofs of the action of water, it may be stated that the great longitudinal valleys follow the outcrop of the softer and more incoherent beds, while ridges or lines of cliff usually occur at those points where the strata are composed of harder stone. Thus, for example, the chalk with flints, together with the subjacent upper greensand, which is often used for building, under the provincial name of "firestone," has been cut into a steep cliff on that side on which the sea encroached. This escarpment bounds a deep valley, excavated chiefly out of the soft argillaceous or marly bed, termed gault (No. 3.). In some places the upper greensand is in a loose and incoherent state, and there it has been as much denuded as the gault; as, for example, near Beachy Head; but farther to the westward it is of great thickness, and contains hard beds of blue chert and calcareous sandstone or firestone. Here, accordingly, we find that it produces a corresponding influence on the scenery of the country; for it runs out like a step beyond the foot of the chalk-hills, and constitutes a lower terrace, varying in breadth from a quarter of a mile to three miles, and following the sinuosities of the chalk escarpment.[248-A]
It is impossible to desire a more satisfactory proof that the escarpment is due to the excavating power of water during the rise of the strata; for I have shown, in my account of the coast of Sicily, in what manner the encroachments of the sea tend to efface that succession of terraces which must otherwise result from the intermittent upheaval of a coast preyed upon by the waves.[248-B] During the interval between two elevatory movements, the lower terrace will usually be destroyed, wherever it is composed of incoherent materials; whereas the sea will not have time entirely to sweep away another part of the same terrace, or lower platform, which happens to be composed of rocks of a harder texture, and capable of offering a firmer resistance to the erosive action of water. As the yielding clay termed gault would be readily washed away, we find its outcrop marked everywhere by a valley which skirts the base of the chalk hills, and which is usually bounded on the opposite side by the lower greensand; but as the upper beds of this last formation are most commonly loose and incoherent, they also have usually disappeared and increased the breadth of the valley. But in those districts where chert, limestone, and other solid materials enter largely into the composition of this formation (No. 4.), they give rise to a range of hills parallel to the chalk, which sometimes rival the escarpment of the chalk itself in height, or even surpass it, as in Leith Hill, near Dorking. This ridge often presents a steep escarpment towards the soft argillaceous deposit called the Weald clay (No. 5.; see the strong lines in fig. 253. p. 243.), which usually forms a broad valley, separating the lower greensand from the Hastings sands or Forest ridge; but where subordinate beds of sandstone of a firmer texture occur, the uniformity of the plain of No. 5. is broken by waving irregularities and hillocks.
It will be easy to show how closely the superficial inequalities agree with those which we might naturally expect to originate during the gradual rise of the Wealden district. Suppose the line of the most energetic movement to have coincided with what is now the central ridge of the Weald valley; in that case the first land which emerged must have been situated where the Forest ridge is now placed. Here many shoals and reefs may first have existed, and islands of chalk devoured in the course of ages by the ocean (see fig. 253.); so that the top of the shattered dome which first appeared above water may have been utterly destroyed, and the masses represented by the fainter lines (fig. 253.) removed.
[2 Illustrations: Fig. 259., Fig. 260.
The dotted lines represent the sea-level.]
The upper greensand is represented (fig. 259.) as forming on the left hand a single precipice with the chalk; while on the right there are two cliffs, with an intervening terrace, as before described in fig. 258. Two strips of land would then remain on each side of a channel, presenting ranges of white cliffs facing each other. A powerful current might then scoop out a channel in the gault (No. 2.). This softer bed would yield with ease in proportion as parts of it were brought up from time to time and exposed to the fury of the waves, so that large spaces occupied by the harder formation or greensand (No. 3.) would be laid bare. This last rock, opposing a more effectual resistance, would next emerge; while the chalk cliffs, at the base of which the gault is rapidly undermined, would recede farther from each other, after which four parallel strips of land, or rows of islands, would be caused, which are represented by the masses which in fig. 260. rise above the dotted line indicating the sea-level. In this diagram, however, the inclination of the upper surface of the formations (Nos. 1. and 3.), is exaggerated. Originally this surface must have been level, like the submarine terraces produced by denudation, and described before (p. 74. and 77.); but they were afterwards more and more tilted by that general movement to which the region of the Weald owes its structure. At length, by the farther elevation of the dome-shaped mass, the clay (No. 4.) would be brought within reach of the waves, which would probably gain the more easy access to the subjacent deposit by the rents which would be caused in No. 3., and in the central part of the ridge where the uplifting force had been exerted with the greatest energy. The opposite cliffs, in which the greensand (No. 3.) terminates, would now begin to recede from each other, having at their base a yielding stratum of clay (No. 4.). Lastly, the sea would penetrate to the sand (No. 5.), and then the state of things indicated in the dark lines of the upper section (fig. 253.), would be consummated.
It was stated that there are many lines of flexure and dislocation, running east and west, or parallel to the central axis of the Wealden. They are numerous in the district of the Hastings sand, and sometimes occur in the chalk itself. One of the latter kind has given rise to the ravine called the Coomb, near Lewes, and was first traced out by Dr. Mantell, in whose company I examined it. This coomb is seen on the eastern side of the valley of the Ouse, in the suburbs of the town of Lewes. The steep declivities on each side are covered with green turf, as is the bottom, which is perfectly dry. No outward signs of disturbance are visible; and the connection of the hollow with subterranean movements would not have been suspected by the geologist, had not the evidence of great convulsions been clearly exposed in the escarpment of the valley of the Ouse, and the numerous chalk pits worked at the termination of the Coomb. By the aid of these we discover that the ravine coincides precisely with a line of fault, on one side of which the chalk with flints (a, fig. 262.), appears at the summit of the hill, while it is thrown down to the bottom on the other.
Mr. Martin, in his work on the geology of Western Sussex, published in 1828, threw much light on the structure of the Wealden by tracing out continuously for miles the direction of many anticlinal lines and cross fractures; and the same course of investigation has since been followed out in greater detail by Mr. Hopkins. The mathematician last-mentioned has shown that the observed direction of the lines of flexure and dislocation in the Weald district coincide with those which might have been anticipated theoretically on mechanical principles, if we assume certain simple conditions under which the strata were lifted up by an expansive subterranean force. He finds by calculation that if this force was applied so as to act uniformly upwards within an elliptic area, the longitudinal fissures thereby produced would nearly coincide with the outlines of the ellipse, forming cracks, which are portions of smaller concentric ellipses, parallel to the margin of the larger one. These longitudinal fissures would also be intercepted by others running at right angles to them, and both lines of fracture may have been produced at the same time.[251-A] In this illustration it is supposed that the expansive force acted simultaneously and with equal intensity at every point within the upheaved area, and not with greater energy along the central axis or region of principal elevation.
The geologist cannot fail to derive great advantage in his speculations from the mathematical investigation of a problem of this kind, where results free from all uncertainty are obtained on the assumption of certain simple conditions. Such results, when once ascertained by mathematical methods, may serve as standard cases, to which others occurring in nature of a more complicated kind may be referred. In order that a uniform force should cause the strata to attain in the centre of the ellipse a height so far exceeding that which they have reached round the margin, it is necessary to assume that the mass of upheaved strata offered originally a very unequal degree of resistance to the subterranean force. This may have happened either from their being more fractured in one place than in another, or from being pressed down by a less weight of incumbent strata; as if we suppose, what is far from improbable, that great denudation had taken place in the middle of the Wealden before the final and principal upheaval occurred. It is suggested that the beds may have been acted upon somewhat in the manner of a carpet spread out loosely on a floor, and nailed down round the edges, which would swell into the shape of a dome if pressed up equally at every point by air admitted from beneath. But when we are reasoning on the particular phenomena of the Weald, we have no geological data for determining whether it be more probable that originally the resistance to be overcome was so extremely unequal in different places, or whether the subterranean force, instead of being everywhere uniform, was not applied with very different degrees of intensity beneath distinct portions of the upraised area.
The opinion that both the longitudinal and transverse lines of fracture may have been produced simultaneously, accords well with that expressed by M. Thurmann, in his work on the anticlinal ridges and valleys of elevation of the Bernese Jura.[252-A] For the accuracy of his map and sections I can vouch, from personal examination, in 1835, of part of the region surveyed by him. Among other results, at which this author arrived, it appears that the breadth of all the numerous anticlinal ridges and dome-shaped masses in the Jura is invariably great in proportion to the number of the formations exposed to view; or, in other words, to the depth to which the superimposed groups of secondary strata have been laid open. (See fig. 71. p. 55. for structure of Jura.) He also remarks, that the anticlinal lines are occasionally oblique and cross each other, in which case the greatest dislocation of the beds takes place. Some of the cross fractures are imagined by him to have been contemporaneous, others subsequent to the longitudinal ones.
I have assumed, in the former part of this chapter, that the rise of the Weald was gradual, whereas many geologists have attributed its elevation to a single effort of subterranean violence. There appears to them such a unity of effect in this and other lines of deranged strata in the south-east of England, such as that of the Isle of Wight, as is inconsistent with the supposition of a great number of separate movements recurring after long intervals of time. But we know that earthquakes are repeated throughout a long series of ages in the same spots, like volcanic eruptions. The oldest lavas of Etna were poured out many thousands, perhaps myriads of years before the newest, and yet they, and the movements accompanying their emission, have produced a symmetrical mountain; and if rivers of melted matter thus continue to flow in the same direction, and towards the same point, for an indefinite lapse of ages, what difficulty is there in conceiving that the subterranean volcanic force, occasioning the rise or fall of certain parts of the earth's crust, may, by reiterated movements, produce the most perfect unity of result?
Alluvium of the Weald.--Our next inquiry may be directed to the alluvium strewed over the surface of the supposed area of denudation. Has any wreck been left behind of the strata removed? To this we may answer, that the chalk downs even on their summits are covered every where with gravel composed of unrounded and partially rounded chalk flints, such as might remain after masses of white chalk had been softened and removed by water. This superficial accumulation of the hard or siliceous materials of the disintegrated strata may be due in some degree to pluvial action; for during extraordinary rains a rush of water charged with calcareous matter, of a milk-white colour, may be seen to descend even gently sloping hills of chalk. If a layer no thicker than the tenth of an inch be removed once in a century, a considerable mass may in the course of indefinite ages melt away, leaving nothing save a layer of flinty nodules to attest its former existence. These unrolled flints may remain mixed with others more or less rounded, which the waves left originally on the surface of the chalk, when it first emerged from the sea. A stratum of fine clay sometimes covers the surface of slight depressions and the bottom of valleys in the white chalk, which may represent the aluminous residue of the rock, after the pure carbonate of lime has been dissolved by rain water, charged with excess of carbonic acid derived from decayed vegetable matter.[253-A]
Although flint gravel is so abundant on the chalk itself, it is usually wanting in the deep longitudinal valleys at the foot of the chalk escarpment, although, in some few instances, the detritus of the chalk has been traced in patches over the gault, and even the lower greensand, for a distance of several miles from the escarpment of the North and South Downs. But no vestige of the chalk and its flints has been seen on the central ridge of the Weald or the Hastings sands, but merely gravel derived from the rocks immediately subjacent. This distribution of alluvium, and especially the absence of chalk detritus in the central district, agrees well with the theory of denudation before set forth; for to return to fig. 259., if the chalk (No. 1.) were once continuous and covered every where with flint gravel, this superficial covering would be the first to be carried away from the highest part of the dome long before any of the gault (No. 2.) was laid bare. Now if some ruins of the chalk remain at first on the gault, these would be, in a great degree, cleared away before any part of the lower greensand (No. 3.) is denuded. Thus in proportion to the number and thickness of the groups removed in succession, is the probability lessened of our finding any remnants of the highest group strewed over the bared surface of the lowest.
As an exception to the general rule of the small distance to which any wreck of the chalk can be traced from the escarpments of the North and South Downs, I may mention a thick bed of chalk flints which occurs near Barcombe, about three miles to the north of Lewes (see fig. 263.), a place which I visited with Dr. Mantell, to whom I am indebted for the accompanying section. Even here it will be seen that the gravel reaches no farther than the Weald Clay. The same section shows one of the minor east and west anticlinal lines before alluded to (p. 244.).
At what period the Weald Valley was denuded.--If we inquire at what geological period the denudation of the Weald was effected, we shall immediately perceive that the question is limited to this point, whether it took place during or subsequent to the deposition of the Eocene strata of the south of England. For in the basins of London and Hampshire the Eocene strata are conformable to the chalk, being horizontal where the beds of chalk are horizontal, and vertical where they are vertical, so that both series of rocks appear to have participated in nearly the same movements. At the eastern extremity of the Isle of Wight, some beds even of the freshwater series have been thrown on their edges, like those of the London clay. Nevertheless we can by no means infer that all the tertiary deposits of the London and Hampshire basins once extended like the chalk over the entire valley of the Weald, because the denudation of the chalk and greensand may have been going on in the centre of that area, while contiguous parts of the sea were sufficiently deep to receive and retain the matter derived from that waste. Thus while the waves and currents were excavating the longitudinal valleys D and C (fig. 264.), the deposits a may have been thrown down to the bottom of the contiguous deep water E, the sediment being drifted through transverse fissures, as before explained. In this case, the rise of the formations Nos. 1, 2, 3, 4, 5, may have been going on contemporaneously with the excavation of the valleys C and D, and with the accumulation of the tertiary strata a.
This idea receives some countenance from the fact of the tertiary strata, near their junction with the chalk of the London and Hampshire basins, often consisting of dense beds of sand and shingle, as at Blackheath and in the Addington Hills near Croydon. They also contain occasionally freshwater shells and the remains of land animals and plants, which indicate the former presence of land at no great distance, some part of which may have occupied the centre of the Weald.
Such masses of well-rolled pebbles occurring in the lowest Eocene strata, or those called "the plastic clay and sands" before described (No. 3. b, Tab. p. 197.), imply the neighbourhood of an ancient shore. They also indicate the destruction of pre-existing chalk with flints. At the same time fossil shells of the genera Melania, Cyclas, and Unio, appearing here and there in beds of the same age, together with plants and the bones of land animals, bear testimony to contiguous land, which probably constituted islands scattered over the space now occupied by the tertiary basins of the Seine and Thames. The stage of denudation represented in fig. 259., p. 249., may explain the state of things prevailing at points where such islands existed. By the alternate rising and sinking of the white chalk and older beds, a large area may have become overspread with gravelly sandy, and clayey beds of fluvio-marine and shallow-water origin, before any of the London clay proper (or Calcaire grossier in France) were superimposed. This may account for the fact that patches of "plastic clay and sand" (No. 3. b, Tab. p. 197.), are scattered over the surface of the chalk, reaching in some places to great heights, and approaching even the edges of the escarpments. We must suppose that subsequently a gradual subsidence took place in certain areas, which allowed the London clay proper to accumulate over the Lower Eocene sands and clays, in a deep sea. During this sinking down (the vertical amount of which equalled 800, and in parts of the Isle of Wight, according to Mr. Prestwich, 1800 feet), the work of denudation would be unceasing, being always however confined to those areas where land or islands existed. At length, when the Bagshot sand had been in its turn thrown down on the London clay, the space covered by these two formations was again upraised from the sea to about the height which it has since retained. During this upheaval, the waves would again exert their power, not only on the white chalk and lower cretaceous and Wealden strata, but also on the Eocene formations of the London basin, excavating valleys and undermining cliffs as the strata emerged from the deep.
There are grounds, as before stated (p. 205.), for presuming that the tertiary area of London was converted into land before that of Hampshire, and for this reason it contains no marine Eocene deposits so modern as those of Barton Cliff, or the still newer freshwater and fluvio-marine beds of Hordwell and the Isle of Wight. These last seem unequivocally to demonstrate the local inequality of the upheaving and depressing movements of the period alluded to; for we find, in spite of the evidence afforded in Alum and White Cliff Bays, of continued depression to the extent of 1800 or 2000 feet, that at the close of the Eocene period a dense formation of freshwater strata was produced. The fossils of these strata bear testimony to rivers draining adjacent lands, and the existence of numerous quadrupeds on those lands. Instead of such phenomena, the signs of an open sea might naturally have been expected as the consequence of so much subsidence, had not the depression been accompanied or followed by upheaval in a region immediately adjoining.
When we attempt to speculate on the geographical changes which took place in the earlier part of the Eocene epoch, and to restore in imagination the former state of the physical geography of the south-east of England, we shall do well to bear in mind that wherever there are proofs of great denudation, there also the greatest area of land has probably existed. In the same space, moreover, the oscillations of level, and the alternate submergence and emergence of coasts, may be presumed to have been most frequent; for these fluctuations facilitate the wasting and removing power of waves, currents, and rivers.
We should also remember that there is always a tendency in the last denuding operations, to efface all signs of preceding denudation, or at least all those marks of waste from which alone a geologist can ascertain the date of the removal of the missing strata within the denuded area. It may often be difficult to settle the chronology even of the last of a series of such acts of removal, but it must be, in the nature of things, almost always impossible to assign a date to each of the antecedent denudations. If we wish to determine the times of the destruction of rocks, we must look any where rather than to the spaces once occupied by the missing rocks. We must inquire to what regions the ruins of the white chalk, greensand, Wealden, and other strata which have disappeared were transported. We are then led at once to the examination of all the deposits newer than the chalk, and first to the oldest of these, the Lower Eocene, and its sand, shingle, and clay. In them, so largely developed in the immediate neighbourhood of the denuded area, we discover the wreck we are in search of, regularly stratified, and inclosing, in some of its layers, organic remains of a littoral, and sometimes fluviatile character. What more can we desire? The shores must have consisted of chalk, greensand, and Wealden, since these were the only superficial rocks in the south-east of England, at the commencement of the Eocene epoch. The waves of the sea, therefore, and the rivers were grinding down chalk-flints and chert from the greensand into shingle and sand, or were washing away calcareous and argillaceous matter from the cretaceous and Wealden beds, during the whole of the Eocene period. Thus we obtain the date of a great part at least of that enormous amount of denudation of which we have such striking monuments in the space intervening between the North and South Downs.
There have been some movements of land on a smaller scale since the Eocene period in the south-east of England. One of the latest of these happened in the Pleistocene, or even perhaps as late as the Post-Pliocene period. The formation called by Dr. Mantell the Elephant Bed, at the foot of the chalk cliffs at Brighton, is not merely a talus of calcareous rubble collected at the base of an inland cliff, but exhibits every appearance of having been spread out in successive horizontal layers by water in motion.
The deposit alluded to skirts the shores between Brighton and Rottingdean, and another mass apparently of the same age occurs at Dover. The phenomena appear to me to suggest the following conclusions:--First, the south-eastern part of England had acquired its actual configuration when the ancient chalk cliff A a was formed, the beach of sand and shingle b having then been thrown up at the base of the cliff. Afterwards the whole coast, or at least that part of it where the elephant bed now extends, subsided to the depth of 50 or 60 feet; and during the period of submergence successive layers of white calcareous rubble c were accumulated, so as to cover the ancient beach b. Subsequently, the coast was again raised, so that the ancient shore was elevated to a level somewhat higher than its original position.[257-A]
FOOTNOTES:
[241-A] An account of these cliffs was read by the author to the British Assoc. at Glasgow, Sept. 1840.
[241-B] Seine-Inferieure, p. 142. and pl. 6. fig. 1.
[243-A] Botley Hill, near Godstone, in Surrey, was found by trigonometrical measurement to be 880 feet above the level of the sea; and Wrotham Hill, near Maidstone, which appears to be next in height of the North Downs, 795 feet.
[243-B] My friend Dr. Mantell has kindly drawn up this scale at my request.
[244-A] Fitton, Geol. of Hastings, p. 55.
[244-B] Conybeare, Outlines of Geol., p. 81.
[245-A] Ibid., p. 145.
[245-B] Geol. of Western Sussex, p. 61.
[247-A] See illustrations of this theory by Dr. Fitton, Geol. Sketch of Hastings.
[248-A] Sir E. Murchison, Geol. Sketch of Sussex, &c., Geol. Trans., Second Series, vol. ii. p. 98.
[248-B] See fig. 94. p. 76.
[251-A] Geol. Soc. Proceed. No. 74. p. 363. 1841, and G. S. Trans. 2 Ser. v. 7.
[251-B] For farther information, see Mantell's Geol. of S. E. of England, p. 352.
[252-A] Soulevemens Jurassiques. Paris, 1832.
[253-A] See above, p. 82.
[257-A] See Mantell's Geol. of S. E. of England, p. 32. After re-examining the elephant bed in 1834, I was no longer in doubt of its having been a regular subaqueous deposit. In 1828, Dr. Mantell discovered in the shingle below the chalk-rubble the jawbone of a whale 12 feet long, which must have belonged to an individual from 60 to 70 feet in length, Medals of Creation, p. 825.
XX. OOLITE AND LIAS
Subdivisions of the Oolitic or Jurassic group--Physical geography of the Oolite in England and France--Upper Oolite--Portland stone and fossils--Lithographic stone of Solenhofen--Middle Oolite, coral rag--Zoophytes--Nerinaean limestone--Diceras limestone--Oxford clay, Ammonites and Belemnites--Lower Oolite, Crinoideans--Great Oolite and Bradford clay--Stonesfield slate--Fossil mammalia, placental and marsupial--Resemblance to an Australian fauna--Doctrine of progressive development--Collyweston slates--Yorkshire Oolitic coal-field--Brora coal--Inferior Oolite and fossils.
OOLITIC OR JURASSIC GROUP.--Below the freshwater group called the Wealden, or, where this is wanting, immediately beneath the Cretaceous formation, a great series of marine strata, commonly called "the Oolite," occurs in England and many other parts of Europe. This group has been so named, because, in the countries where it was first examined, the limestones belonging to it had an oolitic structure (see p. 12.). These rocks occupy in England a zone which is nearly 30 miles in average breadth, and extends across the island, from Yorkshire in the north-east, to Dorsetshire in the south-west. Their mineral characters are not uniform throughout this region; but the following are the names of the principal subdivisions observed in the central and south-eastern parts of England:--
OOLITE.
Upper { a. Portland stone and sand. { b. Kimmeridge clay.
Middle { c. Coral rag. { d. Oxford clay.
Lower { e. Cornbrash and Forest marble. { f. Great Oolite and Stonesfield slate. { g. Fuller's earth. { h. Inferior Oolite.
The Lias then succeeds to the Inferior Oolite.
The Upper oolitic system of the above table has usually the Kimmeridge clay for its base; the Middle oolitic system, the Oxford clay. The Lower system reposes on the Lias, an argillo-calcareous formation, which some include in the Lower Oolite, but which will be treated of separately in the next chapter. Many of these subdivisions are distinguished by peculiar organic remains; and though varying in thickness, may be traced in certain directions for great distances, especially if we compare the part of England to which the above-mentioned type refers with the north-east of France, and the Jura mountains adjoining. In that country, distant above 400 geographical miles, the analogy to the English type, notwithstanding the thinness, or occasional absence of the clays, is more perfect than in Yorkshire or Normandy.
Physical geography.--The alternation, on a grand scale, of distinct formations of clay and limestone, has caused the oolitic and liassic series to give rise to some marked features in the physical outline of parts of England and France. Wide valleys can usually be traced throughout the long bounds of country where the argillaceous strata crop out; and between these valleys the limestones are observed, composing ranges of hills, or more elevated grounds. These ranges terminate abruptly on the side on which the several clays rise up from beneath the calcareous strata.
The annexed diagram will give the reader an idea of the configuration of the surface now alluded to, such as may be seen in passing from London to Cheltenham, or in other parallel lines, from east to west, in the southern part of England. It has been necessary, however, in this drawing, greatly to exaggerate the inclination of the beds, and the height of the several formations, as compared to their horizontal extent. It will be remarked, that the lines of cliff, or escarpment, face towards the west in the great calcareous eminences formed by the Chalk and the Upper, Middle, and Lower Oolites; and at the base of which we have respectively the Gault, Kimmeridge clay, Oxford clay, and Lias. This last forms, generally, a broad vale at the foot of the escarpment of inferior oolite, but where it acquires considerable thickness, and contains solid beds of marlstone, it occupies the lower part of the escarpment.
The external outline of the country which the geologist observes in travelling eastward from Paris to Metz is precisely analogous, and is caused by a similar succession of rocks intervening between the tertiary strata and the Lias; with this difference, however, that the escarpments of Chalk, Upper, Middle, and Lower Oolites, face towards the east instead of the west.
The Chalk crops out from beneath the tertiary sands and clays of the Paris basin, near Epernay, and the Gault from beneath the Chalk and Upper Greensand at Clermont-en-Argonne; and passing from this place by Verdun and Etain to Metz, we find two limestone ranges, with intervening vales of clay, precisely resembling those of southern and central England, until we reach the great plain of Lias at the base of the Inferior Oolite at Metz.
It is evident, therefore, that the denuding causes have acted similarly over an area several hundred miles in diameter, sweeping away the softer clays more extensively than the limestones, and undermining these last so as to cause them to form steep cliffs wherever the harder calcareous rock was based upon a more yielding and destructible clay. This denudation probably occurred while the land was slowly rising out of the sea.[259-A]
Upper Oolite.
The Portland stone has already been mentioned as forming in Dorsetshire the foundation on which the freshwater limestone of the Lower Purbeck reposes (see p. 232.). It supplies the well-known building stone of which St. Paul's and so many of the principal edifices of London are constructed. This upper member, characterized by peculiar marine fossils, rests on a dense bed of sand, called the Portland sand, below which is the Kimmeridge clay. In England these Upper Oolite formations are almost wholly confined to the southern counties. Corals are rare in them, although one species is found plentifully at Tisbury, in Wiltshire, in the Portland sand converted into flint and chert, the original calcareous matter being replaced by silex (fig. 267.).
Among the characteristic fossils of the Upper Oolite, may be mentioned the Ostrea deltoidea (fig. 269.), found in the Kimmeridge clay throughout England and the north of France, and also in Scotland, near Brora. The Gryphaea virgula (fig. 268.), also met with in the same clay near Oxford, is so abundant in the Upper Oolite of parts of France as to have caused the deposit to be termed "marnes a gryphees virgules." Near Clermont, in Argonne, a few leagues from St. Menehould, where these indurated marls crop out from beneath the gault, I have seen them, on decomposing, leave the surface of every ploughed field literally strewed over with this fossil oyster.
[2 Illustrations: Upper Oolite: Kimmeridge clay. 1/4 nat. size.
Fig. 268. Gryphaea virgula.
Fig. 269. Ostrea deltoidea.]
The Kimmeridge clay consists, in great part, of a bituminous shale, sometimes forming an impure coal several hundred feet in thickness. In some places in Wiltshire it much resembles peat; and the bituminous matter may have been, in part at least, derived from the decomposition of vegetables. But as impressions of plants are rare in these shales, which contain ammonites, oysters, and other marine shells, the bitumen may perhaps be of animal origin.
The celebrated lithographic stone of Solenhofen, in Bavaria, belongs to one of the upper divisions of the oolite, and affords a remarkable example of the variety of fossils which may be preserved under favourable circumstances, and what delicate impressions of the tender parts of certain animals and plants may be retained where the sediment is of extreme fineness. Although the number of testacea in this slate is small, and the plants few, and those all marine, Count Munster had determined no less than 237 species of fossils when I saw his collection in 1833; and among them no less than seven species of flying lizards, or pterodactyls, six saurians, three tortoises, sixty species of fish, forty-six of crustacea, and twenty-six of insects. These insects, among which is a libellula, or dragon-fly, must have been blown out to sea, probably from the same land to which the flying lizards, and other contemporaneous reptiles, resorted.
Middle Oolite.
Coral Rag.--One of the limestones of the Middle Oolite has been called the "Coral Rag," because it consists, in part, of continuous beds of petrified corals, for the most part retaining the position in which they grew at the bottom of the sea. They belong chiefly to the genera Caryophyllia (fig. 271.), Agaricia, and Astrea, and sometimes form masses of coral 15 feet thick. In the annexed figure of an Astrea, from this formation, it will be seen that the cup-shaped cavities are deepest on the right-hand side, and that they grow more and more shallow, till those on the left side are nearly filled up. The last-named stars are supposed to be Polyparia of advanced age. These coralline strata extend through the calcareous hills of the N.W. of Berkshire, and north of Wilts, and again recur in Yorkshire, near Scarborough.
One of the limestones of the Jura, referred to the age of the English coral rag, has been called "Nerinaean limestone" (Calcaire a Nerinees) by M. Thirria; Nerinaea being an extinct genus of univalve shells, much resembling the Cerithium in external form. The annexed section (fig. 273.) shows the curious form of the hollow part of each whorl, and also the perforation which passes up the middle of the columella. N. Goodhallii (fig. 274.) is another English species of the same genus, from a formation which seems to form a passage from the Kimmeridge clay to the coral rag.[261-A]
A division of the oolite in the Alps, regarded by most geologists as coeval with the English coral rag, has been often named "Calcaire a Dicerates," or "Diceras limestone," from its containing abundantly a bivalve shell (see fig. 275.) of a genus allied to the Chama.
Oxford Clay.--The coralline limestone, or "coral rag," above described, and the accompanying sandy beds, called "calcareous grits" of the Middle Oolite, rests on a thick bed of clay, called the Oxford clay, sometimes not less than 500 feet thick. In this there are no corals, but great abundance of cephalopoda of the genera Ammonite and Belemnite. (See fig. 277.) In some of the clay of very fine texture ammonites are very perfect, although somewhat compressed, and are seen to be furnished on each side of the aperture with a single horn-like projection (see fig. 278.). These were discovered in the cuttings of the Great Western Railway, near Chippenham, in 1841, and have been described by Mr. Pratt.[262-A]
Similar elongated processes have been also observed to extend from the shells of some belemnites discovered by Dr. Mantell in the same clay (see fig. 279.), who, by the aid of this and other specimens, has been able to throw much light on the structure of this singular extinct form of cuttle-fish.[263-A]
Lower Oolite.
The upper division of this series, which is more extensive than the preceding or Middle Oolite, is called in England the Cornbrash. It consists of clays and calcareous sandstones, which pass downwards into the Forest marble, an argillaceous limestone, abounding in marine fossils. In some places, as at Bradford, this limestone is replaced by a mass of clay. The sandstones of the Forest Marble of Wiltshire are often ripple-marked and filled with fragments of broken shells and pieces of drift-wood, having evidently been formed on a coast. Rippled slabs of fissile oolite are used for roofing, and have been traced over a broad band of country from Bradford, in Wilts, to Tetbury, in Gloucestershire. These calcareous tile-stones are separated from each other by thin seams of clay, which have been deposited upon them, and have taken their form, preserving the undulating ridges and furrows of the sand in such complete integrity, that the impressions of small footsteps, apparently of crabs, which walked over the soft wet sands, are still visible. In the same stone the claws of crabs, fragments of echini, and other signs of a neighbouring beach are observed.[263-B]
Great Oolite.--Although the name of coral-rag has been appropriated, as we have seen, to a member of the Upper Oolite before described, some portions of the Lower Oolite are equally intitled in many places to be called coralline limestones. Thus the Great Oolite near Bath contains various corals, among which the Eunomia radiata (fig. 280.) is very conspicuous, single individuals forming masses several feet in diameter; and having probably required, like the large existing brain-coral (Meandrina) of the tropics, many centuries before their growth was completed.
Different species of Crinoideans, or stone-lilies, are also common in the same rocks with corals; and, like them, must have enjoyed a firm bottom, where their root, or base of attachment, remained undisturbed for years (c, fig. 281.). Such fossils, therefore, are almost confined to the limestones; but an exception occurs at Bradford, near Bath, where they are enveloped in clay. In this case, however, it appears that the solid upper surface of the "Great Oolite" had supported, for a time, a thick submarine forest of these beautiful zoophytes, until the clear and still water was invaded by a current charged with mud, which threw down the stone-lilies, and broke most of their stems short off near the point of attachment. The stumps still remain in their original position; but the numerous articulations once composing the stem, arms, and body of the zoophyte, were scattered at random through the argillaceous deposit in which some now lie prostrate. These appearances are represented in the section b, fig. 281., where the darker strata represent the Bradford clay, which some geologists class with the Forest marble, others with the Great Oolite. The upper surface of the calcareous stone below is completely incrusted over with a continuous pavement, formed by the stony roots or attachments of the Crinoidea; and besides this evidence of the length of time they had lived on the spot, we find great numbers of single joints, or circular plates of the stem and body of the encrinite, covered over with serpulae. Now these serpulae could only have begun to grow after the death of some of the stone-lilies, parts of whose skeletons had been strewed over the floor of the ocean before the irruption of argillaceous mud. In some instances we find that, after the parasitic serpulae were full grown, they had become incrusted over with a coral, called Berenicea diluviana; and many generations of these polyps had succeeded each other in the pure water before they became fossil.
We may, therefore, perceive distinctly that, as the pines and cycadeous plants of the ancient "dirt bed," or fossil forest, of the Lower Purbeck were killed by submergence under fresh water, and soon buried beneath muddy sediment, so an invasion of argillaceous matter put a sudden stop to the growth of the Bradford Encrinites, and led to their preservation in marine strata.[265-A]
Such differences in the fossils as distinguish the calcareous and argillaceous deposits from each other, would be described by naturalists as arising out of a difference in the stations of species; but besides these, there are variations in the fossils of the higher, middle, and lower part of the oolitic series, which must be ascribed to that great law of change in organic life by which distinct assemblages of species have been adapted, at successive geological periods, to the varying conditions of the habitable surface. In a single district it is difficult to decide how far the limitation of species to certain minor formations has been due to the local influence of stations, or how far it has been caused by time or the creative and destroying law above alluded to. But we recognize the reality of the last-mentioned influence, when we contrast the whole oolitic series of England with that of parts of the Jura, Alps, and other distant regions, where there is scarcely any lithological resemblance; and yet some of the same fossils remain peculiar in each country to the Upper, Middle, and Lower Oolite formations respectively. Mr. Thurmann has shown how remarkably this fact holds true in the Bernese Jura, although the argillaceous divisions, so conspicuous in England, are feebly represented there, and some entirely wanting.
The Bradford clay above alluded to is sometimes 60 feet thick, but, in many places, it is wanting; and, in others, where there are no limestones, it cannot easily be separated from the clays of the overlying "forest marble" and underlying "fuller's earth."
The calcareous portion of the Great Oolite consists of several shelly limestones, one of which, called the Bath Oolite, is much celebrated as a building stone. In parts of Gloucestershire, especially near Minchinhampton, the Great Oolite, says Mr. Lycett, "must have been deposited in a shallow sea, where strong currents prevailed, for there are frequent changes in the mineral character of the deposit, and some beds exhibit false stratification. In others, heaps of broken shells are mingled with pebbles of rocks foreign to the neighbourhood, and with fragments of abraded madrepores, dicotyledonous wood, and crabs' claws. The shelly strata, also, have occasionally suffered denudation, and the removed portions have been replaced by clay."[266-A] In such shallow-water beds cephalopoda are rare, and, instead of ammonites and belemnites, numerous genera of carnivorous trachelipods appear. Out of one hundred and forty-two species of univalves obtained from the Minchinhampton beds, Mr. Lycett found no less than forty-one to be carnivorous. They belong principally to the genera Buccinum, Pleurotoma, Rostellaria, Murex, and Fusus, and exhibit a proportion of zoophagous species not very different from that which obtains in warm seas of the recent period. These conchological results are curious and unexpected, since it was imagined that we might look in vain for the carnivorous trachelipods in rocks of such high antiquity as the Great Oolite, and it was a received doctrine that they did not begin to appear in considerable numbers till the Eocene period when those two great families of cephalopoda, the ammonites and belemnites, had become extinct.
Stonesfield slate.--The slate of Stonesfield has been shown by Mr. Lonsdale to lie at the base of the Great Oolite.[266-B] It is a slightly oolitic shelly limestone, forming large spheroidal masses imbedded in sand, only 6 feet thick, but very rich in organic remains. It contains some pebbles of a rock very similar to itself, and which may be portions of the deposit, broken up on a shore at low water or during storms, and redeposited. The remains of belemnites, trigoniae, and other marine shells, with fragments of wood, are common, and impressions of ferns, cycadeae, and other plants. Several insects, also, and, among the rest, the wing-covers of beetles, are perfectly preserved (see fig. 284.), some of them approaching nearly to the genus Buprestis.[267-A] The remains, also, of many genera of reptiles, such as Plesiosaur, Crocodile, and Pterodactyl, have been discovered in the same limestone.
But the remarkable fossils for which the Stonesfield slate is most celebrated, are those referred to the mammiferous class. The student should be reminded that in all the rocks described in the preceding chapters as older than the Eocene, no bones of any land quadruped, or of any cetacean, have been discovered. Yet we have seen that terrestrial plants were not rare in the lower cretaceous formation, and that in the Wealden there was evidence of freshwater sediment on a large scale, containing various plants, and even ancient vegetable soils with the roots and erect stumps of trees. We had also in the same Wealden many land-reptiles and winged-insects, which renders the absence of terrestrial quadrupeds the more striking. The want, however, of any bones of whales, seals, dolphins, and other aquatic mammalia, whether in the chalk or in the upper or middle oolite, is certainly still more remarkable. Formerly, indeed, a bone from the great oolite of Enstone, near Woodstock, in Oxfordshire, was cited, on the authority of Cuvier, as referable to this class. Dr. Buckland, who stated this in his Bridgewater Treatise[267-B], had the kindness to send me the supposed ulna of a whale, that Mr. Owen might examine into its claims to be considered as cetaceous. It is the opinion of that eminent comparative anatomist that it cannot have belonged to the cetacea, because the fore-arm in these marine mammalia is invariably much flatter, and devoid of all muscular depressions and ridges, one of which is so prominent in the middle of this bone, represented in the above cut (fig. 285.). In saurians, on the contrary, such ridges exist for the attachment of muscles; and to some animal of that class the bone is probably referable.
These observations are made to prepare the reader to appreciate more justly the interest felt by every geologist in the discovery in the Stonesfield slate of no less than seven specimens of lower jaws of mammiferous quadrupeds, belonging to three different species and to two distinct genera, for which the names of Amphitherium and Phascolotherium have been adopted. When Cuvier was first shown one of these fossils in 1818, he pronounced it to belong to a small ferine mammal, with a jaw much resembling that of an opossum, but differing from all known ferine genera, in the great number of the molar teeth, of which it had at least ten in a row. Since that period, a much more perfect specimen of the same fossil, obtained by Dr. Buckland (see fig. 286.), has been examined by Mr. Owen, who finds that the jaw contained on the whole twelve molar teeth, with the socket of a small canine, and three small incisors, which are in situ, altogether amounting to sixteen teeth on each side of the lower jaw.
The only question which could be raised respecting the nature of these fossils was, whether they belonged to a mammifer, a reptile, or a fish. Now on this head the osteologist observes that each of the seven half jaws is composed of but one single piece, and not of two or more separate bones, as in fishes and most reptiles, or of two bones, united by a suture, as in some few species belonging to those classes. The condyle, moreover (b, fig. 286.), or articular surface, by which the lower jaw unites with the upper, is convex in the Stonesfield specimens, and not concave as in fishes and reptiles. The coronoid process (a, fig. 286.) is well developed, whereas it is wanting or very small, in the inferior classes of vertebrata. Lastly, the molar teeth in the Amphitherium and Phascolotherium have complicated crowns, and two roots (see d, fig. 286.), instead of being simple and with single fangs.[269-A]
[2 Illustrations: Part of lower jaw of Tupaia Tana; twice natural size.
Fig. 289. End view seen from behind, showing the very slight inflection of the angle at c.
Fig. 290. Side view of same.]
[2 Illustrations: Part of lower jaw of Didelphis Azarae; recent, Brazil. Natural size.
Fig. 291. End view seen from behind, showing the inflection of the angle of the jaw, c. d.
Fig. 292. Side view of same.]
The only question, therefore, which could fairly admit of controversy was limited to this point, whether the fossil mammalia found in the lower oolite of Oxfordshire ought to be referred to the marsupial quadrupeds, or to the ordinary placental series. Cuvier had long ago pointed out a peculiarity in the form of the angular process (c, figs. 291. and 292.) of the lower jaw, as a character of the genus Didelphys; and Mr. Owen has since established its generality in the entire marsupial series. In all these pouched quadrupeds, this process is turned inwards, as at c d, fig. 291. in the Brazilian opossum, whereas in the placental series, as at c, figs. 290. and 289. there is an almost entire absence of such inflection. The Tupaia Tana of Sumatra has been selected by my friend Mr. Waterhouse, for this illustration, because that small insectivorous quadruped bears a great resemblance to those of the Stonesfield Amphitherium. By clearing away the matrix from the specimen of Amphitherium Prevostii above represented (fig. 286.), Mr. Owen ascertained that the angular process (c) bent inwards in a slighter degree than in any of the known marsupialia; in short, the inflection does not exceed that of the mole or hedgehog. This fact turns the scale in favour of its affinities to the placental insectivora. Nevertheless, the Amphitherium offers some points of approximation in its osteology to the marsupials, especially to the Myrmecobius, a small insectivorous quadruped of Australia, which has nine molars on each side of the lower jaw, besides a canine and three incisors.[269-B]
Another species of Amphitherium has been found at Stonesfield (fig. 287. p. 268.), which differs from the former (fig. 286.) principally in being larger.
The second mammiferous genus discovered in the same slates was named originally by Mr. Broderip Didelphys Bucklandi (see fig. 293.), and has since been called Phascolotherium by Owen. It manifests a much stronger likeness to the marsupials in the general form of the jaw, and in the extent and position of its inflected angle, while the agreement with the living genus Didelphys in the number of the premolar and molar teeth, is complete.[270-A]
On reviewing, therefore, the whole of the osteological evidence, it will be seen that we have every reason to presume that the Amphitherium and Phascolotherium of Stonesfield represent both the placental and marsupial classes of mammalia; and if so, they warn us in a most emphatic manner, not to found rash generalizations respecting the non-existence of certain classes of animals at particular periods of the past, on mere negative evidence. The singular accident of our having as yet found nothing but the lower jaws of seven individuals, and no other bones of their skeletons, is alone sufficient to demonstrate the fragmentary manner in which the memorials of an ancient terrestrial fauna are handed down to us. We can scarcely avoid suspecting that the two genera above described, may have borne a like insignificant proportion to the entire assemblage of warm-blooded quadrupeds which flourished in the islands of the oolitic sea.
Mr. Owen has remarked that as the marsupial genera, to which the Phascolotherium is most nearly allied, are now confined to New South Wales and Van Diemen's Land, so also is it in the Australian seas, that we find the Cestracion, a cartilaginous fish which has a bony palate, allied to those called Acrodus and Psammodus (see figs. 307, 308. p. 275.), so common in the oolite and lias. In the same Australian seas, also, near the shore, we find the living Trigonia, a genus of mollusca so frequently met with in the Stonesfield slate. So, also, the Araucarian pines are now abundant, together with ferns, in Australia and its islands, as they were in Europe in the oolitic period. Many botanists incline to the opinion, that the Thuja, Pine, Cycas, Zamia, in short, all the gymnogens, belong to a less highly developed type of flowering plants than do the exogens; but even if this be admitted, no naturalist can ascribe a low standard of organization to the oolitic flora, since we meet with endogens of the most perfect structure in oolitic rocks, both above and below the Stonesfield slate, as, for example, the Podocarya of Buckland, a fruit allied to the Pandanus, found in the Inferior Oolite (see fig. 294.), and the Carpolithes conica of the Coral rag. The doctrine, therefore, of a regular series of progressive development at successive eras in the animal and vegetable kingdoms, from beings of a more simple to those of a more complex organization, receives a check, if not a refutation, from the facts revealed to us by the study of the Lower Oolites.
The Stonesfield slate, in its range from Oxfordshire to the north-east, is represented by flaggy and fissile sandstones, as at Collyweston in Northamptonshire, where, according to the researches of Messrs. Ibbetson and Morris, it contains many shells, such as Trigonia angulata, also found at Stonesfield. But the Northamptonshire strata of this age assume a more marine character, or appear at least to have been formed farther from land. They inclose, however, some fossil ferns, such as Pecopteris polypodioides, of species common to the oolites of the Yorkshire coast[271-A], where rocks of this age put on all the aspect of a true coal-field; thin seams of coal having actually been worked in them for more than a century.
In the north-west of Yorkshire, the formation alluded to consists of an upper and a lower carbonaceous shale, abounding in impressions of plants, divided by a limestone considered by many geologists as the representative of the Great Oolite; but the scarcity of marine fossils makes all comparisons with the subdivisions adopted in the south extremely difficult. A rich harvest of fossil ferns has been obtained from the upper carbonaceous shales and sandstones at Gristhorpe, near Scarborough (see figs. 295, 296.). The lower shales are well exposed in the sea-cliffs at Whitby, and are chiefly characterized by ferns and cycadeae. They contain, also, a species of calamite, and a fossil called Equisetum columnare, which maintains an upright position in sandstone strata over a wide area. Shells of the genus Cypris and Unio, collected by Mr. Bean from these Yorkshire coal-bearing beds, point to the estuary or fluviatile origin of the deposit.
At Brora, in Sutherlandshire, a coal formation, probably coeval with the above, or belonging to some of the lower divisions of the Oolitic period, has been mined extensively for a century or more. It affords the thickest stratum of pure vegetable matter hitherto detected in any secondary rock in England. One seam of coal of good quality has been worked 3-1/2 feet thick, and there are several feet more of pyritous coal resting upon it.
Inferior Oolite.--Between the Great and Inferior Oolite, near Bath, an argillaceous deposit called "the fuller's earth," occurs, but is wanting in the north of England. The Inferior Oolite is a calcareous freestone, usually of small thickness, which sometimes rests upon, or is replaced by, yellow sands, called the sands of the Inferior Oolite. These last, in their turn, repose upon the lias in the south and west of England.
Among the characteristic shells of the Inferior Oolite, I may instance Terebratula spinosa (fig. 297.), and Pholadomya fidicula (fig. 298.). The extinct genus Pleurotomaria is also a form very common in this division as well as in the Oolitic system generally. It resembles the Trochus in form, but is marked by a singular cleft (a, fig. 299.) on the right side of the mouth.
As illustrations of shells having a great vertical range, I may allude to Trigonia clavellata, found in the Upper and Inferior Oolite, and T. costata, common to the Upper, Middle, and Lower Oolite; also Ostrea Marshii (fig. 300.), common to the Cornbrash of Wilts and the Inferior Oolite of Yorkshire; and Ammonites striatulus (fig. 301.) common to the Inferior Oolite and Lias.
Such facts by no means invalidate the general rule, that certain fossils are good chronological tests of geological periods; but they serve to caution us against attaching too much importance to single species, some of which may have a wider, others a more confined vertical range. We have before seen that, in the successive tertiary formations, there are species common to older and newer groups, yet these groups are distinguishable from one another by a comparison of the whole assemblage of fossil shells proper to each.
FOOTNOTES:
[259-A] See Chapters VI. and XIX.
[261-A] Fitton, Geol. Trans., Second Series, vol. iv. pl. 23. fig. 12.
[262-A] S. P. Pratt, Annals of Nat. Hist., November, 1841.
[263-A] See Phil. Trans. 1850, p. 393.
[263-B] P. Scrope, Geol. Proceed., March, 1831.
[265-A] For a fuller account of these Encrinites, see Buckland's Bridgewater Treatise, vol. i. p. 429.
[266-A] Lycett, Quart. Geol. Journ. vol. iv. p. 183.
[266-B] Proceedings Geol. Soc. vol. i. p. 414.
[267-A] See Buckland's Bridgewater Treatise; and Brodie's Fossil Insects, where it is suggested that these elytra may belong to Priomus.
[267-B] Vol. i. p. 115.
[269-A] I have given a figure in the Principles of Geology, chap. ix., of another Stonesfield specimen of Amphitherium Prevostii, in which the sockets and roots of the teeth are finely exposed.
[269-B] A figure of this recent Myrmecobius will be found in the Principles, chap. ix.
[270-A] Owen's British Fossil Mammals, p. 62.
[271-A] Ibbetson and Morris, Report of Brit. Ass., 1847, p. 131.
XXI. OOLITE AND LIAS--continued
Mineral character of Lias--Name of Gryphite limestone--Fossil shells and fish--Ichthyodorulites--Reptiles of the Lias--Ichthyosaur and Plesiosaur--Marine Reptile of the Galapagos Islands--Sudden destruction and burial of fossil animals in Lias--Fluvio-marine beds in Gloucestershire and insect limestone--Origin of the Oolite and Lias, and of alternating calcareous and argillaceous formations--Oolitic coal-field of Virginia, in the United States.
LIAS.--The English provincial name of Lias has been very generally adopted for a formation of argillaceous limestone, marl, and clay, which forms the base of the Oolite, and is classed by many geologists as part of that group. They pass, indeed, into each other in some places, as near Bath, a sandy marl called the marlstone of the Lias being interposed, and partaking of the mineral characters of the upper lias and inferior oolite. These last-mentioned divisions have also some fossils in common, such as the Avicula inaequivalvis (fig. 302.). Nevertheless the Lias may be traced throughout a great part of Europe as a separate and independent group, of considerable thickness, varying from 500 to 1000 feet, containing many peculiar fossils, and having a very uniform lithological aspect. Although usually conformable to the oolite, it is sometimes, as in the Jura, unconformable. In the environs of Lons-le-Saulnier, for instance, in the department of Jura, the strata of lias are inclined at an angle of about 45 deg., while the incumbent oolitic marls are horizontal.
The peculiar aspect which is most characteristic of the Lias in England, France, and Germany, is an alternation of thin beds of blue or grey limestone with a surface becoming light-brown when weathered, these beds being separated by dark-coloured narrow argillaceous partings, so that the quarries of this rock, at a distance, assume a striped and riband-like appearance.[274-A]
Although the prevailing colour of the limestone of this formation is blue, yet some beds of the lower lias are of a yellowish white colour, and have been called white lias. In some parts of France, near the Vosges mountains, and in Luxembourg, M. E. de Beaumont has shown that the lias containing Gryphaea arcuata, Plagiostoma giganteum (see fig. 303.), and other characteristic fossils, becomes arenaceous; and around the Hartz, in Westphalia and Bavaria, the inferior parts of the lias are sandy, and sometimes afford a building stone.
The name of Gryphite limestone has sometimes been applied to the lias, in consequence of the great number of shells which it contains of a species of oyster, or Gryphaea (fig. 304., see also fig. 30. p. 29.). Many cephalopoda, also, such as Ammonite, Belemnite, and Nautilus (fig. 305.), prove the marine origin of the formation.
The fossil fish resemble generically those of the oolite, belonging all, according to M. Agassiz, to extinct genera, and differing remarkably from the ichthyolites of the Cretaceous period. Among them is a species of Lepidotus (L. gigas, Agas.) (fig. 306.), which is found in the lias of England, France, and Germany.[275-A] This genus was before mentioned (p. 229.) as occurring in the Wealden, and is supposed to have frequented both rivers and coasts. The teeth of a species of Acrodus, also, are very abundant in the lias (fig. 307.).
But the remains of fish which have excited more attention than any others, are those large bony spines called ichthyodorulites (a, fig. 308.), which were once supposed by some naturalists to be jaws, and by others weapons, resembling those of the living Balistes and Silurus; but which M. Agassiz has shown to be neither the one nor the other. The spines, in the genera last mentioned, articulate with the backbone, whereas there are no signs of any such articulation in the ichthyodorulites. These last appear to have been bony spines which formed the anterior part of the dorsal fin, like that of the living genera Cestracion and Chimaera (see a, fig. 309.). In both of these genera, the posterior concave face is armed with small spines like that of the fossil Hybodus (fig. 308.), one of the shark family found fossil at Lyme Regis. Such spines are simply imbedded in the flesh, and attached to strong muscles. "They serve," says Dr. Buckland, "as in the Chimaera (fig. 309.), to raise and depress the fin, their action resembling that of a moveable mast, raising and lowering backwards the sail of a barge."[276-A]
Reptiles of the Lias.--It is not, however, the fossil fish which form the most striking feature in the organic remains of the Lias; but the reptiles, which are extraordinary for their number, size, and structure. Among the most singular of these are several species of Ichthyosaurus and Plesiosaurus. The genus Ichthyosaurus, or fish-lizard, is not confined to this formation, but has been found in strata as high as the chalk-marl and gault of England, and as low as the muschelkalk of Germany, a formation which immediately succeeds the lias in the descending order.[276-C] It is evident from their fish-like vertebrae, their paddles, resembling those of a porpoise or whale, the length of their tail, and other parts of their structure, that the habits of the Ichthyosaurs were aquatic. Their jaws and teeth show that they were carnivorous; and the half-digested remains of fishes and reptiles, found within their skeletons, indicate the precise nature of their food.[276-D]
A specimen of the hinder fin or paddle of Ichthyosaurus communis was discovered in 1840 at Barrow-on-Soar, by Sir P. Egerton, which distinctly exhibits on its posterior margin the remains of cartilaginous rays that bifurcate as they approach the edge, like those in the fin of a fish (see a, fig. 312.). It had previously been supposed, says Mr. Owen, that the locomotive organs of the Ichthyosaurus were enveloped, while living, in a smooth integument, like that of the turtle and porpoise, which has no other support than is afforded by the bones and ligaments within; but it now appears that the fin was much larger, expanding far beyond its osseous framework, and deviating widely in its fish-like rays from the ordinary reptilian type. In fig. 312. the posterior bones, or digital ossicles of the paddle, are seen near b; and beyond these is the dark carbonized integument of the terminal half of the fin, the outline of which is beautifully defined.[277-A] Mr. Owen believes that, besides the fore-paddles, these short-and stiff-necked saurians were furnished with a tail-fin without bones and purely tegumentary, expanding in a vertical direction; an organ of motion which enabled them to turn their heads rapidly.[277-B]
Mr. Conybeare was enabled, in 1824, after examining many skeletons nearly perfect, to give an ideal restoration of the osteology of this genus, and of that of the Plesiosaurus.[278-A] (See figs. 310, 311.) The latter animal had an extremely long neck and small head, with teeth like those of the crocodile, and paddles analogous to those of the Ichthyosaurus, but larger. It is supposed to have lived in shallow seas and estuaries, and to have breathed air like the Ichthyosaur, and our modern cetacea.[278-B] Some of the reptiles above mentioned were of formidable dimensions. One specimen of Ichthyosaurus platyodon, from the lias at Lyme, now in the British Museum, must have belonged to an animal more than 24 feet in length; and another of the Plesiosaurus, in the same collection, is 11 feet long. The form of the Ichthyosaurus may have fitted it to cut through the waves like the porpoise; but it is supposed that the Plesiosaurus, at least the long-necked species (fig. 311.), was better suited to fish in shallow creeks and bays defended from heavy breakers.
In many specimens both of Ichthyosaur and Plesiosaur the bones of the head, neck, and tail, are in their natural position, while those of the rest of the skeleton are detached and in confusion. Mr. Stutchburg has suggested that their bodies after death became inflated with gases, and, while the abdominal viscera were decomposing, the bones, though disunited, were retained within the tough dermal covering as in a bag, until the whole, becoming water-logged, sank to the bottom.[278-C] As they belonged to individuals of all ages they are supposed, by Dr. Buckland, to have experienced a violent death; and the same conclusion might also be drawn from their having escaped the attacks of their own predaceous race, or of fishes, found fossil in the same beds.
For the last twenty years, anatomists have agreed that these extinct saurians must have inhabited the sea; and it was argued that, as there are now chelonians, like the tortoise, living in fresh water, and others, as the turtle, frequenting the ocean, so there may have been formerly some saurians proper to salt, others to fresh water. The common crocodile of the Ganges is well known to frequent equally that river and the brackish and salt water near its mouth; and crocodiles are said in like manner to be abundant both in the rivers of the Isla de Pinos (or Isle of Pines), south of Cuba, and in the open sea round the coast. More recently a saurian has been discovered of aquatic habits and exclusively marine. This creature was found in the Galapagos Islands, during the visit of H. M. S. Beagle to that archipelago, in 1835, and its habits were then observed by Mr. Darwin. The islands alluded to are situated under the equator, nearly 600 miles to the westward of the coast of South America. They are volcanic, some of them being 3000 or 4000 feet high; and one of them, Albemarle Island, 75 miles long. The climate is mild; very little rain falls; and, in the whole archipelago, there is only one rill of fresh water that reaches the coast. The soil is for the most part dry and harsh, and the vegetation scanty. The birds, reptiles, plants, and insects are, with very few exceptions, of species found no where else in the world, although all partake, in their general form, of a South American type. Of the mammalia, says Mr. Darwin, one species alone appears to be indigenous, namely, a large and peculiar kind of mouse; but the number of lizards, tortoises, and snakes is so great, that it may be called a land of reptiles. The variety, indeed, of species is small; but the individuals of each are in wonderful abundance. There is a turtle, a large tortoise (Testudo Indicus), four lizards, and about the same number of snakes, but no frogs or toads. Two of the lizards belong to the family Iguanidae of Bell, and to a peculiar genus (Amblyrhynchus) established by that naturalist, and so named from their obtusely truncated head and short snout.[279-A] Of these lizards one is terrestrial in its habits, and burrows in the ground, swarming everywhere on the land, having a round tail, and a mouth somewhat resembling in form that of the tortoise. The other is aquatic, and has its tail flattened laterally for swimming (see fig. 313.). "This marine saurian," says Mr. Darwin, "is extremely common on all the islands throughout the archipelago. It lives exclusively on the rocky sea-beaches, and I never saw one even ten yards inshore. The usual length is about a yard, but there are some even 4 feet long. It is of a dirty black colour, sluggish in its movements on the land; but, when in the water, it swims with perfect ease and quickness by a serpentine movement of its body and flattened tail, the legs during this time being motionless, and closely collapsed on its sides. Their limbs and strong claws are admirably adapted for crawling over the rugged and fissured masses of lava which everywhere form the coast. In such situations, a group of six or seven of these hideous reptiles may oftentimes be seen on the black rocks, a few feet above the surf, basking in the sun with outstretched legs. Their stomachs, on being opened, were found to be largely distended with minced sea-weed, of a kind which grows at the bottom of the sea at some little distance from the coast. To obtain this, the lizards go out to sea in shoals. One of these animals was sunk in salt water, from the ship, with a heavy weight attached to it, and on being drawn up again after an hour it was quite active and unharmed. It is not yet known by the inhabitants where this animal lays its eggs; a singular fact, considering its abundance, and that the natives are well acquainted with the eggs of the terrestrial Amblyrhynchus, which is also herbivorous."[280-A]
In those deposits now forming by the sediment washed away from the wasting shores of the Galapagos Islands the remains of saurians, both of the land and sea, as well as of chelonians and fish, may be mingled with marine shells, without any bones of land quadrupeds or batrachian reptiles; yet even here we should expect the remains of marine mammalia to be imbedded in the new strata, for there are seals, besides several kinds of cetacea, on the Galapagian shores; and, in this respect, the parallel between the modern fauna, above described, and the ancient one of the lias, would not hold good.
Sudden destruction of saurians.--It has been remarked, and truly, that many of the fish and saurians, found fossil in the lias, must have met with sudden death and immediate burial; and that the destructive operation, whatever may have been its nature, was often repeated.
"Sometimes," says Dr. Buckland, "scarcely a single bone or scale has been removed from the place it occupied during life; which could not have happened had the uncovered bodies of these saurians been left, even for a few hours, exposed to putrefaction, and to the attacks of fishes, and other smaller animals at the bottom of the sea."[280-B] Not only are the skeletons of the Ichthyosaurs entire, but sometimes the contents of their stomachs still remain between their ribs, as before remarked, so that we can discover the particular species of fish on which they lived, and the form of their excrements. Not unfrequently there are layers of these coprolites, at different depths in the lias, at a distance from any entire skeletons of the marine lizards from which they were derived; "as if," says Sir H. De la Beche, "the muddy bottom of the sea received small sudden accessions of matter from time to time, covering up the coprolites and other exuviae which had accumulated during the intervals."[281-A] It is farther stated that, at Lyme Regis, those surfaces only of the coprolites which lay uppermost at the bottom of the sea have suffered partial decay, from the action of water before they were covered and protected by the muddy sediment that has afterwards permanently enveloped them.[281-B]
Numerous specimens of the pen-and-ink fish (Sepia loligo, Lin.; Loligo vulgaris, Lam.) have also been met with in the lias at Lyme, with the ink-bags still distended, containing the ink in a dried state, chiefly composed of carbon, and but slightly impregnated with carbonate of lime. These cephalopoda, therefore, must, like the saurians, have been soon buried in sediment; for, if long exposed after death, the membrane containing the ink would have decayed.[281-C]
As we know that river fish are sometimes stifled, even in their own element, by muddy water during floods, it cannot be doubted that the periodical discharge of large bodies of turbid fresh water into the sea may be still more fatal to marine tribes. In the Principles of Geology I have shown that large quantities of mud and drowned animals have been swept down into the sea by rivers during earthquakes, as in Java, in 1699; and that undescribable multitudes of dead fishes have been seen floating on the sea after a discharge of noxious vapours during similar convulsions.[281-D] But, in the intervals between such catastrophes, strata may have accumulated slowly in the sea of the lias, some being formed chiefly of one description of shell, such as ammonites, others of gryphites.
From the above remarks the reader will infer that the lias is for the most part a marine deposit. Some members, however, of the series, especially in the lowest part of it, have an estuary character, and must have been formed within the influence of rivers. In Gloucestershire, where there is a good type of the lias of the West of England, it may be divided into an upper mass of shale with a base of marlstone, and a lower series of shales with underlying limestones and shales. We learn from the researches of the Rev. P. B. Brodie[281-E], that in the superior of these two divisions numerous remains of insects and plants have been detected in several places, mingled with marine shells; but in the inferior division similar fossils are still more plentiful. One band, rarely exceeding a foot in thickness, has been named the "insect limestone." It passes upwards into a shale containing Cypris and Estheria, and is charged with the wing-cases of several genera of coleoptera, and with some nearly entire beetles, of which the eyes are preserved. The nervures of the wings of neuropterous insects (fig. 314.) are beautifully perfect in this bed. Ferns, with leaves of monocotyledonous plants, and freshwater shells, such as Cyclas and Unio, accompany the insects in some places, while in others marine shells predominate, the fossils varying apparently as we examine the bed nearer or farther from the ancient land, or the source whence the fresh water was derived. There are two, or even three, bands of "insect limestone" in several sections, and they have been ascertained by Mr. Brodie to retain the same lithological and zoological characters when traced from the centre of Warwickshire to the borders of the southern part of Wales. After studying 300 specimens of these insects from the lias, Mr. Westwood declares that they comprise both wood-eating and herb-devouring beetles of the Linnean genera Elater, Carabus, &c., besides grasshoppers (Gryllus), and detached wings of dragon-flies and may-flies, or insects referable to the Linnean genera Libellula, Ephemera, Hemerobius, and Panorpa, in all belonging to no less than twenty-four families. The size of the species is usually small, and such as taken alone would imply a temperate climate; but many of the associated organic remains of other classes must lead to a different conclusion.
Fossil plants.--Among the vegetable remains of the Lias, several species of Zamia have been found at Lyme Regis, and the remains of coniferous plants at Whitby. Fragments of wood are common, and often converted into limestone. That some of this wood, though now petrified, was soft when it first lay at the bottom of the sea, is shown by a specimen now in the museum of the Geological Society (see fig. 315.), which has the form of an ammonite indented on its surface.
M. Ad. Brongniart enumerates forty-seven liassic acrogens, most of them ferns; and fifty gymnogens, of which thirty-nine are cycads, and eleven conifers. Among the cycads the predominance of Zamites and Nilsonia, and among the ferns the numerous genera with leaves having reticulated veins (as in fig. 296. p. 272.), are mentioned as botanical characteristics of this era.[282-A]
Origin of the Oolite and Lias.--If we now endeavour to restore, in imagination, the ancient condition of the European area at the period of the Oolite and Lias, we must conceive a sea in which the growth of coral reefs and shelly limestones, after proceeding without interruption for ages, was liable to be stopped suddenly by the deposition of clayey sediment. Then, again, the argillaceous matter, devoid of corals, was deposited for ages, and attained a thickness of hundreds of feet, until another period arrived when the same space was again occupied by calcareous sand, or solid rocks of shell and coral, to be again succeeded by the recurrence of another period of argillaceous deposition. Mr. Conybeare has remarked of the entire group of Oolite and Lias, that it consists of repeated alternations of clay, sandstone, and limestone, following each other in the same order. Thus the clays of the lias are followed by the sands of the inferior oolite, and these again by shelly and coralline limestone (Bath oolite, &c.); so, in the middle oolite, the Oxford clay is followed by calcareous grit and "coral rag;" lastly, in the upper oolite, the Kimmeridge clay is followed by the Portland sand and limestone.[283-A] The clay beds, however, as Sir H. De la Beche remarks, can be followed over larger areas than the sands or sandstones.[283-B] It should also be remembered that while the oolitic system becomes arenaceous, and resembles a coal-field in Yorkshire, it assumes, in the Alps, an almost purely calcareous form, the sands and clays being omitted; and even in the intervening tracts, it is more complicated and variable than appears in ordinary descriptions. Nevertheless, some of the clays and intervening limestones do, in reality, retain a pretty uniform character, for distances of from 400 to 600 miles from east to west and north to south.
According to M. Thirria, the entire oolitic group in the department of the Haute-Saone, in France, may be equal in thickness to that of England; but the importance of the argillaceous divisions is in the inverse ratio to that which they exhibit in England, where they are about equal to twice the thickness of the limestones, whereas, in the part of France alluded to, they reach only about a third of that thickness.[283-C] In the Jura the clays are still thinner; and in the Alps they thin out and almost vanish.
In order to account for such a succession of events, we may imagine, first, the bed of the ocean to be the receptacle for ages of fine argillaceous sediment, brought by oceanic currents, which may have communicated with rivers, or with part of the sea near a wasting coast. This mud ceases, at length, to be conveyed to the same region, either because the land which had previously suffered denudation is depressed and submerged, or because the current is deflected in another direction by the altered shape of the bed of the ocean and neighbouring dry land. By such changes the water becomes once more clear and fit for the growth of stony zoophytes. Calcareous sand is then formed from comminuted shell and coral, or, in some cases, arenaceous matter replaces the clay; because it commonly happens that the finer sediment, being first drifted farthest from coasts, is subsequently overspread by coarse sand, after the sea has grown shallower, or when the land, increasing in extent, whether by upheaval or by sediment filling up parts of the sea, has approached nearer to the spots first occupied by fine mud.
In order to account for another great formation, like the Oxford clay, again covering one of coral limestone, we must suppose a sinking down like that which is now taking place in some existing regions of coral between Australia and South America. The occurrence of subsidences, on so vast a scale, may have caused the bed of the ocean and the adjoining land, throughout great parts of the European area, to assume a shape favourable to the deposition of another set of clayey strata; and this change may have been succeeded by a series of events analogous to that already explained, and these again by a third series in similar order. Both the ascending and descending movements may have been extremely slow, like those now going on in the Pacific; and the growth of every stratum of coral, a few feet of thickness, may have required centuries for its completion, during which certain species of organic beings disappeared from the earth, and others were introduced in their place; so that, in each set of strata, from the Upper Oolite to the Lias, some peculiar and characteristic fossils were embedded.
Oolite and Lias of the United States.
There are large tracts on the globe, as in Russia and the United States, where all the members of the oolitic series are unrepresented. In the state of Virginia, however, at the distance of about 13 miles eastward of Richmond, the capital of that State, there is a regular coal-field occurring in a depression of the granite rocks (see section, fig. 316.), which Professor W. B. Rogers first correctly referred to the age of the lower part of the Jurassic group. This opinion I was enabled to confirm after collecting a large number of fossil plants, fish, and shells, and examining the coal-field throughout its whole area. It extends 26 miles from north to south, and from 4 to 12, from east to west. The plants consist chiefly of zamites, calamites, and equisetums, and these last are very commonly met with in a vertical position more or less compressed perpendicularly. It is clear that they grew in the places where they now lie buried in strata of hardened sand and mud. I found them maintaining their erect attitude, at points many miles distant from others, in beds both above and between the seams of coal. In order to explain this fact we must suppose such shales and sandstones to have been gradually accumulated during the slow and repeated subsidence of the whole region.
It is worthy of remark that the Equisetum columnare of these Virginian rocks appears to be undistinguishable from the species found in the oolitic sandstones near Whitby in Yorkshire, where it also is met with in an upright position. One of the American ferns, Pecopteris Whitbyensis, is also a species common to the Yorkshire oolites.[285-A] These Virginian coal-measures are composed of grits, sandstones, and shales, exactly resembling those of older or primary date in America and Europe, and they rival or even surpass the latter in the richness and thickness of the seams. One of these, the main seam, is in some places from 30 to 40 feet thick, composed of pure bituminous coal. On descending a shaft 800 feet deep, in the Blackheath mines in Chesterfield county, I found myself in a chamber more than 40 feet high, caused by the removal of this coal. Timber props of great strength supported the roof, but they were seen to bend under the incumbent weight. The coal is like the finest kinds shipped at Newcastle, and when analysed yields the same proportions of carbon and hydrogen, a fact worthy of notice when we consider that this fuel has been derived from an assemblage of plants very distinct specifically, and in part generically, from those which have contributed to the formation of the ancient or paleozoic coal.
The fossil fish of these Richmond strata belong to the liassic genus Tetragonolepis, and to a new genus which I have called Dictyopyge. Shells are very rare, as usually in all coal-bearing deposits, but a species of Posidonomya is in such profusion in some shaley beds as to divide them like the plates of mica in micaceous shales (see fig. 317.).
In India, especially in Cutch, a formation occurs clearly referable to the oolitic and liassic type, as shown by the shells, corals, and plants; and there also coal has been procured from one member of the group.
FOOTNOTES:
[274-A] Conyb. and Phil. p. 261.
[275-A] Agassiz, Pois. Fos. vol. ii. tab. 28, 29.
[276-A] Bridgewater Treatise, p. 290.
[276-B] Agassiz, Poissons Fossiles, vol. iii. tab. C. fig. 1.
[276-C] Ibid. p. 168.
[276-D] Ibid. p. 187.
[277-A] Geol. Soc. Proceedings, vol. iii. p. 157. 1839.
[277-B] Geol. Trans. Second Series, vol. v. p. 511.
[278-A] Geol. Trans., Second Series, vol. i. pl. 49.
[278-B] Conybeare and De la Beche. Geol. Trans.; and Buckland, Bridgew. Treat., p. 203.
[278-C] Quart. Geol. Journ. vol. ii. p. 411.
[279-A] +Amblys+, amblys, blunt; and +rhygchos+, rhynchus, snout.
[280-A] Darwin's Journal, chap. xix.
[280-B] Bridgew. Treat., p. 125.
[281-A] Geological Researches, p. 334.
[281-B] Buckland, Bridgew. Treat., p. 307.
[281-C] Ibid.
[281-D] See Principles, Index, Lancerote, Graham Island, Calabria.
[281-E] A History of Fossil Insects, &c. 1845. London.
[282-A] Tableau des Veg. Fos. 1849, p. 105.
[283-A] Con. and Phil., p. 166.
[283-B] Geol. Researches, p. 337.
[283-C] Burat's D'Aubuisson, tom. ii. p. 456.
[285-A] See description of the coal-field by the author, and the plants by C. J. F. Bunbury, Esq., Quart. Geol. Journ., vol. iii. p. 281.
XXII. TRIAS OR NEW RED SANDSTONE GROUP
Distinction between New and Old Red Sandstone--Between Upper and Lower New Red--The Trias and its three divisions--Most largely developed in Germany--Keuper and its fossils--Muschelkalk--Fossil plants of Bunter--Triassic group in England--Bone-bed of Axmouth and Aust--Red Sandstone of Warwickshire and Cheshire--Footsteps of Chirotherium in England and Germany--Osteology of the Labyrinthodon--Identification of this Batrachian with the Chirotherium--Origin of Red Sandstone and Rock-salt--Hypothesis of saline volcanic exhalations--Theory of the precipitation of salt from inland lakes or lagoons--Saltness of the Red Sea--New Red Sandstone in the United States--Fossil footprints of birds and reptiles in the Valley of the Connecticut--Antiquity of the Red Sandstone containing them.
Between the Lias and the Coal, or Carboniferous group, there is interposed, in the midland and western counties of England, a great series of red loams, shales, and sandstones, to which the name of the New Red Sandstone formation was first given, to distinguish it from other shales and sandstones called the "Old Red" (c, fig. 318.), often identical in mineral character, which lie immediately beneath the coal (b).
The name of "Red Marl" has been incorrectly applied to the red clays of this formation, as before explained (p. 13.), for they are remarkably free from calcareous matter. The absence, indeed, of carbonate of lime, as well as the scarcity of organic remains, together with the bright red colour of most of the rocks of this group, causes a strong contrast between it and the Jurassic formations before described.
Before the distinctness of the fossil remains characterizing the upper and lower part of the English New Red had been clearly recognized, it was found convenient to have a common name for all the strata intermediate in position between the Lias and Coal; and the term "Poikilitic" was proposed by Messrs. Conybeare and Buckland[286-A], from +poikilos+, poikilos, variegated, some of the most characteristic strata of this group having been called variegated by Werner, from their exhibiting spots and streaks of light-blue, green, and buff colour, in a red base.
A single term, thus comprehending both Upper and Lower New Red, or the Triassic and Permian groups of modern classifications, may still be useful in describing districts where we have to speak of masses of red sandstone and shale, referable, in part, to both these eras, but which, in the absence of fossils, it is impossible to divide.
Trias or Upper New Red Sandstone Group.
The accompanying table will explain the subdivisions generally adopted for the uppermost of the two systems above alluded to, and the names given to them in England and on the Continent.
Synonyms. German. French.
{ a. Saliferous and } } { gypseous shales and } Keuper } Marnes irisees. Trias or Upper { sandstone } } New Red { Sandstone { b. (wanting in England) } Muschelkalk { Muschelkalk, ou { { calcaire { { coquillier. { { c. Sandstone and } Bunter- } Gres bigarre. { quartzose conglomerate } sandstein }
I shall first describe this group as it occurs in South Western and North Western Germany, for it is far more fully developed there than in England or France. It has been called the Trias by German writers, or the Triple Group, because it is separable into three distinct formations, called the "Keuper," the "Muschelkalk," and the "Bunter-sandstein."
The Keuper, the first or newest of these, is 1000 feet thick in Wuertemberg, and is divided by Alberti into sandstone, gypsum, and carbonaceous slate-clay.[287-A] Remains of Reptiles, called Nothosaurus and Phytosaurus, have been found in it with Labyrinthodon; the detached teeth, also, of placoid fish and of rays, and of the genera Saurichthys and Gyrolepis (figs. 325, 326, p. 289.). The plants of the Keuper are generically very analogous to those of the lias and oolite, consisting of ferns, equisetaceous plants, cycads, and conifers, with a few doubtful monocotyledons. A few species, such as Equisetites columnaris, are common to this group, and the oolite.
The Muschelkalk consists chiefly of a compact, greyish limestone, but includes beds of dolomite in many places, together with gypsum and rock-salt. This limestone, a rock wholly unrepresented in England, abounds in fossil shells, as the name implies. Among the cephalopoda there are no belemnites, and no ammonites with foliated sutures, as in the incumbent lias and oolite, but a genus allied to the Ammonite, called Ceratite by De Haan, in which the descending lobes (see a, b, c, fig. 320.) terminate in a few small denticulations pointing inwards. Among the bivalve shells, the Posidonia minuta, Goldf. (Posidonomya minuta, Bronn) (see fig. 321.), is abundant, ranging through the Keuper, Muschelkalk, and Bunter-sandstein; and Avicula socialis, fig. 322., having a similar range, is very characteristic of the Muschelkalk in Germany, France, and Poland.
The abundance of the heads and stems of lily encrinites, Encrinus liliiformis (or Encrinites moniliformis), show the slow manner in which some beds of this limestone have been formed in clear sea-water.
The Bunter-sandstein consists of various coloured sandstones, dolomites, and red-clays, with some beds, especially in the Hartz, of calcareous pisolite or roe-stone, the whole sometimes attaining a thickness of more than 1000 feet. The sandstone of the Vosges, according to Von Meyer, is proved, by the presence of Labyrinthodon, to belong to this lowest member of the Triassic group. At Sulzbad (or Soultz-les-bains), near Strasburg, on the flanks of the Vosges, many plants have been obtained from the "bunter," especially conifers of the extinct genus Voltzia, peculiar to this period, in which even the fructification has been preserved. (See fig. 323.)
Out of thirty species of ferns, cycads, conifers, and other plants, enumerated by M. Ad. Brongniart, in 1849, as coming from the "gres bigarre," or Bunter, not one is common to the Keuper.[288-A]
The footprints of a reptile (Labyrinthodon) have been observed on the clays of this member of the Trias, near Hildburghausen, in Saxony, impressed on the upper surface of the beds, and standing out as casts in relief from the under sides of incumbent slabs of sandstone. To these I shall again allude in the sequel; they attest, as well as the accompanying ripple-marks, and the cracks which traverse the clays, the gradual formation in shallow water, and sometimes between high and low water, of the beds of this formation.
Triassic group in England.
In England the Lias is succeeded by conformable strata of red and green marl, or clay. There intervenes, however, both in the neighbourhood of Axmouth, in Devonshire, and in the cliffs of Westbury and Aust, in Gloucestershire, on the banks of the Severn, a dark-coloured stratum, well known by the name of the "bone-bed." It abounds in the remains of saurians and fish, and was formerly classed as the lowest bed of the Lias; but Sir P. Egerton has shown that it should be referred to the Upper New Red Sandstone, for it contains an assemblage of fossil fish which are either peculiar to this stratum, or belong to species well known in the Muschelkalk of Germany. These fish belong to the genera Acrodus, Hybodus, Gyrolepis, and Saurichthys.
Among those common to the English bone-bed and the Muschelkalk of Germany are Hybodus plicatilis (fig. 324.), Saurichthys apicalis (fig. 325.), Gyrolepis tenuistriatus (fig. 326.), and G. Albertii. Remains of saurians have also been found in the bone-bed, and plates of an Encrinus.
The strata of red and green marl, which follow the bone-bed in the descending order at Axmouth and Aust, are destitute of organic remains; as is the case, for the most part, in the corresponding beds in almost every part of England. But fossils have lately been found at a few localities in sandstones of this formation, in Worcestershire and Warwickshire, and among them the bivalve shell called Posidonia minuta, Goldf., before mentioned (fig. 321. p. 288.).
The upper member of the English "New Red" containing this shell, in those parts of England, is, according to Messrs. Murchison and Strickland, 600 feet thick, and consists chiefly of red marl or slate, with a band of sandstone. Spines of Hybodus, called ichthyodorulites, teeth of fishes, and footprints of reptiles, with remains of a saurian called Rhyncosaurus, were observed by the same geologists in these strata.[290-A]
In Cheshire and Lancashire the gypseous and saliferous red shales and loams of the Trias are between 1000 and 1500 feet thick. In some places lenticular masses of rock-salt are interpolated between the argillaceous beds, the origin of which will be spoken of in the sequel.
The lower division or English representative of the "Bunter" attains a thickness of 600 feet in the counties last mentioned. Besides red and green shales and red sandstones, it comprises much soft white quartzose sandstone, in which the trunks of silicified trees have been met with at Allesley Hill, near Coventry. Several of them were a foot and a half in diameter, and some yards in length, decidedly of coniferous wood, and showing rings of annual growth.[290-B] Impressions, also, of the footsteps of animals have been detected in Lancashire and Cheshire in this formation. Some of the most remarkable occur a few miles from Liverpool, in the whitish quartzose sandstone of Storton Hill, on the west side of the Mersey. They bear a close resemblance to tracks first observed in a member of the Upper New Red Sandstone, at the village of Hesseberg, near Hildburghausen, in Saxony, to which I have already alluded. For many years these footprints have been referred to a large unknown quadruped, provisionally named Chirotherium by Professor Kaup, because the marks both of the fore and hind feet resembled impressions made by a human hand. (See fig. 327.) The footmarks at Hesseberg are partly concave and partly in relief; the former, or the depressions, are seen upon the upper surface of the sandstone slabs, but those in relief are only upon the lower surfaces, being in fact natural casts, formed in the subjacent footprints as in moulds. The larger impressions, which seem to be those of the hind foot, are generally 8 inches in length, and 5 in width, and one was 12 inches long. Near each large footstep, and at a regular distance (about an inch and a half), before it, a smaller print of a fore foot, 4 inches long and 3 inches wide, occurs. The footsteps follow each other in pairs, each pair in the same line, at intervals of 14 inches from pair to pair. The large as well as the small steps show the great toes alternately on the right and left side; each step makes the print of five toes, the first or great toe being bent inwards like a thumb. Though the fore and hind foot differ so much in size, they are nearly similar in form.
The similar footmarks afterwards observed in a rock of corresponding age at Storton Hill, were imprinted on five thin beds of clay, superimposed one upon the other in the same quarry, and separated by beds of sandstone. On the lower surface of the sandstone strata, the solid casts of each impression are salient, in high relief, and afford models of the feet, toes, and claws of the animals which trod on the clay.
As neither in Germany nor in England any bones or teeth had been met with in the same identical strata as the footsteps, anatomists indulged, for several years, in various conjectures respecting the mysterious animals from which they might have been derived. Professor Kaup suggested that the unknown quadruped might have been allied to the Marsupialia; for in the kangaroo the first toe of the fore foot is in a similar manner set obliquely to the others, like a thumb, and the disproportion between the fore and hind feet is also very great. But M. Link conceived that some of the four species of animals of which the tracks had been found in Saxony might have been gigantic Batrachians; and Dr. Buckland designated some of the footsteps as those of a small web-footed animal, probably crocodilean.
In the course of these discussions several naturalists of Liverpool, in their report on the Storton quarries, declared their opinion that each of the thin seams of clay in which the sandstone casts were moulded had formed successively a surface above water, over which the Chirotherium and other animals walked, leaving impressions of their footsteps, and that each layer had been afterwards submerged by a sinking down of the surface, so that a new beach was formed at low water above the former, on which other tracks were then made. The repeated occurrence of ripple-marks at various heights and depths in the red sandstone of Cheshire had been explained in the same manner. It was also remarked that impressions of such depth and clearness could only have been made by animals walking on the land, as their weight would have been insufficient to make them sink so deeply in yielding clay under water. They must therefore have been air-breathers.
When the inquiry had been brought to this point, the reptilian remains discovered in the Trias, both of Germany and England, were carefully examined by Mr. Owen. He found, after a microscopic investigation of the teeth from the German sandstone called Keuper, and from the sandstone of Warwick and Leamington, that neither of them could be referred to true saurians, although they had been named Mastodonsaurus and Phytosaurus by Jaeger (fig. 329.). It appeared that they were of the Batrachian order, and attested the former existence of frogs of gigantic dimensions in comparison with any now living. Both the Continental and English fossil teeth exhibited a most complicated texture, differing from that previously observed in any reptile, whether recent or extinct, but most nearly analogous to the Ichthyosaurus. A section of one of these teeth exhibits a series of irregular folds, resembling the labyrinthic windings of the surface of the brain; and from this character Mr. Owen has proposed the name Labyrinthodon for the new genus. By his permission, the annexed representation (fig. 330.) of part of one is given from his "Odontography," plate 64. A. The entire length of this tooth is supposed to have been about three inches and a half, and the breadth at the base one inch and a half.
When Mr. Owen had satisfied himself, from an inspection of the cranium, jaws, and teeth, that a gigantic Batrachian had existed at the period of the Trias or Upper New Red Sandstone, he soon found, from the examination of various bones derived from the same formation, that he could define three species of Labyrinthodon, and that in this genus the hind extremities were much larger than the anterior ones. This circumstance, coupled with the fact of the Labyrinthodon having existed at the period when the Chirotherian footsteps were made, was the first step towards the identification of those tracks with the newly discovered Batrachian. It was at the same time observed that the footmarks of Chirotherium were more like those of toads than of any other living animal; and, lastly, that the size of the three species of Labyrinthodon corresponded with the size of three different kinds of footprints which had already been supposed to belong to three distinct Chirotheria. It was moreover inferred, with confidence, that the Labyrinthodon was an air-breathing reptile from the structure of the nasal cavity, in which the posterior outlets were at the back part of the mouth, instead of being directly under the anterior or external nostrils. It must have respired air after the manner of saurians, and may therefore have imprinted on the shore those footsteps, which, as we have seen, could not have originated from an animal walking under water.
It is true that the structure of the foot is still wanting, and that a more connected and complete skeleton is required for demonstration; but the circumstantial evidence above stated is strong enough to produce the conviction that the Chirotherium and Labyrinthodon are one and the same.
In order to show the manner in which one of these formidable Batrachians may have impressed the mark of its feet upon the shore, Mr. Owen has attempted a restoration, of which a reduced copy is annexed.
The only bones of this species at present known are those of the head, the pelvis, and part of the scapula, which are shown by stronger lines in the above figure. There is reason for believing that the head was not smooth externally, but protected by bony scutella.
Origin of Red Sandstone and Rock Salt.
We have seen that, in various parts of the world, red and mottled clays, and sandstones, of several distinct geological epochs, are found associated with salt, gypsum, magnesian limestone, or with one or all of these substances. There is, therefore, in all likelihood, a general cause for such a coincidence. Nevertheless, we must not forget that there are dense masses of red and variegated sandstones and clays, thousands of feet in thickness, and of vast horizontal extent, wholly devoid of saliferous or gypseous matter. There are also deposits of gypsum and of muriate of soda, as in the blue clay formation of Sicily, without any accompanying red sandstone or red clay.
To account for deposits of red mud and red sand, we have simply to suppose the disintegration of ordinary crystalline or metamorphic schists. Thus, in the eastern Grampians of Scotland, as, for example, in the north of Forfarshire, the mountains of gneiss, mica-schist, and clay-slate, are overspread with alluvium, derived from the disintegration of those rocks; and the mass of detritus is stained by oxide of iron, of precisely the same colour as the Old Red Sandstone of the adjoining Lowlands. Now this alluvium merely requires to be swept down to the sea, or into a lake, to form strata of red sandstone and red marl, precisely like the mass of the "Old Red" or New Red systems of England, or those tertiary deposits of Auvergne (see p. 182.), before described, which are in lithological characters quite undistinguishable. The pebbles of gneiss in the Eocene red sandstone of Auvergne point clearly to the rocks from which it has been derived. The red colouring matter may, as in the Grampians, have been furnished by the decomposition of hornblende, or mica, which contain oxide of iron in large quantity.
It is a general fact, and one not yet accounted for, that scarcely any fossil remains are preserved in stratified rocks in which this oxide of iron abounds; and when we find fossils in the New or Old Red Sandstone in England, it is in the grey, and usually calcareous beds, that they occur.
The gypsum and saline matter, occasionally interstratified with such red clays and sandstones of various ages, primary, secondary, and tertiary, have been thought by some geologists to be of volcanic origin. Submarine and subaerial exhalations often occur in regions of earthquakes and volcanos far from points of actual eruption, and charged with sulphur, sulphuric salts, and with common salt or muriate of soda. In a word, they are vents by which all the products which issue in a state of sublimation from the craters of active volcanos, obtain a passage from the interior of the earth to the surface. That such gaseous emanations and mineral springs, impregnated with the ingredients before enumerated, and often intensely heated, continue to flow out unaltered in composition and temperature for ages, is well known. But before we can decide on their real instrumentality in producing in the course of ages beds of gypsum, salt, and dolomite, we require to know what are the chemical changes actually in progress in seas where this volcanic agency is at work.
Yet the origin of rock-salt is a problem of so much interest in theoretical geology as to demand a full discussion of another hypothesis advanced on the subject; namely, that which attributes the precipitation of the salt to evaporation, whether of inland lakes or of lagoons communicating with the ocean.
At Northwich, in Cheshire, two beds of salt, in great part unmixed with earthy matter, attain the extraordinary thickness of 90 and even 100 feet. The upper surface of the highest bed is very uneven, forming cones and irregular figures. Between the two masses there intervenes a bed of indurated clay, traversed with veins of salt. The highest bed thins off towards the south-west, losing 15 feet in thickness in the course of a mile.[295-A] The horizontal extent of these particular masses in Cheshire and Lancashire is not exactly known; but the area, containing saliferous clays and sandstones, is supposed to exceed 150 miles in diameter, while the total thickness of the trias in the same region is estimated by Mr. Ormerod at more than 1700 feet. Ripple-marked sandstones, and the footprints of animals, before described, are observed at so many levels that we may safely assume the whole area to have undergone a slow and gradual depression during the formation of the Red Sandstone. The evidence of such a movement, wholly independent of the presence of salt itself, is very important in reference to the theory under consideration.
In the "Principles of Geology" (chap. 28.), I published a map, furnished to me by the late Sir Alexander Burnes, of that singular flat region called the Runn of Cutch, near the delta of the Indus, which is 7000 square miles in area, or equal in extent to about one-fourth of Ireland. It is neither land nor sea, but is dry during a part of every year, and again covered by salt water during the monsoons. Some parts of it are liable, after long intervals, to be overflowed by river-water. Its surface supports no grass, but is encrusted over, here and there, by a layer of salt, about an inch in depth, caused by the evaporation of sea-water. Certain tracts have been converted into dry land by upheaval during earthquakes since the commencement of the present century, and, in other directions, the boundaries of the Runn have been enlarged by subsidence. That successive layers of salt might be thrown down, one upon the other, over thousands of square miles, in such a region, is undeniable. The supply of brine from the ocean would be as inexhaustible as the supply of heat from the sun to cause evaporation. The only assumption required to enable us to explain a great thickness of salt in such as area is, the continuance, for an indefinite period, of a subsiding movement, the country preserving all the time a general approach to horizontality. Pure salt could only be formed in the central parts of basins, where no sand could be drifted by the wind, or sediment be brought by currents. Should the sinking of the ground be accelerated, so as to let in the sea freely, and deepen the water, a temporary suspension of the precipitation of salt would be the only result. On the other hand, if the area should dry up, ripple-marked sands and the footprints of animals might be formed, where salt had previously accumulated. According to this view the thickness of the salt, as well as of the accompanying beds of mud and sand, becomes a mere question of time, or requires simply a repetition of similar operations.
Mr. Hugh Miller, in an able discussion of this question, refers to Dr. Frederick Parrot's account, in his journey to Ararat (1836), of the salt lakes of Asia. In several of these lakes west of the river Manech, "the water, during the hottest season of the year, is covered on its surface with a crust of salt nearly an inch thick, which is collected with shovels into boats. The crystallization of the salt is effected by rapid evaporation from the sun's heat and the supersaturation of the water with muriate of soda; the lake being so shallow that the little boats trail on the bottom and leave a furrow behind them, so that the lake must be regarded as a wide pan of enormous superficial extent, in which the brine can easily reach the degree of concentration required."
Another traveller, Major Harris, in his "Highlands of Ethiopia," describes a salt lake, called the Bahr Assal, near the Abyssinian frontier, which once formed the prolongation of the Gulf of Tadjara, but was afterwards cut off from the gulf by a broad bar of lava or of land upraised by an earthquake. "Fed by no rivers, and exposed in a burning climate to the unmitigated rays of the sun, it has shrunk into an elliptical basin, seven miles in its transverse axis, half filled with smooth water of the deepest caerulian hue, and half with a solid sheet of glittering snow-white salt, the offspring of evaporation." "If," says Mr. Hugh Miller, "we suppose, instead of a barrier of lava, that sand-bars were raised by the surf on a flat arenaceous coast during a slow and equable sinking of the surface, the waters of the outer gulf might occasionally topple over the bar, and supply fresh brine when the first stock had been exhausted by evaporation.[296-A]
We may add that the permanent impregnation of the waters of a large shallow basin with salt, beyond the proportion which is usual in the ocean, would cause it to be uninhabitable by mollusca or fish, as is the case in the Dead Sea, and the muriate of soda might remain in excess, even though it were occasionally replenished by irruptions of the sea. Should the saline deposit be eventually submerged, it might, as we have seen from the example of the Runn of Cutch, be covered by a freshwater formation containing fluviatile organic remains; and in this way the apparent anomaly of beds of sea-salt and clays devoid of marine fossils, alternating with others of freshwater origin, may be explained.
Dr. G. Buist, in a recent communication to the Bombay Geographical Society (vol. ix.), has asked how it happens that the Red Sea should not exceed the open ocean in saltness, by more than 1/10th per cent. The Red Sea receives no supply of water from any quarter save through the Straits of Babelmandeb; and there is not a single river or rivulet flowing into it from a circuit of 4000 miles of shore. The countries around are all excessively sterile and arid, and composed, for the most part, of burning deserts. From the ascertained evaporation in the sea itself, Dr. Buist computes that nearly 8 feet of pure water must be carried off from the whole of its surface annually, this being probably equivalent to 1/100th part of its whole volume. The Red Sea, therefore, ought to have 1 per cent. added annually to its saline contents; and as these constitute 4 per cent. by weight, or 2-1/2 per cent. in volume of its entire mass, it ought, assuming the average depth to be 800 feet, which is supposed to be far beyond the truth, to have been converted into one solid salt formation in less than 3000 years.[297-A] Does the Red Sea receive a supply of water from the ocean, through the narrow Straits of Babelmandeb, sufficient to balance the loss by evaporation? And is there an undercurrent of heavier saline water annually flowing outwards? If not, in what manner is the excess of salt disposed of? An investigation of this subject by our nautical surveyors may perhaps aid the geologist in framing a true theory of the origin of rock-salt.
On the New Red Sandstone of the valley of the Connecticut River in the United States.
In a depression of the granitic or hypogene rocks in the States of Massachusetts and Connecticut, strata of red sandstone, shale, and conglomerate are found occupying an area more than 150 miles in length from north to south, and about 5 to 10 miles in breadth, the beds dipping to the eastward at angles varying from 5 to 50 degrees. The extreme inclination of 50 degrees is rare, and only observed in the neighbourhood of masses of trap which have been intruded into the red sandstone while it was forming, or before the newer parts of the deposit had been completed. Having examined this series of rocks in many places, I feel satisfied that they were formed in shallow water, and for the most part near the shore, and that some of the beds were from time to time raised above the level of the water, and laid dry, while a newer series, composed of similar sediment, was forming. The red flags of thin-bedded sandstone are often ripple-marked, and exhibit on their under sides casts of cracks formed in the underlying red and green shales. These last must have shrunk by drying before the sand was spread over them. On some shales of the finest texture impressions of rain drops may be seen, and casts of them in the incumbent argillaceous sandstones. Having observed similar markings produced by showers, of which the precise date was known, on the recent red mud of the Bay of Fundy, and casts in relief of the same, on layers of dried mud thrown down by subsequent tides, I feel no doubt in regard to the origin of some of the ancient Connecticut impressions. I have also seen on the mud-flats of the Bay of Fundy the footmarks of birds (Tringa minuta), which daily run along the borders of that estuary at low water, and which I have described in my Travels.[297-B] Similar layers of red mud, now hardened and compressed into shale, are laid open on the banks of the Connecticut, and retain faithfully the impressions and casts of the feet of numerous birds and reptiles which walked over them at the time when they were deposited, probably in the Triassic Period.
According to Professor Hitchcock, the footprints of no less than thirty-two species of bipeds, and twelve of quadrupeds, have been already detected in these rocks. Thirty of these are believed to be those of birds, four of lizards, two of chelonians, and six of batrachians. The tracks have been found in more than twenty places, scattered through an extent of nearly 80 miles from north to south, and they are repeated through a succession of beds attaining at some points a thickness of more than 1000 feet, which may have been thousands of years in forming.[298-A]
As considerable scepticism is naturally entertained in regard to the nature of the evidence derived from footprints, it may be well to enumerate some facts respecting them on which the faith of the geologist may rest. When I visited the United States in 1842, more than 2000 impressions had been observed by Professor Hitchcock, in the district alluded to, and all of them were indented on the upper surface of the layers, while the corresponding casts, standing out in relief, were always on the lower surfaces or planes of the strata. If we follow a single line of marks we find them uniform in size, and nearly uniform in distance from each other, the toes of two successive footprints, turning alternately right and left (see fig. 332.). Such single lines indicate a biped; and there is generally such a deviation from a straight line, in any three successive prints, as we remark in the tracks left by birds. There is also a striking relation between the distance separating two footprints in one series and the size of the impressions; in other words, an obvious proportion between the length of the stride and the dimension of the creature which walked over the mud. If the marks are small, they may be half an inch asunder; if gigantic, as, for example, where the toes are 20 inches long, they are occasionally 4 feet and a half apart. The bipedal impressions are for the most part trifid, and show the same number of joints as exist in the feet of living tridactylous birds. Now such birds have three phalangeal bones for the inner toe, four for the middle and five for the outer one (see fig. 332.); but the impression of the terminal joint is that of the nail only. The fossil footprints exhibit regularly, where the joints are seen, the same number; and we see in each continuous line of tracks the three-jointed and five-jointed toes placed alternately outwards, first on the one side and then on the other. It is not often that the matrix has been fine enough to retain impressions of the integument or skin of the foot; but in one fine specimen found at Turner's Falls on the Connecticut, by Dr. Deane, these markings are well preserved, and have been recognized by Mr. Owen as resembling the skin of the ostrich, and not that of reptiles.[298-B] Much care is required to ascertain the precise layer of a laminated rock on which an animal has walked, because the impression usually extends downwards through several laminae; and if the upper layer originally trodden upon is wanting, one or more joints, or even in some cases an entire toe, which sank less deep into the soft ground, may disappear, and yet the remainder of the footprint be well defined.
The size of several of the fossil impressions of the Connecticut red sandstone so far exceeds that of any living ostrich, that naturalists at first were extremely adverse to the opinion of their having been made by birds, until the bones and almost entire skeleton of the Dinornis and of other feathered giants of New Zealand were discovered. Their dimensions have at least destroyed the force of this particular objection. The magnitude of the impressions of the feet of a heavy animal, which has walked on soft mud, increases for some distance below the surface originally trodden upon. In order, therefore, to guard against exaggeration, the casts rather than the mould are relied on. These casts show that some of the fossil birds had feet four times as large as the ostrich, but not perhaps larger than the Dinornis.
Some of the quadrupedal footprints which accompany those of birds are analogous to European Chirotheria, and with a similar disproportion between the hind and fore feet. Others resemble that remarkable reptile, the Rhyncosaurus of the English Trias, a creature having some relation in its osteology both to chelonians and birds. Other imprints, again, are like those of turtles.
Among the supposed bipedal tracks, a single distinct example only has been observed of feet in which there are four toes directed forwards. In this case a series of four footprints is seen, each 22 inches long and 12 wide, with joints much resembling those in the toes of birds. Professor Agassiz has suggested that it might have belonged to a gigantic bipedal batrachian; but the evidence on this subject is too defective to warrant such a bold conjecture, and if we were to give the reins to our imagination, we might as well conceive a bird having four toes projecting forwards as a huge two-legged frog. Nor should we forget that some quadrupeds place the hind foot so precisely on the spot just quitted by the fore foot, as to produce a single line of imprints like a biped.
No bones have as yet been met with, whether of reptiles or birds, in the rocks of the Connecticut, but there are numerous coprolites; and an ingenious argument has been derived by Mr. Dana, from the analysis of these bodies, and the proportion they contain of uric acid, phosphate of lime, carbonate of lime, and organic matter, to show that, like guano, they are the droppings of birds, rather than of reptiles.[299-A]
Mr. Darwin, in his "Journal of a Voyage in the Beagle," informs us that the "South American ostriches, although they live on vegetable matter, such as roots and grass, are repeatedly seen at Bahia Blanca (lat. 39 deg. S.), on the coast of Buenos Ayres, coming down at low water to the extensive mud-banks which are then dry, for the sake, as the Gauchos say, of feeding on small fish." They readily take to the water, and have been seen at the bay of San Blas, and at Port Valdez, in Patagonia, swimming from island to island.[300-A] It is therefore evident, that in our times a South American mud-bank might be trodden simultaneously by ostriches, alligators, tortoises, and frogs; and the impressions left, in the nineteenth century, by the feet of these various tribes of animals, would not differ from each other more entirely than do those attributed to birds, saurians, chelonians, and batrachians, in the rocks of the Connecticut.
To determine the exact age of the red sandstone and shale containing these ancient footprints in the United States, is not possible at present. No fossil shells have yet been found in the deposit, nor plants in a determinable state. The fossil fish are numerous and very perfect; but they are of a peculiar type, which was originally referred to the genus Palaeoniscus, but has since, with propriety, been ascribed, by Sir Philip Egerton, to a new genus. To this he has given the name of Ischypterus, from the great size and strength of the fulcral rays of the dorsal fin (from +ischys+; strength, and +pteron+, a fin). They differ from Palaeoniscus, as Mr. Redfield first pointed out, by having the vertebral column prolonged to a more limited extent into the upper lobe of the tail, or, in the language of M. Agassiz, they are less heterocercal. The teeth also, according to Sir P. Egerton, who, in 1844, examined for me a fine series of specimens which I procured at Durham, Connecticut, differ from those of Palaeoniscus in being strong and conical.
That the sandstones containing these fish are of older date than the strata containing coal, before described (p. 284.) as occurring near Richmond in Virginia, is highly probable. These were shown to be as old at least as the oolite and lias. The higher antiquity of the Connecticut beds cannot be proved by direct superposition, but may be presumed from the general structure of the country. That structure proves them to be newer than the movements to which the Appalachian or Alleghany chain owes its flexures, and this chain includes the ancient coal formation among its contorted rocks. The unconformable position of this New Red with ornithichnites on the edges of the inclined primary or paleozoic rocks of the Appalachians is seen at 4. of the section, fig. 379. p. 327. The absence of fish with decidedly heterocercal tails may afford an argument against the Permian age of the formation; and the opinion that the red sandstone is triassic, seems, on the whole, the best that we can embrace in the present state of our knowledge.
FOOTNOTES:
[286-A] Buckland, Bridgew. Treat., vol. ii. p. 38.
[287-A] Monog. des Bunten Sandsteins.
[288-A] Tableau des Genres de Veg. Fos., Dict. Univ. 1849.
[290-A] Geol. Trans., Second Series, vol. v.
[290-B] Buckland, Proc. Geol. Soc. vol. ii. p. 439.; and Murchison and Strickland Geol. Trans., Second Ser., vol. v. p. 347.
[295-A] Ormerod, Quart. Geol. Journ. 1848, vol. iv. p. 277.
[296-A] Hugh Miller, First Impressions of England, 1847, pp. 183. 214.
[297-A] Buist, Trans. of Bombay Geograph. Soc. 1850, vol. ix. p. 38.
[297-B] Travels in North America, vol. ii. p. 168.
[298-A] Hitchcock, Mem. of Amer. Acad. New Ser., vol. iii. p. 129.
[298-B] This specimen is now in Dr. Mantell's museum.
[299-A] Amer. Journ. of Sci. vol. xlviii. p. 46.
[300-A] Journal of Voyage of Beagle, &c. 2d edition, p. 89. 1845.
XXIII. PERMIAN OR MAGNESIAN LIMESTONE GROUP
Fossils of Magnesian Limestone and Lower New Red distinct from the Triassic--Term Permian--English and German equivalents--Marine shells and corals of English Magnesian limestone--Palaeoniscus and other fish of the marl slate--Thecodont Saurians of dolomitic conglomerate of Bristol--Zechstein and Rothliegendes of Thuringia--Permian Flora--Its generic affinity to the carboniferous--Psaronites or tree-ferns.
When the use of the term "Poikilitic" was explained in the last chapter, I stated, that in some parts of England it is scarcely possible to separate the red marls and sandstones so called (originally named "the New Red"), into two distinct geological systems. Nevertheless, the progress of investigation, and a careful comparison of English rocks between the lias and the coal with those occupying a similar geological position in Germany and Russia, has enabled geologists to divide the Poikilitic formation; and has even shown that the lowermost of the two divisions is more closely connected, by its fossil remains, with the carboniferous group than with the trias. If, therefore, we are to draw a line between the secondary and primary fossiliferous strata, as between the tertiary and secondary, it must run through the middle of what was once called the "New Red," or Poikilitic group. The inferior half of this group will rank as Primary or Paleozoic, while its upper member will form the base of the Secondary series. For the lower, or Magnesian Limestone division of English geologists, Sir R. Murchison has proposed the name of Permian, from Perm, a Russian government where these strata are more extensively developed than elsewhere, occupying an area twice the size of France, and containing an abundant and varied suite of fossils.
Mr. King, in his valuable monograph, recently published, of the Permian fossils of England, has given a table of the following six members of the Permian system of the north of England, with what he conceives to be the corresponding formations in Thuringia.[301-A]
North of England. Thuringia.
1. Crystalline or concretionary, |1. Stinkstein. and non-crystalline limestone. | 2. Brecciated and pseudo-brecciated |2. Rauchwacke. limestone. | 3. Fossiliferous limestone. |3. Dolomit, or Upper Zechstein. 4. Compact limestone. |4. Zechstein, or Lower Zechstein. 5. Marl-slate. |5. Mergel-schiefer, or Kupferschiefer. 6. Inferior sandstones of various |6. Rothliegendes. colours. |
I shall proceed, therefore, to treat briefly of these subdivisions, beginning with the highest, and referring the reader, for a fuller description of the lithological character of the whole group, as it occurs in the north of England, to a valuable memoir by Professor Sedgwick, published in 1835.[302-A]
Crystalline or concretionary limestone (No. 1.).--This formation is seen upon the coast of Durham and Yorkshire, between the Wear and the Tees. Among its characteristic fossils are Schizodus Schlotheimi (fig. 333.) and Mytilus septifer (fig. 335.).
These shells occur at Hartlepool and Sunderland, where the rock assumes an oolitic and botryoidal character. Some of the beds in this division are ripple-marked; and Mr. King imagines that the absence of corals and the character of the shells indicate shallow water. In some parts of the coast of Durham, where the rock is not crystalline, it contains as much as forty-four per cent. of carbonate of magnesia, mixed with carbonate of lime. In other places,--for it is extremely variable in structure,--it consists chiefly of carbonate of lime, and has concreted into globular and hemispherical masses, varying from the size of a marble to that of a cannon-ball, and radiating from the centre. Occasionally earthy and pulverulent beds pass into compact limestone or hard granular dolomite. The stratification is very irregular, in some places well-defined, in others obliterated by the concretionary action which has re-arranged the materials of the rocks subsequently to their original deposition. Examples of this are seen at Pontefract and Ripon in Yorkshire.
The brecciated limestone (No. 2.) contains no fragments of foreign rocks, but seems composed of the breaking-up of the Permian limestone itself, about the time of its consolidation. Some of the angular masses in Tynemouth Cliff are 2 feet in diameter. This breccia is considered by Professor Sedgwick as one of the forms of the preceding limestone, No. 1., rather than as regularly underlying it. The fragments are angular and never water-worn, and appear to have been re-cemented on the spot where they were formed. It is, therefore, suggested that they may have been due to those internal movements of the mass which produced the concretionary structure; but the subject is very obscure, and after studying the phenomenon in the Marston Rocks, on the coast of Durham, I found it impossible to form any positive opinion on the subject. The well-known brecciated limestones of the Pyrenees appeared to me to present the nearest analogy, but on a much smaller scale.
The fossiliferous limestone (No. 3.) is regarded by Mr. King as a deep-water formation, from the numerous delicate corals which it includes. One of these, Fenestella retiformis (fig. 336.), is a very variable species, and has received many different names. It sometimes attains a large size, measuring 8 inches in width. The same zoophyte is also found abundantly in the Permian of Germany.
Shells of the genera Spirifer and Productus, which do not occur in strata newer than the Permian, are abundant in this division of the series in the ordinary yellow magnesian limestone. (See figs. 337, 338.)
The compact limestone (No. 4.) also contains organic remains, especially corallines, and is intimately connected with the preceding. Beneath it lies the marl-slate (No. 5.), which consists of hard, calcareous shales, marl-slate, and thin-bedded limestones. At East Thickley, in Durham, where it is thirty feet thick, this slate has yielded many fine specimens of fossil fish of the genera Palaeoniscus, Pygopterus, Coelacanthus, and Platysomus, genera which are all found in the coal-measures of the carboniferous epoch, and which therefore, says Mr. King, probably lived at no great distance from the shore. But the Permian species are peculiar, and, for the most part, identical with those found in the marl-slate or copper-slate of Thuringia.
The Palaeoniscus above mentioned belongs to that division of fishes which M. Agassiz has called "Heterocercal," which have their tails unequally bilobate, like the recent shark and sturgeon, and the vertebral column running along the upper caudal lobe. (See fig. 340.) The "Homocercal" fish, which comprise almost all the 8000 species at present known in the living creation, have the tail-fin either single or equally divided; and the vertebral column stops short, and is not prolonged into either lobe. (See fig. 341.)
Now it is a singular fact, first pointed out by Agassiz, that the heterocercal form, which is confined to a small number of genera in the existing creation, is universal in the Magnesian limestone, and all the more ancient formations. It characterizes the earlier periods of the earth's history, when the organization of fishes made a greater approach to that of saurian reptiles than at later epochs. In all the strata above the Magnesian limestone the homocercal tail predominates.
A full description has been given by Sir Philip Egerton of the species of fish characteristic of the marl-slate in Mr. King's monograph before referred to, where figures of the ichthyolites which are very entire and well preserved, will be found. Even a single scale is usually so characteristically marked as to indicate the genus, and sometimes even the particular species. They are often scattered through the beds singly, and maybe useful to a geologist in determining the age of the rock.
[2 Illustrations: Scales of fish. Magnesian limestone.
Fig. 346. Pygopterus mandibularis, Ag. Marl-slate. a. Outside of scale magnified. b. Under surface of same.
Fig. 347. Acrolepis Sedgwickii, Ag. Marl-slate.]
The inferior sandstones (No. 6. Tab. p. 301.), which lie beneath the marl-slate, consist of sandstone and sand, separating the magnesian limestone from the coal, in Yorkshire and Durham. In some instances, red marl and gypsum have been found associated with these beds. They have been classed with the magnesian limestone by Professor Sedgwick, as being nearly co-extensive with it in geographical range, though their relations are very obscure. In some regions we find it stated that the imbedded plants are all specifically identical with those of the carboniferous series; and, if so, they probably belong to that epoch; for the true Permian flora appears, from the researches of MM. Murchison and de Verneuil in Russia, and of Colonel von Gutbier in Saxony, to be, with few exceptions, distinct from that of the coal (see p. 307.).
Dolomitic conglomerate of Bristol.--Near Bristol, in Somersetshire, and in other counties bordering the Severn, the unconformable beds of the Lower New Red, resting immediately upon the Coal, consist of a conglomerate called "dolomitic," because the pebbles of older rocks are cemented together by a red or yellow base of dolomite or magnesian limestone. This conglomerate or breccia, for the imbedded fragments are sometimes angular, occurs in patches over the whole of the downs near Bristol, filling up the hollows and irregularities in the mountain limestone, and being principally composed at every spot of the debris of those rocks on which it immediately rests. At one point we find pieces of coal shale, in another of mountain limestone, recognizable by its peculiar shells and zoophytes. Fractured bones, also, and teeth of saurians, are dispersed through some parts of the breccia.
These saurians (which until the discovery of the Archegosaurus in the coal were the most ancient examples of fossil reptiles) are all distinguished by having the teeth implanted deeply in the jaw-bone, and in distinct sockets, instead of being soldered, as in frogs, to a simple alveolar parapet. In the dolomitic conglomerate near Bristol the remains of species of two distinct genera have been found, called Thecodontosaurus and Palaeosaurus by Dr. Riley and Mr. Stutchbury[306-A]; the teeth of which are conical, compressed, and with finely serrated edges (figs. 348 and 349.).
In Russia, also, Thecodont saurians occur, in beds of the Permian age, of several genera, while others named Protorosaurus are met with in the Zechstein of Thuringia. This family of reptiles is allied to the living monitor, and its appearance in a primary or paleozoic formation, observes Mr. Owen, is opposed to the doctrine of the progressive development of reptiles from fish, or from simpler to more complex forms; for, if they existed at the present day, these monitors would take rank at the head of the Lacertian order.[306-B]
In Russia the Permian rocks are composed of white limestone, with gypsum and white salt; and of red and green grits, with occasionally copper ore; also magnesian limestones, marlstones, and conglomerates.
The country of Mansfeld, in Thuringia, may be called the classic ground of the Lower New Red, or Magnesian Limestone, or Permian formation, on the Continent. It consists there principally of, first, the Zechstein, corresponding to the upper portion of our English series; and, secondly, the marl-slate, with fish of species identical with those of the bed so called in Durham. This slaty marlstone is richly impregnated with copper pyrites, for which it is extensively worked. Magnesian limestone, gypsum, and rock-salt, occur among the superior strata of this group. At its base lies the Rothliegendes, supposed to correspond with the Inferior or Lower New Red Sandstone above mentioned, which occupies a similar place in England between the marl-slate and coal. Its local name of Rothliegendes, red-lyer, or "Roth-todt-liegendes," red-dead-lyer, was given by the workmen in the German mines from its red colour, and because the copper has died out when they reach this rock, which is not metalliferous. It is, in fact, a great deposit of red sandstone and conglomerate, with associated porphyry, basaltic trap, and amygdaloid.
Permian Flora.--We learn from the recent investigation of Colonel von Gutbier, that in the Permian rocks of Saxony no less than sixty species of fossil plants have been met with, forty of which have not yet been found elsewhere. Two or three of these, as Calamites gigas, Sphenopteris erosa, and S. lobata, are also met with in the government of Perm in Russia. Seven others, and among them Neuropteris Loshii, Pecopteris arborescens, and P. similis, with several species of Walchia (Lycopodites), are common to the coal-measures.
Among the genera also enumerated by Colonel Gutbier are Asterophyllites and Annularia, so characteristic of the carboniferous period; also Lepidodendron, which is common to the Permian of Saxony, Thuringia, and Russia, although not abundant. Noeggerathia (see fig. 350.), supposed by A. Brongniart to be allied to Cycas, is another link between the Permian and carboniferous vegetation. Coniferae, of the Araucarian division, also occur; but these are likewise met with both in older and newer rocks. The plants called Sigillaria and Stigmaria, so marked a feature in the carboniferous period, are as yet wanting.
Among the remarkable fossils of the rothliegendes, or lowest part of the Permian in Saxony and Bohemia, are the silicified trunks of tree-ferns called generically Psaronius. Their bark was surrounded by a dense mass of air-roots, which often constituted a great addition to the original stem, so as to double or quadruple its diameter. The same remark holds good in regard to certain living extra-tropical arborescent ferns, particularly those of New Zealand.
Psaronites are also found in the uppermost coal of Autun in France, and in the upper coal-measures of the State of Ohio in the United States, but specifically different from those of the rothliegendes. They serve to connect the Permian flora with the more modern portion of the preceding or carboniferous group. Upon the whole, it is evident that the Permian plants approach nearer to the carboniferous ones than to the triassic; and the same may be said of the Permian fauna.
FOOTNOTES:
[301-A] Palaeontographical Society, 1848, London.
[302-A] Trans. Geol. Soc. Lond., Second Series, vol. iii. p. 37.
[303-A] King's Monograph, pl. 2.
[306-A] See paper by Messrs. Riley and Stutchbury, Geol. Trans., Second Series, vol. v. p. 349., plate 29., figures 2. and 5.
[306-B] Owen, Report on Reptiles, British Assoc., Eleventh Meeting, 1841, p. 197.
[307-A] Murchison's Russia, vol. ii. pl. A. fig. 3.