XXIII. THE COAL OR CARBONIFEROUS GROUP…

XXIII. THE COAL OR CARBONIFEROUS GROUP

Principal Subdivisions of the Carboniferous Group. — Different Thickness of the sedimentary and calcareous Members in Scotland and the South of England. — Coal-measures. — Terrestrial Nature of the Growth of Coal. — Erect fossil Trees. — Uniting of many Coal-seams into one thick Bed. — Purity of the Coal explained. — Conversion of Coal into Anthracite. — Origin of Clay-ironstone. — Marine and brackish-water Strata in Coal. — Fossil Insects. — Batrachian Reptiles. — Labyrinthodont Foot-prints in Coal-measures. — Nova Scotia Coal-measures with successive Growths of erect fossil Trees. — Similarity of American and European Coal. — Air-breathers of the American Coal. — Changes of Condition of Land and Sea indicated by the Carboniferous Strata of Nova Scotia.

Principal Subdivisions of the Carboniferous Group.—The next group which we meet with in the descending order is the Carboniferous, commonly called “The Coal,” because it contains many beds of that mineral, in a more or less pure state, interstratified with sandstones, shales, and limestones. The coal itself, even in Great Britain and Belgium, where it is most abundant, constitutes but an insignificant portion of the whole mass. In South Wales, for example, the thickness of the coal-bearing strata has been estimated at between 11,000 and 12,000 feet, while the various coal seams, about 80 in number, do not, according to Professor Phillips, exceed in the aggregate 120 feet.

The Carboniferous formation assumes various characters in different parts even of the British Islands. It usually comprises two very distinct members: first, the sedimentary beds, usually called the Coal-measures, of mixed fresh-water, terrestrial, and marine origin, often including seams of coal; second, that named in England the Mountain or Carboniferous Limestone, of purely marine origin, and made up chiefly of corals, shells, and encrinites, and resting on shales called the shales of the Mountain Limestone.

In the south-western part of our island, in Somersetshire and South Wales, the three divisions usually spoken of are:

Coal-measures: Strata of shale, sandstone, and grit, from 600 to 12,000 feet thick, with occasional seams of coal.

Millstone grit: A coarse quartzose sandstone passing into a conglomerate, sometimes used for millstones, with beds of shale; usually devoid of coal; occasionally above 600 feet thick.

Mountain or Carboniferous Limestone: A calcareous rock containing marine shells, corals, and encrinites; devoid of coal; thickness variable, sometimes more than 1500 feet.

If the reader will refer to the section in Fig. 85, he will see that the Upper and Lower Coal-measures of the coal-field near Bristol are divided by a micaceous flaggy sandstone called the Pennant Rock. The Lower Coal-measures of the same section rest sometimes, especially in the north part of the basin, on a base of coarse grit called the Millstone Grit (No. 2 on the previous page).

In the South Welsh coal-field Millstone Grit occurs in like manner at the base of the productive coal. It is called by the miners the “Farewell Rock,” as when they reach it they have no longer any hopes of obtaining coal at a greater depth in the same district. In the central and northern coal-fields of England this same grit, including quartz pebbles, with some accompanying sandstones and shales containing coal plants, acquires a thickness of several thousand feet, lying beneath the productive coal-measures, which are nearly 10,000 feet thick.

Below the Millstone Grit is a continuation of similar sandstones and shales called by Professor Phillips the Yoredale series, from Yoredale, in Yorkshire, where they attain a thickness of from 800 to 1000 feet. At several intervals bands of limestone divide this part of the series, one of which, called the Main Limestone or Upper Scar Limestone, composed in great part of encrinites, is 70 feet thick. Thin seams of coal also occur in these lower Yoredale beds in Yorkshire, showing that in the same region there were great alternations in the state of the surface. For at successive periods in the same area there prevailed first terrestrial conditions favourable to the growth of pure coal, secondly, a sea of some depth suited to the formation of Carboniferous Limestone, and, thirdly, a supply of muddy sediment and sand, furnishing the materials for sandstone and shale. There is no clear line of demarkation between the Coal-measures and the Millstone Grit, nor between the Millstone Grit and underlying Yoredale rocks.

On comparing a series of vertical sections in a north-westerly direction from Leicestershire and Warwickshire into North Lancashire, we find, says Mr. Hull, within a distance of 120 miles an augmentation of the sedimentary materials to the extent of 16,000 feet.

Leicestershire and Warwickshire 2,600 feet North Staffordshire 9,000 feet South Lancashire 12,130 feet North Lancashire 18,700 feet

In central England, where the sedimentary beds are reduced to about 3000 feet in all, the Carboniferous Limestone attains an enormous thickness, as much as 4000 feet at Ashbourne, near Derby, according to Mr. Hull’s estimate. To a certain extent, therefore, we may consider the calcareous member of the formation as having originated simultaneously with the accumulation of the materials of grit, sandstone, and shale, with seams of coal; just as strata of mud, sand, and pebbles, several thousand feet thick, with layers of vegetable matter, are now in the process of formation in the cypress swamps and delta of the Mississippi, while coral reefs are forming on the coast of Florida and in the sea of the Bermuda islands. For we may safely conclude that in the ancient Carboniferous ocean those marine animals which were limestone builders were never freely developed in areas where the rivers poured in fresh water charged with sand or clay; and the limestone could only become several thousand feet thick in parts of the ocean which remained perfectly clear for ages.

The calcareous strata of the Scotch coal-fields, those of Lanarkshire, the Lothians, and Fife, for example, are very insignificant in thickness when compared to those of England. They consist of a few beds intercalated between the sandstones and shales containing coal and ironstone, the combined thickness of all the limestones amounting to no more than 150 feet. The vegetation of some of these northern sedimentary beds containing coal may be older than any of the coal-measures of central and southern England, as being coeval with the Mountain Limestone of the south. In Ireland the limestone predominates over the coal-bearing sands and shales. We may infer the former continuity of several of the coal-fields in northern and central England, not only from the abrupt manner in which they are cut off at their outcrop, but from their remarkable correspondence in the succession and character of particular beds. But the limited extent to which these strata are exposed at the surface is not merely owing to their former denudation, but even in a still greater degree to their having been largely covered by the New Red Sandstone, as in Cheshire, and here and there by the Permian strata, as in Durham.

It has long been the opinion of the most eminent geologists that the coal-fields of Yorkshire and Lancashire were once united, the upper Coal-measures and the overlying Millstone Grit and Yoredale rocks having been subsequently removed; but what is remarkable, is the ancient date now assigned to this denudation, for it seems that a thickness of no less than 10,000 feet of the coal-measures had been carried away before the deposition even of the lower Permian rocks which were thrown down upon the already disturbed truncated edges of the coal-strata.[1] The carboniferous strata most productive of workable coal have so often a basin-shaped arrangement that these troughs have sometimes been supposed to be connected with the original conformation of the surface upon which the beds were deposited. But it is now admitted that this structure has been owing to movements of the earth’s crust of upheaval and subsidence, and that the flexure and inclination of the beds has no connection with the original geographical configuration of the district.

COAL-MEASURES.

I shall now treat more particularly of the productive coal-measures, and their mode of origin and organic remains.

Coal formed on Land.—In South Wales, already alluded to, where the coal-measures attain a thickness of 12,000 feet, the beds throughout appear to have been formed in water of moderate depth, during a slow, but perhaps intermittent, depression of the ground, in a region to which rivers were bringing a never-failing supply of muddy sediment and sand. The same area was sometimes covered with vast forests, such as we see in the deltas of great rivers in warm climates, which are liable to be submerged beneath fresh or salt water should the ground sink vertically a few feet.

In one section near Swansea, in South Wales, where the total thickness of strata is 3246 feet, we learn from Sir H. De la Beche that there are ten principal masses of sandstone. One of these is 500 feet thick, and the whole of them make together a thickness of 2125 feet. They are separated by masses of shale, varying in thickness from 10 to 50 feet. The intercalated coal-beds, sixteen in number, are generally from one to five feet thick, one of them, which has two or three layers of clay interposed, attaining nine feet. At other points in the same coal-field the shales predominate over the sandstones. Great as is the diversity in the horizontal extent of individual coal-seams, they all present one characteristic feature, in having, each of them, what is called its underclay. These underclays, co-extensive with every layer of coal, consist of arenaceous shale, sometimes called fire-stone, because it can be made into bricks which stand the fire of a furnace. They vary in thickness from six inches to more than ten feet; and Sir William Logan first announced to the scientific world in 1841 that they were regarded by the colliers in South Wales as an essential accompaniment of each of the eighty or more seams of coal met with in their coal-field. They are said to form the floor on which the coal rests; and some of them have a slight admixture of carbonaceous matter, while others are quite blackened by it.

All of them, as Sir William Logan pointed out, are characterised by inclosing a peculiar species of fossil vegetable called Stigmaria, to the exclusion of other plants. It was also observed that, while in the overlying shales, or “roof” of the coal, ferns and trunks of trees abound without any Stigmariæ, and are flattened and compressed, those singular plants of the underclay most commonly retain their natural forms, unflattened and branching freely, and sending out their slender rootlets, formerly thought to be leaves, through the mud in all directions. Several species of Stigmaria had long been known to botanists, and described by them, before their position under each seam of coal was pointed out, and before their true nature as the roots of trees (some having been actually found attached to the base of Sigillaria stumps) was recognised. It was conjectured that they might be aquatic, perhaps floating plants, which sometimes extended their branches and leaves freely in fluid mud, in which they were finally enveloped.

Now that all agree that these underclays are ancient soils, it follows that in every instance where we find them they attest the terrestrial nature of the plants which formed the overlying coal, which consists of the trunks, branches, and leaves of the same plants. The trunks have generally fallen prostrate in the coal, but some of them still remain at right angles to the ancient soils (see Fig. 440). Professor Goppert, after examining the fossil vegetables of the coal-fields of Germany, has detected, in beds of pure coal, remains of plants of every family hitherto known to occur fossil in the carboniferous rocks. Many seams, he remarks, are rich in Sigillariæ, Lepidodendra, and Stigmariæ, the latter in such abundance as to appear to form the bulk of the coal. In some places, almost all the plants were calamites, in others ferns.[2]

Between the years 1837 and 1840, six fossil trees were discovered in the coal-fields of Lancashire, where it is intersected by the Bolton railway. They were all at right angles to the plane of the bed, which dips about 15 degrees to the south. The distance between the first and the last was more than 100 feet, and the roots of all were imbedded in a soft argillaceous shale. In the same plane with the roots is a bed of coal, eight or ten inches thick, which has been found to extend across the railway, or to the distance of at least ten yards. Just above the covering of the roots, yet beneath the coal-seam, so large a quantity of the Lepidostrobus variabilis was discovered inclosed in nodules of hard clay, that more than a bushel was collected from the small openings around the base of some of the trees (see Fig. 457 of this genus). The exterior trunk of each was marked by a coating of friable coal, varying from one-quarter to three-quarters of an inch in thickness; but it crumbled away on removing the matrix. The dimensions of one of the trees is 15½ feet in circumference at the base, 7½ feet at the top, its height being eleven feet. All the trees have large spreading roots, solid and strong, sometimes branching, and traced to a distance of several feet, and presumed to extend much farther.

In a colliery near Newcastle a great number of Sigillariæ occur in the rock as if they had retained the position in which they grew. No less than thirty, some of them four or five feet in diameter, were visible within an area of 50 yards square, the interior being sandstone, and the bark having been converted into coal. Such vertical stems are familiar to our coal-miners, under the name of coal-pipes. They are much dreaded, for almost every year in the Bristol, Newcastle, and other coal-fields, they are the cause of fatal accidents. Each cylindrical cast of a tree, formed of solid sandstone, and increasing gradually in size towards the base, and being without branches, has its whole weight thrown downward, and receives no support from the coating of friable coal which has replaced the bark. As soon, therefore, as the cohesion of this external layer is overcome, the heavy column falls suddenly in a perpendicular or oblique direction from the roof of the gallery whence coal has been extracted, wounding or killing the workman who stands below. It is strange to reflect how many thousands of these trees fell originally in their native forests in obedience to the law of gravity; and how the few which continued to stand erect, obeying, after myriads of ages, the same force, are cast down to immolate their human victims.

It has been remarked that if, instead of working in the dark, the miner was accustomed to remove the upper covering of rock from each seam of coal, and to expose to the day the soils on which ancient forests grew, the evidence of their former growth would be obvious. Thus in South Staffordshire a seam of coal was laid bare in the year 1844, in what is called an open work at Parkfield colliery, near Wolverhampton. In the space of about a quarter of an acre the stumps of no less than 73 trees with their roots attached appeared, as shown in Fig. 429, some of them more than eight feet in circumference. The trunks, broken off close to the root, were lying prostrate in every direction, often crossing each other. One of them measured 15, another 30 feet in length, and others less. They were invariably flattened to the thickness of one or two inches, and converted into coal. Their roots formed part of a stratum of coal ten inches thick, which rested on a layer of clay two inches thick, below which was a second forest resting on a two-foot seam of coal. Five feet below this, again, was a third forest with large stumps of Lepidodendra, Calamites, and other trees.

Fig. 429: Ground plan of fossil forest, Parkfield Colliery, near Wolverhampton, showing the position of 73 trees in a quarter of an ace. Blending of Coal-seams.—Both in England and North America seams of coal are occasionally observed to be parted from each other by layers of clay and sand, and, after they have been persistent for miles, to come together and blend in one single bed, which is then found to be equal in the aggregate to the thickness of the several seams. I was shown by Mr. H. D. Rogers a remarkable example of this in Pennsylvania. In the Shark Mountain, near Pottsville, in that State, there are thirteen seams of anthracite coal, some of them more than six feet thick, separated by beds of white quartzose grit and a conglomerate of quartz pebbles, often of the size of a hen’s egg. Between Pottsville and the Lehigh Summit Mine, seven of these seams of coal, at first widely separated, are, in the course of several miles, brought nearer and nearer together by the gradual thinning out of the intervening coarse-grained strata and their accompanying shales, until at length they successively unite and form one mass of coal between forty and fifty feet thick, very pure on the whole, though with a few thin partings of clay. This mass of coal I saw quarried in the open air at Mauch Chunk, on the Bear Mountain. The origin of such a vast thickness of vegetable remains, so unmixed, on the whole, with earthy ingredients, can be accounted for in no other way than by the growth, during thousands of years, of trees and ferns in the manner of peat—a theory which the presence of the Stigmaria in situ under each of the seven layers of anthracite fully bears out. The rival hypothesis, of the drifting of plants into a sea or estuary, leaves the non-intermixture of sediment, or of clay, sand, and pebbles, with the pure coal wholly unexplained.

The late Mr. Bowman was the first who gave a satisfactory explanation of the manner in which distinct coal-seams, after maintaining their independence for miles, may at length unite, and then persist throughout another wide area with a thickness equal to that which the separate seams had previously maintained.

Fig. 430: Uniting of distinct coal-seams.

Let A C (Fig. 430) be a three-foot seam of coal originally laid down as a mass of vegetable matter on the level area of an extensive swamp, having an under-clay, f g, through which the Stigmariæ or roots of the trees penetrate as usual. One portion, B C, of this seam of coal is now inclined; the area of the swamp having subsided as much as 25 feet at E C, and become for a time submerged under salt, fresh, or brackish water. Some of the trees of the original forest A B C fell down, others continued to stand erect in the new lagoon, their stumps and part of their trunks becoming gradually enveloped in layers of sand and mud, which at length filled up the new piece of water C E.

When this lagoon has been entirely silted up and converted into land, the forest-covered surface A B will extend once more over the whole area A B E, and a second mass of vegetable matter, D E, forming three feet more of coal, will accumulate. We then find in the region E C two seams of coals, each three feet thick, with their respective under-clays, with erect buried trees based upon the surface of the lower coal, the two seams being separated by 25 feet of intervening shale and sandstone. Whereas in the region A B, where the growth of the forest has never been interrupted by submergence, there will simply be one seam, two yards thick, corresponding to the united thickness of the beds B E and B C. It may be objected that the uninterrupted growth of plants during the interval of time required for the filling up of the lagoon will have caused the vegetable matter in the region D A B to be thicker than the two distinct seams E and C, and no doubt there would actually be a slight excess representing one or more generation of trees and plants forming the undergrowth; but this excess of vegetable matter, when compressed into coal, would be so insignificant in thickness that the miner might still affirm that the seam D A throughout the area D A B was equal to the two seams C and E.

Cause of the Purity of Coal.—The purity of the coal itself, or the absence in it of earthy particles and sand, throughout areas of vast extent, is a fact which appears very difficult to explain when we attribute each coal-seam to a vegetation growing in swamps. It has been asked how, during river inundations capable of sweeping away the leaves of ferns and the stems and roots of Sigillariæ and other trees, could the waters fail to transport some fine mud into the swamps? One generation after another of tall trees grew with their roots in mud, and their leaves and prostrate trunks formed layers of vegetable matter, which was afterwards covered with mud since turned to shale. Yet the coal itself, or altered vegetable matter, remained all the while unsoiled by earthy particles. This enigma, however perplexing at first sight, may, I think, be solved by attending to what is now taking place in deltas. The dense growth of reeds and herbage which encompasses the margins of forest-covered swamps in the valley and delta of the Mississippi is such that the fluviatile waters, in passing through them, are filtered and made to clear themselves entirely before they reach the areas in which vegetable matter may accumulate for centuries, forming coal if the climate be favourable. There is no possibility of the least intermixture of earthy matter in such cases. Thus in the large submerged tract called the “Sunk Country,” near New Madrid, forming part of the western side of the valley of the Mississippi, erect trees have been standing ever since the year 1811-12, killed by the great earthquake of that date; lacustrine and swamp plants have been growing there in the shallows, and several rivers have annually inundated the whole space, and yet have been unable to carry in any sediment within the outer boundaries of the morass, so dense is the marginal belt of reeds and brush-wood. It may be affirmed that generally, in the “cypress swamps” of the Mississippi, no sediment mingles with the vegetable matter accumulated there from the decay of trees and semi-aquatic plants. As a singular proof of this fact, I may mention that whenever any part of a swamp in Louisiana is dried up, during an unusually hot season, and the wood set on fire, pits are burnt into the ground many feet deep, or as far down as the fire can descend without meeting with water, and it is then found that scarcely any residuum or earthy matter is left. At the bottom of all these “cypress swamps” a bed of clay is found, with roots of the tall cypress (Taxodium distichum), just as the under-clays of the coal are filled with Stigmaria.

Conversion of Coal into Anthracite.—It appears from the researches of Liebig and other eminent chemists, that when wood and vegetable matter are buried in the earth exposed to moisture, and partially or entirely excluded from the air, they decompose slowly and evolve carbonic acid gas, thus parting with a portion of their original oxygen. By this means they become gradually converted into lignite or wood-coal, which contains a larger proportion of hydrogen than wood does. A continuance of decomposition changes this lignite into common or bituminous coal, chiefly by the discharge of carbureted hydrogen, or the gas by which we illuminate our streets and houses. According to Bischoff, the inflammable gases which are always escaping from mineral coal, and are so often the cause of fatal accidents in mines, always contain carbonic acid, carbureted hydrogen, nitrogen, and olefiant gas. The disengagement of all these gradually transforms ordinary or bituminous coal into anthracite, to which the various names of glance-coal, coke, hard-coal, culm, and many others, have been given.

There is an intimate connection between the extent to which the coal has in different regions parted with its gaseous contents, and the amount of disturbance which the strata have undergone. The coincidence of these phenomena may be attributed partly to the greater facility afforded for the escape of volatile matter, when the fracturing of the rocks has produced an infinite number of cracks and crevices. The gases and water which are made to penetrate these cracks are probably rendered the more effective as metamorphic agents by increased temperature derived from the interior. It is well known that, at the present period, thermal waters and hot vapours burst out from the earth during earthquakes, and these would not fail to promote the disengagement of volatile matter from the Carboniferous rocks.

In Pennsylvania the strata of coal are horizontal to the westward of the Alleghany Mountains, where the late Professor H. D. Rogers pointed out that they were most bituminous; but as we travel south-eastward, where they no longer remain level and unbroken, the same seams become progressively debitumenized in proportion as the rocks become more bent and distorted. At first, on the Ohio River, the proportion of hydrogen, oxygen, and other volatile matters ranges from forty to fifty per cent. Eastward of this line, on the Monongahela, it still approaches forty per cent, where the strata begin to experience some gentle flexures. On entering the Alleghany Mountains, where the distinct anticlinal axes begin to show themselves, but before the dislocations are considerable, the volatile matter is generally in the proportion of eighteen or twenty per cent. At length, when we arrive at some insulated coal-fields associated with the boldest flexures of the Appalachian chain, where the strata have been actually turned over, as near Pottsville, we find the coal to contain only from six per cent of volatile matter, thus becoming a genuine anthracite.

Clay-ironstone.—Bands and nodules of clay-ironstone are common in coal-measures, and are formed, says Sir H. De la Beche, of carbonate of iron mingled mechanically with earthy matter, like that constituting the shales. Mr. Hunt, of the Museum of Practical Geology, instituted a series of experiments to illustrate the production of this substance, and found that decomposing vegetable matter, such as would be distributed through all coal strata, prevented the further oxidation of the proto-salts of iron, and converted the peroxide into protoxide by taking a portion of its oxygen to form carbonic acid. Such carbonic acid, meeting with the protoxide of iron in solution, would unite with it and form a carbonate of iron; and this mingling with fine mud, when the excess of carbonic acid was removed, might form beds or nodules of argillaceous ironstone.[3]

Intercalated Marine Beds in Coal.—Both in the coal-fields of Europe and America the association of fresh, brackish-water, and marine strata with coal-seams of terrestrial origin is frequently recognised. Thus, for example, a deposit near Shrewsbury, probably formed in brackish water, has been described by Sir R. Murchison as the youngest member of the coal-measures of that district, at the point where they are in contact with the overlying Permian group. It consists of shales and sandstones about 150 feet thick, with coal and traces of plants; including a bed of limestone varying from two to nine feet in thickness, which is cellular, and resembles some lacustrine limestones of France and Germany. It has been traced for 30 miles in a straight line, and can be recognised at still more distant points. The characteristic fossils are a small bivalve, having the form of a Cyclas or Cyrena, also a small entomostracan, Cythere inflata (Fig. 432), and the microscopic shell of an annelid of an extinct genus called Microconchus (Fig. 431), allied to Spirorbis. In the coal-field of Yorkshire there are fresh-water strata, some of which contain shells referred to the family Unionidæ; but in the midst of the series there is one thin but very widely-spread stratum, abounding in fishes and marine shells, such as Goniatites Listeri (Fig. 433), Orthoceras, and Aviculopecten papyraceus, Goldf. (Fig. 434).

Fig. 431: Microconchus (Spirorbis) carbonarius. Fig. 432: Cythere (Leperditia) inflata. Fig. 433: Goniatites Listeri. Fig. 434: Aviculopecten papyraceus. Insects in European Coal.—Articulate animals of the genus Scorpion were found by Count Sternberg in 1835 in the coal-measures of Bohemia, and about the same time in those of Coalbrook Dale by Mr. Prestwich, were also true insects, such as beetles of the family Curculionidæ, a neuropterous insect of the genus Corydalis, and another related to the Phasmidæ, have been found.

From the coal of Wetting, in Westphalia, several specimens of the cockroach or Blatta family, and the wing of a cricket (Acridites) have been described by Germar. Professor Goldenberg published, in 1854, descriptions of no less than twelve species of insects from the nodular clay-ironstone of Saarbrück, near Trèves.[4] Among them are several Blattinæ, three species of Neuroptera, one beetle of the Scarabæus family, a grasshopper or locust, Gryllacris (see Fig. 435), and several white ants or Termites. Professor Goldenberg showed me, in 1864, the wing of a white ant, found low down in the productive coal-measures of Saarbrück, in the interior of a flattened Lepidodendron. It is much larger than that of any known living species of the same genus.

Fig. 435: Wing of a Grasshopper. Gryllacris lithanthraca.

Fig. 436: Archegosaurus minor. Fossil reptile from the coal-measures, Saarbrück. Batrachian Reptiles in Coal.—No vertebrated animals more highly organised than fish were known in rocks of higher antiquity than the Permian until the year 1844, when the Apateon pedestris, Meyer, was discovered in the coal-measures of Munster-Appel in Rhenish Bavaria, and three years later, in 1847, Professor von Dechen found three other distinct species of the same family of Amphibia in the Saarbruck coal-field above alluded to. These were described by the late Professor Goldfuss under the generic name of Archegosaurus. The skulls, teeth, and the greater portions of the skeleton, nay, even a large part of the skin, of two of these reptiles have been faithfully preserved in the centre of spheroidal concretions of clay-ironstone. The largest of these, Archegosaurus Decheni, must have been three feet six inches long. Figure 436 represents the skull and neck bones of the smallest of the three, of the natural size. They were considered by Goldfuss as saurians, but by Herman von Meyer as most nearly allied to the Labyrinthodon before mentioned (p. 371), and the remains of the extremities leave no doubt they were quadrupeds, “provided,” says Von Meyer, “with hands and feet terminating in distinct toes; but these limbs were weak, serving only for swimming or creeping.” The same anatomist has pointed out certain points of analogy between their bones and those of the Proteus anguinus; and Professor Owen has observed that they make an approach to the Proteus in the shortness of their ribs. Two specimens of these ancient reptiles retain a large part of the outer skin, which consisted of long, narrow, wedge-shaped, tile-like, and horny scales, arranged in rows (see Fig. 437).

Fig. 437: Imbricated covering of skin of Archegosaurus medius. In 1865, several species belonging to three different genera of the same family of perennibranchiate Batrachians were found in the coal-field of Kilkenny in bituminous shale at the junction of the coal with the underlying Stigmaria-bearing clay. They were, probably, inhabitants of a marsh, and the large processes projecting from the vertebræ of their tail imply, according to Professor Huxley, great powers of swimming. They were of the Labyrinthodont family, and their association with the fish of the coal, of which so large a proportion are ganoids, reminds us that the living perennibranchiate amphibia of America frequent the same rivers as the ganoid Lepidostei or bony pikes.

Labyrinthodont footprints in coal-measures.—In 1844, the very year when the Apateon, before mentioned, of the coal was first met with in the country between the Moselle and the Rhine, Dr. King published an account of the footprints of a large reptile discovered by him in North America. These occur in the coal-strata of Greensburg, in Westmoreland County, Pennsylvania; and I had an opportunity of examining them when in that country in 1846. The footmarks were first observed standing out in relief from the lower surface of slabs of sandstone, resting on thin layers of fine unctuous clay. I brought away one of these masses, which is represented in Fig. 438. It displays, together with footprints, the casts of cracks (a, a′) of various sizes. The origin of such cracks in clay, and casts of the same, has before been explained, and referred to the drying and shrinking of mud, and the subsequent pouring of sand into open crevices. It will be seen that some of the cracks, as at b, c, traverse the footprints, and produce distortion in them, as might have been expected, for the mud must have been soft when the animal walked over it and left the impressions; whereas, when it afterwards dried up and shrank, it would be too hard to receive such indentations.

Fig. 438: Slab of sandstone from the coal-measures of Pennsylvania, with foot-prints of air-breathing reptile and casts of cracks.

We may assume that the reptile which left these prints on the ancient sands of the coal-measures was an air-breather, because its weight would not have been sufficient under water to have made impressions so deep and distinct. The same conclusion is also borne out by the casts of the cracks above described, for they show that the clay had been exposed to the air and sun, so as to have dried and shrunk.

Nova Scotia Coal-measures.—The sedimentary strata in which thin seams of coal occur attain a thickness, as we have seen, of 18,000 feet in the north of England exclusive of the Mountain Limestone, and are estimated by Von Dechen at over 20,000 feet in Rhenish Prussia. But the finest example in the world of a natural exposure in a continuous section ten miles long, occurs in the sea-cliffs bordering a branch of the Bay of Fundy, in Nova Scotia. These cliffs, called the “South Joggins,” which I first examined in 1842, and afterwards with Dr. Dawson in 1845, have lately been admirably described by the last-mentioned geologist[5] in detail, and his evidence is most valuable as showing how large a portion of this dense mass was formed on land, or in swamps where terrestrial vegetation flourished, or in fresh-water lagoons. His computation of the thickness of the whole series of carboniferous strata as exceeding three miles, agrees with the measurement made independently by Sir William Logan in his survey of this coast.

There is no reason to believe that in this vast succession of strata, comprising some marine as well as many fresh-water and terrestrial formations, there is any repetition of the same beds. There are no faults to mislead the geologist, and cause him to count the same beds over more than once, while some of the same plants have been traced from the top to the bottom of the whole series, and are distinct from the flora of the antecedent Devonian formation of Canada. Eighty-one seams of coal, varying in thickness from an inch to about five feet, have been discovered, and no less than seventy-one of these have been actually exposed in the sea-cliffs.

In the section (Fig. 439), which I examined in 1842, the beds from c to i are seen all dipping the same way, their average inclination being at an angle of 24° S.S.W. The vertical height of the cliffs is from 150 to 200 feet; and between d and g—in which space I observed seventeen trees in an upright position, or, to speak more correctly, at right angles to the planes of stratification—I counted nineteen seams of coal, varying in thickness from two inches to four feet. At low tide a fine horizontal section of the same beds is exposed to view on the beach, which at low tide extends sometimes 200 yards from the base of the cliff. The thickness of the beds alluded to, between d and g, is about 2500 feet, the erect trees consisting chiefly of large Sigillariæ, occurring at ten distinct levels, one above the other. The usual height of the buried trees seen by me was from six to eight feet; but one trunk was about 25 feet high and four feet in diameter, with a considerable bulge at the base. In no instance could I detect any trunk intersecting a layer of coal, however thin; and most of the trees terminated downward in seams of coal. Some few only were based on clay and shale; none of them, except Calamites, on sandstone. The erect trees, therefore, appeared in general to have grown on beds of vegetable matter. In the underclays Stigmaria abounds.

Fig. 439: Section of the cliffs of the South Joggins, near Minudie, Nova Scotia.

These root-bearing beds have been found under all the coal-seams, and such old soils are at present the most destructible masses in the whole cliff, the sandstones and laminated shales being harder and more capable of resisting the action of the waves and the weather. Originally the reverse was doubtless true, for in the existing delta of the Mississippi those clays in which the innumerable roots of the deciduous cypress and other swamp trees ramify in all directions are seen to withstand far more effectually the undermining power of the river, or of the sea at the base of the delta, than do beds of loose sand or layers of mud not supporting trees. It is obvious that if this sand or mud be afterwards consolidated and turned to sandstone and hard shale, it would be the least destructible.

In regard to the plants, they belonged to the same genera, and most of them to the same species, as those met with in the distant coal-fields of Europe. Dr. Dawson has enumerated more than 150 species, two-thirds of which are European, a greater agreement than can be said to exist between the same Nova Scotia flora and that of the coal-fields of the United States. By referring to the section, Fig. 439, the position of the four-foot coal will be perceived, and in Fig. 440 (a section made by me in 1842 of a small portion) that from e to f of the same cliff is exhibited, in order to show the manner of occurrence of erect fossil trees at right angles to the planes of the inclined strata.

Fig. 440: Erect fossil trees, Coal-measures, Nova Scotia. In the sandstone which filled their interiors, I frequently observed fern-leaves, and sometimes fragments of Stigmaria, which had evidently entered together with sediment after the trunk had decayed and become hollow, and while it was still standing under water. Thus the tree, a, Fig. 440, represented in the bed e in the section, Fig. 439, is a hollow trunk five feet eight inches in length, traversing various strata, and cut off at the top by a layer of clay two feet thick, on which rests a seam of coal (b, Fig. 440) one foot thick. On this coal again stood two large trees (c and d), while at a greater height the trees f and g rest upon a thin seam of coal (e), and above them is an underclay, supporting the four-foot coal.

Occasionally the layers of matter in the inside of the tree are more numerous than those without; but it is more common in the coal-measures of all countries to find a cylinder of pure sandstone—the cast of the interior of a tree—intersecting a great many alternating beds of shale and sandstone, which originally enveloped the trunk as it stood erect in the water. Such a want of correspondence in the materials outside and inside, is just what we might expect if we reflect on the difference of time at which the deposition of sediment will take place in the two cases; the imbedding of the tree having gone on for many years before its decay had made much progress. In many places distinct proof is seen that the enveloping strata took years to accumulate, for some of the sandstones surrounding erect sigillarian trunks support at different levels roots and stems of Calamites; the Calamites having begun to grow after the older Sigillariæ had been partially buried.

The general absence of structure in the interior of the large fossil trees of the Coal implies the very durable nature of their bark, as compared with their woody portion. The same difference of durability of bark and wood exists in modern trees, and was first pointed out to me by Dr. Dawson, in the forests of Nova Scotia, where the Canoe Birch (Betula papyracea) has such tough bark that it may sometimes be seen in the swamps looking externally sound and fresh, although consisting simply of a hollow cylinder with all the wood decayed and gone. When portions of such trunks have become submerged in the swamps they are sometimes found filled with mud. One of the erect fossil trees of the South Joggins fifteen feet in height, occurring at a higher level than the main coal, has been shown by Dr. Dawson to have a coniferous structure, so that some Coniferæ of the Coal period grew in the same swamps as Sigillariæ, just as now the deciduous Cypress (Taxodium distichum) abounds in the marshes of Louisiana even to the edge of the sea.

When the carboniferous forests sank below high-water mark, a species of Spirorbis or Serpula (Fig. 431), attached itself to the outside of the stumps and stems of the erect trees, adhering occasionally even to the interior of the bark—another proof that the process of envelopment was very gradual. These hollow upright trees, covered with innumerable marine annelids, reminded me of a “cane-brake,” as it is commonly called, consisting of tall reeds, Arundinaria macrosperma, which I saw in 1846, at the Balize, or extremity of the delta of the Mississippi. Although these reeds are fresh-water plants, they were covered with barnacles, having been killed by an incursion of salt-water over an extent of many acres, where the sea had for a season usurped a space previously gained from it by the river. Yet the dead reeds, in spite of this change, remained standing in the soft mud, enabling us to conceive how easily the larger Sigillariæ, hollow as they were but supported by strong roots, may have resisted an incursion of the sea.

The high tides of the Bay of Fundy, rising more than 60 feet, are so destructive as to undermine and sweep away continually the whole face of the cliffs, and thus a new crop of erect fossil trees is brought into view every three or four years. They are known to extend over a space between two and three miles from north to south, and more than twice that distance from east to west, being seen in the banks of streams intersecting the coal-field.

Structure of Coal.—The bituminous coal of Nova Scotia is similar in composition and structure to that of Great Britain, being chiefly derived from sigillarioid trees mixed with leaves of ferns and of a Lycopodiaceous tree called Cordaites (Noeggerathia, etc., for genus, see Fig. 428), supposed by Dawson to have been deciduous, and which had broad parallel veined leaves without a mid-rib. On the surface of the seams of coal are large quantities of mineral charcoal, which doubtless consist, as Dr. Dawson suggests, of fragments of wood which decayed in the open air, as would naturally be expected in swamps where so many erect trees were preserved. Beds of cannel-coal display, says Dr. Dawson, such a microscopical structure and chemical composition as shows them to have been of the nature of fine vegetable mud such as accumulates in the shallow ponds of modern swamps. The underclays are loamy soils, which must have been sufficiently above water to admit of drainage, and the absence of sulphurets, and the occurrence of carbonate of iron in them, prove that when they existed as soils, rain-water, and not sea-water, percolated them. With the exception, perhaps, of Asterophyllites (see Fig. 461), there is a remarkable absence from the coal-measures of any form of vegetation properly aquatic, the true coal being a sub-aërial accumulation in soil that was wet and swampy but not permanently submerged.

Air-breathers of the Coal.—If we have rightly interpreted the evidence of the former existence at more than eighty different levels of forests of trees, some of them of vast extent, and which lasted for ages, giving rise to a great accumulation of vegetable matter, it is natural to ask whether there were not many air-breathing inhabitants of these same regions. As yet no remains of mammalia or birds have been found, a negative character common at present to all the Palæozoic formations; but in 1852 the osseous remains of a reptile, the first ever met with in the carboniferous strata of the American continent, were found by Dr. Dawson and myself. We detected them in the interior of one of the erect Sigillariæ before alluded to as of such frequent occurrence in Nova Scotia. The tree was about two feet in diameter, and consisted of an external cylinder of bark, converted into coal, and an internal stony axis of black sandstone, or rather mud and sand stained black by carbonaceous matter, and cemented together with fragments of wood into a rock. These fragments were in the state of charcoal, and seem to have fallen to the bottom of the hollow tree while it was rotting away. The skull, jaws, and vertebræ of a reptile, probably about 2½ feet in length (Dendrerpeton Acadianum, Owen), were scattered through this stony matrix. The shell, also, of a Pupa (see Fig. 442), the first land-shell ever met with in the coal or in beds older than the tertiary, was observed in the same stony mass. Dr. Wyman of Boston pronounced the reptile to be allied in structure to Menobranchus and Menopoma, species of batrachians, now inhabiting the North American rivers. The same view was afterwards confirmed by Professor Owen, who also pointed out the resemblance of the cranial plates to those seen in the skull of Archegosaurus and Labyrinthodon.[6] Whether the creature had crept into the hollow tree while its top was still open to the air, or whether it was washed in with mud during a flood, or in whatever other manner it entered, must be matter of conjecture.

Footprints of two reptiles of different sizes had previously been observed by Dr. Harding and Dr. Gesner on ripple-marked flags of the lower coal-measures in Nova Scotia (No. 2, Fig. 447), evidently made by quadrupeds walking on the ancient beach, or out of the water, just as the recent Menopoma is sometimes observed to do. The remains of a second and smaller species of Dendrerpeton, D. Oweni, were also found accompanying the larger one, and still retaining some of its dermal appendages; and in the same tree were the bones of a third small lizard-like reptile, Hylonomus Lyelli, seven inches long, with stout hind limbs, and fore limbs comparatively slender, supposed by Dr. Dawson to be capable of walking and running on land.[7]

Fig. 441: Xylobius Sigillariæ. Coal, Nova Scotia.

In a second specimen of an erect stump of a hollow tree 15 inches in diameter, the ribbed bark of which showed that it was a Sigillaria, and which belonged to the same forest as the specimen examined by us in 1852, Dr. Dawson obtained not only fifty specimens of Pupa vetusta (Fig. 442), and nine skeletons of reptiles belonging to four species, but also several examples of an articulated animal resembling the recent centipede or gally-worm, a creature which feeds on decayed vegetable matter (see Fig. 441). Under the microscope, the head, with the eyes, mandible, and labrum, are well seen. It is interesting, as being the earliest known representative of the myriapods, none of which had previously been met with in rocks older than the oolite or lithographic slate of Germany.

Fig. 442: Pupa vetusta. Some years after the discovery of the first Pupa, Dr. Dawson, carefully examining the same great section containing so many buried forests in the cliffs of Nova Scotia, discovered another bed, separated from the tree containing Dendrerpeton by a mass of strata more than 1200 feet thick. As there were 21 seams of coal in this intervening mass, the length of time comprised in the interval is not to be measured by the mere thickness of the sandstones and shales. This lower bed is an underclay seven feet thick, with stigmarian rootlets, and the small land-shells occurring in it are in all stages of growth. They are chiefly confined to a layer about two inches thick, and are unmixed with any aquatic shells. They were all originally entire when imbedded, but are most of them now crushed, flattened, and distorted by pressure; they must have been accumulated, says Dr. Dawson, in mud deposited in a pond or creek.

Fig. 443: Zonites (Conulus) priseus.

The surface striæ of Pupa vetusta, when magnified 50 diameters, present exactly the same appearance as a portion corresponding in size of the common English Pupa juniperi, and the internal hexagonal cells, magnified 500 diameters, show the internal structure of the fossil and recent Pupa to be identical. In 1866[8] Dr. Dawson discovered in this lower bed, so full of the Pupa, another land-shell of the genus Helix (sub-genus Zonites), see Fig. 443.

None of the reptiles obtained from the coal-measures of the South Joggins are of a higher grade than the Labyrinthodonts, but some of these were of very great size, two caudal vertebræ found by Mr. Marsh in 1862 measuring two and a half inches in diameter, and implying a gigantic aquatic reptile with a powerful swimming tail.

Except some obscure traces of an insect found by Dr. Dawson in a coprolite of a terrestrial reptile occurring in a fossil tree, no specimen of this class has been brought to light in the Joggins. But Mr. James Barnes found in a bed of shale at Little Grace Bay, Cape Breton, the wing of an Ephemera, which must have measured seven inches from tip to tip of the expanded wings—larger than any known living insect of the Neuropterous family.

That we should have made so little progress in obtaining a knowledge of the terrestrial fauna of the Coal is certainly a mystery, but we have no reason to wonder at the extreme rarity of insects, seeing how few are known in the carboniferous rocks of Europe, worked for centuries before America was discovered, and now quarried on so enormous a scale. These European rocks have not yet produced a single land-shell, in spite of the millions of tons of coal annually extracted, and the many hundreds of soils replete with the fossil roots of trees, and the erect trunks and stumps preserved in the position in which they grew. In many large coal-fields we continue as much in the dark respecting the invertebrate air-breathers then living, as if the coal had been thrown down in mid-ocean. The early date of the carboniferous strata cannot explain the enigma, because we know that while the land supported a luxuriant vegetation, the contemporaneous seas swarmed with life—with Articulata, Mollusca, Radiata, and Fishes. The perplexity in which we are involved when we attempt to solve this problem may be owing partly to our want of diligence as collectors, but still more perhaps to ignorance of the laws which govern the fossilisation of land-animals, whether of high or low degree.

Carboniferous Rain-prints.—At various levels in the coal measures of Nova Scotia, ripple-marked sandstones, and shales with rain-prints, were seen by Dr. Dawson and myself, but still more perfect impressions of rain were discovered by Mr. Brown, near Sydney, in the adjoining island of cape Breton. They consist of very delicate markings on greenish slates, accompanied by worm-tracks (a, b, Fig. 444), such as are often seen between high and low water mark on the recent mud of the Bay of Fundy.

The great humidity of the climate of the Coal period had been previously inferred from the number of its ferns and the continuity of its forests for hundreds of miles; but it is satisfactory to have at length obtained such positive proofs of showers of rain, the drops of which resembled in their average size those which now fall from the clouds. From such data we may presume that the atmosphere of the Carboniferous period corresponded in density with that now investing the globe, and that different currents of air varied then as now in temperature, so as to give rise, by their mixture, to the condensation of aqueous vapour.

Fig. 444: Carboniferous rain-prints with worm tracks on green shale, from Cape Breton, Nova Scotia. Fig. 445: Casts of rain-prints on a portion of the same slab (Fig. 444), seen to project on the underside of an incumbent layer of arenaceous shale.

Folding and Denudation of the Beds indicated by the Nova Scotia Coal-strata.—The series of events which are indicated by the great section of the coal-strata in Nova Scotia consist of a gradual and long-continued subsidence of a tract which throughout most of the period was in the state of a delta, though occasionally submerged beneath a sea of moderate depth. Deposits of mud and sand were first carried down into a shallow sea on the low shores of which the footprints of reptiles were sometimes impressed (see p. 407).

Fig. 446: Cone and branch of Lepidodendron corrugatum. Though no regular seams of coal were formed, the characteristic imbedded coal-plants are of the genera Cyclopteris and Alethopteris, agreeing with species occurring at much higher levels, and distinct from those of the antecedent Devonian group. The Lepidodendron corrugatum (see Fig. 446), a plant predominating in the Lower Carboniferous group of Europe, is also conspicuous in these shallow-water beds, together with many fishes and entomostracans. A more rapid rate of subsidence sometimes converted part of the sea into deep clear water, in which there was a growth of coral which was afterwards turned into crystalline limestone, and parts of it, apparently by the action of sulphuric acid, into gypsum. In spite of continued sinking, amounting to several thousand feet, the sea might in time have been rendered shallow by the growth of coral, had not its conversion into land or swampy ground been accelerated by the pouring in of sand and the advance of the delta accompanied with such fluviatile and brackish-water formations as are common in lagoons.

The amount to which the bed of the sea sank down in order to allow of the formation of so vast a thickness of rock of sedimentary and organic origin is expressed by the total thickness of the Carboniferous strata, including the coal-measures, No. 1, and the rocks which underlie them, No. 2, Fig. 447.

Fig. 447: Diagram showing the curvature and supposed denudation of the Carboniferous strata in Nova Scotia.

After the strata No. 2 had been elaborated, the conditions proper to a great delta exclusively prevailed, the subsidence still continuing so that one forest after another grew and was submerged until their under-clays with roots, and usually seams of coal, were left at more than eighty distinct levels. Here and there, also, deposits bearing testimony to the existence of fresh or brackish-water lagoons, filled with calcareo-bituminous mud, were formed. In these beds (h and i, Fig. 439) are found fresh-water bivalves or mussels allied to Anodon, though not identical with that or any living genus, and called Naiadites carbonarius by Dawson. They are associated with small entomostracous crustaceans of the genus Cythere, and scales of small fishes. Occasionally some of the calamite brakes and forests of Sigillariæ and Coniferæ were exposed in the flood season, or sometimes, perhaps, by slight elevatory movements to the denuding action of the river or the sea.

In order to interpret the great coast section exposed to view on the shores of the Bay of Fundy, the student must, in the first place, understand that the newest or last-mentioned coal formations would have been the only ones known to us (for they would have covered all the others), had there not been two great movements in opposite directions, the first consisting of a general sinking of three miles, which took place during the Carboniferous Period, and the second an upheaval of more limited horizontal extent, by which the anticlinal axis A was formed. That the first great change of level was one of subsidence is proved by the fact that there are shallow-water deposits at the base of the Carboniferous series, or in the lowest beds of No. 2.

Subsequent movements produced in the Nova Scotia and the adjoining New Brunswick coal-fields the usual anticlinal and synclinal flexures. In order to follow these, we must survey the country for about thirty miles round the South Joggins, or the region where the erect trees described in the foregoing pages are seen. As we pass along the cliffs for miles in a southerly direction, the beds containing these fossil trees, which were mentioned as dipping about 18° south, are less and less inclined, until they become nearly horizontal in the valley of a small river called the Shoulie, as ascertained by Dr. Dawson. After passing this synclinal line the beds begin to dip in an opposite or north-easterly direction, acquiring a steep dip where they rest unconformably on the edges of the Upper Silurian strata of the Cobequid Hills, as shown in Fig. 447. But if we travel northward towards Minudie from the region of the coal-seams and buried forests, we find the dip of the coal-strata increasing from an angle of 18° to one of more than 40°, lower beds being continually exposed to view until we reach the anticlinal axis A and see the lower Carboniferous formation, No. 2, at the surface. The missing rocks removed by denudation are expressed by the faint lines at A, and thus the student will see that, according to the principles laid down in the seventh chapter, we are enabled, by the joint operations of upheaval and denudation, to look, as it were, about three miles into the interior of the earth without passing beyond the limits of a single formation.

[1] Edward Hull, Quart. Geol. Journ., vol. xxiv, p. 327.

[2] Quart. Geol. Journ., vol. v, Mem., p. 17.

[3] Memoirs of the Geol. Survey, pp. 51, 255, etc.

[4] Dunker and V. Meyer, Palæont., vol. iv, p. 17.

[5] Acadian Geology, 2nd edit., 1868.

[6] Quart. Geol. Journ., vol. ix, p. 58.

[7] Dawson, Air-Breathers of the Coal in Nova Scotia, Montreal, 1863.

[8] Dawson, Acadian Geology, 1868, p. 385.

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XXIV. FLORA AND FAUNA OF THE CARBONIFEROUS PERIOD

Vegetation of the Coal Period. — Ferns, Lycopodiaceæ, Equisetaceæ, Sigillariæ, Stigmariæ, Coniferæ. — Angiosperms. — Climate of the Coal Period. — Mountain Limestone. — Marine Fauna of the Carboniferous Period. — Corals. — Bryozoa, Crinoidea. — Mollusca. — Great Number of fossil Fish. — Foraminifera.

Vegetation of the Coal Period.—In the last chapter we have seen that the seams of coal, whether bituminous or anthracitic, are derived from the same species of plants, and Goppert has ascertained that the remains of every family of plants scattered through the shales and sandstones of the coal-measures are sometimes met with in the pure coal itself—a fact which adds greatly to the geological interest of this flora.

The coal-period was called by Adolphe Brongniart the age of Acrogens,[1] so great appears to have been the numerical preponderance of flowerless or cryptogamic plants of the families of ferns, club-mosses, and horse-tails. He reckoned the known species in 1849 at 500, and the number has been largely increased by recent research in spite of reductions owing to the discovery that different parts of even the same plants had been taken for distinct species. Notwithstanding these changes, Brongniart’s generalisation concerning this flora still holds true, namely, that the state of the vegetable world was then extremely different from that now prevailing, not only because the cryptogamous plants constituted nearly the whole flora, but also because they were, on the whole, more highly developed than any belonging to the same class now existing, and united some forms of structure now only found separately and in distinct orders. The only phænogamous plants were constitute any feature in the coal are the coniferæ; monocotyledonous angiosperms appear to have been very rare, and the dicotyledonous, with one or two doubtful exceptions, were wanting. For this we are in some measure prepared by what we have seen of the Secondary or Mesozoic floras if, consistently with the belief in the theory of evolution, we expect to find the prevalence of simpler and less specialised organisms in older rocks.

Ferns.—We are struck at the first glance with the similarity of the ferns to those now living. In the fossil genus Pecopteris, for example (Fig. 448), it is not easy to decide whether the fossils might not be referred to the same genera as those established for living ferns; whereas, in regard to some of the other contemporary families of plants, with the exception of the fir tribe, it is not easy to guess even the class to which they belong. The ferns of the Carboniferous period are generally without organs of fructification, but in the few instances in which these do occur in a fit state for microscopical investigations they agree with those of the living ferns.

Fig. 448: Pecopteris elliptica. Fig. 449: Caulopteris primæva. Fig. 448: Pecopteris elliptica, Bunbury.[2] Frostburg. Fig. 449: Caulopteris primæva, Lindley.

When collecting fossil specimens from the coal-measures of Frostburg, in Maryland, I found in the iron-shales several species with well-preserved rounded spots or marks of the sori (see Fig. 448). In the general absence of such characters they have been divided into genera distinguished chiefly by the branching of the fronds and the way in which the veins of the leaves are disposed. The larger portion are supposed to have been of the size of ordinary European ferns, but some were decidedly arborescent, especially the group called Caulopteris (see Fig. 449) by Lindley, and the Psaronius of the upper or newest coal-measures, before alluded to (p. 393). All the recent tree-ferns belong to one tribe (Polypodiaceæ), and to a small number only of genera in that tribe, in which the surface of the trunk is marked with scars, or cicatrices, left after the fall of the fronds. These scars resemble those of Caulopteris.

No less than 130 species of ferns are enumerated as having been obtained from the British coal-strata, and this number is more than doubled if we include the Continental and American species. Even if we make some reduction on the ground of varieties which have been mistaken, in the absence of their fructification, for species, still the result is singular, because the whole of Europe affords at present no more than sixty-seven indigenous species.

Living tree-ferns of different genera. Fig. 450: Tree-fern from Isle of Bourbon. Fig. 451: Cyathea glauca, Mauritius. Fig. 452: Tree-fern from Brazil.

Lycopodiaceæ—Lepidodendron.—About forty species of fossil plants of the Coal have been referred to this genus, more than half of which are found in the British coal-measures. They consist of cylindrical stems or trunks, covered with leaf-scars. In their mode of branching, they are always dichotomous (see Fig. 454). They belong to the Lycopodiaceæ, bearing sporangia and spores similar to those of the living representatives of this family (Fig. 457); and although most of the Carboniferous species grew to the size of large trees, Mr. Carruthers has found by careful measurement that the volume of the fossil spores did not exceed that of the recent club-moss, a fact of some geological importance, as it may help to explain the facility with which these seeds may have been transported by the wind, causing the same wide distribution of the species of the fossil forests in Europe and America which we now observe in the geographical distribution of so many living families of cryptogamous plants.

Lepidodendrum Sternbergii. Coal-measures, near Newcastle. Fig. 453: Branching trunk, 49 feet long, supposed to have belonged to L. Sternbergii. Fig. 454: Branching stem with bark and leaves of L. Sternbergii. Fig. 455: Portion of same, nearer the root.

Fig. 456: a. Lycopodium densum. Living species, New Zealand; b. Branch; c. Part of same, magnified. The Figs. 453–455 represent a fossil Lepidodendron, 49 feet long, found in Jarrow Colliery, near Newcastle, lying in shale parallel to the planes of stratification. Fragments of others, found in the same shale, indicate, by the size of the rhomboidal scars which cover them, a still greater magnitude.

The living club-mosses, of which there are about 200 species, are most abundant in tropical climates. They usually creep on the ground, but some stand erect, as the Lycopodium densum from New Zealand (see Fig. 456), which attains a height of three feet.

In the Carboniferous strata of Coalbrook Dale, and in many other coal-fields, elongated cylindrical bodies, called fossil cones, named Lepidostrobus by M. Adolphe Brongniart, are met with. (See Fig. 457.) They often form the nucleus of concretionary balls of clay-ironstone, and are well preserved, exhibiting a conical axis, around which a great quantity of scales were compactly imbricated. The opinion of M. Brongniart that the Lepidostrobus is the fruit of Lepidodendron has been confirmed, for these strobili or fruits have been found terminating the tip of a branch of a well-characterised Lepidodendron in Coalbrook Dale and elsewhere.

Fig. 457: a. Lepidostrobus ornatus; b. Portion of a section, showing the large sporangia in their natural position, and each supported by its bract or scale; c. Spores in these sporangia, highly magnified.

Fig. 458: Calamites Sucowii, common throughout Europe. Fig. 459: Stem of Fig. 458, as retored by Dr. Dawson.

Equisetaceæ.—To this family belong two fossil genera of the coal, Equisetites and Calamites. The Calamites were evidently closely related to the modern horse-tails (Equiseta) differing principally in their great size, the want of sheaths at the joints, and some details of fructification. They grew in dense brakes on sandy and muddy flats in the manner of modern Equisetaceæ, and their remains are frequent in the coal. Seven species of this plant occur in the great Nova Scotia section before described, where the stems of some of them five inches in diameter, and sometimes eight feet high, may be seen terminating downward in a tapering root (see Fig. 460).

Fig. 460: Radical termination of a Calamite. Fig. 461: Asterophyllites foliosus, Coal-measures, Newcastle.

Botanists are not yet agreed whether the Asterophyllites, a species of which is represented in Fig. 461, can form a separate genus from the Calamite, from which, however, according to Dr. Dawson, its foliage is distinguished by a true mid-rib, which is wanting in the leaves known to belong to some Calamites.

Fig. 462: Annularia sphenophylloides.Fig. 463: Sphenophyllum erosum. Figs. 462 and 463 represent leaves of Annularia and Sphenophyllum, common in the coal, and believed by Mr. Carruthers to be leaves of Calamites. Dr. Williamson, who has carefully studied the Calamites, thinks that they had a fistular pith, exogenous woody stem, and thick smooth bark, which last having always disappeared, leaves a fluted stem, as represented in Fig. 459.

Sigillaria.—A large portion of the trees of the Carboniferous period belonged to this genus, of which as many as 28 species are enumerated as British. The structure, both internal and external, was very peculiar, and, with reference to existing types, very anomalous. They were formerly referred, by M. Ad. Brongniart, to ferns, which they resemble in the scalariform texture of their vessels and, in some degree, in the form of the cicatrices left by the base of the leaf-stalks which have fallen off (see Fig. 464). But some of them are ascertained to have had long linear leaves, quite unlike those of ferns. They grew to a great height, from 30 to 60, or even 70 feet, with regular cylindrical stems, and without branches, although some species were dichotomous towards the top. Their fluted trunks, from one to five feet in diameter, appear to have decayed more rapidly in the interior than externally, so that they became hollow when standing; and when thrown prostrate, they were squeezed down and flattened. Hence, we find the bark of the two opposite sides (now converted into bright shining coal) constitute two horizontal layers, one upon the other, half an inch, or an inch, in their united thickness. These same trunks, when they are placed obliquely or vertically to the planes of stratification, retain their original rounded form, and are uncompressed, the cylinder of bark having been filled with sand, which now affords a cast of the interior.

Fig. 464: Sigillaria lævigata. Dr. Hooker inclined to the belief that the Sigillariæ may have been cryptogamous, though more highly developed than any flowerless plants now living. Dr. Dawson having found in some species what he regards as medullary rays, thinks with Brongniart that they have some relation to gymnogens, while Mr. Carruthers leans to the opinion that they belong to the Lycopodiaceæ.

Stigmaria.—This fossil, the importance of which has already been pointed out in p. 398, was originally conjectured to be an aquatic plant. It is now ascertained to be the root of Sigillaria. The connection of the roots with the stem, previously suspected, on botanical grounds, by Brongniart, was first proved, by actual contact, in the Lancashire coal-field, by Mr. Binney. The fact has lately been shown, even more distinctly, by Mr. Richard Brown, in his description of the Stigmariæ occurring in the under-clays of the coal-seams of the Island of Cape Breton, in Nova Scotia. In a specimen of one of these, represented in Fig. 465, the spread of the roots was sixteen feet, and some of them sent out rootlets, in all directions, into the surrounding clay.

Fig. 465: Stigmaria attached to a trunk of Sigillaria.

In the sea-cliffs of the South Joggins in Nova Scotia, I examined several erect Sigillariæ, in company with Dr. Dawson, and we found that from the lower extremities of the trunk they sent out Stigmariæ as roots. All the stools of the fossil trees dug out by us divided into four parts, and these again bifurcated, forming eight roots, which were also dichotomous when traceable far enough. The cylindrical rootlets formerly regarded as leaves are now shown by more perfect specimens to have been attached to the root by fitting into deep cylindrical pits. In the fossil there is rarely any trace of the form of these cavities, in consequence of the shrinkage of the surrounding tissues. Where the rootlets are removed, nothing remains on the surface of the Stigmaria but rows of mammillated tubercles (see Figs. 466, 467), which have formed the base of each rootlet.

Fig. 466: Stigmaria ficoides. Fig. 467: Surface of another individual of same species, showing form of tubercles.

These protuberances may possibly indicate the place of a joint at the lower extremity of the rootlet. Rows of these tubercles are arranged spirally round each root, which have always a medullary axis and woody system much resembling that of Sigillaria, the structure of the vessels being, like it, scalariform.

Coniferæ.—The coniferous trees of this period are referred to five genera; the woody structure of some of them showing that they were allied to the Araucarian division of pines, more than to any of our common European firs. Some of their trunks exceeded forty-four feet in height. Many, if not all of them, seem to have differed from living Coniferæ in having large piths; for Professor Williamson has demonstrated the fossil of the coal-measures called Sternbergia to be the pith of these trees, or rather the cast of cavities formed by the shrinking or partial absorption of the original medullary axis (see Figs. 468, 469). This peculiar type of pith is observed in living plants of very different families, such as the common Walnut and the White Jasmine, in which the pith becomes so reduced as simply to form a thin lining of the medullary cavity, across which transverse plates of pith extend horizontally, so as to divide the cylindrical hollow into discoid interspaces. When these interspaces have been filled up with inorganic matter, they constitute an axis to which, before their true nature was known, the provisional name of Sternbergia (d, d, Fig. 468) was given. In the above specimen the structure of the wood (b, Figs. 468 and 469) is coniferous, and the fossil is referable to Endlicher’s fossil genus Dadoxylon.

Fig. 468: Fragment of coniferous wood. Fig. 468: Fragment of coniferous wood, Dadoxylon, of Endlicher, fractured longitudinally; from Coalbrook Dale. W.C. Williamson[3]

Fig. 469: Magnified portion of Fig. 468; transverse section.

The fossil named Trigonocarpon (Figs. 470 and 471), formerly supposed to be the fruit of a palm, may now, according to Dr. Hooker, be referred, like the Sternbergia, to the Coniferæ. Its geological importance is great, for so abundant is it in the coal-measures, that in certain localities the fruit of some species may be procured by the bushel; nor is there any part of the formation where they do not occur, except the under-clays and limestone. The sandstone, ironstone, shales, and coal itself, all contain them. Mr. Binney has at length found in the clay-ironstone of Lancashire several specimens displaying structure, and from these, says Dr. Hooker, we learn that the Trigonocarpon belonged to that large section of existing coniferous plants which bear fleshy solitary fruits, and not cones. It resembled very closely the fruit of the Chinese genus Salisburia, one of the Yew tribe, or Taxoid conifers.

Fig. 470: Trigonocarpum ovatum.Fig. 471: Trigonocarpum olivæforme. Fig. 472: Antholithes. Angiosperms.—The curious fossils called Antholithes by Lindley have usually been considered to be flower spikes, having what seems a calyx and linear petals (see Fig. 472). Dr. Hooker, after seeing very perfect specimens, also thought that they resembled the spike of a highly-organised plant in full flower, such as one of the Bromeliaceæ, to which Professor Lindley had at first compared them. Mr. Carruthers, who has lately examined a large series in different museums, considers it to be a dicotyledonous angiosperm allied to Orobanche (broom-rape), which grew, not on the soil, but parasitically on the trees of the coal forests.

In the coal-measures of Granton, near Edinburgh, a remarkable fossil (Fig. 473) was found and described in 1840,[4] by Dr. Robert Paterson. It was compressed between layers of bituminous shale, and consists of a stem bearing a cylindrical spike, a, which in the portion preserved in the slate exhibits two subdivisions and part of a third. The spike is covered on the exposed surface with the four-cleft calyces of the flowers arranged in parallel rows. The stem shows, at b, a little below the spike, remains of a lateral appendage, which is supposed to indicate the beginning of the spathe. The fossil has been referred to the Aroidiæ, and there is every probability that it is a true member of this order. There can at least be no doubt as to the high grade of its organisation, and that it belongs to the monocotyledonous angiosperms. Mr. Carruthers has carefully examined the original specimen in the Botanical Museum, Edinburgh, and thinks it may have been an epiphyte.

Fig. 473: Pothocites Grantonii. Climate of the Coal Period.—As to the climate of the Coal, the Ferns and the Coniferæ are perhaps the two classes of plants which may be most relied upon as leading us to safe conclusions, as the genera are nearly allied to living types. All botanists admit that the abundance of ferns implies a moist atmosphere. But the coniferæ, says Hooker, are of more doubtful import, as they are found in hot and dry, and in cold and dry climates; in hot and moist, and in cold and moist regions. In New Zealand the coniferæ attain their maximum in numbers, constituting 1/62 part of all the flowering plants; whereas in a wide district around the Cape of Good Hope they do not form 1/1600 of the phenogamic flora. Besides the conifers, many species of ferns flourish in New Zealand, some of them arborescent, together with many lycopodiums; so that a forest in that country may make a nearer approach to the carboniferous vegetation than any other now existing on the globe.

MARINE FAUNA OF THE CARBONIFEROUS PERIOD.

It has already been stated that the Carboniferous or Mountain Limestone underlies the coal-measures in the South of England and Wales, whereas in the North, and in Scotland, marine calcareous rocks partly of the age of the Mountain Limestone alternate with shales and sandstones, containing seams of coal. In its most calcareous form the Mountain Limestone is destitute of land-plants, and is loaded with marine remains—the greater part, indeed, of the rock being made up bodily of crinoids, corals, and bryozoa with interspersed mollusca.

Corals.—The corals deserve especial notice, as the cup-and-star corals, which have the most massive and stony skeletons, display peculiarities of structure by which they may be distinguished generally, as MM. Milne Edwards and Haime first pointed out, from all species found in strata newer than the Permian. There is, in short, an ancient or Palæozoic, and a modern or Neozoic type, if, by the latter term, we designate (as proposed by Professor E. Forbes) all strata from the triassic to the most modern, inclusive. The accompanying diagrams (Figs. 474, 475) may illustrate these types.

Fig. 474: Palæozoic type of lamelliferous cup-shaped Coral.

Vertical section of Campophyllum flexuosum, (Cyathophyllum, Goldfuss); from the Devonian of the Eifel. The lamellæ are seen around the inside of the cup; the walls consist of cellular tissue; and large transverse plates, called tubulæ, divide the interior into chambers.

Arrangement of the lamellæ in Polycoelia profunda, Germar, sp.; from the Magnesian Limestone, Durham. This diagram shows the quadripartite arrangement of the primary septa, characteristic of palæozoic corals, there being four principal and eight intermediate lamellæ, the whole number in this type being always a multiple of four.

Stauria astræiformis, Milne Edwards. Young group, natural size. Upper Silurian, Gothland. The lamellæ or septal system in each cup are divided by four prominent ridges into four groups.

Fig. 475: Neozoic type of lamelliferous cup-shaped Coral. Parasmilia centralis, Mantell, sp. Vertical section. Upper Chalk, Gravesend. In this type the lamellæ are massive, and extend to the axis or columella composed of loose cellular tissue, without any transverse plates like those in Fig. 474, a.

Cyathina Bowerbankii, Ed. and H. Transverse section, enlarged. Gault, Folkestone. In this coral the primary septa are a multiple of six. The twelve principal plates reach the columella, and between each pair there are three secondaries, in all forty-eight. The short intermediate plates which proceed from the columella are not counted. They are called pali.

Fungia patellaris, Lamarck. Recent; very young state. Diagram of its six primary and six secondary septa, magnified. The sextuple arrangement is always more manifest in the young than in the adult state.

It will be seen that the more ancient corals have what is called a quadripartite arrangement of the chief plates or lamellæ—parts of the skeleton which support the organs of reproduction. The number of these lamellæ in the Palæozoic type is 4, 8, 16, etc.; while in the Neozoic type the number is 6, 12, 24, or some other multiple of six; and this holds good, whether they be simple forms, as in Figs. 474, a, and 475, a, or aggregate clusters of corallites, as in 474, c. But further investigations have shown in this, as in all similar grand generalisations in natural history, that there are excepions to the rule. Thus in the Lower Greensand Holocystis elegans (Ed. and H.) and other forms have the Palæozoic type, and Dr. Duncan has shown to what extent the Neozoic forms penetrate downward into the Carboniferous and Devonian rocks.

Fig. 476: Lithostrotion basaltiforme. Fig. 477: Lonsdaleia floriformis.

From a great number of lamelliferous corals met with in the Mountain Limestone, two species (Figs. 476, 477) have been selected, as having a very wide range, extending from the eastern borders of Russia to the British Isles, and being found almost everywhere in each country. These fossils, together with numerous species of Zaphrentis, Amplexus, Cyathophyllum, Clisiophyllum, Syringopora, and Michelinia,[5] form a group of rugose corals widely different from any that followed them.

Bryozoa and Crinoidea.—Of the Bryozoa, the prevailing forms are Fenestella, Hemitrypa, and Polypora, and these often form considerable beds. Their net-like fronds are easily recognised. Crinoidea are also numerous in the Mountain Limestone (see Figs. 478, 479), two genera, Pentremites and Codonaster, being peculiar to this formation in Europe and North America.

Fig. 478: Cyathocrinus planus. Fig. 479: Cyathocrinus caryocrinoides.

Fig. 480: Palæchinus gigas. In the greater part of them, the cup or pelvis, Figure 479, b, is greatly developed in size in proportion to the arms, although this is not the case in Fig. 478. The genera Poteriocrinus, Cyathocrinus, Pentremites, Actinocrinus, and Platycrinus, are all of them characteristic of this formation. Other Echinoderms are rare, a few Sea-Urchins only being known: these have a complex structure, with many more plates on their surface than are seen in the modern genera of the same group. One genus, the Palæchinus (Fig. 480), is the analogue of the modern Echinus, but has four, five, or six rows of plates in the interambulacral region or area, whereas the modern genera have only two. The other, Archæocidaris, represents, in like manner, the Cidaris of the present seas.

Mollusca.—The British Carboniferous mollusca enumerated by Mr. Etheridge[6] comprise 653 species referable to 86 genera, occurring chiefly in the Mountain Limestone. Of this large number only 40 species are common to the underlying Devonian rocks, 9 of them being Cephalopods, 7 Gasteropods, and the rest bivalves, chiefly Brachiopoda (or Palliobranchiates). This latter group constitutes the larger part of the Carboniferous Mollusca, 157 species being known in Great Britain alone, and it will be found to increase in importance in the fauna of the primary rocks the lower we descend in the series. Perhaps the most characteristic shells of the formation are large species of Productus, such as P. giganteus, p. hemisphericus, P. semireticulatus (Fig. 481), and P. scabriculus. Large plaited spirifers, as Spirifera striata, S. rotundata, and S. trigonalis (Fig. 482), also abound; and smooth species, such as Spirifera glabra (Fig. 483), with its numerous varieties.

Fig. 481: Productus semireticulatus. Fig. 482: Spirifera trigonalis.

Fig. 483: Spirifera glabra. Fig. 484: Terebratula hastata. Fig. 485: Aviculopecten sublobatus. Fig. 486: Pleurotomaria carinata.

Among the brachiopoda, Terebratula hastata (Fig. 484) deserves mention, not only for its wide range, but because it often retains the pattern of the original coloured stripes which ornamented the living shell. These coloured bands are also preserved in several lamellibranchiate bivalves, as in Aviculopecten (Fig. 485), in which dark stripes alternate with a light ground. In some also of the spiral univalves the pattern of the original painting is distinctly retained, as in Pleurotomaria (Fig. 486), which displays wavy blotches, resembling the colouring in many recent trochidæ.

Fig. 487: Euomphalus pentagulatus.

Some few of the carboniferous mollusca, such as Avicula, Nucula (sub-genus Ctenodonta), Solemya, and Lithodomus, belong no doubt to existing genera; but the majority, though often referred to as living types, such as Isocardia, Turritella, and Buccinum, belong really to forms which appear to have become extinct at the close of the Palæozoic epoch. Euomphalus is a characteristic univalve shell of this period. In the interior it is divided into chambers (Fig. 487, d), the septa or partitions not being perforated as in foraminiferous shells, or in those having siphuncles, like the Nautilus. The animal appears to have retreated at different periods of its growth from the internal cavity previously formed, and to have closed all communication with it by a septum. The number of chambers is irregular, and they are generally wanting in the innermost whorl. The animal of the recent Turritella communis partitions off in like manner as it advances in age a part of its spire, forming a shelly septum.

More than twenty species of the genus Bellerophon (see Fig. 488), a shell like the living Argonaut without chambers, occur in the Mountain Limestone. The genus is not met with in strata of later date. It is most generally regarded as belonging to the pelagic Nucleobranchiata and the family Atlantidæ, partly allied to the Glass-Shell, Carinaria; but by some few it is thought to be a simple form of Cephalopod.

Fig. 488: Bellerophon costatus. Fig. 489: Portion of Orthoxeras laterale. Fig. 490: Goniatites crenistra. The carboniferous Cephalopoda do not depart so widely from the living type (the Nautilus) as do the more ancient Silurian representatives of the same order; yet they offer some remarkable forms. Among these is Orthoceras, a siphuncled and chambered shell, like a Nautilus uncoiled and straightened (Fig. 489). Some species of this genus are several feet long. The Goniatite is another genus, nearly allied to the Ammonite, from which it differs in having the lobes of the septa free from lateral denticulations, or crenatures; so that the outline of these is angular, continuous, and uninterrupted. The species represented in Fig. 490 is found in most localities, and presents the zigzag character of the septal lobes in perfection. The dorsal position of the siphuncle, however, clearly distinguishes the Goniatite from the Nautilus, and proves it to have belonged to the family of the Ammonites, from which, indeed, some authors do not believe it to be generically distinct.

Fossil Fish.—The distribution of these is singularly partial; so much so, that M. De Koninck of Liége, the eminent palæontologist, once stated to me that, in making his extensive collection of the fossils of the Mountain Limestone of Belgium, he had found no more than four or five examples of the bones or teeth of fishes. Judging from Belgian data, he might have concluded that this class of vertebrata was of extreme rarity in the Carboniferous seas; whereas the investigation of other countries has led to quite a different result. Thus, near Clifton, on the Avon, as well as at numerous places around the Bristol basin from the Mendip Hills to Tortworth, there is a celebrated “bone-bed,” almost entirely made up of ichthyolites. It occurs at the base of the Lower Limestone shales immediately resting upon the passage beds of the Old Red Sandstone. Similar bone-beds occur in the Carboniferous Limestone of Armagh, in Ireland, where they are made up chiefly of the teeth of fishes of the Placoid order, nearly all of them rolled as if drifted from a distance. Some teeth are sharp and pointed, as in ordinary sharks, of which the genus Cladodus afford an illustration; but the majority, as in Psammodus and Cochliodus, are, like the teeth of the Cestracion of Port Jackson (see Fig. 261), massive palatal teeth fitted for grinding. (See Figs. 491, 492.)

Fig. 491: Psammodus porosus. Fig. 492: Cochliodus controtus. There are upward of seventy other species of fossil fish known in the Mountain Limestone of the British Islands. The defensive fin-bones of these creatures are not infrequent at Armagh and Bristol; those known as Oracanthus, Ctenocanthus, and Onchus are often of a very large size. Ganoid fish, such as Holoptychius, also occur; but these are far less numerous. The great Megalichthys Hibberti appears to range from the Upper Coal-measures to the lowest Carboniferous strata.

Foraminifera.—In the upper part of the Mountain Limestone group in the S.W. of England, near Bristol, limestones having a distinct oolitic structure alternate with shales. In these rocks the nucleus of every minute spherule is seen, under the microscope, to consist of a small rhizopod or foraminifer. This division of the lower animals, which is represented so fully at later epochs by the Nummulites and their numerous minute allies, appears in the Mountain Limestone to be restricted to a very few species, among which Textularia, Nodosaria, Endothyra, and Fusulina (Fig. 493), have been recognised. The first two genera are common to this and all the after periods; the third has been found in the Upper Silurian, but is not known above the Carboniferous strata; the fourth (Fig. 493) is characteristic of the Mountain Limestone in the United States, Arctic America, Russia, and Asia Minor, but is also known in the Permian.

Fig. 493: Fusulina cylindrica.

[1] For botanical nomenclature see p. 304.

[2] Sir C. Bunbury, Quart. Geol. Journ., vol. ii, 1845.

[3] Manchester Phil. Mem., vol. ix, 1851.

[4] Trans. of Bot. Soc. of Edinburgh, vol. i, 1844.

[5] For figures of these corals, see Palæontographical Society’s Monographs, 1852.

[6] Quart. Geol. Journ., vol. xxiii, p. 674, 1867.

Spotted a typo or a problem in this chapter? Tell us

XXV. DEVONIAN OR OLD RED SANDSTONE GROUP

Classification of the Old Red Sandstone in Scotland and in Devonshire. — Upper Old Red Sandstone in Scotland, with Fish and Plants. — Middle Old Red Sandstone. — Classification of the Ichthyolites of the Old Red, and their Relation to Living Types. — Lower Old Red Sandstone, with Cephalaspis and Pterygotus. — Marine or Devonian Type of Old Red Sandstone. — Table of Devonian Series. — Upper Devonian Rocks and Fossils. — Middle. — Lower. — Eifel Limestone of Germany. — Devonian of Russia. — Devonian Strata of the United States and Canada. — Devonian Plants and Insects of Canada.

Classification of the two Types of Old Red Sandstone.—We have seen that the Carboniferous strata are surmounted by the Permian and Trias, both originally included in England under the name “New Red Sandstone,” from the prevailing red colour of the strata. Under the coal came other red sandstones and shales which were distinguished by the title of “Old Red Sandstone.” Afterwards the name of “Devonian” was given by Sir R. Murchison and Professor Sedgwick to marine fossiliferous strata which, in the south of England, occupy a similar position between the overlying coal and the underlying Silurian formations.

It may be truly said that in the British Isles the rocks of this age present themselves in their mineral aspect, and even to some extent in their fossil contents, under two very different forms; the one as distinct from the other as are often lacustrine or fluviatile from marine strata. It has indeed been suggested that by far the greater part of the deposits belonging to what may be termed the Old Red Sandstone type are of fresh-water origin. The number of land-plants, the character of the fishes, and the fact that the only shell yet discovered belongs to the genus Anodonta, must be allowed to lend no small countenance to this opinion. In this case the difficulty of classification when the strata of this type are compared in different regions, even where they are contiguous, may arise partly from their having been formed in distinct hydrographical basins, or in the neighbourhood of the land in shallow parts of the sea into which large bodies of fresh-water entered, and where no marine mollusca or corals could flourish. Under such geographical conditions the limited extent of some kinds of sediment, as well as the absence of those marine forms by which we are able to identify or contrast marine formations, may be explained, while the great thickness of the rocks, which might seem at first sight to require a corresponding depth of water, can often be shown to have been due to the gradual sinking down of the bottom of the estuary or sea where the sediment was accumulated.

Another active cause of local variation in Scotland was the frequency of contemporaneous volcanic eruptions; some of the rocks derived from this source, as between the Grampians and the Tay, having formed islands in the sea, and having been converted into shingle and conglomerate, before the upper portions of the red shales and sandstones were superimposed.

The dearth of calcareous matter over wide areas is characteristic of the Old Red Sandstone. This is, no doubt, in great part due to the absence of shells and corals; but why should these be so generally wanting in all sedimentary rocks the colour of which is determined by the red oxide of iron? Some geologists are of opinion that the waters impregnated with this oxide were prejudicial to living beings, others that strata permeated with this oxide would not preserve such fossil remains.

In regard to the two types, the Old Red Sandstone and the Devonian, I shall first treat of them separately, and then allude to the proofs of their having been to a great extent contemporaneous. That they constitute a series of rocks intermediate in date between the lowest Carboniferous and the uppermost Silurian is not disputed by the ablest geologists; and it can no longer be contended that the Upper, Middle, and Lower Old Red Sandstone preceded in date the three divisions to which, by aid of the marine shells, the Devonian rocks have been referred, while, on the other hand, we have not yet data for enabling us to affirm to what extent the subdivisions of the one series may be the equivalents in time of those of the other.

Upper Old Red Sandstone.—The highest beds of the series in Scotland, lying immediately below the coal in Fife, are composed of yellow sandstone well seen at Dura Den, near Coupar, in Fife, where, although the strata contain no mollusca, fish have been found abundantly, and have been referred to the genera Holoptychius, Pamphractus, Glyptopomus, and many others. In the county of Cork, in Ireland, a similar yellow sandstone occurs containing fish of genera characteristic of the Scotch Old Red Sandstone, as for example Coccosteus (a form represented by many species in the Old Red Sandstone and by one only in the Carboniferous group), and Glytolepis and Asterolepis, both exclusively confined to the “Old Red.” In the same Irish sandstone at Kiltorkan has been found an Anodonta or fresh-water mussel, the only shell hitherto discovered in the Old Red Sandstone of the British Isles (see Fig. 494).

Fig. 494: Anodonta Jukesii. Fig. 495: Bifurcating branch of Lepidendron Griffithsii. Fig. 496: Palæopteris Hibernia. In the same formation are found the fern (Fig. 496) and the Lepidodendron (Fig. 495), and other species of plants, some of which, Professor Heer remarks, agree specifically with species from the lower carboniferous beds. This induces him to lean to the opinion long ago advocated by Sir Richard Griffiths, that the yellow sandstone, in spite of its fish remains, should be classed as Lower Carboniferous, an opinion which I am not yet prepared to adopt. Between the Mountain Limestone and the yellow sandstone in the south-west of Ireland there intervenes a formation no less than 5000 feet thick, called the “Carboniferous slate,” and at the base of this, in some places, are local deposits, such as the Glengariff Grits, which appear to be beds of passage between the Carboniferous and Old Red Sandstone groups.

It is a remarkable result of the recent examination of the fossil flora of Bear Island, latitude 74° 30′ N., that Professor Heer has described as occurring in that part of the Arctic region (nearly twenty-six degrees to the north of the Irish locality) a flora agreeing in several of its species with that of the yellow sandstones of Ireland. This Bear Island flora is believed by Professor Heer to comprise species of plants some of which ascend even to the higher stages of the European Carboniferous formation, or as high as the Mountain Limestone and Millstone Grit. Palæontologists have long maintained that the same species which have a wide range in space are also the most persistent in time, which may prepare us to find that some plants having a vast geographical range may also have endured from the period of the Upper Devonian to that of the Millstone Grit.

Fig. 497: Scale of Holoptychius nobilissimus.

Outliers of the Upper “Old Red” occur unconformably on older members of the group, and the formation represented at Whiteness, near Arbroath, a, Fig. 55, may probably be one of these outliers, though the want of organic remains renders this uncertain. It is not improbable that the beds given in this section as Nos. 1, 2, and 3, may all belong to the early part of the period of the Upper Old Red, as some scales of Holoptychius nobilissimus have been found scattered through these beds, No. 2, in Strathmore. Another nearly allied Holoptychius occurs in Dura Den, see Fig. 498 of this fish and also Fig. 497 of one of its scales, as these last are often the only parts met with; being scattered in Forfarshire through red-coloured shales and sandstones, as are scales of a large species of the same genus in a corresponding matrix in Herefordshire.[1] The number of fish obtained from the British Upper Old Red Sandstone amounts to fifteen species referred to eleven genera.

Fig. 498: Holoptychius, as restored by Professor Huxley.

Sir R. Murchison groups with this upper division of the Old Red of Scotland certain light-red and yellow sandstones and grits which occur in the northernmost part of the mainland, and extend also into the Orkney and Shetland Islands. They contain Calamites and other plants which agree generically with Carboniferous forms.

Middle Old Red Sandstone.—In the northern part of Scotland there occur a great series of bituminous schists and flagstones, to the fossil fish of which attention was first called by the late Hugh Miller. They were afterwards described by Agassiz, and the rocks containing them were examined by Sir R. Murchison and Professor Sedgwick, in Caithness, Cromarty, Moray, Nairn, Gamrie in Banff, and the Orkneys and Shetlands, in which great numbers of fossil fish have been found. These were at first supposed to be the oldest known vertebrate animals, as in Cromarty the beds in which they occur seem to form the base of the Old Red system resting almost immediately on the crystalline or metamorphic rocks. But in fact these fish-bearing beds, when they are traced from north to south, or to the central parts of Scotland, thin out, so that their relative age to the Lower Old Red Sandstone, presently to be mentioned, was not at first detected, the two formations not appearing in superposition in the same district. In Caithness, however, many hundred feet below the fish-zone of the middle division, remains of Pteraspis were found by Mr. Peach in 1861. This genus has never yet been found in either of the two higher divisions of the Old Red Sandstone, and confirms Sir R. Murchison’s previous suspicion that the rocks in which it occurs belong to the Lower “Old Red,” or agree in age with the Arbroath paving-stone.[2]

Fossil Fish of the Middle Old Red Sandstone.—The Devonian fish were referred by Agassiz to two of his great orders, namely, the Placoids and Ganoids. Of the first of these, which in the Recent period comprise the shark, the dog-fish, and the ray, no entire skeletons are preserved, but fin-spines, called ichthyodorulites, and teeth occur. On such remains the genera Onchus, Odontacanthus, and Ctenodus, a supposed cestraciont, and some others, have been established.

By far the greater number of the Old Red Sandstone fishes belong to a sub-order of Ganoids instituted by Huxley in 1861, and for which he has proposed the name of Crossopterygidæ,[3] or the fringe-finned, in consideration of the peculiar manner in which the fin-rays of the paired fins are arranged so as to form a fringe round a central lobe, as in the Polypterus (see a, Fig. 499), a genus of which there are several species now inhabiting the Nile and other African rivers. The reader will at once recognise in Osteolepis (Fig. 500), one of the common fishes of the Old Red Sandstone, many points of analogy with Polypterus. They not only agree in the structure of the fin, at first pointed out by Huxley, but also in the position of the pectoral, ventral, and anal fins, and in having an elongated body and rhomboidal scales. On the other hand, the tail is more symmetrical in the recent fish, which has also an apparatus of dorsal finlets of a very abnormal character, both as to number and structure. As to the dorsals of Osteolepis, they are regular in structure and position, having nothing remarkable about them, except that there are two of them, which is comparatively unusual in living fish.

Fig. 499: Polypterus. Living in the Nile and other rivers.

Fig. 500: Restoration of Osteolepis.

Among the “fringe-finned” Ganoids we find some with rhomboidal scales, such as Osteolepis, Fig. 500; others with cycloidal scales, as Holoptychius, before mentioned (see Fig. 498). In the genera Dipterus and Diplopterus, as Hugh Miller pointed out, and in several other of the fringe-finned genera, as in Gyroptychius and Glyptolepis, the two dorsals are placed far backward, or directly over the ventral and anal fins. The Asterolepis was a ganoid fish of gigantic dimensions. A. Asmusii, Eichwald, a species characteristic of the Old Red Sandstone of Russia, as well as that of Scotland, attained the length of between twenty and thirty feet. It was clothed with strong bony armour, embossed with star-like tubercles, but it had only a cartilaginous skeleton. The mouth was furnished with two rows of teeth, the outer ones small and fish-like, the inner larger and with a reptilian character. The Asterolepis occurs also in the Devonian rocks of North America.

If we except the Placoids already alluded to, and a few other families of doubtful affinities, all the Old Red Sandstone fishes are Ganoids, an order so named by Agassiz from the shining outer surface of their scales; but Professor Huxley has also called our attention to the fact that, while a few of the primary and the great majority of the secondary Ganoids resemble the living bony pike, Lepidosteus, or the Amia, genera now found in North American rivers, and one of them, Lepidosteus, extending as far south as Guatemala, the Crossopterygii, or fringe-finned Ichthyolites, of the Old Red are closely related to the African Polypterus, which is represented by five or six species now inhabiting the Nile and the rivers of Senegal. These North American and African Ganoids are quite exceptional in the living creation; they are entirely confined to the northern hemisphere, unless some species of Polypterus range to the south of the line in Africa; and, out of about 9000 living species of fish known to M. Günther, and of which more than 6000 are now preserved in the British Museum, they probably constitute no more than nine.

Fig. 501: Pterichthys. Upper side, showing mouth. If many circumstances favour the theory of the fresh-water origin of the Old Red Sandstone, this view of its nature is not a little confirmed by our finding that it is in Llake Superior and the other inland Canadian seas of fresh water, and in the Mississippi and African rivers, that we at present find those fish which have the nearest affinity to the fossil forms of this ancient formation.

Among the anomalous forms of Old Red fishes not referable to Huxley’s Crossopterygii is the Pterichthys, of which five species have been found in the middle division of the Old Red of Scotland. Some writers have compared their shelly covering to that of Crustaceans, with which, however, they have no real affinity. The wing-like appendages, whence the genus is named, were first supposed by Hugh Miller to be paddles, like those of the turtle; and there can now be no doubt that they do really correspond with the pectoral fins.

The number of species of fish already obtained from the middle division of the Old Red Sandstone in Great Britain is about 70, and the principal genera, besides Osteolepis and Pterichthys, already mentioned, are Glyptolepis, Diplacanthus, Dendrodus, Coccosteus, Cheirancanthus, and Acanthoides.

Fig. 502: Cephalapsis Lyellii.

Lower Old Red Sandstone.—The third or lowest division south of the Grampians consists of grey paving-stone and roofing-slate, with associated red and grey shales; these strata underlie a dense mass of conglomerate. In these grey beds several remarkable fish have been found of the genus named by Agassiz Cephalaspis, or “buckler-headed,” from the extraordinary shield which covers the head (see Fig. 502), and which has often been mistaken for that of a trilobite, such as Asaphus. A species of Pteraspis, of the same family, has also been found by the Reverend Hugh Mitchell in beds of corresponding age in Perthshire; and Mr. Powrie enumerates no less than five genera of the family Acanthodidæ, the spines, scales, and other remains of which have been detected in the grey flaggy sandstones.[4]

Fig. 503: Pteygotus anglicus. In the same formation at Carmylie, in Forfarshire, commonly known as the Arbroath paving-stone, fragments of a huge crustacean have been met with from time to time. They are called by the Scotch quarrymen the “Seraphim,” from the wing-like form and feather-like ornament of the thoracic appendage, the part most usually met with. Agassiz, having previously referred some of these fragments to the class of fishes, was the first to recognise their crustacean character, and, although at the time unable correctly to determine the true relation of the several parts, he figured the portions on which he founded his opinion, in the first plate of his “Poissons Fossiles du Vieux Grès Rouge.”

Fig. 504: Pterygotus anglicus. Ventral aspect. Carapace, showing the large sessile eyes at the anterior angles.

The metastoma or post-oral plate (serving the office of a lower lip).

Chelate appendages (antennules).

First pair of simple palpi (antennæ).

Second pair of simple palpi (mandibles).

Third pair of simple palpi (first maxillæ).

Pair of swimming feet with their broad basal joints, whose serrated edges serve the office of maxillæ.

Thoracic plate covering the first two thoracic segments, which are indicated by the figures 1, 2, and a dotted line. 1-6. Thoracic segments. 7-12. Abdominal segments. 13. Telson, or tail-plate.)

A restoration in correct proportion to the size of the fragments of P. anglicus (Fig. 504), from the Lower Old Red Sandstone of Perthshire and Forfarshire, would give us a creature measuring from five to six feet in length, and more than one foot across.

The largest crustaceans living at the present day are the Inachus Kaempferi, of De Haan, from Japan (a brachyurous or short-tailed crab), chiefly remarkable for the extraordinary length of its limbs; the fore-arm measuring four feet in length, and the others in proportion, so that it covers about 25 square feet of ground; and the Limulus Moluccanus, the great King Crab of China and the Eastern seas, which, when adult, measures 1½ foot across its carapace, and is three feet in length.

Besides some species of Pterygotus, several of the allied genus Eurypterus occur in the Lower Old Red Sandstone, and with them the remains of grass-like plants so abundant in Forfarshire and Kincardineshire as to be useful to the geologist by enabling him to identify the inferior strata at distant points. Some botanists have suggested that these plants may be of the family Fluviales, and of fresh-water genera. They are accompanied by fossils, called “berries” by the quarrymen, which they compared to a compressed blackberry (see Figs. 505, 506), and which were called “Parka” by Dr. Fleming. They are now considered by Mr. Powrie to be the eggs of crustaceans, which is highly probable, for they have not only been found with Pterygotus anglicus in Forfarshire and Perthshire, but also in the Upper Silurian strata of England, in which species of the same genus, Pterygotus, occur.

Fig. 505: Parka decipiens. In sandstone of lower beds of Old Red, Ley’s Mill, Forfarshire. Fig. 506: Parka decipiens. In shale of Lower Old Red, Park Hill, Fife.

Fig. 507: Shale of Old Red Sandstone. Forfarshire. With impression of plants and eggs of Crustaceans. The grandest exhibitions, says Sir R. Murchison, of the Old Red Sandstone in England and Wales appear in the escarpments of the Black Mountains and in the Fans of Brecon and Carmarthen, the one 2862, and the other 2590 feet above the sea. The mass of red and brown sandstone in these mountains is estimated at not less than 10,000 feet, clearly intercalated between the Carboniferous and Silurian strata. No shells or corals have ever been found in the whole series, not even where the beds are calcareous, forming irregular courses of concretionary lumps called “corn-stones,” which may be described as mottled red and green earthy limestones. The fishes of this lowest English Old Red are Cephalaspis and Pteraspis, specifically different from species of the same genera which occur in the uppermost Ludlow or Silurian tilestones. Crustaceans also of the genus Eurypterus are met with.

Marine or Devonian Type.—We may now speak of the marine type of the British strata intermediate between the Carboniferous and Silurian, in treating of which we shall find it much more easy to identify the Upper, Middle, and Lower divisions with strata of the same age in other countries. It was not until the year 1836 that Sir R. Murchison and Professor Sedgwick discovered that the culmiferous or anthracitic shales and sandstones of North Devon, several thousand feet thick, belonged to the coal, and that the beds below them, which are of still greater thickness, and which, like the carboniferous strata, had been confounded under the general name “graywacke,” occupied a geological position corresponding to that of the Old Red Sandstone already described. In this reform they were aided by a suggestion of Mr. Lonsdale, who, after studying the Devonshire fossils, perceived that they belonged to a peculiar palæontological type of intermediate character between the Carboniferous and Silurian.

It is in the north of Devon that these formations may best be studied, where they have been divided into an Upper, Middle, and Lower Group, and where, although much contorted and folded, they have for the most part escaped being altered by intrusive trap-rocks and by granite, which in Dartmoor and the more southern parts of the same county have often reduced them to a crystalline or metamorphic state.

DEVONIAN SERIES IN NORTH DEVON.

UPPER DEVONIAN OR PILTON GROUP (a) Sandy slates and schists with fossils, 36 species out of 110 common to the Carboniferous group (Pilton, Barnstaple, etc.), resting on soft schists in which fossils are very abundant (Croyde, etc.), and which pass down into (b) Yellow, brown, and red sandstone, with land plants (Cyclopteris, etc.) and marine shells. One zone, characterised by the abundance of cucullæa (Baggy Point, Marwood, Sloly, etc.) resting on hard grey and reddish sandstone and micaceous flags, no fossils yet found (Dulverton, Pickwell, Down, etc.) MIDDLE DEVONIAN OR ILFRACOMBE GROUP. (a) Green glossy slates of considerable thickness, no fossils yet recorded from these beds (Mortenoe, Lee Bay, etc.). (b) Slates and schists, with several irregular courses of limestone containing shells and corals like those of the Plymouth Limestone (Combe Martin, Ilfracombe, etc.). LOWER DEVONIAN OR LYNTON GROUP. (a) Hard, greenish, red, and purple sandstone—no fossils yet found (Hangman Hill, etc.). (b) Soft slates with subordinate sandstones—fossils numerous at various horizons—Orthis, Corals, Encrinites, etc. (Valley of Rocks, Lynmouth, etc.).

The above table exhibits the sequence of the strata or subdivisions as seen both on the sea-coast of the British Channel and in the interior of Devon. It will be seen that in all main points it agrees with the table drawn up in 1864 for the sixth edition of my “Elements.” Mr. Etheridge[5] has since published an excellent account of the different subdivisions of the rocks and their fossils, and has also pointed out their relation to the corresponding marine strata of the Continent. The slight modifications introduced in my table since 1864 are the result of a tour made in 1870 in company with Mr. T. Mck. Hughes, when we had the advantage of Mr. Etheridge’s memoir as our guide.

The place of the sandstones of the Foreland is not yet clearly made out, as they are cut off by a great fault and disturbance.

Fig. 508: Spirifera disjuncta. Fig. 509: Phacops latifrons. Upper Devonian Rocks.—The slates and sandstones of Barnstaple (a and b of the preceding section) contain the shell Spirifera disjuncta, Sowerby (S. Verneuilii, Murch.), (see Fig. 508), which has a very wide range in Europe, Asia Minor, and even China; also Strophalosia caperata, together with the large trilobite Phacops latifrons, Bronn. (See Fig. 509), which is all but world-wide in its distribution. The fossils are numerous, and comprise about 150 species of mollusca, a fifth of which pass up into the overlying Carboniferous rocks. To this Upper Devonian belong a series of limestones and slates well developed at Petherwyn, in Cornwall, where they have yielded 75 species of fossils. The genus of Cephalopoda called Clymenia (Fig. 510) is represented by no less than eleven species, and strata occupying the same position in Germany are called Clymenien-Kalk, or sometimes Cypridinen-Schiefer, on account of the number of minute bivalve shells of the crustacean called Cypridina serrato-striata (Fig. 511), which is found in these beds, in the Rhenish provinces, the Harz, Saxony, and Silesia, as well as in Cornwall and Belgium.

Middle Devonian Rocks.—We come next to the most typical portion of the Devonian system, including the great limestones of Plymouth and Torbay, replete with shells, trilobites, and corals. Of the corals 51 species are enumerated by Mr. Etheridge, none of which pass into the Carboniferous formation. Among the genera we find Favosites, Heliolites, and Cyathophyllum. The two former genera are very frequent in Silurian rocks: some few even of the species are said to be common to the Devonian and Silurian groups, as, for example, Favosites cervicornis (Fig. 513), one of the commonest of all the Devonshire fossils. The Cyathophyllum cæspitosum (Fig. 514) and Heliolites pyriformis (Fig. 512) are species peculiar to this formation.

Fig. 510: Clymenia linearis. Fig. 511: Cypridina serrato-striata.

Fig. 512: Heliolites porosa. Fig. 513: Favosites cervicornis. Fig. 514: Cyathophyllum cæspitosum.

Fig. 515: Stringocephalus Burtini. Fig. 516: Uncites Gryphus.

With the above are found no less than eleven genera of stone-lilies or crinoids, some of them, such as Cupressocrinites, distinct from any Carboniferous forms. The mollusks, also, are no less characteristic; of 68 species of Brachiopoda, ten only are common to the Carboniferous Limestone. The Stringocephalus Burtini (Fig. 515) and Uncites Gryphus (Fig. 516) may be mentioned as exclusively Middle Devonian genera, and extremely characteristic of the same division in Belgium. The Stringocephalus is also so abundant in the Middle Devonian of the banks of the Rhine as to have suggested the name of Stringocephalus Limestone.

Fig. 517: Megalodon cucullatus. The only two species of Brachiopoda common to the Silurian and Devonian formations are Atrypa reticularis (Fig. 532), which seems to have been a cosmopolite species, and Strophomena rhomboidalis.

Among the peculiar lamellibranchiate bivalves common to the Plymouth limestone of Devonshire and the Continent, we find the Megalodon (Fig. 517). There are also twelve genera of Gasteropods which have yielded 36 species, four of which pass to the Carboniferous group, namely Macrocheilus, Acroculia, Euomphalus, and Murchisonia. Pteropods occur, such as Conularia (Fig. 518), and Cephalopods, such as Cyrtoceras, Gyroceras, Orthoceras, and others, nearly all of genera distinct from those prevailing in the Upper Devonian Limestone, or Clymenien-kalk of the Germans already mentioned. Although but few species of Trilobites occur, the characteristic Bronteus flabellifer (Fig. 519) is far from rare, and all collectors are familiar with its fan-like tail. In this same group, called, as before stated, the Stringocephalus, or Eifel Limestone, in Germany, several fish remains have been detected, and among others the remarkable genus Coccosteus, covered with its tuberculated bony armour; and these ichthyolites serve, as Sir R. Murchison observes (Siluria, p. 362), to identify this middle marine Devonian with the Old Red Sandstone of Britain and Russia.

Fig. 518: Conularia ornata.Fig. 519: Bronteus flabellifer. Fig. 520: Calceola sandalina. Beneath the Eifel Limestone (the great central and typical member of “the Devonian” on the Continent) lie certain schists called by German writers “Calceola-schiefer,” because they contain in abundance a fossil body of very curious structure, Calceola sandalina (Fig. 520), which has been usually considered a brachiopod, but which some naturalists have lately referred to a Goniophyllum, supposing it to be an abnormal form of the order Zoantharia rugosa (see Fig. 474), differing from all other corals in being furnished with a strong operculum. This is by no means a rare fossil in the slaty limestone of South Devon, and, like the Eifel form, is confined to the middle group of this country.

Lower Devonian Rocks.—A great series of sandstones and glossy slates, with Crinoids, Brachiopods, and some corals, occurring on the coast at Lynmouth and the neighbourhood, and called the Lynton Group (see Table p. 449, form the lowest member of the Devonian in North Devon. Among the 18 species of all classes enumerated by Mr. Etheridge, two-thirds are common to the Middle Devonian, but only one, the ubiquitous Atrypa reticularis, can with certainty be identified with Silurian species. Among the characteristic forms are Alveolites suborbicularis, also common to this formation in the Rhine, and Orthis arcuata, very widely spread in the North Devon localities. But we may expect a large addition to the number of fossils whenever these strata shall have been carefully searched. The Spirifer Sandstone of Sandberger, as exhibited in the rocks bordering the Rhine between Coblentz and Caub, belong to this Lower division, and the same broad-winged Spirifers distinguish the Devonian strata of North America.

Fig. 521: Spirifora mucronata. Fig. 522: Homalonotus armatus. Among the Trilobites of this era several large species of Homalonotus (Fig. 522) are conspicuous. The genus is still better known as a Silurian form, but the spinose species appear to belong exclusively to the “Lower Devonian,” and are found in Britain, Europe, and the Cape of Good Hope.

Devonian of Russia.—The Devonian strata of Russia extend, according to Sir R. Murchison, over a region more spacious than the British Isles; and it is remarkable that, where they consist of sandstone like the “Old Red” of Scotland and Central England, they are tenanted by fossil fishes often of the same species and still oftener of the same genera as the British, whereas when they consist of limestone they contain shells similar to those of Devonshire, thus confirming, as Sir Roderick has pointed out, the contemporaneous origin which had been previously assigned to formations exhibiting two very distinct mineral types in different parts of Britain.[6] The calcareous and the arenaceous rocks of Russia above alluded to alternate in such a manner as to leave no doubt of their having been deposited in different parts of the same great period.

Fig. 523: Psilophyton princeps. Devonian Strata in the United States and Canada.—Between the Carboniferous and Silurian strata there intervenes, in the United States and Canada, a great series of formations referable to the Devonian group, comprising some strata of marine origin abounding in shells and corals, and others of shallow-water and littoral origin in which terrestrial plants abound. The fossils, both of the deep and shallow water strata, are very analogous to those of Europe, the species being in some cases the same. In Eastern Canada Sir W. Logan has pointed out that in the peninsula of Gaspe, south of the estuary of St. Lawrence, a mass of sandstone, conglomerate, and shale referable to this period occurs, rich in vegetable remains, together with some fish-spines. Far down in the sandstones of Gaspe, Dr. Dawson found, in 1869, an entire specimen of the genus Cephalaspis, a form so characteristic, as we have already seen, of the Scotch Lower Old Red Sandstone. Some of the sandstones are ripple-marked, and towards the upper part of the whole series a thin seam of coal has been observed, measuring, together with some associated carbonaceous shale, about three inches in thickness. It rests on an under-clay in which are the roots of Psilophyton (see Fig. 523). At many other levels rootlets of this same plant have been shown by Principal Dawson to penetrate the clays, and to play the same part as do the rootlets of Stigmaria in the coal formation.

We had already learnt from the works of Göppert, Unger, and Bronn that the European plants of the Devonian epoch resemble generically, with few exceptions, those already known as Carboniferous; and Dr. Dawson, in 1859, enumerated 32 genera and 69 species which he had then obtained from the State of New York and Canada. A perusal of his catalogue,[7] comprising Coniferæ, Sigillariæ, Calamites, Asterophyllites, Lepidodendra, and ferns of the genera Cyclopteris, Neuropteris, Sphenopteris, and others, together with fruits, such as Cardiocarpum and Trigonocarpum, might dispose geologists to believe that they were presented with a list of Carboniferous fossils, the difference of the species from those of the coal-measures, and even a slight admixture of genera unknown in Europe, being naturally ascribed to geographical distribution and the distance of the New from the Old World. But fortunately the coal formation is fully developed on the other side of the Atlantic, and is singularly like that of Europe, both lithologically and in the species of its fossil plants. There is also the most unequivocal evidence of relative age afforded by superposition, for the Devonian strata in the United States are seen to crop out from beneath the Carboniferous on the borders of Pennsylvania and New York, where both formations are of great thickness.

The number of American Devonian plants has now been raised by Dr. Dawson to 120, to which we may add about 80 from the European flora of the same age, so that already the vegetation of this period is beginning to be nearly half as rich as that of the coal-measures which have been studied for so much longer a time and over so much wider an area. The Psilophyton above alluded to is believed by Dr. Dawson to be a lycopodiaceous plant, branching dichotomously (see P. princeps, Fig. 523), with stems springing from a rhizome, which last has circular areoles, much resembling those of Stigmaria, and like it sending forth cylindrical rootlets. The extreme points of some of the branchlets are rolled up so as to resemble the croziers or circinate vernation of ferns; the leaves or bracts, a, supposed to belong to the same plant, are described by Dawson as having inclosed the fructification. The remains of Psilophyton princeps have been traced through all the members of the Devonian series in America, and Dr. Dawson has lately recognised it in specimens of Old Red Sandstone from the north of Scotland.

The monotonous character of the Carboniferous flora might be explained by imagining that we have only the vegetation handed down to us of one set of stations, consisting of wide swampy flats. But Dr. Dawson supposes that the geographical conditions under which the Devonian plants grew were more varied, and had more of an upland character. If so, the limitation of this more ancient flora, represented by so many genera and species, to the gymnospermous and cryptogamous orders, and the absence or extreme rarity of plants of higher grade, lead us naturally to speculate on the theory of progressive development, however difficult it may be to avail ourselves of this explanation, so long as we meet with even a few exceptional cases of what may seem to be monocotyledonous or dicotyledonous exogens.

Devonian Insects of Canada.—The earliest known insects were brought to light in 1865 in the Devonian strata of St. John’s, New Brunswick, and are referred by Mr. Scudder to four species of Neuroptera. One of them is a gigantic Ephemera, and measured five inches in expanse of wing.

Like many other ancient animals, says Dr. Dawson, they show a remarkable union of characters now found in distinct orders of insects, or constitute what have been named “synthetic types.” Of this kind is a stridulating or musical apparatus like that of the cricket in an insect otherwise allied to the Neuroptera. This structure, as Dr. Dawson observes, if rightly interpreted by Mr. Scudder, introduces us to the sounds of the Devonian woods, bringing before our imagination the trill and hum of insect life that enlivened the solitudes of these strange old forests.

[1] Siluria, 4th ed., p. 265.

[2] Siluria, 4th ed., p. 258.

[3] Abridged from crossotos, a fringe, and pteryx, a fin.

[4] Powrie, Geol. Quart. Journ., vol. xx, p. 417.

[5] Quart. Geol. Journ., vol. xxiii., 1867.

[6] Murchison’s Siluria, p. 329.

[7] Quart. Geol. Journ., vol. xv, p. 477, 1859; also vol. xviii, p. 296, 1862.

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XXVI. SILURIAN GROUP

Classification of the Silurian Rocks. — Ludlow Formation and Fossils. — Bone-bed of the Upper Ludlow. — Lower Ludlow Shales with Pentamerus. — Oldest known Remains of fossil Fish. — Table of the progressive Discovery of Vertebrata in older Rocks. — Wenlock Formation, Corals, Cystideans and Trilobites. — Llandovery Group or Beds of Passage. — Lower Silurian Rocks. — Caradoc and Bala Beds. — Brachiopoda. — Trilobites. — Cystideæ. — Graptolites. — Llandeilo Flags. — Arenig or Stiper-stones Group. — Foreign Silurian Equivalents in Europe. — Silurian Strata of the United States. — Canadian Equivalents. — Amount of specific Agreement of Fossils with those of Europe.

Classification of the Silurian Rocks.—We come next in descending order to that division of Primary or Palæozoic rocks which immediately underlie the Devonian group or Old Red Sandstone. For these strata Sir Roderick Murchison first proposed the name of Silurian when he had studied and classified them in that part of Wales and some of the contiguous counties of England which once constituted the kingdom of the Silures, a tribe of ancient Britons. The following table will explain the two principal divisions, Upper and Lower, of the Silurian rocks, and the minor subdivisions usually adopted, comprehending all the strata originally embraced in the Silurian system by Sir Roderick Murchison. The formations below the Arenig or Stiper-stones group are treated of in the next chapter, when the “Primordial” or Cambrian group is described.

UPPER SILURIAN ROCKS. Thickness in feet 1. LUDLOW FORMATION: a. Upper Ludlow beds 780 b. Lower Ludlow beds: 1,050 2. WENLOCK FORMATION: a. Wenlock limestone and shale above 4,000 b. Woolhope limestone and shale, and Denbighshire grits: 3. LLANDOVERY FORMATION (Beds of passage between Upper and Lower Silurian): a. Upper Llandovery (May-Hill beds): 800 b. Lower Llandovery: 600–1,000 LOWER SILURIAN ROCKS. 1. BALA AND CARADOC BEDS, including volcanic rocks: 12,000 2. LLANDEILO FLAGS, including volcanic rocks: 4,500 3. ARENIG OR STIPER-STONES GROUP, including volcanic rocks: above 10,000

UPPER SILURIAN ROCKS.

1. Ludlow Formation.—This member of the Upper Silurian group, as will be seen by above table, is of great thickness, and subdivided into two parts—the Upper Ludlow and the Lower Ludlow. Each of these may be distinguished near the town of Ludlow, and at other places in Shropshire and Herefordshire, by peculiar organic remains; but out of more than 500 species found in the Ludlow formation as a whole, not more than five species per hundred are common to the overlying Devonian. The student may refer to the excellent tables given in the last edition of Sir R. Murchison’s Siluria for a list of the organic remains of all classes distributed through the different subdivisions of the Upper and Lower Silurian.

a. Upper Ludlow: Downton Sandstone.—At the top of this subdivision there occur beds of fine-grained yellowish sandstone and hard reddish grits which were formerly referred by Sir R. Murchison to the Old Red Sandstone, under the name of “Tilestones.” In mineral character this group forms a transition from the Silurian to the Old Red Sandstone, the strata of both being conformable; but it is now ascertained that the fossils agree in great part specifically, and in general character entirely, with those of the underlying Upper Ludlow rocks. Among these are Orthoceras bullatum, Platyschisma helicites, Bellerophon trilobatus, Chonetes lata, etc., with numerous defenses of fishes.

These beds, therefore, now generally called the “Downton Sandstone,” are classed as the newest member of the Upper Silurian. They are well seen at Downton Castle, near Ludlow, where they are quarried for building, and at Kington, in Herefordshire. In the latter place, as well as at Ludlow, crustaceans of the genera Pterygotus (for genus see Fig. 504) and Eurypterus are met with.

Bone-bed of the Upper Ludlow.—At the base of the Downton sandstones there occurs a bone-bed which deserves especial notice as affording the most ancient example of fossil fish occurring in any considerable quantity. It usually consists of one or two thin layers of brown bony fragments near the junction of the Old Red Sandstone and the Ludlow rocks, and was first observed by Sir R. Murchison near the town of Ludlow, where it is three or four inches thick. It has since been traced to a distance of 45 miles from that point into Gloucestershire and other counties, and is commonly not more than an inch thick, but varies to nearly a foot. Near Ludlow two bone-beds are observable, with 14 feet of intervening strata full of Upper Ludlow fossils.[1] At that point immediately above the upper fish-bed numerous small globular bodies have been found, which were determined by Dr. Hooker to be the sporangia of a cryptogamic land-plant, probably lycopodiaceous.

Fig. 524: Onchus tenuistriatus. Fig. 525: Shagreen-scales of a placoid fish, Thelodus parvidens.

Most of the fish have been referred by Agassiz to his placoid order, some of them to the genus Onchus, to which the spine (Fig. 524) and the minute scales (Fig. 525) are supposed to belong. It has been suggested, however, that Onchus may be one of those Acanthodian fish referred by Agassiz to his Ganoid order, which are so characteristic of the base of the Old Red Sandstone in Forfarshire, although the species of the Old Red are all different from these of the Silurian beds now under consideration.

Fig. 526: Plectrodus mirabilis. The jaw and teeth of another predaceous genus (Fig. 526) have also been detected, together with some specimens of Pteraspis Ludensis. As usual in bone-beds, the teeth and bones are, for the most part, fragmentary and rolled.

Fig. 527: Orthis elegantula.Fig. 528: Rhynchonella navicula. Grey Sandstone and Mudstone, etc.—The next subdivision of the Upper Ludlow consists of grey calcareous sandstone, or very commonly a micaceous stone, decomposing into soft mud, and contains, besides the shells mentioned aon page 459, Lingula cornea, Orthis orbicularis, a round variety of O. elegantula, Modiolopsis platyphylla, Grammysia cingulata, all characteristic of the Upper Ludlow. The lowest or mud-stone beds contain Rhynchonella navicula (Fig. 528), which is common to this bed and the Lower Ludlow. As usual in Palæozoic strata older than the coal, the brachiopodous mollusca greatly outnumber the lamellibranchiate (see p. 470); but the latter are by no means unrepresented. Among other genera, for example, we observe Avicula and Pterinea, Cardiola, Ctenodonta (sub-genus of Nucula), Orthonota, Modiolopsis, and Palæarca.

Some of the Upper Ludlow sandstones are ripple-marked, thus affording evidence of gradual deposition; and the same may be said of the accompanying fine argillaceous shales, which are of great thickness, and have been provincially named “mud-stones.” In some of these shales stems of crinoidea are found in an erect position, having evidently become fossil on the spots where they grew at the bottom of the sea. The facility with which these rocks, when exposed to the weather, are resolved into mud, proves that, notwithstanding their antiquity, they are nearly in the state in which they were first thrown down.

Fig. 529: Pentamerus Knightii.

b. Lower Ludlow Beds.—The chief mass of this formation consists of a dark grey argillaceous shale with calcareous concretions, having a maximum thickness of 1000 feet. In some places, and especially at Aymestry, in Herefordshire, a subcrystalline and argillaceous limestone, sometimes 50 feet thick, overlies the shale. Sir R. Murchison therefore classes this Aymestry limestone as holding an intermediate position between the Upper and Lower Ludlow, but Mr. Lightbody remarks that at Mocktrie, near Leintwardine, the Lower Ludlow shales, with their characteristic fossils, occur both above and below a similar limestone. This limestone around Aymestry and Sedgeley is distinguished by the abundance of Pentamerus Knightii, Sowerby (Fig. 529), also found in the Lower Ludlow and Wenlock shale. This genus of brachiopoda was first found in Silurian strata, and is exclusively a palæozoic form. The name was derived from pente, five, and meros, a part, because both valves are divided by a central septum, making four chambers, and in one valve the septum itself contains a small chamber, making five. The size of these septa is enormous compared with those of any other brachiopod shell; and they must nearly have divided the animal into two equal halves; but they are, nevertheless, of the same nature as the septa or plates which are found in the interior of Spirifera, Terebratula, and many other shells of this order. Messrs. Murchison and De Verneuil discovered this species dispersed in myriads through a white limestone of Upper Silurian age, on the banks of the Is, on the eastern flank of the Urals in Russia, and a similar species is frequent in Sweden.

Fig. 530: Lingula Lewisii. Three other abundant shells in the Aymestry limestone are, first, Lingula Lewisii (Fig. 530); second, Rhynchonella Wilsoni, Sowerby (Fig. 531), which is also common to the Lower Ludlow and Wenlock limestone; third, Atrypa reticularis, Linn. (Fig. 532), which has a very wide range, being found in every part of the Upper Silurian system, and even ranging up into the Middle Devonian series.

Fig. 531: Rhynchonella (Terebratula) Wilsoni.

The Aymestry Limestone contains many shells, especially brachiopoda, corals, trilobites, and other fossils, amounting on the whole to 74 species, all except three or four being common to the beds either above or below.

Fig. 532: Atrypa reticularis. The Lower Ludlow Shale contains, among other fossils, many large cephalopoda not known in newer rocks, as the Phragmoceras of Broderip, and the Lituites of Breynius (see Figs. 533, 534). The latter is partly straight and partly convoluted in a very flat spire. The Orthoceras Ludense (Fig. 535), as well as the cephalopod last mentioned, occurs in this member of the species.

Fig. 533: Phragmoceras ventricosum.

A species of Graptolite, G. priodon, Bronn (Fig. 545), occurs plentifully in the Lower Ludlow. This fossil, referred, though somewhat doubtfully, to a form of hydrozoid or sertularian polyp, has not yet been met with in strata above the Silurian.

Star-fish, as Sir R. Murchison points out, are by no means rare in the Lower Ludlow rock. These fossils, of which six extinct genera are now known in the Ludlow series, represented by 18 species, remind us of various living forms now found in our British seas, both of the families Asteriadæ and Ophiuridæ.

Fig. 534: Lituites (Trochoceras) giganteus. Fig. 535: Fragment of Orthoceras Ludense.

Oldest known Fossil Fish.—Until 1859 there was no example of a fossil fish older than the bone-bed of the Upper Ludlow, but in that year a specimen of Pteraspis was found at Church Hill, near Leintwardine, in Shropshire, by Mr. J. E. Lee of Caerleon, F.G.S., in shale below the Aymestry limestone, associated with fossil shells of the Lower Ludlow formation—shells which differ considerably from those characterising the Upper Ludlow already described. This discovery is of no small interest as bearing on the theory of progressive development, because, according to Professor Huxley, the genus Pteraspis is allied to the sturgeon, and therefore by no means of low grade in the piscine class.

It is a fact well worthy of notice that no remains of vertebrata have yet been met with in any strata older than the Lower Ludlow.

When we reflect on the hundreds of Mollusks, Echinoderms, Trilobites, Corals, and other fossils already obtained from more ancient Silurian formations, Upper, Middle, and Lower, we may well ask whether any set of fossiliferous rocks newer in the series were ever studied with equal diligence, and over so vast an area, without yielding a single ichthyolite. Yet we must hesitate before we accept, even on such evidence, so sweeping a conclusion, as that the globe, for ages after it was inhabited by all the great classes of invertebrata, remained wholly untenanted by vertebrate animals.

Dates of the Discovery of different Classes of Fossil Vertebrata; showing the gradual progress made in tracing them to rocks of higher antiquity.

Year Formations Geographical localities Mammalia 1798 Upper Eocene Paris (Gypsum of Montmartre).1 1818 Lower Oolite Stonesfield.2 1847 Upper Trias Stuttgart.3 Aves 1782 Upper Eocene Paris (Gypsum of Montmartre).4 1839 Lower Eocene Isle of Sheppey (London Clay).5 1854 Lower Eocene Woolwich Beds.6 1855 Lower Eocene Mendon (Plastic Clay).7 1858 Chloritic Series, or Upper Greensand Cambridge.8 1863 Upper Oolite Solenhofen.9 Reptilia 1710 Permian (or Zechstein) Thuringia.10 1844 Carboniferous Saarbrück, near Trèves.11 Pisces 1709 Permian (or Kupferschiefer) Thuringia.12 1793 Carboniferous (Mountain Limestone) Glasgow.13 1828 Devonian Caithness.14 1840 Upper Ludlow Ludlow.15 1859 Lower Ludlow Leintwardine.16

1. George Cuvier, Bulletin Soc. Philom. xx. 2. In 1818, Cuvier, visiting the Museum of Oxford, decided on the mammalian character of a jaw from Stonesfield. See also p. 347. 3. Prof. Plieninger. See p. 368. 4. Cuvier, Ossemens Foss. Art. “Oiseaux.” 5. Prof. Owen, Geol. Trans., 2nd series, vol. vi, p. 203, 1839. 6. Upper part of the Woolwich beds. Prestwich, Quart. Geol. Journ., vol. x, p. 157. 7. Gastornis Parisiensis. Owen, Quart. Geol. Journ., vol. xii, p. 204, 1856. 8. Coprolitic bed, in the Upper Greensand. See p. 299. 9. The Archæopteryx macrura, Owen. See p. 338. 10. The fossil monitor of Thuringia (Protosaurus Speneri, V. Meyer) was figured by Spener of Berlin in 1810. (Miscel. Berlin.) 11. See p. 406. 12. Memorabilia Saxoniæ Subterr., Leipsic, 1709. 13. History of Rutherglen by Rev. David Ure, 1793. 14. Sedgwick and Murchison, Geol. Trans., 2nd series, vol. ii, p. 141, 1828. 15. Sir R. Murchison. See p. 459. 16. See p. 461.

Obs.—The evidence derived from foot-prints, though often to be relied on, is omitted in the above table, as being less exact than that founded on bones and teeth.

In the preceding Table a few dates are set before the reader of the discovery of different classes of animals in ancient rocks, to enable him to perceive at a glance how gradual has been our progress in tracing back the signs of vertebrata to formations of high antiquity. Such facts may be useful in warning us not to assume too hastily that the point which our retrospect may have reached at the present moment can be regarded as fixing the date of the first introduction of any one class of beings upon the earth.

2. Wenlock Formation.—We next come to the Wenlock formation, which has been divided (see Table, p. 458) into Wenlock limestone, Wenlock shale, and Woolhope limestone and Denbighshire grits.

Fig. 536: Halysites catenularius.

a. Wenlock Limestone.—This limestone, otherwise well known to collectors by the name of the Dudley Limestone, forms a continuous ridge in Shropshire, ranging for about 20 miles from S.W. to N.E., about a mile distant from the nearly parallel escarpment of the Aymestry limestone. This ridgy prominence is due to the solidity of the rock, and to the softness of the shales above and below it. Near Wenlock it consists of thick masses of grey subcrystalline limestone, replete with corals, encrinites, and trilobites. It is essentially of a concretionary nature; and the concretions, termed “ball-stones” in Shropshire, are often enormous, even 80 feet in diameter. They are of pure carbonate of lime, the surrounding rock being more or less argillaceous[2] Sometimes in the Malvern Hills this limestone, according to Professor Phillips, is oolitic.

Fig. 537: Favosites Gothlandica. Among the corals, in which this formation is so rich, 53 species being known, the “chain-coral,” Halysites catenularius (Fig. 536), may be pointed out as one very easily recognised, and widely spread in Europe, ranging through all parts of the Silurian group, from the Aymestry limestone to near the bottom of the Llandeilo rocks. Another coral, the Favosites Gothlandica (Fig. 537), is also met with in profusion in large hemispherical masses, which break up into columnar and prismatic fragments, like that here figured (Fig. 537, b). Another common form in the Wenlock limestone is the Omphyma turbinatum (Fig. 538), which, like many of its modern companions, reminds us of some cup-corals; but all the Silurian genera belong to the palæozoic type before mentioned (p. 432), exhibiting the quadripartite arrangement of the septalamellæ within the cup.

Fig. 538: Omphyma turbinatum.Fig. 539: Pseudocrinites bifasciatus.

Among the numerous Crinoids, several peculiar species of Cyathocrinus (for genus see Figs. 478, 479) contribute their calcareous stems, arms, and cups towards the composition of the Wenlock limestone. Of Cystideans there are a few very remarkable forms, most of them peculiar to the Upper Silurian formation, as, for example, the Pseudocrinites, which was furnished with pinnated fixed arms,[3] as represented in Fig. 539.

Fig. 540: Strophomena (Leptæna) depressa. The Brachiopoda are, many of them, of the same species as those of the Aymestry limestone; as, for example, Atrypa reticularis (Fig. 532), and Strophomena depressa (Fig. 540); but the latter species ranges also from the Ludlow rocks, through the Wenlock shale, to the Caradoc Sandstone.

Fig. 541: Calymene Blumenbachii. The crustaceans are represented almost exclusively by Trilobites, which are very conspicuous, 22 being peculiar. The Calymene Blumenbachii (Fig. 541), called the ”Dudley Trilobite,” was known to collectors long before its true place in the animal kingdom was ascertained. It is often found coiled up like the common Oniscus or wood-louse, and this is so usual a circumstance among certain genera of trilobites as to lead us to conclude that they must have habitually resorted to this mode of protecting themselves when alarmed. The other common species is the Phacops caudatus (Asaphus caudatus), Brong. (see Fig. 542), and this is conspicuous for its large size and flattened form. Sphærexochus mirus (Fig. 543) is almost a globe when rolled up, the forehead or glabellum of this species being extremely inflated. The Homalonotus, a form of Trilobite in which the tripartite division of the dorsal crust is almost lost (see Fig. 544), is very characteristic of this division of the Silurian series.

Fig. 542: Phacops (Asaphus) caudatus.Fig. 543: Sphærexochus mirus. Wenlock Shale.—This, observes Sir R. Murchison, is infinitely the largest and most persistent member of the Wenlock formation, for the limestone often thins out and disappears. The shale, like the Lower Ludlow, often contains elliptical concretions of impure earthy limestone.

Fig. 544: Homalonotus delphinocephalus. In the Malvern district it is a mass of finely levigated argillaceous matter, attaining, according to Professor Phillips, a thickness of 640 feet, but it is sometimes more than 1000 feet thick in Wales, and is worked for flag-stones and slates. The prevailing fossils, besides corals and trilobites, and some crinoids, are several small species of Orthis, Cardiola, and numerous thin-shelled species of Orthoceratites.

About six species of Graptolite, a peculiar group of sertularian fossils before alluded to (p. 463) as being confined to Silurian rocks, occur in this shale. Of fossils of this genus, which is very characteristic of the Lower Silurian, I shall again speak in the sequel (p. 474).

Fig. 545: Graptolithus priodon. b. Woolhope Beds.—Though not always recognised as a separate subdivision of the Wenlock, the Woolhope beds, which underlie the Wenlock shale, are of great importance. Usually they occur as massive or nodular limestones, underlaid by a fine shale or flag-stone; and in other cases, as in the noted Denbighshire sandstones, as a coarse grit of very great thickness. This grit forms mountain ranges through North and South Wales, and is generally marked by the great sterility of the soil where it occurs. It contains the usual Wenlock fossils, but with the addition of some common in the uppermost Ludlow rock, such as Chonetes lata and Bellerophon trilobatus. The chief fossils of the Woolhope limestone are Illænus Barriensis, Homalonotus delphinocephalus (Fig. 544), Strophomena imbrex, and Rhynchonella Wilsoni (Fig. 531). The latter attains in the Woolhope beds an unusual size for the species, the specimens being sometimes twice as large as those found in the Wenlock limestone.

In some places below the Wenlock formation there are shales of a pale or purple colour, which near Tarannon attain a thickness of about 1000 feet; they can be traced through Radnor and Montgomery to North Wales, according to Messrs. Jukes and Aveline. By the latter geologist they have been identified with certain shales above the May-Hill Sandstone, near Llandovery, but, owing to the extreme scarcity of fossils, their exact position remains doubtful.

3. Llandovery Group—Beds of Passage.—We now come to beds respecting the classification of which there has been much difference of opinion, and which in fact must be considered as beds of passage between Upper and Lower Silurian. I formerly adopted the plan of those who class them as Middle Silurian, but they are scarcely entitled to this distinction, since after about 1400 Silurian species have been compared the number peculiar to the group in question only gives them an importance equal to such minor subdivisions as the Ludlow or Bala groups. I therefore prefer to regard them as the base of the Upper Silurian, to which group they are linked by more than twice as many species as to the Lower Silurian. By this arrangement the line of demarkation between the two great divisions, though confessedly arbitrary, is less so than by any other. They are called Llandovery Rocks, from a town in South Wales, in the neighbourhood of which they are well developed, and where, especially at a hill called Noeth Grug, in spite of several faults, their relations to one another can be clearly seen.

a. Upper Llandovery or May-Hill Sandstone.—The May-Hill group, which has also been named ”Upper Llandovery,” by Sir R. Murchison, ranges from the west of the Longmynd to Builth, Llandovery, and Llandeilo, and to the sea in Marlow’s Bay, where it is seen in the cliffs. It consists of brownish and yellow sandstones with calcareous nodules, having sometimes a conglomerate at the base derived from the waste of the Lower Silurian rocks. These May-Hill beds were formerly supposed to be part of the Caradoc formation, but their true position was determined by Professor Sedgwick[4] to be at the base of the Upper Silurian proper. The more calcareous portions of the rock have been called the Pentamerus limestone, because Pentamerus oblongus (Fig. 546) is very abundant in them. It is usually accompanied by P. (Stricklandinia) lirata (Fig. 547); both forms have a wide geographical range, being also met with in the same part of the Silurian series in Russia and the United States.

Fig. 546: Pentamerus oblongus. About 228 species of fossils are known in the May-Hill division, more than half of which are Wenlock species. They consist of trilobites of the genera Illænus and Calymene; Brachiopods of the genera Orthis, Atrypa, Leptæna, Pentamerus, Strophomena, and others; Gasteropods of the genera Turbo, Murchisonia (for genus, see Fig. 567), and Bellerophon; and Pteropods of the genus Conularia. The Brachiopods, of which there are 66 species, are almost all Upper Silurian.

Fig. 547: Stricklandinia (Pentamerus) lirata. Fig. 548: Tentaculites annulatus. Among the fossils of the May-Hill shelly sandstone at Malvern, Tentaculites annulatus (Fig. 548), an annelid, probably allied to Serpula, is found.

Lower Llandovery Rocks.—Below the May-Hill Group are the Lower Llandovery Rocks, which consist chiefly of hard slaty rocks, and beds of conglomerate from 600 to 1000 feet in thickness. The fossils, which are somewhat rare in the lower beds, consist of 128 known species, only eleven of which are peculiar, 83 being common to the May-Hill group above, and 93 common to the rocks below. Stricklandinia (Pentamerus) levis, which is common in the Lower Llandovery, becomes rare in the Upper, while Pentamerus oblongus (Fig. 546), which is the characteristic shell of the Upper Llandovery, occurs but seldom in the Lower.

LOWER SILURIAN ROCKS.

The Lower Silurian has been divided into, first, the Bala Group; second, the Llandeilo flags; and, third, the Arenig or Lower Llandeilo formation.

Bala and Caradoc Beds.—The Caradoc sandstone was originally so named by Sir R. I. Murchison from the mountain called Caer Caradoc, in Shropshire; it consists of shelly sandstones of great thickness, and sometimes containing much calcareous matter. The rock is frequently laden with the beautiful trilobite called by Murchison Trinucleus Caractaci (see Fig. 553), which ranges from the base to the summit of the formation, usually accompanied by Strophomena grandis (see Fig. 551), and Orthis vespertilio (Fig. 550), with many other fossils.

Fig. 549: Orthis tricenaria. Fig. 550: Orthis vespertilio. Fig. 551: Orthis (Strophomena) grandis.

Brachiopoda.—Nothing is more remarkable in these beds, and in the Silurian strata generally of all countries, than the preponderance of brachiopoda over other forms of mollusca. Their proportional numbers can by no means be explained by supposing them to have inhabited seas of great depth, for the contrast between the palæozoic and the present state of things has not been essentially altered by the late discoveries made in our deep-sea dredgings. We find the living brachiopoda so rare as to form about one forty-fourth of the whole bivalve fauna, whereas in the Lower Silurian rocks of which we are now about to treat, and where the brachiopoda reach their maximum, they are represented by more than twice as many species as the Lamellibranchiate bivalves.

There may, indeed, be said to be a continued decrease of the proportional number of this lower tribe of mollusca as we proceed from older to newer rocks. In the British Devonian, for example, the Brachiopoda number 99, the Lamellibranchiata 58; while in the Carboniferous their proportions are more than reversed, the Lamellibranchiata numbering 334 species, and the Brachiopoda only 157. In the Secondary or Cainozoic formations the preponderance of the higher grade of bivalves becomes more and more marked, till in the tertiary strata it approaches that observed in the living creation.

While on this subject, it may be useful to the student to know that a Brachiopod differs from ordinary bivalves, mussels, cockles, etc., in being always equal-sided and never quite equi-valved; the form of each valve being symmetrical, it may be divided into two equal parts by a line drawn from the apex to the centre of the margin.

Trilobites.—In the Bala and Caradoc beds the trilobites reach their maximum, being represented by 111 species referred to 23 genera.

Burmeister, in his work on the organisation of trilobites, supposes that they swam at the surface of the water in the open sea and near coasts, feeding on smaller marine animals, and to have had the power of rolling themselves into a ball as a defence against injury. He was also of opinion that they underwent various transformations analogous to those of living crustaceans. M. Barrande, author of an admirable work on the Silurian rocks of Bohemia, confirms the doctrine of their metamorphosis, having traced more than twenty species through different stages of growth from the young state just after its escape from the egg to the adult form. He has followed some of them from a point in which they show no eyes, no joints, or body rings, and no distinct tail, up to the complete form with the full number of segments. This change is brought about before the animal has attained a tenth part of its full dimensions, and hence such minute and delicate specimens are rarely met with. Some of his figures of the metamorphoses of the common Trinucleus are copied in Figs. 552 and 553. It was not till 1870 that Mr. Billings was enabled, by means of a specimen found in Canada, to prove that the trilobite was provided with eight legs.

It has been ascertained that a great thickness of slaty and crystalline rocks of South Wales, as well as those of Snowdon and Bala, in North Wales, which were first supposed to be of older date than the Silurian sandstones and mudstones of Shropshire, are in fact identical in age, and contain the same organic remains. At Bala, in Merionethshire, a limestone rich in fossils occurs, in which two genera of star-fish, Protaster and Palæaster, are found; the fossil specimen of the latter (Fig. 554) being almost as uncompressed as if found just washed up on the sea-beach. Besides the star-fish there occur abundance of those peculiar bodies called Cystideæ. They are the Sphæronites of old authors, and were considered by Professor E. Forbes as intermediate between the crinoids and echinoderms. The Echinosphæronite here represented (Fig. 555) is characteristic of the Caradoc beds in Wales, and of their equivalents in Sweden and Russia.

Fig. 552: Young individuals of Trinucleus concentricus. Fig. 553: Trinucleus concentricus.

Fig. 554: Palæaster asperimus. Fig. 555: Echinosphæronites ballicus. With it have been found several other genera of the same family, such as Sphæronites, Hemicosmites, etc. Among the mollusca are Pteropods of the genus Conularia of large size (for genus, see Fig. 518). About eleven species of Graptolite are reckoned as belonging to this formation; they are chiefly found in peculiar localities where black mud abounded. The formation, when traced into South Wales and Ireland, assumes a greatly altered mineral aspect, but still retains its characteristic fossils. The known fauna of the Bala group comprises 565 species, 352 of which are peculiar, and 93, as before stated, are common to the overlying Llandovery rocks. It is worthy of remark that, when it occurs under the form of trappean tuff (volcanic ashes of De la Beche), as in the crest of Snowdon, the peculiar species which distinguish it from the Llandeilo beds are still observable. The formation generally appears to be of shallow-water origin, and in that respect is contrasted with the group next to be described. Professor Ramsay estimates the thickness of the Bala Beds, including the contemporaneous volcanic rocks, stratified and unstratified, as being from 10,000 to 12,000 feet.

Fig. 556: Didymograpsus (Graptolites) Murchisonii. Llandeilo Flags.—The Lower Silurian strata were originally divided by Sir R. Murchison into the upper group already described, under the name of Caradoc Sandstone, and a lower one, called, from a town in Carmarthenshire, the Llandeilo flags. The last mentioned strata consist of dark-coloured micaceous flags, frequently calcareous, with a great thickness of shales, generally black, below them. The same beds are also seen at Builth, in Radnorshire, where they are interstratified with volcanic matter.

A still lower part of the Llandeilo rocks consists of a black carbonaceous slate of great thickness, frequently containing sulphate of alumina, and sometimes, as in Dumfriesshire, beds of anthracite. It has been conjectured that this carbonaceous matter may be due in great measure to large quantities of imbedded animal remains, for the number of Graptolites included in these slates was certainly very great. In Great Britain eleven genera and about 40 species of Graptolites occur in the Llandeilo flags and underlying Arenig beds. The double Graptolites, or those with two rows of cells, such as Diplograpsus (Fig. 557), are conspicuous.

Fig. 557: Diplograpsus pristis. Fig. 558: Rastrites peregrinus.

Fig. 559: Diplograpsus folium. The brachiopoda of the Llandeilo flags, which number 47 species, are in the main the same as those of the Caradoc Sandstone, but the other mollusca are in great part of different species.

In Europe generally, as, for example, in Sweden and Russia, no shells are so characteristic of this formation as Orthoceratites, usually of great size, and with a wide siphuncle placed on one side instead of being central (see Fig. 560).

Fig. 560: Orthoceras duplex.

Among other Cephalopods in the Llandeilo flags is Cyrtoceras; in the same beds also are found Bellerophon (see Fig. 488) and some Pteropod shells (Conularia, Theca, etc.), also in spots where sand abounded, lamellibranchiate bivalves of large size. The Crustaceans were plentifully represented by the Trilobites, which appear to have swarmed in the Silurian seas just as crabs and shrimps do in our own; no less than 263 species have been found in the British Silurian fauna. The genera Asaphus (Fig. 561), Ogygia (Fig. 562), and Trinucleus (Figs. 552 and 553) form a marked feature of the rich and varied Trilobitic fauna of this age.

Fig. 561: Asaphus tyrannus. Fig. 562: Ogygia Buchii.

Beneath the black slates above described of the Llandeilo formation, Graptolites are still found in great variety and abundance, and the characteristic genera of shells and trilobites of the Lower Silurian rocks are still traceable downward, in Shropshire, Cumberland, and North and South Wales, through a vast depth of shaly beds, in some districts interstratified with trappean formations of contemporaneous origin; these consist of tuffs and lavas, the tuffs being formed of such materials as are ejected from craters and deposited immediately on the bed of the ocean, or washed into it from the land. According to Professor Ramsay, their thickness is about 3300 feet in North Wales, including those of the Lower Llandeilo. The lavas are feldspathic, and of porphyritic structure, and, according to the same authority, of an aggregate thickness of 2500 feet.

Fig. 563: Arenicolites linearis. Arenig or Stiper-Stones Group (Lower Llandeilo of Murchison).—Next in the descending order are the shales and sandstones in which the quartzose rocks called Stiper-Stones in Shropshire occur. Originally these Stiper-Stones were only known as arenaceous quartzose strata in which no organic remains were conspicuous, except the tubular burrows of annelids (see Fig. 563, Arenicolites linearis), which are remarkably common in the Lowest Silurian in Shropshire, and in the State of New York, in America. They have already been alluded to as occurring by thousands in the Silurian strata unconformably overlying the Cambrian, in the mountain of Queenaig, in Sutherlandshire (Fig. 82). I have seen similar burrows now made on the retiring of the tides in the sands of the Bristol Channel, near Minehead, by lob-worms which are dug out by fishermen and used as bait. When the term Silurian was given by Sir R. Murchison, in 1835, to the whole series, he considered the Stiper-Stones as the base of the Silurian system, but no fossil fauna had then been obtained, such as could alone enable the geologist to draw a line between this member of the series and the Llandeilo flags above, or a vast thickness of rock below, which was seen to form the Longmynd hills, and was called ”unfossiliferous graywacke.” Professor Sedgwick had described, in 1843, strata now ascertained to be of the same age as largely developed in the Arenig mountain, in Merionethshire; and the Skiddaw slates in the Lake-District of Cumberland, studied by the same author, were of corresponding date, though the number of fossils was, in both cases, too few for the determination of their true chronological relations. The subsequent researches of Messrs. Sedgwick and Harkness, in Cumberland, and of Sir R. I. Murchison and the Government surveyors in Shropshire, have increased the species to more than sixty. These were examined by Mr. Salter, and shown in the third edition of ”Siluria” (p. 52, 1859) to be quite distinct from the fossils of the overlying Llandeilo flags. Among these the Obolella plumbea, Æglina binodosa, Ogygia Selwynii, and Didymograpsus geminus (Fig. 564), and D. Hirundo, are characteristic.

Fig. 564: Didymograpsus geminus.

But, although the species are distinct, the genera are the same as those which characterise the Silurian rocks above, and none of the characteristic primordial or Cambrian forms, presently to be mentioned, are intermixed. The same may be said of a set of beds underlying the Arenig rocks at Ramsay Island and other places in the neighbourhood of St. David’s. These beds, which have only lately become known to us through the labours of Dr. Hicks,[5] present already twenty new species, the greater part of them allied generically to the Arenig rocks. This Arenig group may therefore be conveniently regarded as the base of the great Silurian system, a system which, by the thickness of its strata and the changes in animal life of which it contains the record, is more than equal in value to the Devonian, or Carboniferous, or other principal divisions, whether of primary or secondary date.

It would be unsafe to rely on the mere thickness of the strata, considered apart from the great fluctuations in organic life which took place between the era of the Llandeilo and that of the Ludlow formation, especially as the enormous pile of Silurian rocks observed in Great Britain (in Wales more particularly) is derived in great part from igneous action, and is not confined to the ordinary deposition of sediment from rivers or the waste of cliffs.

In volcanic archipelagoes, such as the Canaries, we see the most active of all known causes, aqueous and igneous, simultaneously at work to produce great results in a comparatively moderate lapse of time. The outpouring of repeated streams of lava—the showering down upon land and sea of volcanic ashes—the sweeping seaward of loose sand and cinders, or of rocks ground down to pebbles and sand, by rivers and torrents descending steeply inclined channels—the undermining and eating away of long lines of sea-cliff exposed to the swell of a deep and open ocean—these operations combine to produce a considerable volume of superimposed matter, without there being time for any extensive change of species. Nevertheless, there would seem to be a limit to the thickness of stony masses formed even under such favourable circumstances, for the analogy of tertiary volcanic regions lends no countenance to the notion that sedimentary and igneous rocks 25,000, much less 45,000 feet thick, like those of Wales, could originate while one and the same fauna should continue to people the earth. If, then, we allow that about 25,000 feet of matter may be ascribed to one system, such as the Silurian, as above described, we may be prepared to discover in the next series of subjacent rocks a distinct assemblage of species, or even in great part of genera, of organic remains. Such appears to be the fact, and I shall therefore conclude with the Arenig beds my enumeration of the Silurian formations in Great Britain, and proceed to say something of their foreign equivalents, before treating of rocks older than the Silurian.

Silurian Strata of the Continent of Europe.—When we turn to the continent of Europe, we discover the same ancient series occupying a wide area, but in no region as yet has it been observed to attain great thickness. Thus, in Norway and Sweden, the total thickness of strata of Silurian age is considerably less than 1000 feet, although the representatives both of the Upper and Lower Silurian of England are not wanting there. In Russia the Silurian strata, so far as they are yet known, seem to be even of smaller vertical dimensions than in Scandinavia, and they appear to consist chiefly of the Llandovery group, or of a limestone containing Pentamerus oblongus, below which are strata with fossils corresponding to those of the Llandeilo beds of England. The lowest rock with organic remains yet discovered is ”the Ungulite or Obolus grit” of St. Petersburg, probably coeval with the Llandeilo flags of Wales.

The shales and grits near St. Petersburg, above alluded to, contain green grains in their sandy layers, and are in a singularly unaltered state, taking into account their high antiquity. The prevailing Brachiopods consist of the Obolus Shells of the lowest known Fossiliferous Beds in Russia.

Fig. 565: Siphonotreta unguiculata. Fig. 566: Obolus Apollinis.

or Ungulite of Pander, and a Siphonotreta (Figs. 565, 566). Notwithstanding the antiquity of this Russian formation, it should be stated that both of these genera of brachiopods have been also found in the Upper Silurian of England, i.e., in the Wenlock limestone.

Among the green grains of the sandy strata above-mentioned, Professor Ehrenberg announced in 1854 his discovery of remains of foraminifera. These are casts of the cells; and among five or six forms three are considered by him as referable to existing genera (e.g., Textularia, Rotalia, and Guttulina).

Silurian Strata of the United States.—The Silurian formations can be advantageously studied in the States of New York, Ohio, and other regions north and south of the great Canadian lakes. Here they are often found, as in Russia, nearly in horizontal position, and are more rich in well-preserved fossils than in almost any spot in Europe. In the State of New York, where the succession of the beds and their fossils have been most carefully worked out by the Government surveyors, the subdivisions given in the first column of the table below have been adopted.

_Subdivisions of the Silurian Strata of New York. (Strata below the Oriskany sandstone or base of the Devonian.)_

New York Names British equivalents 1. Upper Pentamerus Limestone 2. Encrinal Limestone 3. Delthyris Shaly Limestone 4. Pentamerus and Tentaculite Limestones 5. Water Lime Group 6. Onondaga Salt Group 7. Niagara Group Upper Silurian (or Ludlow and Wenlock formations 8. Clinton Group 9. Medina Sandstone 10. Oneida Conglomerate 11. Gray Sandstone Beds of Passage, Llandovery Group. 12. Hudson River Group 13. Trenton Limestone 14. Black-River Limestone 15. Bird’s-eye Limestone 16. Chazy Limestone 17. Calciferous Sandstone Lower Silurian (or Caradoc and Bala, Llandeilo and Arenig Formations).

In the second column of the same table I have added the supposed British equivalents. All Palæontologists, European and American, such as MM. De Verneuil, D. Sharpe, Professor Hall, E. Billings, and others, who have entered upon this comparison, admit that there is a marked general correspondence in the succession of fossil forms, and even species, as we trace the organic remains downward from the highest to the lowest beds; but it is impossible to parallel each minor subdivision.

That the Niagara Limestone, over which the river of that name is precipitated at the great cataract, together with its underlying shales, corresponds to the Wenlock limestone and shale of England there can be no doubt. Among the species common to this formation in America and Europe are Calymene Blumenbachii, Homalonotus delphinocephalus (Fig. 544), with several other trilobites; Rhynchonella Wilsoni, Fig. 531, and Retzia cuneata; Orthis elegantula, Pentamerus galeatus, with many more brachiopods; Orthoceras annulatum, among the cephalopodous shells; and Favosites gothlandica, with other large corals.

Fig. 567: Murchisonia gracilis. The Clinton Group, containing Pentamerus oblongus and Stricklandinia, and related more nearly by its fossil species with the beds above than with those below, is the equivalent of the Llandovery Group or beds of passage.

The Hudson River Group, and the Trenton Limestone, agree palæontologically with the Caradoc or Bala group, containing in common with them several species of trilobites, such as Asaphus (Isotelus) gigas, Trinucleus concentricus (Fig. 553); and various shells, such as Orthis striatula, Orthis biforata (or O. lynx), O. porcata (O. occidentalis of Hall), and Bellerophon bilobatus. In the Trenton limestone occurs Murchisonia gracilis, Fig. 567, a fossil also common to the Llandeilo beds in England.

Mr. D. Sharpe, in his report on the mollusca collected by me from these strata in North America,[6] has concluded that the number of species common to the Silurian rocks on both sides of the Atlantic is between 30 and 40 per cent; a result which, although no doubt liable to future modification, when a larger comparison shall have been made, proves, nevertheless, that many of the species had a wide geographical range. It seems that comparatively few of the gasteropods and lamellibranchiate bivalves of North America can be identified specifically with European fossils, while no less than two-fifths of the brachiopoda, of which my collection chiefly consisted, are the same. In explanation of these facts, it is suggested that most of the recent brachiopoda (especially the orthidiform ones) are inhabitants of deep water, and that they may have had a wider geographical range than shells living near shore. The predominance of bivalve mollusca of this peculiar class has caused the Silurian period to be sometimes styled ”the age of brachiopods.”

In Canada, as in the State of New York, the Potsdam Sandstone underlies the above-mentioned calcareous rocks, but contains a different suite of fossils, as will be hereafter explained. In parts of the globe still more remote from Europe the Silurian strata have also been recognised, as in South America, Australia, and India. In all these regions the facies of the fauna, or the types of organic life, enable us to recognise the contemporaneous origin of the rocks; but the fossil species are distinct, showing that the old notion of a universal diffusion throughout the ”primæval seas” of one uniform specific fauna was quite unfounded, geographical provinces having evidently existed in the oldest as in the most modern times.

[1] Murchison’s Siluria, p. 140.

[2] Murchison’s Siluria, chap. vi.

[3] E. Forbes, Mem. Geol. Surv., vol. ii, p. 496.

[4] Quart. Geol. Journ., vol. iv, p. 215, 1853.

[5] Trans. Brit. Assoc., 1866. Proc. Liverpool Geol. Soc., 1869.

[6] Quart. Geol. Journ., vol. iv.

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XXVII. CAMBRIAN AND LAURENTIAN GROUPS

Classification of the Cambrian Group, and its Equivalent in Bohemia. — Upper Cambrian Rocks. — Tremadoc Slates and their Fossils. — Lingula Flags. — Lower Cambrian Rocks. — Menevian Beds. — Longmynd Group. — Harlech Grits with large Trilobites. — Llanberis Slates. — Cambrian Rocks of Bohemia. — Primordial Zone of Barrande. — Metamorphosis of Trilobites. — Cambrian Rocks of Sweden and Norway. — Cambrian Rocks of the United States and Canada. — Potsdam Sandstone. — Huronian Series. — Laurentian Group, upper and lower. — Eozoon Canadense, oldest known Fossil. — Fundamental Gneiss of Scotland.

CAMBRIAN GROUP.

The characters of the Upper and Lower Silurian rocks were established so fully, both on stratigraphical and palæontological data, by Sir Roderick Murchison after five years’ labour, in 1839, when his “Silurian System” was published, that these formations could from that period be recognised and identified in all other parts of Europe and in North America, even in countries where most of the fossils differed specifically from those of the classical region in Britain, where they were first studied.

While Sir R. I. Murchison was exploring in 1833, in Shropshire and the borders of Wales, the strata which in 1835 he first called Silurian, Professor Sedgwick was surveying the rocks of North Wales, which both these geologists considered at that period as of older date, and for which in 1836 Sedgwick proposed the name of Cambrian. It was afterwards found that a large portion of the slaty rocks of North Wales, which had been considered as more ancient than the Llandeilo beds and Stiper-Stones before alluded to, were, in reality, not inferior in position to those Lower Silurian beds of Murchison, but merely extensive undulations of the same, bearing fossils identical in species, though these were generally rarer and less perfectly preserved, owing to the changes which the rocks had undergone from metamorphic action. To such rocks the term “Cambrian” was no longer applicable, although it continued to be appropriate to strata inferior to the Stiper-Stones, and which were older than those of the Lower Silurian group as originally defined. It was not till 1846 that fossils were found in Wales in the Lingula flags, the place of which will be seen in the table below. By this time Barrande had already published an account of a rich collection of fossils which he had discovered in Bohemia, portions of which he recognised as of corresponding age with Murchison’s Upper and Lower Silurian, while others were more ancient, to which he gave the name of “Primordial,” for the fossils were sufficiently distinct to entitle the rocks to be referred to a new period. They consisted chiefly of trilobites of genera distinct from those occurring in the overlying Silurian formations. These peculiar genera were afterwards found in rocks holding a corresponding position in Wales, and I shall retain for them the term Cambrian, as recent discoveries in our own country seem to carry the first fauna of Barrande, or his primordial type, even into older strata than any which he found to be fossiliferous in Bohemia.

The term primordial was intended to express M. Barrande’s own belief that the fossils of the rocks so-called afforded evidence of the first appearance of vital phenomena on this planet, and that consequently no fossiliferous strata of older date would or could ever be discovered. The acceptance of such a nomenclature would seem to imply that we despaired of extending our discoveries of new and more ancient fossil groups at some future day when vast portions of the globe, hitherto unexplored, should have been thoroughly surveyed. Already the discovery of the Laurentian Eozoon in Canada, presently to be mentioned, discountenances such views.

The following table will show the succession of the strata in England and Wales which belong to the Cambrian group or the fossiliferous rocks older than the Arenig or Lower Llandeilo rocks:

UPPER CAMBRIAN TREMADOC SLATES (Primordial of Barrande in part) LINGULA FLAGS (Primordial of Barrande) LOWER CAMBRIAN MENEVIAN BEDS (Primordial of Barrande) LONGMYND GROUP a. Harlech Grits b. Llanberis Slates

Tremadoc Slates.—The Tremadoc slates of Sedgwick are more than 1000 feet in thickness, and consist of dark earthy slates occurring near the little town of Tremadoc, situated on the north side of Cardigan Bay, in Carnarvonshire. These slates were first examined by Sedgwick in 1831, and were re-examined by him and described in 1846,[1] after some fossils had been found in the underlying Lingula flags by Mr. Davis. The inferiority in position of these Lingula flags to the Tremadoc beds was at the same time established. The overlying Tremadoc beds were traced by their pisolitic ore from Tremadoc to Dolgelly. No fossils proper to the Tremadoc slates were then observed, but subsequently, thirty-six species of all classes have been found in them, thanks to the researches of Messrs. Salter, Homfray, and Ash. We have already seen that in the Arenig or Stiper-Stones group, where the species are distinct, the genera agree with Silurian types; but in these Tremadoc slates, where the species are also peculiar, there is about an equal admixture of Silurian types with those which Barrande has termed “primordial.” Here, therefore, it may truly be said that we are entering upon a new domain of life in our retrospective survey of the past. The trilobites of new species, but of Lower Silurian genera, belong to Ogygia, Asaphus, and Cheirurus; whereas those belonging to primordial types, or Barrande’s first fauna as well as to the Lingula flags of Wales, comprise Dikelocephalus, Conocoryphe (for genera see Fig. 577 and 581),[2] Olenus, and Angelina.

Fig. 568: Theca (Cleidotheca operculata. In the Tremadoc slates are found Bellerophon, Orthoceras, and Cyrtoceras, all specifically distinct from Lower Silurian fossils of the same genera: the Pteropods Theca (Fig. 568) and Conularia range throughout these slates; there are no Graptolites. The Lingula (Lingulella) Davisii ranges from the top to the bottom of the formation, and links it with the zone next to be described. The Tremadoc slates are very local, and seem to be confined to a small part of North Wales; and Professor Ramsay supposes them to lie unconformably on the Lingula flags, and that a long interval of time elapsed between these formations. Cephalopoda have not yet been found lower than this group, but it will be observed that they occur here associated with genera of Trilobites considered by Barrande as characteristically Primordial, some of which belong to all the divisions of the British Cambrian about to be mentioned. This renders the absence of cephalopoda of less importance as bearing on the theory of development.

Lingula Flags.—Next below the Tremadoc slates in North Wales lie micaceous flagstones and slates, in which, in 1846, Mr. E. Davis discovered the Lingula (Lingulella), Fig. 570, named after him, and from which was derived the name of Lingula flags. These beds, which are palæontologically the equivalents of Barrande’s primordial zone, are represented by more than 5000 feet of strata, and have been studied chiefly in the neighbourhood of Dolgelly, Ffestiniog, and Portmadoc in North Wales, and at St. David’s in South Wales. They have yielded about forty species of fossils, of which six only are common to the overlying Tremadoc rocks, but the two formations are closely allied by having several characteristic “primordial” genera in common. Dikelocephalus, Olenus (Fig. 571), and Conocoryphe are prominent forms, as is also Hymenocaris (Fig. 569), a genus of phyllopod crustacean entirely confined to the Lingula Flags. According to Mr. Belt, who has devoted much attention to these beds, there are already palæontological data for subdividing the Lingula Flags into three sections.[3]

“Lingula Flags” of Dolgelly, and Ffestiniog; N. Wales.

In Merionethshire, according to Professor Ramsay, the Lingula Flags attain their greatest development; in Carnarvonshire they thin out so as to have lost two-thirds of their thickness in eleven miles, while in Anglesea and on the Menai Straits both they and the Tremadoc beds are entirely absent, and the Lower Silurian rests directly on Lower Cambrian strata.

LOWER CAMBRIAN.

Menevian Beds.—Immediately beneath the Lingula Flags there occurs a series of dark grey and black flags and slates alternating at the upper part with some beds of sandstone, the whole reaching a thickness of from 500 to 600 feet. These beds were formerly classed, on purely lithological grounds, as the base of the Lingula Flags, but Messrs. Hicks and Salter, to whose exertions we owe almost all our knowledge of the fossils, have pointed out[4] that the most characteristic genera found in them are quite unknown in the Lingula Flags, while they possess many of the strictly Lower Cambrian genera, such as Microdiscus and Paradoxides. They therefore proposed to place them, and it seems to me with good reason, at the top of the Lower Cambrian under the term “Menevian,” Menevia being the classical name of St. David’s. The beds are well exhibited in the neighbourhood of St. David’s in South Wales, and near Dolgelly and Maentwrog in North Wales. They are the equivalents of the lowest part of Barrande’s Primordial Zone (Étage C). More than forty species have been found in them, and the group is altogether very rich in fossils for so early a period.

Fig. 572: Paradoxides Davidis. The trilobites are of large size; Paradoxides Davidis (see Fig. 572), the largest trilobite known in England, 22 inches or nearly two feet long, is peculiar to the Menevian Beds. By referring to the Bohemian trilobite of the same genus (Fig. 576), the reader will at once see how these fossils (though of such different dimensions) resemble each other in Bohemia and Wales, and other closely allied species from the two regions might be added, besides some which are common to both countries. The Swedish fauna, presently to be mentioned, will be found to be still more nearly connected with the Welsh Menevian. In all these countries there is an equally marked difference between the Cambrian fossils and those of the Upper and Lower Silurian rocks. The trilobite with the largest number of rings, Erinnys venulosa, occurs here in conjunction with Agnostus and Microdiscus, the genera with the smallest number. Blind trilobites are also found as well as those which have the largest eyes, such as Microdiscus on the one hand, and Anoplenus on the other.

LONGMYND GROUP.

Older than the Menevian Beds are a thick series of olive green, purple, red and grey grits and conglomerates found in North and South Wales, Shropshire, and parts of Ireland and Scotland. They have been called by Professor Sedgwick the Longmynd or Bangor Group, comprising, first, the Harlech and Barmouth sandstones; and secondly, the Llanberis slates.

Fig. 573: Histioderma Hibernica. Harlech Grits.—The sandstones of this period attain in the Longmynd hills a thickness of no less than 6000 feet without any interposition of volcanic matter; in some places in Merionethshire they are still thicker. Until recently these rocks possessed but a very scanty fauna.

With the exception of five species of annelids (see Fig. 460) brought to light by Mr. Salter in Shropshire, and Dr. Kinahan in Wicklow, and an obscure crustacean form, Palæopyge Ramsayi, they were supposed to be barren of organic remains. Now, however, through the labours of Mr. Hicks,[5] they have yielded at St. David’s a rich fauna of trilobites, brachiopods, phyllopods, and pteropods, showing, together with other fossils, a by no means low state of organisation at this early period. Already the fauna amounts to 20 species referred to 17 genera.

A new genus of trilobite called Plutonia Sedgwickii, not yet figured and described, has been met with in the Harlech grits. It is comparable in size to the large Paradoxides Davidis before mentioned, has well-developed eyes, and is covered all over with tubercles. In the same strata occur other genera of trilobites, namely, Conocoryphe, Paradoxides, Microdiscus, and the Pteropod Theca (Fig. 568), all represented by species peculiar to the Harlech grits. The sands of this formation are often rippled, and were evidently left dry at low tides, so that the surface was dried by the sun and made to shrink and present sun-cracks. There are also distinct impressions of rain-drops on many surfaces, like those in Fig. 444 and 445.

Lanberis Slates.—The slates of Llanberis and Penrhyn in Carnarvonshire, with their associated sandy strata, attain a great thickness, sometimes about 3000 feet. They are perhaps not more ancient than the Harlech and Barmouth beds last mentioned, for they may represent the deposits of fine mud thrown down in the same sea, on the borders of which the sands above-mentioned were accumulating. In some of these slaty rocks in Ireland, immediately opposite Anglesea and Carnarvon, two species of fossils have been found, to which the late Professor E. Forbes gave the name of Oldhamia. The nature of these organisms is still a matter of discussion among naturalists.

Fig. 574: Oldhamia radiata. Fig. 575: Oldhamia antiqua. Cambrian Rocks of Bohemia (Primordial zone of Barrande).—In the year 1846, as before stated, M. Joachim Barrande, after ten years’ exploration of Bohemia, and after collecting more than a thousand species of fossils, had ascertained the existence in that country of three distinct faunas below the Devonian. To his first fauna, which was older than any then known in this country, he gave the name of Étage C; his two first stages A and B consisting of crystalline and metamorphic rocks and unfossiliferous schists. This Étage C or primordial zone proved afterwards to be the equivalent of those subdivisions of the Cambrian groups which have been above described under the names of Menevian and Lingula Flags. The second fauna tallies with Murchison’s Lower Silurian, as originally defined by him when no fossils had been discovered below the Stiper-Stones. The third fauna agrees with the Upper Silurian of the same author. Barrande, without government assistance, had undertaken single-handed the geological survey of Bohemia, the fossils previously obtained from that country having scarcely exceeded 20 in number, whereas he had already acquired, in 1850, no less than 1100 species, namely, 250 crustaceans (chiefly Trilobites), 250 Cephalopods, 160 gasteropods and pteropods, 130 acephalous mollusks, 210 brachiopods, and 110 corals and other fossils. These numbers have since been almost doubled by subsequent investigations in the same country.

In the primordial zone C, he discovered trilobites of the genera Paradoxides, Conocoryphe, Ellipsocephalus, Sao, Arionellus, Hydrocephalus, and Agnostus. M. Barrande pointed out that these primordial trilobites have a peculiar facies of their own dependent on the multiplication of their thoracic segments and the diminution of their caudal shield or pygidium.

_Fossils of the lowest Fossiliferous Beds in Bohemia, or “Primordial Zone” of Barrande._

Fig. 576: Paradoxides Bohemicus. Fig. 577: Conocoryphe striata. Fig. 578: Agnostus integer. Fig. 579: Agnostus Rex. Fig. 580: Sao hirsuta in its various stages of growth.

One of the “primordial” or Upper Cambrian Trilobites of the genus Sao, a form not found as yet elsewhere in the world, afforded M. Barrande a fine illustration of the metamorphosis of these creatures, for he traced them through no less than twenty stages of their development. A few of these changes have been selected for representation in Figure 580, that the reader may learn the gradual manner in which different segments of the body and the eyes make their appearance.

In Bohemia the primordial fauna of Barrande derived its importance exclusively from its numerous and peculiar trilobites. Besides these, however, the same ancient schists have yielded two genera of brachiopods, Orthis and Orbicula, a Pteropod of the genus Theca, and four echinoderms of the cystidean family.

Cambrian of Sweden and Norway.—The Cambrian beds of Wales are represented in Sweden by strata the fossils of which have been described by a most able naturalist, M. Angelin, in his “Palæontologica Suecica” (1852-4). The “alum-schists,” as they are called in Sweden, are horizontal argillaceous rocks which underlie conformably certain Lower Silurian strata in the mountain called Kinnekulle, south of the great Wener Lake in Sweden. These schists contain trilobites belonging to the genera Paradoxides, Olenus, Agnostus, and others, some of which present rudimentary forms, like the genus last mentioned, without eyes, and with the body segments scarcely developed, and others, again, have the number of segments excessively multiplied, as in Paradoxides. Such peculiarities agree with the characters of the crustaceans met with in the Cambrian strata of Wales; and Dr. Torell has recently found in Sweden the Paradoxides Hicksii, a well-known Lower Cambrian fossil.

At the base of the Cambrian strata in Sweden, which in the neighbourhood of Lake Wener are perfectly horizontal, lie ripple-marked quartzose sandstones with worm-tracks and annelid borings, like some of those found in the Harlech grits of the Longmynd. Among these are some which have been referred doubtfully to plants. These sandstones have been called in Sweden “fucoid sandstones.” The whole thickness of the Cambrian rocks of Sweden does not exceed 300 feet from the equivalents of the Tremadoc beds to these sandstones, which last seem to correspond with the Longmynd, and are regarded by Torell as older than any fossiliferous primordial rocks in Bohemia.

Cambrian of the United States and Canada (Potsdam Sandstone).—This formation, as we learn from Sir W. Logan, is 700 feet thick in Canada; the upper part consists of sandstone containing fucoids, and perforated by small vertical holes, which are very characteristic of the rock, and appear to have been made by annelids (Scolithus linearis). The lower portion is a conglomerate with quartz pebbles. I have seen the Potsdam sandstone on the banks of the St. Lawrence, and on the borders of Lake Champlain, where, as at Keesville, it is a white quartzose fine-grained grit, almost passing into quartzite. It is divided into horizontal ripple-marked beds, very like those of the Lingula Flags of Britain, and replete with a small round-shaped Obolella, in such numbers as to divide the rock into parallel planes, in the same manner as do the scales of mica in some micaceous sandstones. Among the shells of this formation in Wisconsin are species of Lingula and Orthis, and several trilobites of the primordial genus Dikelocephalus (Fig. 581). On the banks of the St. Lawrence, near Beauharnois and elsewhere, many fossil footprints have been observed on the surface of the rippled layers. They are supposed by Professor Owen to be the trails of more than one species of articulate animal, probably allied to the King Crab, or Limulus.

Fig. 581: Dikelocephalus Minnesotensis. Recent investigations by the naturalists of the Canadian survey have rendered it certain that below the level of the Potsdam Sandstone there are slates and schists extending from New York to Newfoundland, occupied by a series of trilobitic forms similar in genera, though not in species, to those found in the European Upper Cambrian strata.

Huronian Series.—Next below the Upper Cambrian occur strata called the Huronian by Sir W. Logan, which are of vast thickness, consisting chiefly of quartzite, with great masses of greenish chloritic slate, which sometimes include pebbles of crystalline rocks derived from the Laurentian formation, next to be described. Limestones are rare in this series, but one band of 300 feet in thickness has been traced for considerable distances to the north of Lake Huron. Beds of greenstone are intercalated conformably with the quartzose and argillaceous members of this series. No organic remains have yet been found in any of the beds, which are about 18,000 feet thick, and rest unconformably on the Laurentian rocks.

LAURENTIAN GROUP.

In the course of the geological survey carried on under the direction of Sir W.E. Logan, it has been shown that, northward of the river St. Lawrence, there is a vast series of crystalline rocks of gneiss, mica-schist, quartzite, and limestone, more than 30,000 feet in thickness, which have been called Laurentian, and which are already known to occupy an area of about 200,000 square miles. They are not only more ancient than the fossiliferous Cambrian formations above described, but are older than the Huronian last mentioned, and had undergone great disturbing movements before the Potsdam sandstone and the other “primordial” or Cambrian rocks were formed. The older half of this Laurentian series is unconformable to the newer portion of the same.

Upper Laurentian or Labrador Series.—The Upper Group, more than 10,000 feet thick, consists of stratified crystalline rocks in which no organic remains have yet been found. They consist in great part of feldspars, which vary in composition from anorthite to andesine, or from those kinds in which there is less than one per cent of potash and soda to those in which there is more than seven per cent of these alkalies, the soda preponderating greatly. These feldsparites sometimes form mountain masses almost without any admixture of other minerals; but at other times they include augite, which passes into hypersthene. They are often granitoid in structure. One of the varieties is the same as the apolescent labradorite rock of Labrador. The Adirondack Mountains in the State of New York are referred to the same series, and it is conjectured that the hypersthene rocks of Skye, which resemble this formation in mineral character, may be of the same geological age.

Lower Laurentian.—This series, about 20,000 feet in thickness, is, as before stated, unconformable to that last mentioned; it consists in great part of gneiss of a reddish tint with orthoclase feldspar. Beds of nearly pure quartz, from 400 to 600 feet thick, occur in some places. Hornblendic and micaceous schists are often interstratified, and beds of limestone, usually crystalline. Beds of plumbago also occur. That this pure carbon may have been of organic origin before metamorphism has naturally been conjectured.

There are several of these limestones which have been traced to great distances, and one of them is from 700 to 1500 feet thick. In the most massive of them Sir W. Logan observed, in 1859, what he considered to be an organic body much resembling the Silurian fossil called Stromatopora rugosa. It had been obtained the year before by Mr. J. MacMullen at the Grand Calumet, on the river Ottawa. This fossil was examined in 1864 by Dr. Dawson of Montreal, who detected in it, by aid of the microscope, the distinct structure of a Rhizopod or Foraminifer. Dr. Carpenter and Professor T. Rupert Jones have since confirmed this opinion, comparing the structure to that of the well-known nummulite. It appears to have grown one layer over another, and to have formed reefs of limestone as do the living coral-building polyp animals. Parts of the original skeleton, consisting of carbonate of lime, are still preserved; while certain inter-spaces in the calcareous fossil have been filled up with serpentine and white augite. On this oldest of known organic remains Dr. Dawson has conferred the name of Eozoon Canadense (see Figs. 582, 583); its antiquity is such that the distance of time which separated it from the Upper Cambrian period, or that of the Potsdam sandstone, may, says Sir W. Logan, be equal to the time which elapsed between the Potsdam sandstone and the nummulitic limestones of the Tertiary period. The Laurentian and Huronian rocks united are about 50,000 feet in thickness, and the Lower Laurentian was disturbed before the newer series was deposited. We may naturally expect the other proofs of unconformability will hereafter be detected at more than one point in so vast a succession of strata.

Fig. 582 and 583: Eozoon Canadense.

Fig. 582. a. Chambers of lower tier communicating at +, and separated from adjoining chambers at O by an intervening septum, traversed by passages. b. Chambers of an upper tier. c. Walls of the chambers traversed by fine tubules. (These tubules pass with uniform parallelism from the inner to the outer surface, opening at regular distances from each other.) d. Intermediate skeleton, composed of homogeneous shell substance, traversed by f. Stoloniferous passages connecting the chambers of the two tiers. e. Canal system in intermediate skeleton, showing the arborescent saceodic prolongations. (Fig. 583 shows these bodies in a decalcified state.) f. Stoloniferous passages. Fig. 583. Decalcified portion of natural rock, showing canal system and the several layers; the acuteness of the planes prevents more than one or two parallel tiers being observed.

The mineral character of the Upper Laurentian differs, as we have seen, from that of the Lower, and the pebbles of gneiss in the Huronian conglomerates are thought to prove that the Laurentian strata were already in a metamorphic state before they were broken up to supply materials for the Huronian. Even if we had not discovered the Eozoon, we might fairly have inferred from analogy that as the quartzites were once beds of sand, and the gneiss and mica-schist derived from shales and argillaceous sandstones, so the calcareous masses, from 400 to 1000 feet and more in thickness, were originally of organic origin. This is now generally believed to have been the case with the Silurian, Devonian, Carboniferous, Oolitic, and Cretaceous limestones and those nummulitic rocks of tertiary date which bear the closest affinity to the Eozoon reefs of the Lower Laurentian. The oldest stratified rock in Scotland is that called by Sir R. Murchison “the fundamental gneiss,” which is found in the north-west of Ross-shire, and in Sutherlandshire (see Fig. 82), and forms the whole of the adjoining island of Lewis, in the Hebrides. It has a strike from north-west to south-east, nearly at right angles to the metamorphic strata of the Grampians. On this Laurentian gneiss, in parts of the western Highlands, the Lower Cambrian and various metamorphic rocks rest unconformably. It seems highly probable that this ancient gneiss of Scotland may correspond in date with part of the great Laurentian group of North America.

[1] Quart. Geol. Journ., vol. iii, p. 156.

[2] This genus has been substituted for Barrande’s Conocephalus, as the latter term had been preoccupied by the entomologists.

[3] Geol. Mag., vol iv.

[4] British Association Report 1865, 1866, 1868 and Quart. Geol. Journ., vols. xxi, xxv.

[5] Brit. Assoc. Report, 1868.

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