Opening
A MANUAL OF ELEMENTARY GEOLOGY: OR, THE ANCIENT CHANGES OF THE EARTH AND ITS INHABITANTS
AS ILLUSTRATED BY GEOLOGICAL MONUMENTS.
BY SIR CHARLES LYELL, M.A. F.R.S.
AUTHOR OF "PRINCIPLES OF GEOLOGY," "TRAVELS IN NORTH AMERICA," "A SECOND VISIT TO THE UNITED STATES," ETC. ETC.
"It is a philosophy which never rests--its law is progress: a point which yesterday was invisible is its goal to-day, and will be its starting post to-morrow." EDINBURGH REVIEW, No. 132. p. 83. July, 1837.
FOURTH AND ENTIRELY REVISED EDITION. ILLUSTRATED WITH 500 WOODCUTS.
LONDON: JOHN MURRAY, ALBEMARLE STREET. 1852.
LONDON: SPOTTISWOODES and SHAW, New-street-Square.
PREFACE TO THE FOURTH EDITION.
In consequence of the rapid sale of the third edition of the "Manual," of which 2000 copies were printed in January last, a new edition has been called for in less than a twelvemonth. Even in this short interval some new facts of unusual importance in palaeontology have come to light, or have been verified for the first time. Instead of introducing these new discoveries into the body of the work, which would render them inaccessible to the purchasers of the former edition, I have given them in a postscript to this Preface (printed and sold separately), and have pointed out at the same time their bearing on certain questions of the highest theoretical interest.[v-A]
As on former occasions, I shall take this opportunity of stating that the "Manual" is not an epitome of the "Principles of Geology," nor intended as introductory to that work. So much confusion has arisen on this subject, that it is desirable to explain fully the different ground occupied by the two publications. The first five editions of the "Principles" comprised a 4th book, in which some account was given of systematic geology, and in which the principal rocks composing the earth's crust and their organic remains were described. In subsequent editions this book was omitted, it having been expanded, in 1838, into a separate treatise called the "Elements of Geology," first re-edited in 1842, and again recast and enlarged in 1851, and entitled "A Manual of Elementary Geology."
Although the subjects of both treatises relate to geology, as their titles imply, their scope is very different; the "Principles" containing a view of the modern changes of the earth and its inhabitants, while the "Manual" relates to the monuments of ancient changes. In separating the one from the other, I have endeavoured to render each complete in itself, and independent; but if asked by a student which he should read first, I would recommend him to begin with the "Principles," as he may then proceed from the known to the unknown, and be provided beforehand with a key for interpreting the ancient phenomena, whether of the organic or inorganic world, by reference to changes now in progress.
Owing to the former incorporation of the two subjects in one work, and the supposed identity of their subject matter, it may be useful to give here a brief abstract of the contents of the "Principles," for the sake of comparison.
Abstract of the "Principles of Geology," Eighth Edition.
Book I
1. Historical sketch of the early progress of geology, chaps. i. to iv.
2. Circumstances which combined to make the first cultivators of the science regard the former course of nature as different from the present, and the former changes of the earth's surface as the effects of agents different in kind and degree from those now acting, chap. v.
3. Whether the former variations in climate established by geology are explicable by reference to existing causes, chaps. vi. to viii.
4. Theory of the progressive development of organic life in former ages, and the introduction of man into the earth, chap. ix.
5. Supposed former intensity of aqueous and igneous causes considered, chaps. x. and xi.
6. How far the older rocks differ in texture from those now forming, chap. xii.
7. Supposed alternate periods of repose and disorder, chap. xiii.
II. CHANGES NOW IN PROGRESS IN THE INORGANIC WORLD
8. Aqueous causes now in action: Floods--Rivers--Carrying power of ice--Springs and their deposits--Deltas--Waste of cliffs and strata produced by marine currents: chaps. xiv. to xxii.
9. Permanent effects of igneous causes now in operation: Active volcanos and earthquakes--their effects and causes: chaps. xxiii. to xxxiii.
III. CHANGES OF THE ORGANIC WORLD NOW IN PROGRESS
10. Doctrine of the transmutation of species controverted, chaps. xxxiv. and xxxv.
11. Whether species have a real existence in nature, chaps. xxxvi. and xxxvii.
12. Laws which regulate the geographical distribution of species, chaps. xxxviii. to xl.
13. Creation and extinction of species, chaps. xli. to xliv.
14. Imbedding of organic bodies, including the remains of man and his works, in strata now forming, chaps. xlv. to l.
15. Formation of coral reefs, chap. li.
It will be seen on comparing this analysis of the contents of the "Principles" with the headings of the chapters of the present work (see p. xxiii.), that the two treatises have but little in common; or, to repeat what I have said in the Preface to the 8th edition of the "Principles," they have the same kind of connection which Chemistry bears to Natural Philosophy, each being subsidiary to the other, and yet admitting of being considered as different departments of science.[vi-A] CHARLES LYELL. 11 Harley Street, London, December 10. 1851.
FOOTNOTES:
[v-A] Postscript to 4th edition of the Manual, price 6_d._
[vi-A] As it is impossible to enable the reader to recognize rocks and minerals at sight by aid of verbal descriptions or figures, he will do well to obtain a well-arranged collection of specimens, such as may be procured from Mr. Tennant (149. Strand), teacher of Mineralogy at King's College, London.
Postscript
Tracks of a Lower Silurian reptile in Canada--Chelonian footprints in Old Red Sandstone, Morayshire--Skeleton of a reptile in the same formation in Scotland--Eggs of Batrachians (?) in a lower division of the "Old Red," or Devonian--Footprints of Lower Carboniferous reptiles in the United States--Fossil rain-marks of the Carboniferous period in Nova Scotia--Triassic Mammifer from the Keuper of Stuttgart--Cretaceous Gasteropoda--Dicotyledonous leaves in Lower Cretaceous strata--Bearing of the recent discoveries above-mentioned on the theory of the progressive development of animal life.
Tracks of a Lower Silurian reptile in Canada.--In the year 1847, Mr. Robert Abraham announced in the Montreal Gazette, of which he was editor, that the track of a freshwater tortoise had been observed on the surface of a stratum of sandstone in a quarry opened on the banks of the St. Lawrence at Beauharnais in Upper Canada. The inhabitants of the parish being perfectly familiar with the track of the amphibious mud-turtles or terrapins of their country, assured Mr. Abraham that the fossil impressions closely resembled those left by the recent species on sand or mud. Having satisfied himself of the truth of their report, he was struck with the novelty and geological interest of the phenomenon. Imagining the rock to be the lowest member of the old red sandstone, he was aware that no traces had as yet been found of a reptile in strata of such high antiquity.
He was soon informed by Mr. Logan, at that time engaged in the geological survey of Canada, that the white sandstone above Montreal was really much older than the "Old Red," or Devonian. It had in fact been ascertained many years before, by the State surveyors of New York (who called it the "Potsdam Sandstone"), to lie at the base of the whole Silurian series. As such it had been pointed out to me in 1841, in the valley of the Mohawk, by Mr. James Hall[vii-A], and its position was correctly described by Mr. Emmons, on the borders of Lake Champlain, where I examined it in 1842. It has there the character of a shallow-water deposit, ripple-marked throughout a considerable thickness, and full of a species of Lingula. The flat valves of this shell, of a dark colour, are so numerous, and so arranged in horizontal layers, as to play the part of mica, causing the rock to divide into laminae, as in some micaceous sandstones.
When I mentioned this rock in my Travels[vii-B] as occurring between Kingston and Montreal, (the same in which the Chelonian foot-prints have since been found,) I spoke of it as the lowest member of the Lower Silurian series; but no traces of any organic being of a higher grade than the Lingula had then been seen in it, and I called attention to the singular fact, that the oldest fossil form then known in the world, was a marine shell strictly referable to a genus now existing.
Early in the year 1851, Mr. Logan laid before the Geological Society of London a slab of this sandstone from Beauharnais, containing no less than twenty-eight foot-prints of the fore and hind feet of a quadruped, and six casts in plaster of Paris, exhibiting a continuation of the same trail. Each cast contained from twenty-six to twenty-eight impressions with a median channel equidistant from the two parallel rows of foot-prints, the one made by the feet of the right side, the other by those of the left. In these specimens a greater number of successive foot-marks belonging to one and the same series were displayed than had ever before been observed in any rock ancient or modern. Mr. Abraham has inferred that the breadth of the quadruped was from five to seven inches. A detailed account of the trail was published by Professor Owen, in April 1851, from which the following extracts are made.
"The foot-prints are in pairs, and the pairs extend in two parallel series, with a channel exactly midway between the right and left series. The pairs of the same side succeed each other at intervals, varying from one inch and a half to two inches and a half, the common distance being about two inches. The interval between the right and left pairs, measured from the inner border of the small prints, is three inches and a half, and from the outer border of the exterior or large prints, is seven inches. The shallow median track is one inch and a quarter in breadth, varying in depth, but not in its relative position to the right and left foot prints."
"The inference to be deduced from these characters is, that the impressions were made by a quadruped with the hind feet larger and somewhat wider apart than the fore feet, with both hind and fore feet either very short, or prevented by some other part of the animal's structure from making long steps; and with the limbs of the right side wide apart from those of the left; consequently, that the quadruped had a broad trunk in proportion to its length, supported on limbs either short, or capable only of short steps, and with rounded and stumpy feet, not provided with long claws. There are faint traces of a fine reticulate pattern of the cuticle of the sole at the bottom of some of the foot-prints on one portion of the sandstone; and the surface of the sand is generally smoother there than where not impressed, which, with the rising of the sand at the border of the prints, indicates the weight of the impressing body. The median groove may be interpreted as due either to the abdomen or the tail of the animal; but as there is no indication of any bending or movement of a tail from side to side, it was probably scooped out of the soft sand by a hard breast-plate or plastron. If this were so, it may be inferred that the species was a freshwater or estuary tortoise rather than a land tortoise."[viii-A]
Previously to this discovery, the trias was the oldest stratum in which any remains or signs of a Chelonian had been detected. Numerous other trails have since been observed (1850-51) in various localities in Canada, all in the same very ancient fossiliferous rock; and Mr. Logan, who has visited the spots, will shortly publish a description of the phenomena.
Chelonian foot-prints in Old Red Sandstone, Morayshire.--Captain Lambart Brickenden has just communicated to the Geological Society of London a drawing and description of a continuous series of no less than thirty-four foot-prints of a quadruped observed in the course of last year (1850), on a slab of sandstone quarried at Cummingstone, near Elgin, in Morayshire, a rock which has always been considered as an upper member of the Devonian or "Old Red."[ix-A] A part of the track, the course of which was from A to B, is represented in the annexed woodcut, fig. 521. The foot-prints are in pairs, forming two parallel rows, which are somewhat less distant from each other than those of the Lower Silurian tortoise of Canada above mentioned. The stride, on the other hand, is four inches, or twice that of the Beauharnais Chelonian. The hind foot is exactly of the same size, being one inch in diameter, and larger than the fore foot in the proportion of four to three.
Skeleton of a reptile, allied to the Batrachians, in the Old Red Sandstone of Morayshire.--Mr. Patrick Duff, author of a "Sketch of the Geology of Morayshire" (Elgin, 1842), obtained recently (October, 1851), from the rock above alluded to, the first example ever seen of the skeleton of a reptile in the Old Red Sandstone. He has kindly allowed me to give a figure of this fossil, of which Dr. Mantell has drawn up a detailed osteological account for publication in the "Journal of the Geological Society of London." The bones in this specimen have decomposed, but the natural position of almost all of them can be seen, and nearly perfect casts of their form taken from the hollow moulds which they have left. The matrix is a fine-grained, whitish sandstone, with a cement of carbonate of lime. The skeleton exhibits the general characters of the Lacertians, blended with peculiarities that are Batrachian. Hence Dr. Mantell infers that this reptile was either a freshwater Batrachian, resembling the Triton, or a small terrestrial Lizard. Slight indications are visible of very minute conical teeth. Captain Brickenden, who is well acquainted with the geology of that part of Scotland, informs me that this fossil was found in the Hill of Spynie, north of the town of Elgin, in a rock quarried for building, and the same in which the Chelonian foot-prints, alluded to in the last page, occur. The skeleton is about four and a half inches in length, but part of the tail is concealed in the rock. Dr. Mantell has proposed for it the generic name of Telerpeton, from +tele+, afar off, and +herpeton+, a reptile; while the specific name Elginense commemorates the principal place near which it was obtained.
Eggs of Batrachians (?) in the Old Red Sandstone of Scotland.--At page 344. of this work I have given two figures (figs. 397 and 398.) of small groups of eggs, very common in the shales and sandstones of the "Old Red" of Kincardineshire, Forfarshire, and Fife. I threw out as a conjecture, that they might belong to gasteropodous mollusca, like those represented in fig. 399. p. 345.; but Dr. Mantell, some years ago, showed me a small bundle of the dried-up eggs of the common English frog (see fig. 524 a.), black and carbonaceous, and so identical in appearance with the fossils in question, that he suggested the probability of these last being of Batrachian origin. The plants by which they are often accompanied (fig. 398. p. 344.), I formerly supposed to be Fuci, but Mr. Bunbury tells me that their grass-like form agrees well with the idea of their being freshwater, and of the family Fluviales.
The absence of all shells, so far as our researches have yet gone, in the slates and sandstones of Scotland above alluded to, raises a presumption against their marine origin, and a still stronger one against the fossil eggs being those of Gasteropoda. It is well known that a single female of the Batrachian tribe ejects annually an astonishing quantity of spawn. Mr. Newport, author of many accurate researches into the metamorphoses of the Amphibia, having examined my fossils from Forfarshire, concurs in Dr. Mantell's opinion that the clusters of eggs (figs. 397. 398. p. 344.) may be those of frogs; while other larger ones, occurring singly or in pairs in the same slates, and often attached to a leaf, may be the ova of a gigantic Triton or Salamander. (See figs. 523, 524, 525.) I may observe that the subdivision of the Old Red Sandstone, in which these plants and ova occur (No. 4. of the section, fig. 62. p. 48.), is considerably lower in position than the rock in which the Telerpeton of Elgin is imbedded.
Foot-prints of Lower Carboniferous reptiles in the United States.--I have stated, at p. 340., that in 1849, Mr. Isaac Lea observed the foot-marks of a large reptile in the lowest beds of the coal formation at Pottsville, about seventy miles N.E. of Philadelphia. These researches have since been carried farther by Professor H. D. Rogers, in the same region of anthracitic coal, lying on the eastern flank of the Alleghany Mountains. Beneath the productive coal-measures of that country occurs a dense mass of red shales and sandstones, which correspond nearly in position to the millstone grit and Mountain Limestone of the south-east of England. In these beds foot-prints, referred to three species of quadrupeds, have lately been detected, all of them five-toed and in double rows, with an opposite symmetry, as if made by right and left feet, while they likewise display the alternation of fore foot and hind foot. One species, the largest of the three, presents a diameter for each foot-print of about two inches, and shows the fore and hind feet to be nearly equal in dimensions. It exhibits a length of stride of about nine inches, and a breadth between the right and left treads of nearly four inches. The impressions of the hind feet are but little in the rear of the fore feet. The animal which made them is supposed to have been allied to a Saurian, rather than to a Batrachian or Chelonian; but more information is required before so difficult a point can be decided. With these foot-marks were seen shrinkage cracks, such as are caused by the sun's heat in mud, and rain-spots, with the signs of the trickling of water on a wet, sandy beach; all confirming the conclusion derived from the foot-prints, that the quadrupeds belonged to air-breathers, and not to aquatic races.[xii-A] The Cheirotherian foot-prints, figured by me at p. 338., in which the fore and hind feet are very unequal in size, betoken a distinct genus, and occur in the midst of the productive coal measures, being consequently less ancient.
On Fossil Rain-marks of the Carboniferous Period in North America.--Having alluded to the spots left by rain on the surface of carboniferous strata in the Alleghanies, on which quadrupedal foot-prints are seen, I may mention that similar rain-prints are conspicuous in the coal measures of Cape Breton, in Nova Scotia, in which Mr. Richard Brown has described Stigmariae and erect trunks of trees, and where there are proofs, as stated at p. 324., of many fossil forests ranged one above the other. In such a region, if anywhere, might we expect to detect evidence of the fall of rain on a sea-beach, so repeatedly must the conditions of the same area have oscillated between land and sea. The intercalation of deposits, containing shells of marine or brackish water, indicate the constant proximity of a body of salt water when the clays which supported the upright trees were formed. In the course of 1851, Mr. Brown had the kindness to send me some greenish slates from Sydney, Cape Breton, on which are imprinted very delicate impressions of rain-drops, with several worm-tracks (a, b, fig. 526.), such as usually accompany rain-marks on the recent mud of the Bay of Fundy, and other modern beaches.[xii-B]
The casts of rain-prints, in figs. 527. and 528., project from the under side of two layers, occurring at different levels, the one a sandy shale, resting on the green shale (fig. 526.), the other a sandstone presenting a similar warty or blistered surface, on which are also observable some small ridges as at a, which stand out in relief, and afford evidence of cracks formed by the shrinkage of subjacent clay, on which rain had fallen. Many of the associated sandstones are described by Mr. Brown as ripple-marked.
The great humidity of the climate of the coal period had been previously inferred from the nature of its vegetation 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.
Triassic Mammifer (Microlestes antiquus Plieninger.)--In the year 1847, Professor Plieninger, of Stuttgart, published a description of two fossil molar teeth, referred by him to a warm-blooded quadruped[xiii-A], which he obtained from a bone-breccia in Wuertemberg occurring between the lias and the keuper. As the announcement of so novel a fact has never met with the attention it deserved, we are indebted to Dr. Jaeger, of Stuttgart, for having recently reminded us of it in his Memoir on the Fossil Mammalia of Wuertemberg.[xiii-B]
Fig. 529. represents the tooth first found, taken from the plate published in 1847, by Professor Plieninger; and fig. 530. is a drawing of the same executed from the original by Mr. Hermann von Meyer, which he has been kind enough to send me. Fig. 529. is a second and larger molar, copied from Dr. Jaeger's plate lxxi., fig. 15.
Professor Plieninger inferred in 1847, from the double fangs of this tooth and their unequal size, and from the form and number of the protuberances or cusps on the flat crowns, that it was the molar of a Mammifer; and considering it as predaceous, probably insectivorous, he called it Microlestes, from +mikros+, little, and +lestes+, a beast of prey. Soon afterwards, he found the second tooth also, at the same locality, Diegerloch, about two miles to the south-east of Stuttgart. Some of its cusps are broken, but there seem to have been six of them originally. From its agreement in general characters, it is supposed by Professor Plieninger to be referable to the same animal, but as it is four times as big, it may perhaps have belonged to another allied species. This molar is attached to the matrix consisting of sandstone, whereas the tooth, No. 529., is isolated. Several fragments of bone, differing in structure from that of the associated saurians and fish, and believed to be mammiferous, were imbedded near them in the same rock.
Mr. Waterhouse, of the British Museum, after studying the annexed figs. 529. 531. and the descriptions of Prof. Plieninger, observes, that not only the double roots of the teeth and their crowns presenting several cusps, resemble those of Mammalia, but the cingulum also, or ridge surrounding the base of that part of the body of the tooth which was exposed or above the gum, is a character distinguishing them from fish and reptiles. "The arrangement of the six cusps or tubercles in two rows, in fig. 529., with a groove or depression between them and the oblong form of the tooth, lead him, he says, to regard it as a molar of the lower jaw. Both the teeth differ from those of the Stonesfield Mammalia[xiv-A], but do not supply sufficient data for determining to what order they belonged. Even in regard to the Stonesfield jaws, where we possess so much ampler materials, we cannot safely pronounce on the order."
Professor Plieninger has sent me a cast of the smaller tooth, which exhibits well the characteristic mammalian test, the double fang; but Mr. Owen, to whom I have shown it, is not able to recognize its affinity with any mammalian type, recent or extinct, known to him.
It has already been stated that the stratum in which the above-mentioned fossils occur is intermediate between the lias and the uppermost member of the trias. That it is really triassic may be deduced from the following considerations. In Wuertemberg there are two "bone-beds," one of great extent, and very rich in the remains of fish and reptiles, which intervenes between the muschelkalk and keuper, the other, containing the Microlestes, less extensive and fossiliferous, which rests on the keuper, or superior member of the trias, and is covered by the sandstone of the lias. The last-mentioned breccia therefore occupies the same place as the well-known English "bone-bed" of Axmouth and Aust-cliff near Bristol, which is shown[xv-A] to include characteristic species of muschelkalk fish, of the genus Saurichthys, Hybodus, and Gyrolepis. In both the Wuertemberg bone-beds these three genera are also found, and one of the species, Saurichthys Mougeotii, is common to both the lower and upper breccias, as is also a remarkable reptile called Nothosaurus mirabilis. The Saurian called Belodon by H. Von Meyer of the Thecodont family, is another Triassic form, associated at Diegerloch with Microlestes.
Previous to this discovery of Professor Plieninger, the most ancient of known fossil Mammalia were those of the Stonesfield slate, a subdivision of the Lower Oolite[xv-B] no representative of this class having as yet been met with in the Fuller's earth, or inferior Oolite (see Table, p. 258.), nor in any member of the lias.
Thecodont Saurians.--This family of reptiles is common to the Trias and Permian groups in Germany, and the geologists employed in the government survey of Great Britain have come to the conclusion, that the rock containing the two species alluded to at p. 306., and of which the teeth are represented in figs. 348, 349., ought rather to be referred to the Trias than to the Permian group.
CRETACEOUS GASTEROPODA.
In speaking of the chalk of Faxoe in Denmark (p. 210.) or the highest member of the Cretaceous series, I have remarked that it is characterized by univalve Mollusca, both spiral and patelliform, which are wanting or rare in the white chalk of Europe. This last statement requires, I find, some modification. It holds true in regard to certain forms, such as Cypraea and Oliva, found at Faxoe; but M. A. d'Orbigny enumerates 24 species of Gasteropoda from the white chalk (Terrain Senonien) of France alone. The same author describes 134 French species of Gasteropoda from the chloritic chalk marl and upper greensand (Turonien), 77 from the gault, and 90 from the lower greensand (Neocomien), in all 325 species of Gasteropoda, from the cretaceous group below the Maestricht beds. Among these he refers 1 to the genus Mitra, 17 to Fusus, 17 to Trochus, 4 to Emarginula, and 36 to Cerithium. Notwithstanding, therefore, the peculiarity of the chambered univalves of various genera, so abundant in the chalk, the Mollusca of the period approximate in character to the tertiary and recent Fauna far more than was formerly supposed.
DICOTYLEDONOUS LEAVES IN LOWER CRETACEOUS STRATA.
M. Adolphe Brongniart when founding his classification of the fossiliferous strata in reference to their imbedded fossil plants, has placed the cretaceous group in the same division with the tertiary, that is to say, in his "Age of Angiosperms."[xvi-A] This arrangement is based on the fact, that the cretaceous plants display a transition character from the vegetation of the secondary to that of the tertiary periods. Coniferae and Cycadeae still flourished as in the preceding oolitic and triassic epochs; but with these fossils, some well-marked leaves of dicotyledonous trees referred to several species of the genus Credneria, had been found in Germany in the Quadersandstein and Plaener-kalk. Still more recently, Dr. Debey of Aix-la-Chapelle has met with a great variety of other leaves of dicotyledonous plants in the cretaceous flora[xvi-B], of which he enumerates no less than 26 species, some of the leaves being from four to six inches in length, and in a beautiful state of preservation. In the absence of the organs of fructification and of fossil fruits, the number of species may be exaggerated; but we may nevertheless affirm, reasoning from our present data, that in the lower chalk of Aix-la-Chapelle, Dicotyledonous Angiosperms flourished nearly in equal proportions with Gymnosperms; a fact of great significance, as some geologists had wished to connect the rarity of dicotyledonous trees with a peculiarity in the state of the atmosphere in the earlier ages of the planet, imagining that a denser air and noxious gases, especially carbonic acid in excess, were adverse to the prevalence, not only of the quick-breathing classes of animals, (mammalia and birds,) but to a flora like that now existing, while it favoured the predominance of reptile life, and a cryptogamic and gymnospermous flora. The co-existence, therefore, of dicotyledonous angiosperms in abundance with Cycads and Coniferae, and with a rich reptilian fauna comprising the Iguanodon, Ichthyosaurus, Pliosaurus, and Pterodactyl, in the lower cretaceous series tends, like the oolitic mammalia of Stonesfield and Stuttgart, and the triassic birds of Connecticut, to dispel the idea of a meteorological state of things in the secondary periods widely distinct from that now prevailing.
General remarks.--In the preliminary chapters of "The Principles of Geology," in the first and subsequent editions, I have considered the question, how far the changes of the earth's crust in past times confirm or invalidate the popular hypothesis of a gradual improvement in the habitable condition of the planet, accompanied by a contemporaneous development and progression in organic life. It had long been a favourite theory, that in the earlier ages to which we can carry back our geological researches, the earth was shaken by more frequent and terrible earthquakes than now, and that there was no certainty nor stability in the order of the natural world. A few sea-weeds and zoophytes, or plants and animals of the simplest organization, were alone capable of existing in a state of things so unfixed and unstable. But in proportion as the conditions of existence improved, and great convulsions and catastrophes became rarer and more partial, flowering plants were added to the cryptogamic class, and by the introduction of more and more perfect species, a varied and complex flora was at last established. In like manner, in the animal kingdom, the zoophyte, the brachiopod, the cephalopod, the fish, the reptile, the bird, and the warm-blooded quadruped made their entrance into the earth, one after the other, until finally, after the close of the tertiary period, came the quadrumanous mammalia, most nearly resembling man in outward form and internal structure, and followed soon afterwards, if not accompanied at first, by the human race itself.
The objections which, in 1830, I urged against this doctrine[xvii-A], in so far as relates to the passage of the earth from a chaotic to a more settled condition, have since been embraced by a large and steadily increasing school of geologists; and in reference to the animate world, it will be seen, on comparing the present state of our knowledge with that which we possessed twenty years ago, how fully I was justified in declaring the insufficiency of the data on which such bold generalizations, respecting progressive development, were based. Speaking of the absence, from the tertiary formations, of fossil Quadrumana, I observed, in 1830, that "we had no right to expect to have detected any remains of tribes which live in trees, until we knew more of those quadrupeds which frequent marshes, rivers, and the borders of lakes, such being usually first met with in a fossil state."[xvii-B] I also added, "if we are led to infer, from the presence of crocodiles and turtles in the London clay, and from the cocoa-nuts and spices found in the isle of Sheppey, that at the period when our older tertiary strata were formed, the climate was hot enough for the Quadrumana, we nevertheless could not hope to discover any of their skeletons, until we had made considerable progress in ascertaining what were the contemporary Pachydermata; and not one of these has been discovered as yet in any strata of this epoch in England."
Nine years afterwards, when these fossil Pachyderms had been found in the London clay, and in the sandy strata at its base, the remains of a monkey, of the genus Macacus, were detected near Woodbridge, in Suffolk; and other Quadrumana had been met with, a short time previously, in different stages of the tertiary series, in India, France, and Brazil.
When we consider the small area of the earth's surface hitherto examined geologically, and our scanty acquaintance with the fossil Vertebrata, even of the environs of great European capitals, it is truly surprising that any naturalist should be rash enough to assume that the Lower Eocene deposits mark the era of the first creation of Quadrumana. It is, however, still more unphilosophical to infer from a single extinct species of this order, obtained in a latitude far from the tropics, that the Eocene Quadrumana had not attained as high a grade of organization as those of our own times, when the naturalist is acquainted with all, or nearly all, the species of monkeys, apes and orangs which are contemporary with man.
To return to the year 1830, Mammalia had not then been traced to rocks of higher antiquity than the Stonesfield Oolite, whereas we have just seen that memorials of this class have at length made their appearance in the Trias of Germany. In 1830 birds had been discovered no lower in the series than the Paris gypsum, or Middle Eocene. Their bones have now been found both in England and the Swiss Alps in the Lower Eocene, and their existence has been established by foot-prints in the triassic epoch in North America (p. 297.). Reptiles in 1830 had not been detected in rocks older than the Magnesian limestone, or Permian formation; whereas the skeletons of four species have since been brought to light (see p. 336.) in the coal-measures, and one in the Old Red sandstone, of Europe, while the footprints of three or four more have been observed in carboniferous rocks of North America, not to mention the chelonian trail above described, from the most ancient of the fossiliferous rocks of Canada, the "Potsdam Sandstone," which lies at the base of the Lower Silurian system. (See above, p. vii.)
Lastly, the remains of fish, which in 1830 were scarcely recognized in deposits older than the coal, have now been found plentifully in the Devonian, and sparingly in the Silurian, strata; though not in any formation of such high antiquity as the Chelonian of Montreal.
Previously to the discovery last mentioned, it was by no means uncommon for paleontologists to speak with confidence of fish as having been created before reptiles. It was deemed reasonable to suppose that the introduction of a particular class or order of beings into the planet coincided, in date, with the age of the oldest rock to which the remains of that class or order happened then to have been traced back. To be consistent with themselves, the same naturalists ought now to take for granted that reptiles were called into existence before fish. This they will not do, because such a conclusion would militate against their favourite hypothesis of an ascending scale, according to which Nature "evolved the organic world," rendering it more and more perfect in the lapse of ages.
In our efforts to arrive at sound theoretical views on such a question, it would seem most natural to turn to the marine invertebrate animals as to a class affording the most complete series of monuments that have come down to us, and where we can find corresponding terms of comparison, in strata of every age. If, in this more complete series of her archives, Nature had really exhibited a more simple grade of organization in fossils of the remotest antiquity, we might have suspected that there was some foundation of facts in the theory of successive development. But what do we find? In the Lower Silurian there is a full representation of the Radiata, Mollusca, and Articulata proper to the sea. The marine Fauna, indeed, in those three classes, is so rich as almost to imply a more perfect development than that which now peoples the ocean. Thus, in the great division of the Radiata, we find asteroid and helianthoid zoophytes, besides crinoid and cystidean echinoderms. In the Mollusca of the same most ancient epoch M. Barrande enumerates, in Bohemia alone, the astonishing number of 253 species of Cephalopoda. In the Articulata we have the crustaceans, represented by more than 200 species of Trilobites, not to mention other genera.
It is only then, in reference to the Vertebrata, that the argument of degeneracy in proportion as we trace fossils back to older formations can be maintained; and the dogma rests mainly for its support on negative evidence, whether deduced from the entire absence of the fossil representatives of certain classes in particular rocks, or the low grade of the first few species of a class which chance has thrown in our way.
The scarcity of all memorials of birds in strata below the Eocene, has been a subject of surprise to some geologists. The bones formerly referred to birds in the Wealden and Chalk, are now admitted to have belonged to flying reptiles, of various sizes, one of them from the Kentish chalk so large as to have measured 16 feet 6 inches from tip to tip of its outstretched wings. Whether some elongated bones of the Stonesfield Oolite should be referred to birds, which they seem greatly to resemble in microscopic structure or to Pterodactyles, is a point now under investigation. If it should be proved that no osseous remains of the class Aves have hitherto been derived from any secondary or primary formation, we must not too hastily conclude that birds were even scarce in these periods. The rarity of such fossils in the Eocene marine strata is very striking. In 1846, Professor Owen, in his "History of the Fossil Mammalia and Birds of Great Britain," was unable to obtain more than four or five fragments of bones and skulls of birds from the London Clay, by the aid of which four species were recognized. Even so recently, therefore, as 1846, as much was known of the Mammalia of the Stonesfield Oolite, as of the ornithic Fauna of our English Eocene deposits.
To reason correctly on the value of negative facts in this branch of Paleontology, we must first have ascertained how far the relics of birds are now becoming preserved in new strata, whether marine, fluviatile, or lacustrine. I have explained, in the "Principles of Geology," that the imbedding of the bones of living birds in deposits now in progress in inland lakes appears to be extremely rare. In the shell-marl of Scotland, which is made up bodily of the shells of the genera Limneus, Planorbis, Succinea, and Valvata, and in which the skeletons of deer and oxen abound, we find no bones of birds. Yet we know that, before the lakes were drained which yield this marl used in agriculture, the surface of the water and the bordering swamps were covered with wild ducks, herons, and other fowl. They left no memorials behind them, because, if they perished on the land, their bodies decomposed or became the prey of carnivorous animals; if on the water, they were buoyant and floated till they were devoured by predaceous fish or birds. The same causes of obliteration have no power to efface the foot-prints which the same creatures may leave, under favourable circumstances, imprinted on an ancient mud-bank or shore, on which new strata may be from time to time thrown down. In the red mud of recent origin spread over wide areas by the high tides of the bay of Fundy, innumerable foot-tracks of recent birds (Tringa minuta) are preserved in successive layers, and hardened by the sun. Yet none of the bones of these birds, though diligently searched for, have yet been discovered in digging trenches through the red mud. It is true that, in a few spots, the bones of birds have been met with plentifully in the older tertiary strata, but always in rocks of freshwater origin, such as the Paris gypsum or the lacustrine limestone of the Limagne d'Auvergne. In strata of the same age, in Belgium and other European countries, or in the United States, where no less careful search has been made, few, if any, fossil birds have come to light.
We ought, therefore, most clearly to perceive that it is no part of the plan of Nature to hand down to after times a complete or systematic record of the former history of the animate world. The preservation of the relics, even of aquatic tribes of animals, is an exception to the general rule, although time may so multiply exceptional cases that they may seem to constitute the rule; and may thus impose upon the imagination, leading us to infer the non-existence of creatures of which no monuments are extant. Hitherto our acquaintance with the birds, and even the Mammalia, of the Eocene period has depended, almost everywhere, on single specimens, or on a few individuals found in one spot. It has therefore depended on what we commonly call chance; and we must not wonder if the casual discovery of a tertiary, secondary, or primary rock, rich in fossil impressions of the foot-prints of birds or quadrupeds, should modify or suddenly overthrow all theories based on negative facts.
The chief reason why we meet more readily with the remains of every class in tertiary than in secondary strata, is simply that the older rocks are more and more exclusively marine in proportion as we depart farther and farther from periods during which the existing continents were built up. The secondary and primary formations are, for the most part, marine,--not because the ocean was more universal in past times, but because the epochs which preceded the Eocene were so distant from our own, that entire continents have been since submerged.
I have alluded at p. 299. to Mr. Darwin's account of the South American Ostriches, seen on the coast of Buenos Ayres, walking at low water over extensive mud-banks, which are then dry, for the sake of feeding on small fish. Perhaps no bird of such perfect organization as the eagle or vulture may ever accompany these ostriches. Certainly, we cannot expect the condor of the Andes to leave its trail on such a shore; and no traveller, after searching for footprints along the whole eastern coast of South America, would venture to speculate, from the results of such an inquiry, on the extent, variety, or development of the feathered Fauna of the interior of that continent.
The absence of Cetacea from rocks older than the Eocene has been frequently adduced as lending countenance to the theory of the late appearance of the highest class of Vertebrata on the earth. That we have hitherto failed to detect them in the Oolite or Trias, does not imply, as we have now seen, that Mammalia were not then created. Even in the Eocene strata of Europe, the discovery of Cetaceans has never kept pace with that of land quadrupeds. The only instance cited in Great Britain is a species of Monodon, from the London clay, of doubtful authenticity as to its geological position. On the other hand, the gigantic Zeuglodon of North America (see p. 207.), occurs abundantly in the Middle Eocene strata of Georgia and Alabama, from which as yet no bones of land-quadrupeds have been obtained.
Professor Sedgwick states in a recent work[xxi-A], that he possesses in the Woodwardian Museum, a mass of anchylosed cervical vertebrae of a whale which he found near Ely, and which he believes to have been washed out of the Kimmeridge clay, a member of the Upper Oolite; but its true geological site is not well determined. It differs, says Professor Owen, from any other known fossil or recent whale.
In the present imperfect state then of our information, we can scarcely say more than that the Cetacea may have been scarce, in the secondary and primary periods. It is quite conceivable that when aquatic saurians, some of them carnivorous, like the Ichthyosaurus, were swarming in the sea, and when there were large herbivorous reptiles, like the Iguanodon, on the land, such reptiles may, to a certain extent, have superseded the Cetacea, and discharged their functions in the animal economy.
The views which I proposed originally in the Principles of Geology in opposition to the theory of progressive development may be thus briefly explained. From the earliest period at which plants and animals can be proved to have existed, there has been a continual change going on in the position of land and sea, accompanied by great fluctuations of climate. To these ever-varying geographical and climatal conditions the state of the animate world has been unceasingly adapted. No satisfactory proof has yet been discovered of the gradual passage of the earth from a chaotic to a more habitable state, nor of a law of progressive development governing the extinction and renovation of species, and causing the Fauna and Flora to pass from an embryonic to a more perfect condition, from a simple to a more complex organization.
The principle of adaptation above alluded to, appears to have been analogous to that which now peoples the arctic, temperate, and tropical regions contemporaneously with distinct assemblages of species and genera, or which independently of mere temperature gives rise to a predominance of the marsupial tribe of quadrupeds in Australia, and of the placental tribe in Asia and Europe, or to a profusion of reptiles without mammalia in the Galapagos Archipelago, and of mammalia without reptiles in Greenland.[xxii-A]
This theory implies, almost necessarily, a very unequal representation at successive periods of the principal classes and orders of plants and animals, if not in the whole globe, at least throughout very wide areas. Thus, for example, the proportional number of genera, species, and individuals in the vertebrate class may differ, in two different and distinct epochs, to an extent unparalleled by any two contemporaneous Faunas, because in the course of millions of ages, the contrast of climate and geographical conditions may exceed the difference now observable in polar and equatorial latitudes.
I shall conclude by observing, that if the doctrine of successive development had been paleontologically true, as the new discoveries above enumerated show that it is not; if the sponge, the cephalopod, the fish, the reptile, the bird, and the mammifer had followed each other in regular chronological order--the creation of each class being separated from the other by vast intervals of time; and if it were admitted that Man was created last of all, still we should by no means be able to recognize, in his entrance upon the earth, the last term of one and the same series of progressive developments. For the superiority of Man, as compared to the irrational mammalia, is one of kind, rather than of degree, consisting in a rational and moral nature, with an intellect capable of indefinite progression, and not in the perfection of his physical organization, or those instincts in which he resembles the brutes. He may be considered as a link in the same unbroken chain of being, if we regard him simply as a new species--a member of the animal kingdom--subject, like other species, to certain fixed and invariable laws, and adapted like them to the state of the animate and inanimate world prevailing at the time of his creation. Physically considered, he may form part of an indefinite series of terrestrial changes past, present, and to come; but morally and intellectually he may belong to another system of things--of things immaterial--a system which is not permitted to interrupt or disturb the course of the material world, or the laws which govern its changes.[xxii-B]
FOOTNOTES:
[vii-A] Travels in North America by the Author, vol. ii. chap. 22.
[vii-B] Ibid. 1842.
[viii-A] Quart. Journ. Geol. Soc. 1851, vol. vii. p. 250.
[ix-A] The generally received determination of the age of this rock is probably correct; but as there are no overlying coal-measures and no well-known Devonian fossils in the whitish stone of Elgin, and as I have not personally explored the geology of that district, I cannot speak as confidently as in regard to the age of the Montreal Chelonian.
[xii-A] H. D. Rogers, Proceedings of Amer. Assoc. of Science, Albany, 1851.
[xii-B] See Memoir by the Author, Quart. Journ. Geol. Soc., vol. vii. p. 240.
[xiii-A] Wuertembergisch. Naturwissen. Jahreshefte, 3 Jahr. Stuttgart, 1847.
[xiii-B] Nov. Act. Acad. Caesar. Leopold. Nat. Cur. 1850, p. 902. For figures, see ibid. plate xxi. figs. 14, 15, 16, 17.
[xiv-A] See Manual, p. 268.
[xv-A] Manual, p. 289.
[xv-B] Ibid. p. 268.
[xvi-A] For Terminology, see Note, p. 223.
[xvi-B] Quart. Journ. vol. vii. Memoirs, p. 111.
[xvii-A] Principles, 1st ed. chaps. v. and ix.
[xvii-B] Ibid. p. 153.
[xxi-A] Preface to 5th ed. of Studies of University of Cambridge.
[xxii-A] Principles, 4th ed. 1835, vol. i. p. 231, and vol. i. chap. 9. subsequent ed.
[xxii-B] In my Anniversary Address, for 1851, to the Geological Society, the reader will find a full discussion of the facts and arguments which bear on the theory of progressive development.--Quart. Journ. Geol. Soc., vol. vii.
CONTENTS.
I. ON THE DIFFERENT CLASSES OF ROCKS
Geology defined--Successive formation of the earth's crust--Classification of rocks according to their origin and age--Aqueous rocks--Their stratification and imbedded fossils--Volcanic rocks, with and without cones and craters--Plutonic rocks, and their relation to the volcanic--Metamorphic rocks and their probable origin--The term primitive, why erroneously applied to the crystalline formations--Leading division of the work Page 1
Chapter II
AQUEOUS ROCKS--THEIR COMPOSITION AND FORMS OF STRATIFICATION.
Mineral composition of strata--Arenaceous rocks--Argillaceous--Calcareous--Gypsum--Forms of stratification--Original horizontality--Thinning out--Diagonal arrangement--Ripple mark 10
III. ARRANGEMENT OF FOSSILS IN STRATA--FRESHWATER AND MARINE
Successive deposition indicated by fossils--Limestones formed of corals and shells--Proofs of gradual increase of strata derived from fossils--Serpula attached to spatangus--Wood bored by Teredina--Tripoli and semi-opal formed of infusoria--Chalk derived principally from organic bodies--Distinction of freshwater from marine formations--Genera of freshwater and land shells--Rules for recognizing marine testacea--Gyrogonite and chara--Freshwater fishes--Alternation of marine and freshwater deposits--Lym-Fiord 21
IV. CONSOLIDATION OF STRATA AND PETRIFACTION OF FOSSILS
Chemical and mechanical deposits--Cementing together of particles--Hardening by exposure to air--Concretionary nodules--Consolidating effects of pressure--Mineralization of organic remains--Impressions and casts how formed--Fossil wood--Goeppert's experiments--Precipitation of stony matter most rapid where putrefaction is going on--Source of lime in solution--Silex derived from decomposition of felspar--Proofs of the lapidification of some fossils soon after burial, of others when much decayed 33
Chapter V
ELEVATION OF STRATA ABOVE THE SEA--HORIZONTAL AND INCLINED STRATIFICATION.
Why the position of marine strata, above the level of the sea, should be referred to the rising up of the land, not to the going down of the sea--Upheaval of extensive masses of horizontal strata--Inclined and vertical stratification--Anticlinal and synclinal lines--Bent strata in east of Scotland--Theory of folding by lateral movement--Creeps--Dip and strike--Structure of the Jura--Various forms of outcrop--Rocks broken by flexure--Inverted position of disturbed strata--Unconformable stratification--Hutton and Playfair on the same--Fractures of strata--Polished surfaces--Faults--Appearance of repeated alternations produced by them--Origin of great faults Page 44
VI. DENUDATION
Denudation defined--Its amount equal to the entire mass of stratified deposits in the earth's crust--Horizontal sandstone denuded in Ross-shire--Levelled surface of countries in which great faults occur--Coalbrook Dale--Denuding power of the ocean during the emergence of land--Origin of Valleys--Obliteration of sea-cliffs--Inland sea-cliffs and terraces in the Morea and Sicily--Limestone pillars at St. Mihiel, in France--in Canada--in the Bermudas 66
VII. ALLUVIUM
Alluvium described--Due to complicated causes--Of various ages, as shown in Auvergne--How distinguished from rocks in situ--River-terraces--Parallel roads of Glen Roy--Various theories respecting their origin 79
VIII. CHRONOLOGICAL CLASSIFICATION OF ROCKS
Aqueous, plutonic, volcanic, and metamorphic rocks, considered chronologically--Lehman's division into primitive and secondary--Werner's addition of a transition class--Neptunian theory--Hutton on igneous origin of granite--How the name of primary was still retained for granite--The term "transition," why faulty--The adherence to the old chronological nomenclature retarded the progress of geology--New hypothesis invented to reconcile the igneous origin of granite to the notion of its high antiquity--Explanation of the chronological nomenclature adopted in this work, so far as regards primary, secondary, and tertiary periods 89
IX. ON THE DIFFERENT AGES OF THE AQUEOUS ROCKS
On the three principal tests of relative age--superposition, mineral character, and fossils--Change of mineral character and fossils in the same continuous formation--Proofs that distinct species of animals and plants have lived at successive periods--Distinct provinces of indigenous species--Great extent of single provinces--Similar laws prevailed at successive geological periods--Relative importance of mineral and palaeontological characters--Test of age by included fragments--Frequent absence of strata of intervening periods--Principal groups of strata in western Europe 96
X. CLASSIFICATION OF TERTIARY FORMATIONS.--POST-PLIOCENE GROUP
General principles of classification of tertiary strata--Detached formations scattered over Europe--Strata of Paris and London--More modern groups--Peculiar difficulties in determining the chronology of tertiary formations--Increasing proportion of living species of shells in strata of newer origin--Terms Eocene, Miocene, and Pliocene--Post-Pliocene strata--Recent or human period--Older Post-Pliocene formations of Naples, Uddevalla, and Norway--Ancient upraised delta of the Mississippi--Loess of the Rhine Page 104
XI. NEWER PLIOCENE PERIOD.--BOULDER FORMATION
Drift of Scandinavia, northern Germany, and Russia--Its northern origin--Not all of the same age--Fundamental rocks polished, grooved, and scratched--Action of glaciers and icebergs--Fossil shells of glacial period--Drift of eastern Norfolk--Associated freshwater deposit--Bent and folded strata lying on undisturbed beds--Shells on Moel Tryfane--Ancient glaciers of North Wales--Irish drift 121
XII. BOULDER FORMATION--continued
Difficulty of interpreting the phenomena of drift before the glacial hypothesis was adopted--Effects of intense cold in augmenting the quantity of alluvium--Analogy of erratics and scored rocks in North America and Europe--Bayfield on shells in drift of Canada--Great subsidence and re-elevation of land from the sea, required to account for glacial appearances--Why organic remains so rare in northern drift--Mastodon giganteus in United States--Many shells and some quadrupeds survived the glacial cold--Alps an independent centre of dispersion of erratics--Alpine blocks on the Jura--Recent transportation of erratics from the Andes to Chiloe--Meteorite in Asiatic drift 131
XIII. NEWER PLIOCENE STRATA AND CAVERN DEPOSITS
Chronological classification of Pleistocene formations, why difficult--Freshwater deposits in valley of Thames--In Norfolk cliffs--In Patagonia--Comparative longevity of species in the mammalia and testacea--Fluvio-marine crag of Norwich--Newer Pliocene strata of Sicily--Limestone of great thickness and elevation--Alternation of marine and volcanic formations--Proofs of slow accumulation--Great geographical changes in Sicily since the living fauna and flora began to exist--Osseous breccias and cavern deposits--Sicily--Kirkdale--Origin of stalactite--Australian cave-breccias--Geographical relationship of the provinces of living vertebrata and those of the fossil species of the Pliocene periods--Extinct struthious birds of New Zealand--Teeth of fossil quadrupeds 146
XIV. OLDER PLIOCENE AND MIOCENE FORMATIONS
Strata of Suffolk termed Red and Coralline crag--Fossils, and proportion of recent species--Depth of sea and climate--Reference of Suffolk crag to the older Pliocene period--Migration of many species of shells southwards during the glacial period--Fossil whales--Subapennine beds--Asti, Sienna, Rome--Miocene formations--Faluns of Touraine--Depth of sea and littoral character of fauna--Tropical climate implied by the testacea--Proportion of recent species of shells--Faluns more ancient than the Suffolk crag--Miocene strata of Bordeaux and Piedmont--Molasse of Switzerland--Tertiary strata of Lisbon--Older Pliocene and Miocene formations in the United States--Sewalik Hills in India 161
XV. UPPER EOCENE FORMATIONS
Eocene areas in England and France--Tabular view of French Eocene strata--Upper Eocene group of the Paris basin--Same beds in Belgium and at Berlin--Mayence tertiary strata--Freshwater upper Eocene of Central France--Series of geographical changes since the land emerged in Auvergne--Mineral character an uncertain test of age--Marls containing Cypris--Oolite of Eocene period--Indusial limestone and its origin--Fossil mammalia of the upper Eocene strata in Auvergne--Freshwater strata of the Cantal, calcareous and siliceous--Its resemblance to chalk--Proofs of gradual deposition of strata 174
XVI. EOCENE FORMATIONS--continued
Subdivisions of the Eocene group in the Paris basin--Gypseous series--Extinct quadrupeds--Impulse given to geology by Cuvier's osteological discoveries--Shelly sands called sables moyens--Calcaire grossier--Miliolites--Calcaire siliceux--Lower Eocene in France--Lits coquilliers--Sands and plastic clay--English Eocene strata--Freshwater and fluvio-marine beds--Barton beds--Bagshot and Bracklesham division--Large ophidians and saurians--Lower Eocene and London Clay proper--Fossil plants and shells--Strata of Kyson in Suffolk--Fossil monkey and opossum--Mottled clays and sand below London Clay--Nummulitic formation of Alps and Pyrenees--Its wide geographical extent--Eocene strata in the United States--Section at Claiborne, Alabama--Colossal cetacean--Orbitoid limestone--Burr stone 190
XVII. CRETACEOUS GROUP
Divisions of the cretaceous series in North-Western Europe--Upper cretaceous strata--Maestricht beds--Chalk of Faxoe--White chalk--Characteristic fossils--Extinct cephalopoda--Sponges and corals of the chalk--Signs of open and deep sea--White area of white chalk--Its origin from corals and shells--Single pebbles in chalk--Siliceous sandstone in Germany contemporaneous with white chalk--Upper greensand and gault--Lower cretaceous strata--Atherfield section, Isle of Wight--Chalk of South of Europe--Hippurite limestone--Cretaceous Flora--Chalk of United States 209
XVIII. WEALDEN GROUP
The Wealden divisible into Weald Clay, Hastings Sand, and Purbeck Beds--Intercalated between two marine formations--Weald clay and Cypris-bearing strata--Iguanodon--Hastings sands--Fossil fish--Strata formed in shallow water--Brackish water-beds--Upper, middle, and lower Purbeck--Alternations of brackish water, freshwater, and land--Dirt-bed, or ancient soil--Distinct species of fossils in each subdivision of the Wealden--Lapse of time implied--Plants and insects of Wealden--Geographical extent of Wealden--Its relation to the cretaceous and oolitic periods--Movements in the earth's crust to which it owed its origin and submergence 225
XIX. DENUDATION OF THE CHALK AND WEALDEN
Physical geography of certain districts composed of Cretaceous and Wealden strata--Lines of inland chalk-cliffs on the Seine in Normandy--Outstanding pillars and needles of chalk--Denudation of the chalk and Wealden in Surrey, Kent, and Sussex--Chalk once continuous from the North to the South Downs--Anticlinal axis and parallel ridges--Longitudinal and transverse valleys--Chalk escarpments--Rise and denudation of the strata gradual--Ridges formed by harder, valleys by softer beds--Why no alluvium, or wreck of the chalk, in the central district of the Weald--At what periods the Weald valley was denuded--Land has most prevailed where denudation has been greatest--Elephant bed, Brighton 238
XX. OOLITE AND LIAS
Subdivisions of the Oolitic or Jurassic group--Physical geography of the Oolite in England and France--Upper Oolite--Portland stone and fossils--Lithographic stone of Solenhofen--Middle Oolite, coral rag--Zoophytes--Nerinaean limestone--Diceras limestone--Oxford clay, Ammonites and Belemnites--Lower Oolite, Crinoideans--Great Oolite and Bradford clay--Stonesfield slate--Fossil mammalia, placental and marsupial--Resemblance to an Australian fauna--Doctrine of progressive development--Collyweston slates--Yorkshire Oolitic coal-field--Brora coal--Inferior Oolite and fossils 257
XXI. OOLITE AND LIAS--continued
Mineral character of Lias--Name of Gryphite limestone--Fossil shells and fish--Ichthyodorulites--Reptiles of the Lias--Ichthyosaur and Plesiosaur--Marine Reptile of the Galapagos Islands--Sudden destruction and burial of fossil animals in Lias--Fluvio-marine beds in Gloucestershire and insect limestone--Origin of the Oolite and Lias, and of alternating calcareous and argillaceous formations--Oolitic coal-field of Virginia, in the United States 273
XXII. TRIAS OR NEW RED SANDSTONE GROUP
Distinction between New and Old Red Sandstone--Between Upper and Lower New Red--The Trias and its three divisions--Most largely developed in Germany--Keuper and its fossils--Muschelkalk--Fossil plants of Bunter--Triassic group in England--Bone-bed of Axmouth and Aust--Red Sandstone of Warwickshire and Cheshire--Footsteps of Chirotherium in England and Germany--Osteology of the Labyrinthodon--Identification of this Batrachian with the Chirotherium--Origin of Red Sandstone and rock-salt--Hypothesis of saline volcanic exhalations--Theory of the precipitation of salt from inland lakes or lagoons--Saltness of the Red Sea--New Red Sandstone in the United States--Fossil footprints of birds and reptiles in the Valley of the Connecticut--Antiquity of the Red Sandstone containing them 286
XXIII. PERMIAN OR MAGNESIAN LIMESTONE GROUP
Fossils of Magnesian Limestone and Lower New Red distinct from the Triassic--Term Permian--English and German equivalents--Marine shells and corals of English Magnesian limestone--Palaeoniscus and other fish of the marl slate--Thecodont Saurians of dolomitic conglomerate of Bristol--Zechstein and Rothliegendes of Thuringia--Permian Flora--Its generic affinity to the carboniferous--Psaronites or tree-ferns 301
XXIV. THE COAL OR CARBONIFEROUS GROUP
Carboniferous strata in the south-west of England--Superposition of Coal-measures to Mountain limestone--Departure from this type in north of England and Scotland--Section in South Wales--Underclays with Stigmaria--Carboniferous Flora--Ferns, Lepidodendra, Calamites, Asterophyllites, Sigillariae, Stigmariae,--Coniferae--Endogens--Absence of Exogens--Coal, how formed--Erect fossil trees--Parkfield Colliery--St. Etienne, Coal-field--Oblique trees or snags--Fossil forests in Nova Scotia--Brackish water and marine strata--Origin of Clay-iron-stone 308
XXV. CARBONIFEROUS GROUP--continued
Coal-fields of the United States--Section of the country between the Atlantic and Mississippi--Position of land in the carboniferous period eastward of the Alleghanies--Mechanically formed rocks thinning out westward, and limestones thickening--Uniting of many coal-seams into one thick one--Horizontal coal at Brownsville, Pennsylvania--Vast extent and continuity of single seams of coal--Ancient river-channel in Forest of Dean coal-field--Absence of earthy matter in coal--Climate of carboniferous period--Insects in coal--Rarity of air-breathing animals--Great number of fossil fish--First discovery of the skeletons of fossil reptiles--Footprints of reptilians--Mountain limestone--Its corals and marine shells 326
XXVI. OLD RED SANDSTONE, OR DEVONIAN GROUP
Old Red Sandstone of Scotland, and borders of Wales--Fossils usually rare--"Old Red" in Forfarshire--Ichthyolites of Caithness--Distinct lithological type of Old Red in Devon and Cornwall--Term "Devonian"--Organic remains of intermediate character between those of the Carboniferous and Silurian systems--Corals and shells--Devonian strata of Westphalia, the Eifel, Russia, and the United States--Coral reef at Falls of the Ohio--Devonian Flora 342
XXVII. SILURIAN GROUP
Silurian strata formerly called transition--Term grauwacke--Subdivisions of Upper and Lower Silurian--Ludlow formation and fossils--Wenlock formation, corals and shells--Caradoc and Llandeilo beds--Graptolites--Lingula--Trilobites--Cystideae--Vast thickness of Silurian strata in North Wales--Unconformability of Caradoc sandstone--Silurian strata of the United States--Amount of specific agreement of fossils with those of Europe--Great number of brachiopods--Deep-sea origin of Silurian strata--Absence of fluviatile formations--Mineral character of the most ancient fossiliferous rocks 350
XXVIII. VOLCANIC ROCKS
Trap rocks--Name, whence derived--Their igneous origin at first doubted--Their general appearance and character--Volcanic cones and craters, how formed--Mineral composition and texture of volcanic rocks--Varieties of felspar--Hornblende and augite--Isomorphism--Rocks, how to be studied--Basalt, greenstone, trachyte, porphyry, scoria, amygdaloid, lava, tuff--Alphabetical list, and explanation of names and synonyms, of volcanic rocks--Table of the analyses of minerals most abundant in the volcanic and hypogene rocks 366
XXIX. VOLCANIC ROCKS--continued
Trap dike--sometimes project--sometimes leave fissures vacant by decomposition--Branches and veins of trap--Dikes more crystalline in the centre--Foreign fragments of rock imbedded--Strata altered at or near the contact--Obliteration of organic remains--Conversion of chalk into marble--and of coal into coke--Inequality in the modifying influence of dikes--Trap interposed between strata--Columnar and globular structure--Relation of trappean rocks to the products of active volcanos--Submarine lava and ejected matter correspond generally to ancient trap--Structure and physical features of Palma and some other extinct volcanos 378
XXX. ON THE DIFFERENT AGES OF THE VOLCANIC ROCKS
Tests of relative age of volcanic rocks--Test by superposition and intrusion--Dike of Quarrington Hill, Durham--Test by alteration of rocks in contact--Test by organic remains--Test of age by mineral character--Test by included fragments--Volcanic rocks of the Post-Pliocene period--Basalt of Bay of Trezza in Sicily--Post-Pliocene volcanic rocks near Naples--Dikes of Somma--Igneous formations of the Newer Pliocene period--Val di Noto in Sicily 397
XXXI. ON THE DIFFERENT AGES OF THE VOLCANIC ROCKS--continued
Volcanic rocks of the Older Pliocene period--Tuscany--Rome--Volcanic region of Olot in Catalonia--Cones and lava-currents--Ravines and ancient gravel-beds--Jets of air called Bufadors--Age of the Catalonian volcanos--Miocene period--Brown-coal of the Eifel and contemporaneous trachytic breccias--Age of the brown-coal--Peculiar characters of the volcanos of the upper and lower Eifel--Lake craters--Trass--Hungarian volcanos 408
XXXII. ON THE DIFFERENT AGES OF THE VOLCANIC ROCKS--continued
Volcanic rocks of the Pliocene and Miocene periods continued--Auvergne--Mont Dor--Breccias and alluviums of Mont Perrier, with bones of quadrupeds--River dammed up by lava-current--Range of minor cones from Auvergne to the Vivarais--Monts Dome--Puy de Come--Puy de Pariou--Cones not denuded by general flood--Velay--Bones of quadrupeds buried in scoriae--Cantal--Eocene volcanic rocks--Tuffs near Clermont--Hill of Gergovia--Trap of Cretaceous period--Oolitic period--New Red Sandstone period--Carboniferous period--Old Red Sandstone period--"Rock and Spindle" near St. Andrews--Silurian period--Cambrian volcanic rocks 422
XXXIII. PLUTONIC ROCKS--GRANITE
General aspect of granite--Decomposing into spherical masses--Rude columnar structure--Analogy and difference of volcanic and plutonic formations--Minerals in granite, and their arrangement--Graphic and porphyritic granite--Mutual penetration of crystals of quartz and felspar--Occasional minerals--Syenite--Syenitic, talcose, and schorly granites--Eurite--Passage of granite into trap--Examples near Christiania and in Aberdeenshire--Analogy in composition of trachyte and granite--Granite veins in Glen Tilt, Cornwall, the Valorsine, and other countries--Different composition of veins from main body of granite--Metalliferous veins in strata near their junction with granite--Apparent isolation of nodules of granite--Quartz veins--Whether plutonic rocks are ever overlying--Their exposure at the surface due to denudation 436
XXXIV. ON THE DIFFERENT AGES OF THE PLUTONIC ROCKS
Difficulty in ascertaining the precise age of a plutonic rock--Test of age by relative position--Test by intrusion and alteration--Test by mineral composition--Test by included fragments--Recent and Pliocene plutonic rocks, why invisible--Tertiary plutonic rocks in the Andes--Granite altering Cretaceous rocks--Granite altering Lias in the Alps and in Skye--Granite of Dartmoor altering Carboniferous strata--Granite of the Old Red Sandstone period--Syenite altering Silurian strata in Norway--Blending of the same with gneiss--Most ancient plutonic rocks--Granite protruded in a solid form--On the probable age of the granites of Arran, in Scotland 449
XXXV. METAMORPHIC ROCKS
General character of metamorphic rocks--Gneiss--Hornblende-schist --Mica-schist--Clay-slate--Quartzite--Chlorite-schist--Metamorphic limestone--Alphabetical list and explanation of other rocks of this family--Origin of the metamorphic strata--Their stratification is real and distinct from cleavage--Joints and slaty cleavage--Supposed causes of these structures--how far connected with crystalline action 463
XXXVI. METAMORPHIC ROCKS--continued
Strata near some intrusive masses of granite converted into rocks identical with different members of the metamorphic series--Arguments hence derived as to the nature of plutonic action--Time may enable this action to pervade denser masses--From what kinds of sedimentary rock each variety of the metamorphic class may be derived--Certain objections to the metamorphic theory considered--Lamination of trachyte and obsidian due to motion--Whether some kinds of gneiss have become schistose by a similar action 473
XXXVII. ON THE DIFFERENT AGES OF THE METAMORPHIC ROCKS
Age of each set of metamorphic strata twofold--Test of age by fossils and mineral character not available--Test by superposition ambiguous--Conversion of dense masses of fossiliferous strata into metamorphic rocks--Limestone and shale of Carrara--Metamorphic strata of modern periods in the Alps of Switzerland and Savoy--Why the visible crystalline strata are none of them very modern--Order of succession in metamorphic rocks--Uniformity of mineral character--Why the metamorphic strata are less calcareous than the fossiliferous 481
XXXVIII. MINERAL VEINS
Werner's doctrine that mineral veins were fissures filled from above--Veins of segregation--Ordinary metalliferous veins or lodes--Their frequent coincidence with faults--Proofs that they originated in fissures in solid rock--Veins shifting other veins--Polishing of their walls--Shells and pebbles in lodes--Evidence of the successive enlargement and re-opening of veins--Fournet's observations in Auvergne--Dimensions of veins--Why some alternately swell out and contract--Filling of lodes by sublimation from below--Chemical and electrical action--Relative age of the precious metals--Copper and lead veins in Ireland older than Cornish tin--Lead vein in lias, Glamorganshire--Gold in Russia--Connection of hot springs and mineral veins--Concluding remarks 488
* * * * *
Dates of the successive Editions of the "Principles" and "Elements" (or Manual) of Geology, by the Author.
Principles, 1st vol. in octavo, published in - - - Jan. 1830.
----, 2d vol. do. - - - - - - - - - - - - - - - - Jan. 1832.
----, 1st vol. 2d edition in octavo - - - - - - - 1832.
----, 2d vol. 2d edition do. - - - - - - - - - - Jan. 1833.
----, 3d vol. 1st edition do. - - - - - - - - - - May 1833.
----, New edition (called the 3d) of the whole work in 4 vols. 12mo. - - - - - - - - - - - - - - - - - - - May 1834.
----, 4th edition, 4 vols. 12mo. - - - - - - - - - June 1835.
----, 5th edition, do. do. - - - - - - - - - - - - Mar. 1837.
Elements, 1st edition in one vol. - - - - - - - - July 1838.
Principles, 6th edition, 3 vols. 12mo. - - - - - - June 1840.
Elements, 2d edition in 2 vols. 12mo. - - - - - - July 1841.
Principles, 7th edition in one vol. 8vo. - - - - - Feb. 1847.
----, 8th edition, now published in one vol. 8vo. - May 1850.
Manual of Elementary Geology (or "Elements," 3d edition), now published in one vol. 8vo. - - - - - - - - - - - - Jan. 1851.
Works by Sir Charles Lyell.
I.
TRAVELS IN NORTH AMERICA,--1841-2. With Geological Observations on the United States, Canada, and Nova Scotia. With large coloured geological Map and Plates. 2 vols. post 8vo. 21_s._
II.
A SECOND VISIT TO THE UNITED STATES,--1845-6. Second Edition. 2 vols. post 8vo. 18_s._
III.
PRINCIPLES OF GEOLOGY; or the Modern Changes of the Earth and its Inhabitants considered, as illustrative of Geology. Eighth Edition, thoroughly revised. With Maps, Plates, and Woodcuts. 8vo. 18_s._
IV.
A MANUAL OF ELEMENTARY GEOLOGY; or the ANCIENT CHANGES of the Earth and its Inhabitants, as illustrated by Geological Monuments. Fourth Edition. Thoroughly revised. With 531 Woodcuts and Plates. 8vo. 12_s._
MANUAL OF ELEMENTARY GEOLOGY.
I. ON THE DIFFERENT CLASSES OF ROCKS
Geology defined--Successive formation of the earth's crust--Classification of rocks according to their origin and age--Aqueous rocks--Their stratification and imbedded fossils--Volcanic rocks, with and without cones and craters--Plutonic rocks, and their relation to the volcanic--Metamorphic rocks and their probable origin--The term primitive, why erroneously applied to the crystalline formations--Leading division of the work.
Of what materials is the earth composed, and in what manner are these materials arranged? These are the first inquiries with which Geology is occupied, a science which derives its name from the Greek +ge+, ge, the earth, and +logos+, logos, a discourse. Previously to experience we might have imagined that investigations of this kind would relate exclusively to the mineral kingdom, and to the various rocks, soils, and metals, which occur upon the surface of the earth, or at various depths beneath it. But, in pursuing such researches, we soon find ourselves led on to consider the successive changes which have taken place in the former state of the earth's surface and interior, and the causes which have given rise to these changes; and, what is still more singular and unexpected, we soon become engaged in researches into the history of the animate creation, or of the various tribes of animals and plants which have, at different periods of the past, inhabited the globe.
All are aware that the solid parts of the earth consist of distinct substances, such as clay, chalk, sand, limestone, coal, slate, granite, and the like; but previously to observation it is commonly imagined that all these had remained from the first in the state in which we now see them,--that they were created in their present form, and in their present position. The geologist soon comes to a different conclusion, discovering proofs that the external parts of the earth were not all produced in the beginning of things, in the state in which we now behold them, nor in an instant of time. On the contrary, he can show that they have acquired their actual configuration and condition gradually, under a great variety of circumstances, and at successive periods, during each of which distinct races of living beings have flourished on the land and in the waters, the remains of these creatures still lying buried in the crust of the earth.
By the "earth's crust," is meant that small portion of the exterior of our planet which is accessible to human observation, or on which we are enabled to reason by observations made at or near the surface. These reasonings may extend to a depth of several miles, perhaps ten miles; and even then it may be said, that such a thickness is no more than 1/400 part of the distance from the surface to the centre. The remark is just; but although the dimensions of such a crust are, in truth, insignificant when compared to the entire globe, yet they are vast, and of magnificent extent in relation to man, and to the organic beings which people our globe. Referring to this standard of magnitude, the geologist may admire the ample limits of his domain, and admit, at the same time, that not only the exterior of the planet, but the entire earth, is but an atom in the midst of the countless worlds surveyed by the astronomer.
The materials of this crust are not thrown together confusedly; but distinct mineral masses, called rocks, are found to occupy definite spaces, and to exhibit a certain order of arrangement. The term rock is applied indifferently by geologists to all these substances, whether they be soft or stony, for clay and sand are included in the term, and some have even brought peat under this denomination. Our older writers endeavoured to avoid offering such violence to our language, by speaking of the component materials of the earth as consisting of rocks and soils. But there is often so insensible a passage from a soft and incoherent state to that of stone, that geologists of all countries have found it indispensable to have one technical term to include both, and in this sense we find roche applied in French, rocca in Italian, and felsart in German. The beginner, however, must constantly bear in mind, that the term rock by no means implies that a mineral mass is in an indurated or stony condition.
The most natural and convenient mode of classifying the various rocks which compose the earth's crust, is to refer, in the first place, to their origin, and in the second to their relative age. I shall therefore begin by endeavouring briefly to explain to the student how all rocks may be divided into four great classes by reference to their different origin, or, in other words, by reference to the different circumstances and causes by which they have been produced.
The first two divisions, which will at once be understood as natural, are the aqueous and volcanic, or the products of watery and those of igneous action at or near the surface.
Aqueous rocks.--The aqueous rocks, sometimes called the sedimentary, or fossiliferous, cover a larger part of the earth's surface than any others. These rocks are stratified, or divided into distinct layers, or strata. The term stratum means simply a bed, or any thing spread out or strewed over a given surface; and we infer that these strata have been generally spread out by the action of water, from what we daily see taking place near the mouths of rivers, or on the land during temporary inundations. For, whenever a running stream charged with mud or sand, has its velocity checked, as when it enters a lake or sea, or overflows a plain, the sediment, previously held in suspension by the motion of the water, sinks, by its own gravity, to the bottom. In this manner layers of mud and sand are thrown down one upon another.
If we drain a lake which has been fed by a small stream, we frequently find at the bottom a series of deposits, disposed with considerable regularity, one above the other; the uppermost, perhaps, may be a stratum of peat, next below a more dense and solid variety of the same material; still lower a bed of shell-marl, alternating with peat or sand, and then other beds of marl, divided by layers of clay. Now, if a second pit be sunk through the same continuous lacustrine formation, at some distance from the first, nearly the same series of beds is commonly met with, yet with slight variations; some, for example, of the layers of sand, clay, or marl, may be wanting, one or more of them having thinned out and given place to others, or sometimes one of the masses first examined is observed to increase in thickness to the exclusion of other beds.
The term "formation," which I have used in the above explanation, expresses in geology any assemblage of rocks which have some character in common, whether of origin, age, or composition. Thus we speak of stratified and unstratified, freshwater and marine, aqueous and volcanic, ancient and modern, metalliferous and non-metalliferous formations.
In the estuaries of large rivers, such as the Ganges and the Mississippi, we may observe, at low water, phenomena analogous to those of the drained lakes above mentioned, but on a grander scale, and extending over areas several hundred miles in length and breadth. When the periodical inundations subside, the river hollows out a channel to the depth of many yards through horizontal beds of clay and sand, the ends of which are seen exposed in perpendicular cliffs. These beds vary in colour, and are occasionally characterized by containing drift-wood or shells. The shells may belong to species peculiar to the river, but are sometimes those of marine testacea, washed into the mouth of the estuary during storms.
The annual floods of the Nile in Egypt are well known, and the fertile deposits of mud which they leave on the plains. This mud is stratified, the thin layer thrown down in one season differing slightly in colour from that of a previous year, and being separable from it, as has been observed in excavations at Cairo, and other places.[3-A]
When beds of sand, clay, and marl, containing shells and vegetable matter, are found arranged in a similar manner in the interior of the earth, we ascribe to them a similar origin; and the more we examine their characters in minute detail, the more exact do we find the resemblance. Thus, for example, at various heights and depths in the earth, and often far from seas, lakes, and rivers, we meet with layers of rounded pebbles composed of different rocks mingled together. They are like the shingle of a sea-beach, or pebbles formed in the beds of torrents and rivers, which are carried down into the ocean wherever these descend from high grounds bordering a coast. There the gravel is spread out by the waves and currents over a considerable space; but during seasons of drought the torrents and rivers are nearly dry, and have only power to convey fine sand or mud into the sea. Hence, alternate layers of gravel and fine sediment accumulate under water, and such alternations are found by geologists in the interior of every continent.[4-A]
If a stratified arrangement, and the rounded forms of pebbles, are alone sufficient to lead us to the conclusion that certain rocks originated under water, this opinion is farther confirmed by the distinct and independent evidence of fossils, so abundantly included in the earth's crust. By a fossil is meant any body, or the traces of the existence of any body, whether animal or vegetable, which has been buried in the earth by natural causes. Now the remains of animals, especially of aquatic species, are found almost everywhere imbedded in stratified rocks, and sometimes, in the case of limestone, they are in such abundance as to constitute the entire mass of the rock itself. Shells and corals are the most frequent, and with them are often associated the bones and teeth of fishes, fragments of wood, impressions of leaves, and other organic substances. Fossil shells, of forms such as now abound in the sea, are met with far inland, both near the surface, and at great depths below it. They occur at all heights above the level of the ocean, having been observed at elevations of 8000 feet in the Pyrenees, 10,000 in the Alps, 13,000 in the Andes, and above 16,000 feet in the Himalayas.[4-B]
These shells belong mostly to marine testacea, but in some places exclusively to forms characteristic of lakes and rivers. Hence it is concluded that some ancient strata were deposited at the bottom of the sea, and others in lakes and estuaries.
When geology was first cultivated, it was a general belief, that these marine shells and other fossils were the effects and proofs of the deluge of Noah; but all who have carefully investigated the phenomena have long rejected this doctrine. A transient flood might be supposed to leave behind it, here and there upon the surface, scattered heaps of mud, sand, and shingle, with shells confusedly intermixed; but the strata containing fossils are not superficial deposits, and do not simply cover the earth, but constitute the entire mass of mountains. Nor are the fossils mingled without reference to the original habits and natures of the creatures of which they are the memorials; those, for example, being found associated together which lived in deep or in shallow water, near the shore or far from it, in brackish or in salt water.
It has, moreover, been a favourite notion of some modern writers, who were aware that fossil bodies could not all be referred to the deluge, that they, and the strata in which they are entombed, might have been deposited in the bed of the ocean during the period which intervened between the creation of man and the deluge. They have imagined that the antediluvian bed of the ocean, after having been the receptacle of many stratified deposits, became converted, at the time of the flood, into the lands which we inhabit, and that the ancient continents were at the same time submerged, and became the bed of the present sea. This hypothesis, although preferable to the diluvial theory before alluded to, since it admits that all fossiliferous strata were successively thrown down from water, is yet wholly inadequate to explain the repeated revolutions which the earth has undergone, and the signs which the existing continents exhibit, in most regions, of having emerged from the ocean at an era far more remote than four thousand years from the present time. Ample proofs of these reiterated revolutions will be given in the sequel, and it will be seen that many distinct sets of sedimentary strata, each several hundreds or thousands of feet thick, are piled one upon the other in the earth's crust, each containing peculiar fossil animals and plants which are distinguishable with few exceptions from species now living. The mass of some of these strata consists almost entirely of corals, others are made up of shells, others of plants turned into coal, while some are without fossils. In one set of strata the species of fossils are marine; in another, lying immediately above or below, they as clearly prove that the deposit was formed in a brackish estuary or lake. When the student has more fully examined into these appearances, he will become convinced that the time required for the origin of the rocks composing the actual continents must have been far greater than that which is conceded by the theory above alluded to; and likewise that no one universal and sudden conversion of sea into land will account for geological appearances.
We have now pointed out one great class of rocks, which, however they may vary in mineral composition, colour, grain, or other characters, external and internal, may nevertheless be grouped together as having a common origin. They have all been formed under water, in the same manner as modern accumulations of sand, mud, shingle, banks of shells, reefs of coral, and the like, and are all characterized by stratification or fossils, or by both.
Volcanic rocks.--The division of rocks which we may next consider are the volcanic, or those which have been produced at or near the surface whether in ancient or modern times, not by water, but by the action of fire or subterranean heat. These rocks are for the most part unstratified, and are devoid of fossils. They are more partially distributed than aqueous formations, at least in respect to horizontal extension. Among those parts of Europe where they exhibit characters not to be mistaken, I may mention not only Sicily and the country round Naples, but Auvergne, Velay, and Vivarais, now the departments of Puy de Dome, Haute Loire, and Ardeche, towards the centre and south of France, in which are several hundred conical hills having the forms of modern volcanos, with craters more or less perfect on many of their summits. These cones are composed moreover of lava, sand, and ashes, similar to those of active volcanos. Streams of lava may sometimes be traced from the cones into the adjoining valleys, where they have choked up the ancient channels of rivers with solid rock, in the same manner as some modern flows of lava in Iceland have been known to do, the rivers either flowing beneath or cutting out a narrow passage on one side of the lava. Although none of these French volcanos have been in activity within the period of history or tradition, their forms are often very perfect. Some, however, have been compared to the mere skeletons of volcanos, the rains and torrents having washed their sides, and removed all the loose sand and scoriae, leaving only the harder and more solid materials. By this erosion, and by earthquakes, their internal structure has occasionally been laid open to view, in fissures and ravines; and we then behold not only many successive beds and masses of porous lava, sand, and scoriae, but also perpendicular walls, or dikes, as they are called, of volcanic rock, which have burst through the other materials. Such dikes are also observed in the structure of Vesuvius, Etna, and other active volcanos. They have been formed by the pouring of melted matter, whether from above or below, into open fissures, and they commonly traverse deposits of volcanic tuff, a substance produced by the showering down from the air, or incumbent waters, of sand and cinders, first shot up from the interior of the earth by the explosions of volcanic gases.
Besides the parts of France above alluded to, there are other countries, as the north of Spain, the south of Sicily, the Tuscan territory of Italy, the lower Rhenish provinces, and Hungary, where spent volcanos may be seen, still preserving in many cases a conical form, and having craters and often lava-streams connected with them.
There are also other rocks in England, Scotland, Ireland, and almost every country in Europe, which we infer to be of igneous origin, although they do not form hills with cones and craters. Thus, for example, we feel assured that the rock of Staffa, and that of the Giant's Causeway, called basalt, is volcanic, because it agrees in its columnar structure and mineral composition with streams of lava which we know to have flowed from the craters of volcanos. We find also similar basaltic and other igneous rocks associated with beds of tuff in various parts of the British Isles, and forming dikes, such as have been spoken of; and some of the strata through which these dikes cut are occasionally altered at the point of contact, as if they had been exposed to the intense heat of melted matter.
The absence of cones and craters, and long narrow streams of superficial lava, in England and many other countries, is principally to be attributed to the eruptions having been submarine, just as a considerable proportion of volcanos in our own times burst out beneath the sea. But this question must be enlarged upon more fully in the chapters on Igneous Rocks, in which it will also be shown, that as different sedimentary formations, containing each their characteristic fossils, have been deposited at successive periods, so also volcanic sand and scoriae have been thrown out, and lavas have flowed over the land or bed of the sea, at many different epochs, or have been injected into fissures; so that the igneous as well as the aqueous rocks may be classed as a chronological series of monuments, throwing light on a succession of events in the history of the earth.
Plutonic rocks (Granite, &c.).--We have now pointed out the existence of two distinct orders of mineral masses, the aqueous and the volcanic: but if we examine a large portion of a continent, especially if it contain within it a lofty mountain range, we rarely fail to discover two other classes of rocks, very distinct from either of those above alluded to, and which we can neither assimilate to deposits such as are now accumulated in lakes or seas, nor to those generated by ordinary volcanic action. The members of both these divisions of rocks agree in being highly crystalline and destitute of organic remains. The rocks of one division have been called plutonic, comprehending all the granites and certain porphyries, which are nearly allied in some of their characters to volcanic formations. The members of the other class are stratified and often slaty, and have been called by some the crystalline schists, in which group are included gneiss, micaceous-schist (or mica-slate), hornblende-schist, statuary marble, the finer kinds of roofing slate, and other rocks afterwards to be described.
As it is admitted that nothing strictly analogous to these crystalline productions can now be seen in the progress of formation on the earth's surface, it will naturally be asked, on what data we can find a place for them in a system of classification founded on the origin of rocks. I cannot, in reply to this question, pretend to give the student, in a few words, an intelligible account of the long chain of facts and reasonings by which geologists have been led to infer the analogy of the rocks in question to others now in progress at the surface. The result, however, may be briefly stated. All the various kinds of granite, which constitute the plutonic family, are supposed to be of igneous origin, but to have been formed under great pressure, at considerable depths in the earth, or sometimes, perhaps, under a certain weight of incumbent water. Like the lava of volcanos, they have been melted, and have afterwards cooled and crystallized, but with extreme slowness, and under conditions very different from those of bodies cooling in the open air. Hence they differ from the volcanic rocks, not only by their more crystalline texture, but also by the absence of tuffs and breccias, which are the products of eruptions at the earth's surface, or beneath seas of inconsiderable depth. They differ also by the absence of pores or cellular cavities, to which the expansion of the entangled gases gives rise in ordinary lava.
Although granite has often pierced through other strata, it has rarely, if ever, been observed to rest upon them, as if it had overflowed. But as this is continually the case with the volcanic rocks, they have been styled, from this peculiarity, "overlying" by Dr. MacCulloch; and Mr. Necker has proposed the term "underlying" for the granites, to designate the opposite mode in which they almost invariably present themselves.
Metamorphic, or stratified crystalline rocks.--The fourth and last great division of rocks are the crystalline strata and slates, or schists, called gneiss, mica-schist, clay-slate, chlorite-schist, marble, and the like, the origin of which is more doubtful than that of the other three classes. They contain no pebbles, or sand, or scoriae, or angular pieces of imbedded stone, and no traces of organic bodies, and they are often as crystalline as granite, yet are divided into beds, corresponding in form and arrangement to those of sedimentary formations, and are therefore said to be stratified. The beds sometimes consist of an alternation of substances varying in colour, composition, and thickness, precisely as we see in stratified fossiliferous deposits. According to the Huttonian theory, which I adopt as most probable, and which will be afterwards more fully explained, the materials of these strata were originally deposited from water in the usual form of sediment, but they were subsequently so altered by subterranean heat, as to assume a new texture. It is demonstrable, in some cases at least, that such a complete conversion has actually taken place, fossiliferous strata having exchanged an earthy for a highly crystalline texture for a distance of a quarter of a mile from their contact with granite. In some cases, dark limestones, replete with shells and corals, have been turned into white statuary marble, and hard clays into slates called mica-schist and hornblende-schist, all signs of organic bodies having been obliterated.
Although we are in a great degree ignorant of the precise nature of the influence exerted in these cases, yet it evidently bears some analogy to that which volcanic heat and gases are known to produce; and the action may be conveniently called plutonic, because it appears to have been developed in those regions where plutonic rocks are generated, and under similar circumstances of pressure and depth in the earth. Whether hot water or steam permeating stratified masses, or electricity, or any other causes have co-operated to produce the crystalline texture, may be matter of speculation, but it is clear that the plutonic influence has sometimes pervaded entire mountain masses of strata.
In accordance with the hypothesis above alluded to, I proposed in the first edition of the Principles of Geology (1833), the term "Metamorphic" for the altered strata, a term derived from +meta+, meta, trans, and +morphe+, morphe, forma.
Hence there are four great classes of rocks considered in reference to their origin,--the aqueous, the volcanic, the plutonic, and the metamorphic. In the course of this work it will be shown, that portions of each of these four distinct classes have originated at many successive periods. They have all been produced contemporaneously, and may even now be in the progress of formation. It is not true, as was formerly supposed, that all granites, together with the crystalline or metamorphic strata, were first formed, and therefore entitled to be called "primitive," and that the aqueous and volcanic rocks were afterwards superimposed, and should, therefore, rank as secondary in the order of time. This idea was adopted in the infancy of the science, when all formations, whether stratified or unstratified, earthy or crystalline, with or without fossils, were alike regarded as of aqueous origin. At that period it was naturally argued, that the foundation must be older than the superstructure; but it was afterwards discovered, that this opinion was by no means in every instance a legitimate deduction from facts; for the inferior parts of the earth's crust have often been modified, and even entirely changed, by the influence of volcanic and other subterranean causes, while superimposed formations have not been in the slightest degree altered. In other words, the destroying and renovating processes have given birth to new rocks below, while those above, whether crystalline or fossiliferous, have remained in their ancient condition. Even in cities, such as Venice and Amsterdam, it cannot be laid down as universally true, that the upper parts of each edifice, whether of brick or marble, are more modern than the foundations on which they rest, for these often consist of wooden piles, which may have rotted and been replaced one after the other, without the least injury to the buildings above; meanwhile, these may have required scarcely any repair, and may have been constantly inhabited. So it is with the habitable surface of our globe, in its relation to large masses of rock immediately below: it may continue the same for ages, while subjacent materials, at a great depth, are passing from a solid to a fluid state, and then reconsolidating, so as to acquire a new texture.
As all the crystalline rocks may, in some respects, be viewed as belonging to one great family, whether they be stratified or unstratified, plutonic or metamorphic, it will often be convenient to speak of them by one common name. It being now ascertained, as above stated, that they are of very different ages, sometimes newer than the strata called secondary, the term primary, which was formerly used for the whole, must be abandoned, as it would imply a manifest contradiction. It is indispensable, therefore, to find a new name, one which must not be of chronological import, and must express, on the one hand, some peculiarity equally attributable to granite and gneiss (to the plutonic as well as the altered rocks), and, on the other, must have reference to characters in which those rocks differ, both from the volcanic and from the unaltered sedimentary strata. I proposed in the Principles of Geology (first edition, vol. iii.), the term "hypogene" for this purpose, derived from +hypo+, under, and +ginomai+, to be, or to be born; a word implying the theory that granite, gneiss, and the other crystalline formations are alike nether-formed rocks, or rocks which have not assumed their present form and structure at the surface. This occurs in the lowest place in the order of superposition. Even in regions such as the Alps, where some masses of granite and gneiss can be shown to be of comparatively modern date, belonging, for example, to the period hereafter to be described as tertiary, they are still underlying rocks. They never repose on the volcanic or trappean formations, nor on strata containing organic remains. They are hypogene, as "being under" all the rest.
From what has now been said, the reader will understand that each of the four great classes of rocks may be studied under two distinct points of view; first, they may be studied simply as mineral masses deriving their origin from particular causes, and having a certain composition, form, and position in the earth's crust, or other characters both positive and negative, such as the presence or absence of organic remains. In the second place, the rocks of each class may be viewed as a grand chronological series of monuments, attesting a succession of events in the former history of the globe and its living inhabitants.
I shall accordingly proceed to treat of each family of rocks; first, in reference to those characters which are not chronological, and then in particular relation to the several periods when they were formed.
FOOTNOTES:
[3-A] See Principles of Geology, by the Author, Index, "Nile," "Rivers," &c.
[4-A] See p. 18.
[4-B] See Geograph. Journ. vol. iv. p. 64.
Chapter II
AQUEOUS ROCKS--THEIR COMPOSITION AND FORMS OF STRATIFICATION.
Mineral composition of strata--Arenaceous rocks--Argillaceous--Calcareous--Gypsum--Forms of stratification--Original horizontality--Thinning out--Diagonal arrangement--Ripple mark.
In pursuance of the arrangement explained in the last chapter, we shall begin by examining the aqueous or sedimentary rocks, which are for the most part distinctly stratified, and contain fossils. We may first study them with reference to their mineral composition, external appearance, position, mode of origin, organic contents, and other characters which belong to them as aqueous formations, independently of their age, and we may afterwards consider them chronologically or with reference to the successive geological periods when they originated.
I have already given an outline of the data which led to the belief that the stratified and fossiliferous rocks were originally deposited under water; but, before entering into a more detailed investigation, it will be desirable to say something of the ordinary materials of which such strata are composed. These may be said to belong principally to three divisions, the arenaceous, the argillaceous, and the calcareous, which are formed respectively of sand, clay, and carbonate of lime. Of these, the arenaceous, or sandy masses, are chiefly made up of siliceous or flinty grains; the argillaceous, or clayey, of a mixture of siliceous matter, with a certain proportion, about a fourth in weight, of aluminous earth; and, lastly, the calcareous rocks or limestones consist of carbonic acid and lime.
Arenaceous or siliceous rocks.--To speak first of the sandy division: beds of loose sand are frequently met with, of which the grains consist entirely of silex, which term comprehends all purely siliceous minerals, as quartz and common flint. Quartz is silex in its purest form; flint usually contains some admixture of alumine and oxide of iron. The siliceous grains in sand are usually rounded, as if by the action of running water. Sandstone is an aggregate of such grains, which often cohere together without any visible cement, but more commonly are bound together by a slight quantity of siliceous or calcareous matter, or by iron or clay.
Pure siliceous rocks may be known by not effervescing when a drop of nitric, sulphuric, or other acid is applied to them, or by the grains not being readily scratched or broken by ordinary pressure. In nature there is every intermediate gradation, from perfectly loose sand, to the hardest sandstone. In micaceous sandstones mica is very abundant; and the thin silvery plates into which that mineral divides, are often arranged in layers parallel to the planes of stratification, giving a slaty or laminated texture to the rock.
When sandstone is coarse-grained, it is usually called grit. If the grains are rounded, and large enough to be called pebbles, it becomes a conglomerate, or pudding-stone, which may consist of pieces of one or of many different kinds of rock. A conglomerate, therefore, is simply gravel bound together by a cement.
Argillaceous rocks.--Clay, strictly speaking, is a mixture of silex or flint with a large proportion, usually about one fourth, of alumine, or argil; but, in common language, any earth which possesses sufficient ductility, when kneaded up with water, to be fashioned like paste by the hand, or by the potter's lathe, is called a clay; and such clays vary greatly in their composition, and are, in general, nothing more than mud derived from the decomposition or wearing down of various rocks. The purest clay found in nature is porcelain clay, or kaolin, which results from the decomposition of a rock composed of felspar and quartz, and it is almost always mixed with quartz.[11-A] Shale has also the property, like clay, of becoming plastic in water: it is a more solid form of clay, or argillaceous matter, condensed by pressure. It usually divides into irregular laminae.
One general character of all argillaceous rocks is to give out a peculiar, earthy odour when breathed upon, which is a test of the presence of alumine, although it does not belong to pure alumine, but, apparently, to the combination of that substance with oxide of iron.[11-B]
Calcareous rocks.--This division comprehends those rocks which, like chalk, are composed chiefly of lime and carbonic acid. Shells and corals are also formed of the same elements, with the addition of animal matter. To obtain pure lime it is necessary to calcine these calcareous substances, that is to say, to expose them to heat of sufficient intensity to drive off the carbonic acid, and other volatile matter, without vitrifying or melting the lime itself. White chalk is often pure carbonate of lime; and this rock, although usually in a soft and earthy state, is sometimes sufficiently solid to be used for building, and even passes into a compact stone, or a stone of which the separate parts are so minute as not to be distinguishable from each other by the naked eye.
Many limestones are made up entirely of minute fragments of shells and coral, or of calcareous sand cemented together. These last might be called "calcareous sandstones;" but that term is more properly applied to a rock in which the grains are partly calcareous and partly siliceous, or to quartzose sandstones, having a cement of carbonate of lime.
The variety of limestone called "oolite" is composed of numerous small egg-like grains, resembling the roe of a fish, each of which has usually a small fragment of sand as a nucleus, around which concentric layers of calcareous matter have accumulated.
Any limestone which is sufficiently hard to take a fine polish is called marble. Many of these are fossiliferous; but statuary marble, which is also called saccharine limestone, as having a texture resembling that of loaf-sugar, is devoid of fossils, and is in many cases a member of the metamorphic series.
Siliceous limestone is an intimate mixture of carbonate of lime and flint, and is harder in proportion as the flinty matter predominates.
The presence of carbonate of lime in a rock may be ascertained by applying to the surface a small drop of diluted sulphuric, nitric, or muriatic acids, or strong vinegar; for the lime, having a greater chemical affinity for any one of these acids than for the carbonic, unites immediately with them to form new compounds, thereby becoming a sulphate, nitrate, or muriate of lime. The carbonic acid, when thus liberated from its union with the lime, escapes in a gaseous form, and froths up or effervesces as it makes its way in small bubbles through the drop of liquid. This effervescence is brisk or feeble in proportion as the limestone is pure or impure, or, in other words, according to the quantity of foreign matter mixed with the carbonate of lime. Without the aid of this test, the most experienced eye cannot always detect the presence of carbonate of lime in rocks.
The above-mentioned three classes of rocks, the siliceous, argillaceous, and calcareous, pass continually into each other, and rarely occur in a perfectly separate and pure form. Thus it is an exception to the general rule to meet with a limestone as pure as ordinary white chalk, or with clay as aluminous as that used in Cornwall for porcelain, or with sand so entirely composed of siliceous grains as the white sand of Alum Bay in the Isle of Wight, or sandstone so pure as the grit of Fontainebleau, used for pavement in France. More commonly we find sand and clay, or clay and marl, intermixed in the same mass. When the sand and clay are each in considerable quantity, the mixture is called loam. If there is much calcareous matter in clay it is called marl; but this term has unfortunately been used so vaguely, as often to be very ambiguous. It has been applied to substances in which there is no lime; as, to that red loam usually called red marl in certain parts of England. Agriculturists were in the habit of calling any soil a marl, which, like true marl, fell to pieces readily on exposure to the air. Hence arose the confusion of using this name for soils which, consisting of loam, were easily worked by the plough, though devoid of lime.
Marl slate bears the same relation to marl which shale bears to clay, being a calcareous shale. It is very abundant in some countries, as in the Swiss Alps. Argillaceous or marly limestone is also of common occurrence.
There are few other kinds of rock which enter so largely into the composition of sedimentary strata as to make it necessary to dwell here on their characters. I may, however, mention two others,--magnesian limestone or dolomite, and gypsum. Magnesian limestone is composed of carbonate of lime and carbonate of magnesia; the proportion of the latter amounting in some cases to nearly one half. It effervesces much more slowly and feebly with acids than common limestone. In England this rock is generally of a yellowish colour; but it varies greatly in mineralogical character, passing from an earthy state to a white compact stone of great hardness. Dolomite, so common in many parts of Germany and France, is also a variety of magnesian limestone, usually of a granular texture.
Gypsum.--Gypsum is a rock composed of sulphuric acid, lime, and water. It is usually a soft whitish-yellow rock, with a texture resembling that of loaf-sugar, but sometimes it is entirely composed of lenticular crystals. It is insoluble in acids, and does not effervesce like chalk and dolomite, because it does not contain carbonic acid gas, or fixed air, the lime being already combined with sulphuric acid, for which it has a stronger affinity than for any other. Anhydrous gypsum is a rare variety, into which water does not enter as a component part. Gypseous marl is a mixture of gypsum and marl. Alabaster is a granular and compact variety of gypsum found in masses large enough to be used in sculpture and architecture. It is sometimes a pure snow-white substance, as that of Volterra in Tuscany, well known as being carved for works of art in Florence and Leghorn. It is a softer stone than marble, and more easily wrought.
Forms of stratification.--A series of strata sometimes consists of one of the above rocks, sometimes of two or more in alternating beds. Thus, in the coal districts of England, for example, we often pass through several beds of sandstone, some of finer, others of coarser grain, some white, others of a dark colour, and below these, layers of shale and sandstone or beds of shale, divisible into leaf-like laminae, and containing beautiful impressions of plants. Then again we meet with beds of pure and impure coal, alternating with shales and sandstones, and underneath the whole, perhaps, are calcareous strata, or beds of limestone, filled with corals and marine shells, each bed distinguishable from another by certain fossils, or by the abundance of particular species of shells or zoophytes.
This alternation of different kinds of rock produces the most distinct stratification; and we often find beds of limestone and marl, conglomerate and sandstone, sand and clay, recurring again and again, in nearly regular order, throughout a series of many hundred strata. The causes which may produce these phenomena are various, and have been fully discussed in my treatise on the modern changes of the earth's surface.[14-A] It is there seen that rivers flowing into lakes and seas are charged with sediment, varying in quantity, composition, colour, and grain according to the seasons; the waters are sometimes flooded and rapid, at other periods low and feeble; different tributaries, also, draining peculiar countries and soils, and therefore charged with peculiar sediment, are swollen at distinct periods. It was also shown that the waves of the sea and currents undermine the cliffs during wintry storms, and sweep away the materials into the deep, after which a season of tranquillity succeeds, when nothing but the finest mud is spread by the movements of the ocean over the same submarine area.
It is not the object of the present work to give a description of these operations, repeated as they are, year after year, and century after century; but I may suggest an explanation of the manner in which some micaceous sandstones have originated, those in which we see innumerable thin layers of mica dividing layers of fine quartzose sand. I observed the same arrangement of materials in recent mud deposited in the estuary of La Roche St. Bernard in Brittany, at the mouth of the Loire. The surrounding rocks are of gneiss, which, by its waste, supplies the mud: when this dries at low water, it is found to consist of brown laminated clay, divided by thin seams of mica. The separation of the mica in this case, or in that of micaceous sandstones, may be thus understood. If we take a handful of quartzose sand, mixed with mica, and throw it into a clear running stream, we see the materials immediately sorted by the water, the grains of quartz falling almost directly to the bottom, while the plates of mica take a much longer time to reach the bottom, and are carried farther down the stream. At the first instant the water is turbid, but immediately after the flat surfaces of the plates of mica are seen alone reflecting a silvery light, as they descend slowly, to form a distinct micaceous lamina. The mica is the heavier mineral of the two; but it remains longer suspended, owing to its great extent of surface. It is easy, therefore, to perceive that where such mud is acted upon by a river or tidal current, the thin plates of mica will be carried farther, and not deposited in the same places as the grains of quartz; and since the force and velocity of the stream varies from time to time, layers of mica or of sand will be thrown down successively on the same area.
Original horizontality.--It has generally been said that the upper and under surfaces of strata, or the planes of stratification, as they are termed, are parallel. Although this is not strictly true, they make an approach to parallelism, for the same reason that sediment is usually deposited at first in nearly horizontal layers. The reason of this arrangement can by no means be attributed to an original evenness or horizontality in the bed of the sea; for it is ascertained that in those places where no matter has been recently deposited, the bottom of the ocean is often as uneven as that of the dry land, having in like manner its hills, valleys, and ravines. Yet if the sea should sink, or the water be removed near the mouth of a large river where a delta has been forming, we should see extensive plains of mud and sand laid dry, which, to the eye, would appear perfectly level, although, in reality, they would slope gently from the land towards the sea.
This tendency in newly-formed strata to assume a horizontal position arises principally from the motion of the water, which forces along particles of sand or mud at the bottom, and causes them to settle in hollows or depressions, where they are less exposed to the force of a current than when they are resting on elevated points. The velocity of the current and the motion of the superficial waves diminish from the surface downwards, and are least in those depressions where the water is deepest.
A good illustration of the principle here alluded to may be sometimes seen in the neighbourhood of a volcano, when a section, whether natural or artificial, has laid open to view a succession of various-coloured layers of sand and ashes, which have fallen in showers upon uneven ground. Thus let A B (fig. 1.) be two ridges, with an intervening valley. These original inequalities of the surface have been gradually effaced by beds of sand and ashes c, d, e, the surface at e being quite level. It will be seen that although the materials of the first layers have accommodated themselves in a great degree to the shape of the ground A B, yet each bed is thickest at the bottom. At first a great many particles would be carried by their own gravity down the steep sides of A and B, and others would afterwards be blown by the wind as they fell off the ridges, and would settle in the hollow, which would thus become more and more effaced as the strata accumulated from c to e. This levelling operation may perhaps be rendered more clear to the student by supposing a number of parallel trenches to be dug in a plain of moving sand, like the African desert, in which case the wind would soon cause all signs of these trenches to disappear, and the surface would be as uniform as before. Now, water in motion can exert this levelling power on similar materials more easily than air, for almost all stones lose in water more than a third of the weight which they have in air, the specific gravity of rocks being in general as 2-1/2 when compared to that of water, which is estimated at 1. But the buoyancy of sand or mud would be still greater in the sea, as the density of salt water exceeds that of fresh.
Yet, however uniform and horizontal may be the surface of new deposits in general, there are still many disturbing causes, such as eddies in the water, and currents moving first in one and then in another direction, which frequently cause irregularities. We may sometimes follow a bed of limestone, shale, or sandstone, for a distance of many hundred yards continuously; but we generally find at length that each individual stratum thins out, and allows the beds which were previously above and below it to meet. If the materials are coarse, as in grits and conglomerates, the same beds can rarely be traced many yards without varying in size, and often coming to an end abruptly. (See fig. 2.)
Diagonal or Cross Stratification.--There is also another phenomenon of frequent occurrence. We find a series of larger strata, each of which is composed of a number of minor layers placed obliquely to the general planes of stratification. To this diagonal arrangement the name of "false or cross stratification" has been given. Thus in the annexed section (fig. 3.) we see seven or eight large beds of loose sand, yellow and brown, and the lines a, b, c, mark some of the principal planes of stratification, which are nearly horizontal. But the greater part of the subordinate laminae do not conform to these planes, but have often a steep slope, the inclination being sometimes towards opposite points of the compass. When the sand is loose and incoherent, as in the case here represented, the deviation from parallelism of the slanting laminae cannot possibly be accounted for by any re-arrangement of the particles acquired during the consolidation of the rock. In what manner then can such irregularities be due to original deposition? We must suppose that at the bottom of the sea, as well as in the beds of rivers, the motions of waves, currents, and eddies often cause mud, sand, and gravel to be thrown down in heaps on particular spots, instead of being spread out uniformly over a wide area. Sometimes, when banks are thus formed, currents may cut passages through them, just as a river forms its bed. Suppose the bank A (fig. 4.) to be thus formed with a steep sloping side, and the water being in a tranquil state, the layer of sediment No. 1. is thrown down upon it, conforming nearly to its surface. Afterwards the other layers, 2, 3, 4, may be deposited in succession, so that the bank B C D is formed. If the current then increases in velocity, it may cut away the upper portion of this mass down to the dotted line e (fig. 4.), and deposit the materials thus removed farther on, so as to form the layers 5, 6, 7, 8. We have now the bank B C D E (fig. 5.), of which the surface is almost level, and on which the nearly horizontal layers, 9, 10, 11, may then accumulate. It was shown in fig. 3. that the diagonal layers of successive strata may sometimes have an opposite slope. This is well seen in some cliffs of loose sand on the Suffolk coast. A portion of one of these is represented in fig. 6., where the layers, of which there are about six in the thickness of an inch, are composed of quartzose grains. This arrangement may have been due to the altered direction of the tides and currents in the same place.
The description above given of the slanting position of the minor layers constituting a single stratum is in certain cases applicable on a much grander scale to masses several hundred feet thick, and many miles in extent. A fine example may be seen at the base of the Maritime Alps near Nice. The mountains here terminate abruptly in the sea, so that a depth of many hundred fathoms is often found within a stone's throw of the beach, and sometimes a depth of 3000 feet within half a mile. But at certain points, strata of sand, marl, or conglomerate, intervene between the shore and the mountains, as in the annexed fig. 7., where a vast succession of slanting beds of gravel and sand may be traced from the sea to Monte Calvo, a distance of no less than 9 miles in a straight line. The dip of these beds is remarkably uniform, being always southward or towards the Mediterranean, at an angle of about 25 deg.. They are exposed to view in nearly vertical precipices, varying from 200 to 600 feet in height, which bound the valley through which the river Magnan flows. Although in a general view, the strata appear to be parallel and uniform, they are nevertheless found, when examined closely, to be wedge-shaped, and to thin out when followed for a few hundred feet or yards, so that we may suppose them to have been thrown down originally upon the side of a steep bank, where a river or alpine torrent discharged itself into a deep and tranquil sea, and formed a delta, which advanced gradually from the base of Monte Calvo to a distance of 9 miles from the original shore. If subsequently this part of the Alps and bed of the sea were raised 700 feet, the coast would acquire its present configuration, the delta would emerge, and a deep channel might then be cut through it by a river.
It is well known that the torrents and streams, which now descend from the alpine declivities to the shore, bring down annually, when the snow melts, vast quantities of shingle and sand, and then, as they subside, fine mud, while in summer they are nearly or entirely dry; so that it may be safely assumed, that deposits like those of the valley of the Magnan, consisting of coarse gravel alternating with fine sediment, are still in progress at many points, as, for instance, at the mouth of the Var. They must advance upon the Mediterranean in the form of great shoals terminating in a steep talus; such being the original mode of accumulation of all coarse materials conveyed into deep water, especially where they are composed in great part of pebbles, which cannot be transported to indefinite distances by currents of moderate velocity. By inattention to facts and inferences of this kind, a very exaggerated estimate has sometimes been made of the supposed depth of the ancient ocean. There can be no doubt, for example, that the strata a, fig. 7., or those nearest to Monte Calvo, are older than those indicated by b, and these again were formed before c; but the vertical depth of gravel and sand in any one place cannot be proved to amount even to 1000 feet, although it may perhaps be much greater, yet probably never exceeding at any point 3000 or 4000 feet. But were we to assume that all the strata were once horizontal, and that their present dip or inclination was due to subsequent movements, we should then be forced to conclude, that a sea 9 miles deep had been filled up with alternate layers of mud and pebbles thrown down one upon another.
In the locality now under consideration, situated a few miles to the west of Nice, there are many geological data, the details of which cannot be given in this place, all leading to the opinion, that when the deposit of the Magnan was formed, the shape and outline of the alpine declivities and the shore greatly resembled what we now behold at many points in the neighbourhood. That the beds, a, b, c, d, are of comparatively modern date is proved by this fact, that in seams of loamy marl intervening between the pebbly beds are fossil shells, half of which belong to species now living in the Mediterranean.
Ripple mark.--The ripple mark, so common on the surface of sandstones of all ages (see fig. 8.), and which is so often seen on the sea-shore at low tide, seems to originate in the drifting of materials along the bottom of the water, in a manner very similar to that which may explain the inclined layers above described. This ripple is not entirely confined to the beach between high and low water mark, but is also produced on sands which are constantly covered by water. Similar undulating ridges and furrows may also be sometimes seen on the surface of drift snow and blown sand. The following is the manner in which I once observed the motion of the air to produce this effect on a large extent of level beach, exposed at low tide near Calais. Clouds of fine white sand were blown from the neighbouring dunes, so as to cover the shore, and whiten a dark level surface of sandy mud, and this fresh covering of sand was beautifully rippled. On levelling all the small ridges and furrows of this ripple over an area of several yards square, I saw them perfectly restored in about ten minutes, the general direction of the ridges being always at right angles to that of the wind. The restoration began by the appearance here and there of small detached heaps of sand, which soon lengthened and joined together, so as to form long sinuous ridges with intervening furrows. Each ridge had one side slightly inclined, and the other steep; the lee-side being always steep, as b, c,--d, e; the windward-side a gentle slope, as a, b,--c, d, fig. 9. When a gust of wind blew with sufficient force to drive along a cloud of sand, all the ridges were seen to be in motion at once, each encroaching on the furrow before it, and, in the course of a few minutes, filling the place which the furrows had occupied. The mode of advance was by the continual drifting of grains of sand up the slopes a b and c d, many of which grains, when they arrived at b and d, fell over the scarps b c and d e, and were under shelter from the wind; so that they remained stationary, resting, according to their shape and momentum, on different parts of the descent, and a few only rolling to the bottom. In this manner each ridge was distinctly seen to move slowly on as often as the force of the wind augmented. Occasionally part of a ridge, advancing more rapidly than the rest, overtook the ridge immediately before it, and became confounded with it, thus causing those bifurcations and branches which are so common, and two of which are seen in the slab, fig. 8. We may observe this configuration in sandstones of all ages, and in them also, as now on the sea-coast, we may often detect two systems of ripples interfering with each other; one more ancient and half effaced, and a newer one, in which the grooves and ridges are more distinct, and in a different direction. This crossing of two sets of ripples arises from a change of wind, and the new direction in which the waves are thrown on the shore.
The ripple mark is usually an indication of a sea-beach, or of water from 6 to 10 feet deep, for the agitation caused by waves even during storms extends to a very slight depth. To this rule, however, there are some exceptions, and recent ripple marks have been observed at the depth of 60 or 70 feet. It has also been ascertained that currents or large bodies of water in motion may disturb mud and sand at the depth of 300 or even 450 feet.[21-A]
FOOTNOTES:
[11-A] The kaolin of China consists of 71.15 parts of silex, 15.86 of alumine, 1.92 of lime, and 6.73 of water (W. Phillips, Mineralogy, p. 33.); but other porcelain clays differ materially, that of Cornwall being composed, according to Boase of nearly equal parts of silica and alumine, with 1 per cent. of magnesia. (Phil. Mag. vol. x. 1837.)
[11-B] See W. Phillips's Mineralogy, "Alumine."
[14-A] Consult Index to Principles of Geology, "Stratification," "Currents," "Deltas," "Water," &c.
[21-A] Siau. Edin. New Phil. Journ. vol. xxxi.; and Darwin, Volc. Islands, p. 134.
III. ARRANGEMENT OF FOSSILS IN STRATA--FRESHWATER AND MARINE
Successive deposition indicated by fossils--Limestones formed of corals and shells Proofs of gradual increase of strata derived from fossils--Serpula attached to spatangus--Wood bored by Teredina--Tripoli and semi-opal formed of infusoria--Chalk derived principally from organic bodies--Distinction of freshwater from marine formations--Genera of freshwater and land shells--Rules for recognizing marine testacea--Gyrogonite and chara--Freshwater fishes--Alternation of marine and freshwater deposits--Lym-Fiord.
Having in the last chapter considered the forms of stratification so far as they are determined by the arrangement of inorganic matter, we may now turn our attention to the manner in which organic remains are distributed through stratified deposits. We should often be unable to detect any signs of stratification or of successive deposition, if particular kinds of fossils did not occur here and there at certain depths in the mass. At one level, for example, univalve shells of some one or more species predominate; at another, bivalve shells; and at a third, corals; while in some formations we find layers of vegetable matter, commonly derived from land plants, separating strata.
It may appear inconceivable to a beginner how mountains, several thousand feet thick, can have become filled with fossils from top to bottom; but the difficulty is removed, when he reflects on the origin of stratification, as explained in the last chapter, and allows sufficient time for the accumulation of sediment. He must never lose sight of the fact that, during the process of deposition, each separate layer was once the uppermost, and covered immediately by the water in which aquatic animals lived. Each stratum in fact, however far it may now lie beneath the surface, was once in the state of shingle, or loose sand or soft mud at the bottom of the sea, in which shells and other bodies easily became enveloped.
By attending to the nature of these remains, we are often enabled to determine whether the deposition was slow or rapid, whether it took place in a deep or shallow sea, near the shore or far from land, and whether the water was salt, brackish, or fresh. Some limestones consist almost exclusively of corals, and in many cases it is evident that the present position of each fossil zoophyte has been determined by the manner in which it grew originally. The axis of the coral, for example, if its natural growth is erect, still remains at right angles to the plane of stratification. If the stratum be now horizontal, the round spherical heads of certain species continue uppermost, and their points of attachment are directed downwards. This arrangement is sometimes repeated throughout a great succession of strata. From what we know of the growth of similar zoophytes in modern reefs, we infer that the rate of increase was extremely slow, and some of the fossils must have flourished for ages like forest trees, before they attained so large a size. During these ages, the water remained clear and transparent, for such corals cannot live in turbid water.
In like manner, when we see thousands of full-grown shells dispersed every where throughout a long series of strata, we cannot doubt that time was required for the multiplication of successive generations; and the evidence of slow accumulation is rendered more striking from the proofs, so often discovered, of fossil bodies having lain for a time on the floor of the ocean after death before they were imbedded in sediment. Nothing, for example, is more common than to see fossil oysters in clay, with serpulae, or barnacles (acorn-shells), or corals, and other creatures, attached to the inside of the valves, so that the mollusk was certainly not buried in argillaceous mud the moment it died. There must have been an interval during which it was still surrounded with clear water, when the testacea, now adhering to it, grew from an embryo state to full maturity. Attached shells which are merely external, like some of the serpulae (a) in the annexed figure (fig. 10.), may often have grown upon an oyster or other shell while the animal within was still living; but if they are found on the inside, it could only happen after the death of the inhabitant of the shell which affords the support. Thus, in fig. 10., it will be seen that two serpulae have grown on the interior, one of them exactly on the place where the adductor muscle of the Gryphaea (a kind of oyster) was fixed.
Some fossil shells, even if simply attached to the outside of others, bear full testimony to the conclusion above alluded to, namely, that an interval elapsed between the death of the creature to whose shell they adhere, and the burial of the same in mud or sand. The sea-urchins or Echini, so abundant in white chalk, afford a good illustration. It is well known that these animals, when living, are invariably covered with numerous spines, which serve as organs of motion, and are supported by rows of tubercles, which last are only seen after the death of the sea-urchin, when the spines have dropped off. In fig. 12. a living species of Spatangus, common on our coast, is represented with one half of its shell stripped of the spines. In fig. 11. a fossil of the same genus from the white chalk of England shows the naked surface which the individuals of this family exhibit when denuded of their bristles. The full-grown Serpula, therefore, which now adheres externally, could not have begun to grow till the Spatangus had died, and the spines were detached.
Now the series of events here attested by a single fossil may be carried a step farther. Thus, for example, we often meet with a sea-urchin in the chalk (see fig. 13.), which has fixed to it the lower valve of a Crania, a genus of bivalve mollusca. The upper valve (b, fig. 13.) is almost invariably wanting, though occasionally found in a perfect state of preservation in white chalk at some distance. In this case, we see clearly that the sea-urchin first lived from youth to age, then died and lost its spines, which were carried away. Then the young Crania adhered to the bared shell, grew and perished in its turn; after which the upper valve was separated from the lower before the Echinus became enveloped in chalky mud.
It may be well to mention one more illustration of the manner in which single fossils may sometimes throw light on a former state of things, both in the bed of the ocean and on some adjoining land. We meet with many fragments of wood bored by ship-worms at various depths in the clay on which London is built. Entire branches and stems of trees, several feet in length, are sometimes dug out, drilled all over by the holes of these borers, the tubes and shells of the mollusk still remaining in the cylindrical hollows. In fig. 15. e, a representation is given of a piece of recent wood pierced by the Teredo navalis, or common ship-worm, which destroys wooden piles and ships. When the cylindrical tube d has been extracted from the wood, a shell is seen at the larger extremity, composed of two pieces, as shown at c. In like manner, a piece of fossil wood (a, fig. 14.) has been perforated by an animal of a kindred but extinct genus, called Teredina by Lamarck. The calcareous tube of this mollusk was united and as it were soldered on to the valves of the shell (b), which therefore cannot be detached from the tube, like the valves of the recent Teredo. The wood in this fossil specimen is now converted into a stony mass, a mixture of clay and lime; but it must once have been buoyant and floating in the sea, when the Teredinae lived upon it, perforating it in all directions. Again, before the infant colony settled upon the drift wood, the branch of a tree must have been floated down to the sea by a river, uprooted, perhaps, by a flood, or torn off and cast into the waves by the wind: and thus our thoughts are carried back to a prior period, when the tree grew for years on dry land, enjoying a fit soil and climate.
[2 Illustrations: Fossil and recent wood drilled by perforating Mollusca.
Fig. 14. a. Fossil wood from London clay, bored by Teredina. b. Shell and tube of Teredina personata, the right-hand figure the ventral, the left the dorsal view.
Fig. 15. e. Recent wood bored by Teredo. d. Shell and tube of Teredo navalis, from the same. c. Anterior and posterior view of the valves of same detached from the tube.]
It has been already remarked that there are rocks in the interior of continents, at various depths in the earth, and at great heights above the sea, almost entirely made up of the remains of zoophytes and testacea. Such masses may be compared to modern oyster-beds and coral reefs; and, like them, the rate of increase must have been extremely gradual. But there are a variety of stony deposits in the earth's crust, now proved to have been derived from plants and animals, of which the organic origin was not suspected until of late years, even by naturalists. Great surprise was therefore created by the recent discovery of Professor Ehrenberg of Berlin, that a certain kind of siliceous stone, called tripoli, was entirely composed of millions of the remains of organic beings, which the Prussian naturalist refers to microscopic Infusoria, but which most others now believe to be plants. They abound in freshwater lakes and ponds in England and other countries, and are termed Diatomaceae by those naturalists who believe in their vegetable origin. The substance alluded to has long been well known in the arts, being used in the form of powder for polishing stones and metals. It has been procured, among other places, from Bilin, in Bohemia, where a single stratum, extending over a wide area, is no less than 14 feet thick. This stone, when examined with a powerful microscope, is found to consist of the siliceous plates or frustules of the above-mentioned Diatomaceae, united together without any visible cement. It is difficult to convey an idea of their extreme minuteness; but Ehrenberg estimates that in the Bilin tripoli there are 41,000 millions of individuals of the Gaillonella distans (see fig. 17.) in every cubic inch, which weighs about 220 grains, or about 187 millions in a single grain. At every stroke, therefore, that we make with this polishing powder, several millions, perhaps tens of millions, of perfect fossils are crushed to atoms.
[3 Illustrations: These figures are magnified nearly 300 times, except the lower figure of G. ferruginea (fig. 18. a), which is magnified 2000 times.
Fig. 16. Bacillaria vulgaris?
Fig. 17. Gaillonella distans.
Fig. 18. Gaillonella ferruginea.]
[2 Illustrations: Fragment of semi-opal from the great bed of Tripoli, Bilin.
Fig. 19. Natural size.
Fig. 20. The same magnified, showing circular articulations of a species of Gaillonella, and spiculae of Spongilla.]
The remains of these Diatomaceae are of pure silex, and their forms are various, but very marked and constant in particular genera and species. Thus, in the family Bacillaria (see fig. 16.), the fossils preserved in tripoli are seen to exhibit the same divisions and transverse lines which characterize the living species of kindred form. With these, also, the siliceous spiculae or internal supports of the freshwater sponge, or Spongilla of Lamarck, are sometimes intermingled (see the needle-shaped bodies in fig. 20.). These flinty cases and spiculae, although hard, are very fragile, breaking like glass, and are therefore admirably adapted, when rubbed, for wearing down into a fine powder fit for polishing the surface of metals.
Besides the tripoli, formed exclusively of the fossils above described, there occurs in the upper part of the great stratum at Bilin another heavier and more compact stone, a kind of semi-opal, in which innumerable parts of Diatomaceae and spiculae of the Spongilla are filled with, and cemented together by, siliceous matter. It is supposed that the siliceous remains of the most delicate Diatomaceae have been dissolved by water, and have thus given rise to this opal in which the more durable fossils are preserved like insects in amber. This opinion is confirmed by the fact that the organic bodies decrease in number and sharpness of outline in proportion as the opaline cement increases in quantity.
In the Bohemian tripoli above described, as in that of Planitz in Saxony, the species of Diatomaceae (or Infusoria, as termed by Ehrenberg) are freshwater; but in other countries, as in the tripoli of the Isle of France, they are of marine species, and they all belong to formations of the tertiary period, which will be spoken of hereafter.
A well-known substance, called bog-iron ore, often met with in peat-mosses, has also been shown by Ehrenberg to consist of innumerable articulated threads, of a yellow ochre colour, composed partly of flint and partly of oxide of iron. These threads are the cases of a minute microscopic body, called Gaillonella ferruginea (fig. 18.).
[4 Illustrations: Cytheridae and Foraminifera from the chalk.
Fig. 21. Cythere, Muell. Cytherina, Lam.
Fig. 22. Portion of Nodosaria.
Fig. 23. Cristellaria rotulata.
Fig. 24. Rosalina.]
It is clear that much time must have been required for the accumulation of strata to which countless generations of Diatomaceae have contributed their remains; and these discoveries lead us naturally to suspect that other deposits, of which the materials have usually been supposed to be inorganic, may in reality have been derived from microscopic organic bodies. That this is the case with the white chalk, has often been imagined, this rock having been observed to abound in a variety of marine fossils, such as shells, echini, corals, sponges, crustacea, and fishes. Mr. Lonsdale, on examining, in Oct. 1835, in the museum of the Geological Society of London, portions of white chalk from different parts of England, found, on carefully pulverizing them in water, that what appear to the eye simply as white grains were, in fact, well preserved fossils. He obtained above a thousand of these from each pound weight of chalk, some being fragments of minute corallines, others entire Foraminifera and Cytheridae. The annexed drawings will give an idea of the beautiful forms of many of these bodies. The figures a a represent their natural size, but, minute as they seem, the smallest of them, such as a, fig. 24., are gigantic in comparison with the cases of Diatomaceae before mentioned. It has, moreover, been lately discovered that the chambers into which these Foraminifera are divided are actually often filled with thousands of well-preserved organic bodies, which abound in every minute grain of chalk, and are especially apparent in the white coating of flints, often accompanied by innumerable needle-shaped spiculae of sponges. After reflecting on these discoveries, we are naturally led on to conjecture that, as the formless cement in the semi-opal of Bilin has been derived from the decomposition of animal and vegetable remains, so also even those parts of chalk flints in which no organic structure can be recognized may nevertheless have constituted a part of microscopic animalcules.
"The dust we tread upon was once alive!"--BYRON.
How faint an idea does this exclamation of the poet convey of the real wonders of nature! for here we discover proofs that the calcareous and siliceous dust of which hills are composed has not only been once alive, but almost every particle, albeit invisible to the naked eye, still retains the organic structure which, at periods of time incalculably remote, was impressed upon it by the powers of life.
Freshwater and marine fossils.--Strata, whether deposited in salt or fresh water, have the same forms; but the imbedded fossils are very different in the two cases, because the aquatic animals which frequent lakes and rivers are distinct from those inhabiting the sea. In the northern part of the Isle of Wight a formation of marl and limestone, more than 50 feet thick, occurs, in which the shells are principally, if not all, of extinct species. Yet we recognize their freshwater origin, because they are of the same genera as those now abounding in ponds and lakes, either in our own country or in warmer latitudes.
In many parts of France, as in Auvergne, for example, strata of limestone, marl, and sandstone are found, hundreds of feet thick, which contain exclusively freshwater and land shells, together with the remains of terrestrial quadrupeds. The number of land shells scattered through some of these freshwater deposits is exceedingly great; and there are districts in Germany where the rocks scarcely contain any other fossils except snail-shells (helices); as, for instance, the limestone on the left bank of the Rhine, between Mayence and Worms, at Oppenheim, Findheim, Budenheim, and other places. In order to account for this phenomenon, the geologist has only to examine the small deltas of torrents which enter the Swiss lakes when the waters are low, such as the newly-formed plain where the Kander enters the Lake of Thun. He there sees sand and mud strewed over with innumerable dead land shells, which have been brought down from valleys in the Alps in the preceding spring, during the melting of the snows. Again, if we search the sands on the borders of the Rhine, in the lower part of its course, we find countless land shells mixed with others of species belonging to lakes, stagnant pools, and marshes. These individuals have been washed away from the alluvial plains of the great river and its tributaries, some from mountainous regions, others from the low country.
Although freshwater formations are often of great thickness, yet they are usually very limited in area when compared to marine deposits, just as lakes and estuaries are of small dimensions in comparison with seas.
We may distinguish a freshwater formation, first, by the absence of many fossils almost invariably met with in marine strata. For example, there are no sea-urchins, no corals, and scarcely any zoophytes; no chambered shells, such as the nautilus, nor microscopic Foraminifera. But it is chiefly by attending to the forms of the mollusca that we are guided in determining the point in question. In a freshwater deposit, the number of individual shells is often as great, if not greater, than in a marine stratum; but there is a smaller variety of species and genera. This might be anticipated from the fact that the genera and species of recent freshwater and land shells are few when contrasted with the marine. Thus, the genera of true mollusca according to Blainville's system, excluding those of extinct species and those without shells, amount to about 200 in number, of which the terrestrial and freshwater genera scarcely form more than a sixth.[28-A]
Almost all bivalve shells, or those of acephalous mollusca, are marine, about ten only out of ninety genera being freshwater. Among these last, the four most common forms, both recent and fossil, are Cyclas, Cyrena, Unio, and Anodonta (see figures); the two first and two last of which are so nearly allied as to pass into each other.
Lamarck divided the bivalve mollusca into the Dimyary, or those having two large muscular impressions in each valve, as a b in the Cyclas, fig. 25., and the Monomyary, such as the oyster and scallop, in which there is only one of these impressions, as is seen in fig. 30. Now, as none of these last, or the unimuscular bivalves, are freshwater, we may at once presume a deposit in which we find any of them to be marine.
The univalve shells most characteristic of freshwater deposits are, Planorbis, Lymnea, and Paludina. (See figures.) But to these are occasionally added Physa, Succinea, Ancylus, Valvata, Melanopsis, Melania, and Neritina. (See figures.)
In regard to one of these, the Ancylus (fig. 35.), Mr. Gray observes that it sometimes differs in no respect from the marine Siphonaria, except in the animal. The shell, however, of the Ancylus is usually thinner.[29-A]
Some naturalists include Neritina (fig. 42.) and the marine Nerita (fig. 43.) in the same genus, it being scarcely possible to distinguish the two by good generic characters. But, as a general rule, the fluviatile species are smaller, smoother, and more globular than the marine; and they have never, like the Neritae, the inner margin of the outer lip toothed or crenulated. (See fig. 43.)
A few genera, among which Cerithium (fig. 44.) is the most abundant, are common both to rivers and the sea, having species peculiar to each. Other genera, like Auricula (fig. 38.), are amphibious, frequenting marshes, especially near the sea.
The terrestrial shells are all univalves. The most abundant genera among these, both in a recent and fossil state, are Helix (fig. 45.), Cyclostoma (fig. 46.), Pupa (fig. 47.), Clausilia (fig. 48.), Bulimus (fig. 49.), and Achatina; which two last are nearly allied and pass into each other.
The Ampullaria (fig. 50.) is another genus of shells, inhabiting rivers and ponds in hot countries. Many fossil species have been referred to this genus, but they have been found chiefly in marine formations, and are suspected by some conchologists to belong to Natica and other marine genera.
All univalve shells of land and freshwater species, with the exception of Melanopsis (fig. 41.), and Achatina, which has a slight indentation, have entire mouths; and this circumstance may often serve as a convenient rule for distinguishing freshwater from marine strata; since, if any univalves occur of which the mouths are not entire, we may presume that the formation is marine. The aperture is said to be entire in such shells as the Ampullaria and the land shells (figs. 45-49.), when its outline is not interrupted by an indentation or notch, such as that seen at b in Ancillaria (fig. 52.); or is not prolonged into a canal, as that seen at a in Pleurotoma (fig. 51.).
The mouths of a large proportion of the marine univalves have these notches or canals, and almost all such species are carnivorous; whereas nearly all testacea having entire mouths, are plant-eaters; whether the species be marine, freshwater, or terrestrial.
There is, however, one genus which affords an occasional exception to one of the above rules. The Cerithium (fig. 44.), although provided with a short canal, comprises some species which inhabit salt, others brackish, and others fresh water, and they are said to be all plant-eaters.
Among the fossils very common in freshwater deposits are the shells of Cypris, a minute crustaceous animal, having a shell much resembling that of the bivalve mollusca.[31-A] Many minute living species of this genus swarm in lakes and stagnant pools in Great Britain; but their shells are not, if considered separately, conclusive as to the freshwater origin of a deposit, because the majority of species in another kindred genus of the same order, the Cytherina of Lamarck (see above, fig. 21. p. 26.), inhabit salt water; and, although the animal differs slightly, the shell is scarcely distinguishable from that of the Cypris.
The seed-vessels and stems of Chara, a genus of aquatic plants, are very frequent in freshwater strata. These seed-vessels were called, before their true nature was known, gyrogonites, and were supposed to be foraminiferous shells. (See fig. 53. a.)
The Charae inhabit the bottom of lakes and ponds, and flourish mostly where the water is charged with carbonate of lime. Their seed-vessels are covered with a very tough integument, capable of resisting decomposition; to which circumstance we may attribute their abundance in a fossil state. The annexed figure (fig. 54.) represents a branch of one of many new species found by Professor Amici in the lakes of northern Italy. The seed-vessel in this plant is more globular than in the British Charae, and therefore more nearly resembles in form the extinct fossil species found in England, France, and other countries. The stems, as well as the seed-vessels, of these plants occur both in modern shell marl and in ancient freshwater formations. They are generally composed of a large tube surrounded by smaller tubes; the whole stem being divided at certain intervals by transverse partitions or joints. (See b, fig. 53.)
It is not uncommon to meet with layers of vegetable matter, impressions of leaves, and branches of trees, in strata containing freshwater shells; and we also find occasionally the teeth and bones of land quadrupeds, of species now unknown. The manner in which such remains are occasionally carried by rivers into lakes, especially during floods, has been fully treated of in the "Principles of Geology."[32-A]
The remains of fish are occasionally useful in determining the freshwater origin of strata. Certain genera, such as carp, perch, pike, and loach (Cyprinus, Perca, Esox, and Cobitis), as also Lebias, being peculiar to freshwater. Other genera contain some freshwater and some marine species, as Cottus, Mugil, and Anguilla, or eel. The rest are either common to rivers and the sea, as the salmon; or are exclusively characteristic of salt water. The above observations respecting fossil fishes are applicable only to the more modern or tertiary deposits; for in the more ancient rocks the forms depart so widely from those of existing fishes, that it is very difficult, at least in the present state of science, to derive any positive information from ichthyolites respecting the element in which strata were deposited.
The alternation of marine and freshwater formations, both on a small and large scale, are facts well ascertained in geology. When it occurs on a small scale, it may have arisen from the alternate occupation of certain spaces by river water and the sea; for in the flood season the river forces back the ocean and freshens it over a large area, depositing at the same time its sediment; after which the salt water again returns, and, on resuming its former place, brings with it sand, mud, and marine shells.
There are also lagoons at the mouths of many rivers, as the Nile and Mississippi, which are divided off by bars of sand from the sea, and which are filled with salt and fresh water by turns. They often communicate exclusively with the river for months, years, or even centuries; and then a breach being made in the bar of sand, they are for long periods filled with salt water.
The Lym-Fiord in Jutland offers an excellent illustration of analogous changes; for, in the course of the last thousand years, the western extremity of this long frith, which is 120 miles in length, including its windings, has been four times fresh and four times salt, a bar of sand between it and the ocean having been as often formed and removed. The last irruption of salt water happened in 1824, when the North Sea entered, killing all the freshwater shells, fish, and plants; and from that time to the present, the sea-weed Fucus vesiculosus, together with oysters and other marine mollusca, have succeeded the Cyclas, Lymnea, Paludina, and Charae.[33-A]
But changes like these in the Lym-Fiord, and those before mentioned as occurring at the mouths of great rivers, will only account for some cases of marine deposits of partial extent resting on freshwater strata. When we find, as in the south-east of England, a great series of freshwater beds, 1000 feet in thickness, resting upon marine formations and again covered by other rocks, such as the cretaceous, more than 1000 feet thick, and of deep-sea origin, we shall find it necessary to seek for a different explanation of the phenomena.[33-B]
FOOTNOTES:
[28-A] See Synoptic Table in Blainville's Malacologie.
[29-A] Gray, Phil. Trans., 1835, p. 302.
[31-A] For figures of recent species, see below, p. 183., and figs. of fossils, see p. 228.
[32-A] See Index of Principles, "Fossilization."
[33-A] See Principles, Index, "Lym-Fiord."
[33-B] See below, Chap. XVIII., on the Wealden.
IV. CONSOLIDATION OF STRATA AND PETRIFACTION OF FOSSILS
Chemical and mechanical deposits--Cementing together of particles--Hardening by exposure to air--Concretionary nodules--Consolidating effects of pressure--Mineralization of organic remains--Impressions and casts how formed--Fossil wood--Goeppert's experiments--Precipitation of stony matter most rapid where putrefaction is going on--Source of lime in solution--Silex derived from decomposition of felspar--Proofs of the lapidification of some fossils soon after burial, of others when much decayed.
Having spoken in the preceding chapters of the characters of sedimentary formations, both as dependent on the deposition of inorganic matter and the distribution of fossils, I may next treat of the consolidation of stratified rocks, and the petrifaction of imbedded organic remains.
Chemical and mechanical deposits.--A distinction has been made by geologists between deposits of a chemical, and those of a mechanical, origin. By the latter name are designated beds of mud, sand, or pebbles produced by the action of running water, also accumulations of stones and scoriae thrown out by a volcano, which have fallen into their present place by the force of gravitation. But the matter which forms a chemical deposit has not been mechanically suspended in water, but in a state of solution until separated by chemical action. In this manner carbonate of lime is often precipitated upon the bottom of lakes and seas in a solid form, as may be well seen in many parts of Italy, where mineral springs abound, and where the calcareous stone, called travertin, is deposited. In these springs the lime is usually held in solution by an excess of carbonic acid, or by heat if it be a hot spring, until the water, on issuing from the earth, cools or loses part of its acid. The calcareous matter then falls down in a solid state, encrusting shells, fragments of wood and leaves, and binding them together.[34-A]
In coral reefs, large masses of limestone are formed by the stony skeletons of zoophytes; and these, together with shells, become cemented together by carbonate of lime, part of which is probably furnished to the sea-water by the decomposition of dead corals. Even shells of which the animals are still living, on these reefs, are very commonly found to be encrusted over with a hard coating of limestone.[34-B]
If sand and pebbles are carried by a river into the sea, and these are bound together immediately by carbonate of lime, the deposit may be described as of a mixed origin, partly chemical, and partly mechanical.
Now, the remarks already made in Chapter II. on the original horizontality of strata are strictly applicable to mechanical deposits, and only partially to those of a mixed nature. Such as are purely chemical may be formed on a very steep slope, or may even encrust the vertical walls of a fissure, and be of equal thickness throughout; but such deposits are of small extent, and for the most part confined to veinstones.
Cementing of particles.--It is chiefly in the case of calcareous rocks that solidification takes place at the time of deposition. But there are many deposits in which a cementing process comes into operation long afterwards. We may sometimes observe, where the water of ferruginous or calcareous springs has flowed through a bed of sand or gravel, that iron or carbonate of lime has been deposited in the interstices between the grains or pebbles, so that in certain places the whole has been bound together into a stone, the same set of strata remaining in other parts loose and incoherent.
Proofs of a similar cementing action are seen in a rock at Kelloway in Wiltshire. A peculiar band of sandy strata, belonging to the group called Oolite by geologists, may be traced through several counties, the sand being for the most part loose and unconsolidated, but becoming stony near Kelloway. In this district there are numerous fossil shells which have decomposed, having for the most part left only their casts. The calcareous matter hence derived has evidently served, at some former period, as a cement to the siliceous grains of sand, and thus a solid sandstone has been produced. If we take fragments of many other argillaceous grits, retaining the casts of shells, and plunge them into dilute muriatic or other acid, we see them immediately changed into common sand and mud; the cement of lime, derived from the shells, having been dissolved by the acid.
Traces of impressions and casts are often extremely faint. In some loose sands of recent date we meet with shells in so advanced a stage of decomposition as to crumble into powder when touched. It is clear that water percolating such strata may soon remove the calcareous matter of the shell; and, unless circumstances cause the carbonate of lime to be again deposited, the grains of sand will not be cemented together; in which case no memorial of the fossil will remain. The absence of organic remains from many aqueous rocks may be thus explained; but we may presume that in many of them no fossils were ever imbedded, as there are extensive tracts on the bottoms of existing seas even of moderate depth on which no fragment of shell, coral, or other living creature can be detected by dredging. On the other hand, there are depths where the zero of animal life has been approached; as, for example, in the Mediterranean, at the depth of about 230 fathoms, according to the researches of Prof. E. Forbes. In the AEgean Sea a deposit of yellowish mud of a very uniform character, and closely resembling chalk, is going on in regions below 230 fathoms, and this formation must be wholly devoid of organic remains.[35-A]
In what manner silex and carbonate of lime may become widely diffused in small quantities through the waters which permeate the earth's crust will be spoken of presently, when the petrifaction of fossil bodies is considered; but I may remark here that such waters are always passing in the case of thermal springs from hotter to colder parts of the interior of the earth; and as often as the temperature of the solvent is lowered, mineral matter has a tendency to separate from it and solidify. Thus a stony cement is often supplied to any sand, pebbles, or fragmentary mixture. In some conglomerates, like the pudding-stone of Hertfordshire, pebbles of flint and grains of sand are united by a siliceous cement so firmly, that if a block be fractured the rent passes as readily through the pebbles as through the cement.
It is probable that many strata became solid at the time when they emerged from the waters in which they were deposited, and when they first formed a part of the dry land. A well-known fact seems to confirm this idea: by far the greater number of the stones used for building and road-making are much softer when first taken from the quarry than after they have been long exposed to the air; and these, when once dried, may afterwards be immersed for any length of time in water without becoming soft again. Hence it is found desirable to shape the stones which are to be used in architecture while they are yet soft and wet, and while they contain their "quarry-water," as it is called; also to break up stone intended for roads when soft, and then leave it to dry in the air for months that it may harden. Such induration may perhaps be accounted for by supposing the water, which penetrates the minutest pores of rocks, to deposit, on evaporation, carbonate of lime, iron, silex, and other minerals previously held in solution, and thereby to fill up the pores partially. These particles, on crystallizing, would not only be themselves deprived of freedom of motion, but would also bind together other portions of the rock which before were loosely aggregated. On the same principle wet sand and mud become as hard as stone when frozen; because one ingredient of the mass, namely, the water, has crystallized, so as to hold firmly together all the separate particles of which the loose mud and sand were composed.
Dr. MacCulloch mentions a sandstone in Skye, which may be moulded like dough when first found; and some simple minerals, which are rigid and as hard as glass in our cabinets, are often flexible and soft in their native beds; this is the case with asbestos, sahlite, tremolite, and chalcedony, and it is reported also to happen in the case of the beryl.[36-A]
The marl recently deposited at the bottom of Lake Superior, in North America, is soft, and often filled with freshwater shells; but if a piece be taken up and dried, it becomes so hard that it can only be broken by a smart blow of the hammer. If the lake therefore was drained, such a deposit would be found to consist of strata of marlstone, like that observed in many ancient European formations, and like them containing freshwater shells.[36-B]
It is probable that some of the heterogeneous materials which rivers transport to the sea may at once set under water, like the artificial mixture called pozzolana, which consists of fine volcanic sand charged with about 20 per cent. of oxide of iron, and the addition of a small quantity of lime. This substance hardens, and becomes a solid stone in water, and was used by the Romans in constructing the foundations of buildings in the sea.
Consolidation in these cases is brought about by the action of chemical affinity on finely comminuted matter previously suspended in water. After deposition similar particles seem to exert a mutual attraction on each other, and congregate together in particular spots, forming lumps, nodules, and concretions. Thus in many argillaceous deposits there are calcareous balls, or spherical concretions, ranged in layers parallel to the general stratification; an arrangement which took place after the shale or marl had been thrown down in successive laminae; for these laminae are often traced in the concretions, remaining parallel to those of the surrounding unconsolidated rock. (See fig. 55.) Such nodules of limestone have often a shell or other foreign body in the centre.[37-A]
Among the most remarkable examples of concretionary structure are those described by Professor Sedgwick as abounding in the magnesian limestone of the north of England. The spherical balls are of various sizes, from that of a pea to a diameter of several feet, and they have both a concentric and radiated structure, while at the same time the laminae of original deposition pass uninterruptedly through them. In some cliffs this limestone resembles a great irregular pile of cannon balls. Some of the globular masses have their centre in one stratum, while a portion of their exterior passes through to the stratum above or below. Thus the larger spheroid in the annexed section (fig. 56.) passes from the stratum b upwards into a. In this instance we must suppose the deposition of a series of minor layers, first forming the stratum b, and afterwards the incumbent stratum a; then a movement of the particles took place, and the carbonates of lime and magnesia separated from the more impure and mixed matter forming the still unconsolidated parts of the stratum. Crystallization, beginning at the centre, must have gone on forming concentric coats, around the original nucleus without interfering with the laminated structure of the rock.
When the particles of rocks have been thus re-arranged by chemical forces, it is sometimes difficult or impossible to ascertain whether certain lines of division are due to original deposition or to the subsequent aggregation of similar particles. Thus suppose three strata of grit, A, B, C, are charged unequally with calcareous matter, and that B is the most calcareous. If consolidation takes place in B, the concretionary action may spread upwards into a part of A, where the carbonate of lime is more abundant than in the rest; so that a mass, d, e, f, forming a portion of the superior stratum, becomes united with B into one solid mass of stone. The original line of division d, e, being thus effaced, the line d, f, would generally be considered as the surface of the bed B, though not strictly a true plane of stratification.
Pressure and heat.--When sand and mud sink to the bottom of a deep sea, the particles are not pressed down by the enormous weight of the incumbent ocean; for the water, which becomes mingled with the sand and mud, resists pressure with a force equal to that of the column of fluid above. The same happens in regard to organic remains which are filled with water under great pressure as they sink, otherwise they would be immediately crushed to pieces and flattened. Nevertheless, if the materials of a stratum remain in a yielding state, and do not set or solidify, they will be gradually squeezed down by the weight of other materials successively heaped upon them, just as soft clay or loose sand on which a house is built may give way. By such downward pressure particles of clay, sand, and marl, may become packed into a smaller space, and be made to cohere together permanently.
Analogous effects of condensation may arise when the solid parts of the earth's crust are forced in various directions by those mechanical movements afterwards to be described, by which strata have been bent, broken, and raised above the level of the sea. Rocks of more yielding materials must often have been forced against others previously consolidated, and, thus compressed, may have acquired a new structure. A recent discovery may help us to comprehend how fine sediment derived from the detritus of rocks may be solidified by mere pressure. The graphite or "black lead" of commerce having become very scarce, Mr. Brockedon contrived a method by which the dust of the purer portions of the mineral found in Borrowdale might be recomposed into a mass as dense and compact as native graphite. The powder of graphite is first carefully prepared and freed from air, and placed under a powerful press on a strong steel die, with air-tight fittings. It is then struck several blows, each of a power of 1000 tons; after which operation the powder is so perfectly solidified that it can be cut for pencils, and exhibits when broken the same texture as native graphite.
But the action of heat at various depths in the earth is probably the most powerful of all causes in hardening sedimentary strata. To this subject I shall refer again when treating of the metamorphic rocks, and of the slaty and jointed structure.
Mineralization of organic remains.--The changes which fossil organic bodies have undergone since they were first imbedded in rocks, throw much light on the consolidation of strata. Fossil shells in some modern deposits have been scarcely altered in the course of centuries, having simply lost a part of their animal matter. But in other cases the shell has disappeared, and left an impression only of its exterior, or a cast of its interior form, or thirdly, a cast of the shell itself, the original matter of which has been removed. These different forms of fossilization may easily be understood if we examine the mud recently thrown out from a pond or canal in which there are shells. If the mud be argillaceous, it acquires consistency on drying, and on breaking open a portion of it we find that each shell has left impressions of its external form. If we then remove the shell itself, we find within a solid nucleus of clay, having the form of the interior of the shell. This form is often very different from that of the outer shell. Thus a cast such as a, fig. 58., commonly called a fossil screw, would never be suspected by an inexperienced conchologist to be the internal shape of the fossil univalve, b, fig. 58. Nor should we have imagined at first sight that the shell a and the cast b, fig. 59., were different parts of the same fossil. The reader will observe, in the last-mentioned figure (b, fig. 59.), that an empty space shaded dark, which the shell itself once occupied, now intervenes between the enveloping stone and the cast of the smooth interior of the whorls. In such cases the shell has been dissolved and the component particles removed by water percolating the rock. If the nucleus were taken out a hollow mould would remain, on which the external form of the shell with its tubercles and striae, as seen in a, fig. 59., would be seen embossed. Now if the space alluded to between the nucleus and the impression, instead of being left empty, has been filled up with calcareous spar, flint, pyrites, or other mineral, we then obtain from the mould an exact cast both of the external and internal form of the original shell. In this manner silicified casts of shells have been formed; and if the mud or sand of the nucleus happen to be incoherent, or soluble in acid, we can then procure in flint an empty shell, which in shape is the exact counterpart of the original. This cast may be compared to a bronze statue, representing merely the superficial form, and not the internal organization; but there is another description of petrifaction by no means uncommon, and of a much more wonderful kind, which may be compared to certain anatomical models in wax, where not only the outward forms and features, but the nerves, blood-vessels, and other internal organs are also shown. Thus we find corals, originally calcareous, in which not only the general shape, but also the minute and complicated internal organization are retained in flint.
Such a process of petrifaction is still more remarkably exhibited in fossil wood, in which we often perceive not only the rings of annual growth, but all the minute vessels and medullary rays. Many of the minute pores and fibres of plants, and even those spiral vessels which in the living vegetable can only be discovered by the microscope, are preserved. Among many instances, I may mention a fossil tree, 72 feet in length, found at Gosforth near Newcastle, in sandstone strata associated with coal. By cutting a transverse slice so thin as to transmit light, and magnifying it about fifty-five times, the texture seen in fig. 60. is exhibited. A texture equally minute and complicated has been observed in the wood of large trunks of fossil trees found in the Craigleith quarry near Edinburgh, where the stone was not in the slightest degree siliceous, but consisted chiefly of carbonate of lime, with oxide of iron, alumina, and carbon. The parallel rows of vessels here seen are the rings of annual growth, but in one part they are imperfectly preserved, the wood having probably decayed before the mineralizing matter had penetrated to that portion of the tree.
In attempting to explain the process of petrifaction in such cases, we may first assume that strata are very generally permeated by water charged with minute portions of calcareous, siliceous, and other earths in solution. In what manner they become so impregnated will be afterwards considered. If an organic substance is exposed in the open air to the action of the sun and rain, it will in time putrefy, or be dissolved into its component elements, which consist chiefly of oxygen, hydrogen, and carbon. These will readily be absorbed by the atmosphere or be washed away by rain, so that all vestiges of the dead animal or plant disappear. But if the same substances be submerged in water, they decompose more gradually; and if buried in earth, still more slowly, as in the familiar example of wooden piles or other buried timber. Now, if as fast as each particle is set free by putrefaction in a fluid or gaseous state, a particle equally minute of carbonate of lime, flint, or other mineral, is at hand and ready to be precipitated, we may imagine this inorganic matter to take the place just before left unoccupied by the organic molecule. In this manner a cast of the interior of certain vessels may first be taken, and afterwards the more solid walls of the same may decay and suffer a like transmutation. Yet when the whole is lapidified, it may not form one homogeneous mass of stone or metal. Some of the original ligneous, osseous, or other organic elements may remain mingled in certain parts, or the lapidifying substance itself may be differently coloured at different times, or so crystallized as to reflect light differently, and thus the texture of the original body may be faithfully exhibited.
The student may perhaps ask whether, on chemical principles, we have any ground to expect that mineral matter will be thrown down precisely in those spots where organic decomposition is in progress? The following curious experiments may serve to illustrate this point. Professor Goeppert of Breslau attempted recently to imitate the natural process of petrifaction. For this purpose he steeped a variety of animal and vegetable substances in waters, some holding siliceous, others calcareous, others metallic matter in solution. He found that in the period of a few weeks, or even days, the organic bodies thus immersed were mineralized to a certain extent. Thus, for example, thin vertical slices of deal, taken from the Scotch fir (Pinus sylvestris), were immersed in a moderately strong solution of sulphate of iron. When they had been thoroughly soaked in the liquid for several days they were dried and exposed to a red-heat until the vegetable matter was burnt up and nothing remained but an oxide of iron, which was found to have taken the form of the deal so exactly that casts even of the dotted vessels peculiar to this family of plants were distinctly visible under the microscope.
Another accidental experiment has been recorded by Mr. Pepys in the Geological Transactions.[41-A] An earthen pitcher containing several quarts of sulphate of iron had remained undisturbed and unnoticed for about a twelvemonth in the laboratory. At the end of this time when the liquor was examined an oily appearance was observed on the surface, and a yellowish powder, which proved to be sulphur, together with a quantity of small hairs. At the bottom were discovered the bones of several mice in a sediment consisting of small grains of pyrites, others of sulphur, others of crystallized green sulphate of iron, and a black muddy oxide of iron. It was evident that some mice had accidentally been drowned in the fluid, and by the mutual action of the animal matter and the sulphate of iron on each other, the metallic sulphate had been deprived of its oxygen; hence the pyrites and the other compounds were thrown down. Although the mice were not mineralized, or turned into pyrites, the phenomenon shows how mineral waters, charged with sulphate of iron, may be deoxydated on coming in contact with animal matter undergoing putrefaction, so that atom after atom of pyrites may be precipitated, and ready, under favourable circumstances, to replace the oxygen, hydrogen, and carbon into which the original body would be resolved.
The late Dr. Turner observes, that when mineral matter is in a "nascent state," that is to say, just liberated from a previous state of chemical combination, it is most ready to unite with other matter, and form a new chemical compound. Probably the particles or atoms just set free are of extreme minuteness, and therefore move more freely, and are more ready to obey any impulse of chemical affinity. Whatever be the cause, it clearly follows, as before stated, that where organic matter newly imbedded in sediment is decomposing, there will chemical changes take place most actively.
An analysis was lately made of the water which was flowing off from the rich mud deposited by the Hooghly river in the Delta of the Ganges after the annual inundation. This water was found to be highly charged with carbonic acid gas holding lime in solution.[41-B] Now if newly-deposited mud is thus proved to be permeated by mineral matter in a state of solution, it is not difficult to perceive that decomposing organic bodies, naturally imbedded in sediment, may as readily become petrified as the substances artificially immersed by Professor Goeppert in various fluid mixtures.
It is well known that the water of springs, or that which is continually percolating the earth's crust, is rarely free from a slight admixture either of iron, carbonate of lime, sulphur, silica, potash, or some other earthy, alkaline, or metallic ingredient. Hot springs in particular are copiously charged with one or more of these elements; and it is only in their waters that silex is found in abundance. In certain cases, therefore, especially in volcanic regions, we may imagine the flint of silicified wood and corals to have been supplied by the waters of thermal springs. In other instances, as in tripoli and chalk-flint, it may have been derived in great part, if not wholly, from the decomposition of infusoria or diatomaceae, sponges, and other bodies. But even if this be granted, we have still to inquire whence a lake or the ocean can be constantly replenished with the calcareous and siliceous matter so abundantly withdrawn from it by the secretions of these zoophytes.
In regard to carbonate of lime there is no difficulty, because not only are calcareous springs very numerous, but even rain-water has the power of dissolving a minute portion of the calcareous rocks over which it flows. Hence marine corals and mollusca may be provided by rivers with the materials of their shells and solid supports. But pure silex, even when reduced to the finest powder and boiled, is insoluble in water, except at very high temperatures. Nevertheless Dr. Turner has well explained, in an essay on the chemistry of geology[42-A], how the decomposition of felspar may be a source of silex in solution. He has remarked that the siliceous earth, which constitutes more than half the bulk of felspar, is intimately combined with alumine, potash, and some other elements. The alkaline matter of the felspar has a chemical affinity for water, as also for the carbonic acid which is more or less contained in the waters of most springs. The water therefore carries away alkaline matter, and leaves behind a clay consisting of alumine and silica. But this residue of the decomposed mineral, which in its purest state is called porcelain clay, is found to contain a part only of the silica which existed in the original felspar. The other part, therefore, must have been dissolved and removed; and this can be accounted for in two ways; first, because silica when combined with an alkali is soluble in water; secondly, because silica in what is technically called its nascent state is also soluble in water. Hence an endless supply of silica is afforded to rivers and the waters of the sea. For the felspathic rocks are universally distributed, constituting, as they do, so large a proportion of the volcanic, plutonic, and metamorphic formations. Even where they chance to be absent in mass, they rarely fail to occur in the superficial gravel or alluvial deposits of the basin of every large river.
The disintegration of mica also, another mineral which enters largely into the composition of granite and various sandstones, may yield silica which may be dissolved in water, for nearly half of this mineral consists of silica, combined with alumine, potash, and about a tenth part of iron. The oxidation of this iron in the air is the principal cause of the waste of mica.
We have still, however, much to learn before the conversion of fossil bodies into stone is fully understood. Some phenomena seem to imply that the mineralization must proceed with considerable rapidity, for stems of a soft and succulent character, and of a most perishable nature, are preserved in flint; and there are instances of the complete silicification of the young leaves of a palm-tree when just about to shoot forth, and in that state which in the West Indies is called the cabbage of the palm.[43-A] It may, however, be questioned whether in such cases there may not have been some antiseptic quality in the water which retarded putrefaction, so that the soft parts of the buried substance may have remained for a long time without disintegration, like the flesh of bodies imbedded in peat.
Mr. Stokes has pointed out examples of petrifactions in which the more perishable, and others where the more durable portions of wood are preserved. These variations, he suggests, must doubtless have depended on the time when the lapidifying mineral was introduced. Thus, in certain silicified stems of palm-trees, the cellular tissue, that most destructible part, is in good condition, while all signs of the hard woody fibre have disappeared, the spaces once occupied by it being hollow or filled with agate. Here, petrifaction must have commenced soon after the wood was exposed to the action of moisture, and the supply of mineral matter must then have failed, or the water must have become too much diluted before the woody fibre decayed. But when this fibre is alone discoverable, we must suppose that an interval of time elapsed before the commencement of lapidification, during which the cellular tissue was obliterated. When both structures, namely, the cellular and the woody fibre, are preserved, the process must have commenced at an early period, and continued without interruption till it was completed throughout.[43-B]
FOOTNOTES:
[34-A] See Principles, Index, "Calcareous Springs," &c.
[34-B] Ibid. "Travertin," "Coral Reefs," &c.
[35-A] Report Brit. Ass. 1843, p. 178.
[36-A] Dr. MacCulloch, Syst. of Geol. vol. i. p. 123.
[36-B] Princ. of Geol., Index, "Superior Lake."
[37-A] De la Beche, Geol. Researches, p. 95., and Geol. Observer (1851), p. 686.
[41-A] Vol. i. p. 399. first series.
[41-B] Piddington, Asiat. Research. vol. xviii. p. 226.
[42-A] Jam. Ed. New Phil. Journ. No. 30. p. 246.
[43-A] Stokes, Geol. Trans., vol. v. p. 212. second series.
[43-B] Ibid.