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diff --git a/34350-8.txt b/34350-8.txt new file mode 100644 index 0000000..057d60e --- /dev/null +++ b/34350-8.txt @@ -0,0 +1,29264 @@ +Project Gutenberg's A Manual of Elementary Geology, by Charles Lyell + +This eBook is for the use of anyone anywhere at no cost and with +almost no restrictions whatsoever. You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at www.gutenberg.org + + +Title: A Manual of Elementary Geology + or, The Ancient Changes of the Earth and its Inhabitants + as Illustrated by Geological Monuments + +Author: Charles Lyell + +Release Date: November 17, 2010 [EBook #34350] + +Language: English + +Character set encoding: ISO-8859-1 + +*** START OF THIS PROJECT GUTENBERG EBOOK A MANUAL OF ELEMENTARY GEOLOGY *** + + + + +Produced by Julia Miller, Iris Schröder-Gehring and the +Online Distributed Proofreading Team at http://www.pgdp.net +(This file was produced from images generously made +available by The Internet Archive/American Libraries.) + + + + + + + + + +[Illustration: From a Painting by James Hall, Esq. Engraved by +S. Williams. + +STRATA OF RED SANDSTONE, SLIGHTLY INCLINED, RESTING ON VERTICAL SCHIST, AT +THE SICCAR POINT, BERWICKSHIRE. + +TO ILLUSTRATE UNCONFORMABLE STRATIFICATION. See page 60. + +_"The mind seemed to grow giddy by looking so far into the abyss of time; +and while we listened with earnestness and admiration to the philosopher +who was now unfolding to us the order and series of these wonderful events, +we became sensible how much farther reason may sometimes go than +imagination can venture to follow."_--PLAYFAIR, Biography of Hutton.] + + + + + 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 palæontology 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. + + +BOOK 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. + + +BOOK 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 laminæ, 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. + +[Illustration: Fig. 521. Scale one-sixth the original size. + +Part of the trail of a (Chelonian?) quadruped from the Old Red Sandstone of +Cummingstone, near Elgin, Morayshire.--Captain Brickenden.] + +_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 +têle+, afar off, and +herpeton+, a +reptile; while the specific name Elginense commemorates the principal place +near which it was obtained. + +[Illustration: Fig. 522. Natural size. _Telerpeton Elginense._ (Mantell.) + +Reptile of Old Red Sandstone, from near Elgin, Morayshire.] + +_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. + +[Illustration: Fig. 523. Fossil.--Old Red. + +Fig. 523. Slab of Old Red Sandstone, Forfarshire, with eggs of Batrachians. + + _a._ Ova in a carbonized state. + _b._ Egg cells; the ova shed.] + +[Illustration: Fig. 524. Recent. + +Fig. 524. Eggs of the common frog, _Rana temporaria_, in a carbonized +state, from a dried-up pond in Clapham Common. + + _a._ The ova. + _b._ A transverse section of the mass exhibiting the form of the + egg-cells.] + +[Illustration: Fig. 525. Eggs of Batrachians.--Old Red Sandstone. + +Fig. 525. Shale of Old Red Sandstone, or Devonian, Forfarshire, with +impression of plants and eggs of Batrachians. + + _a._ Two pair of ova resembling those large Salamanders or Tritons + on the same leaf. + _b b._ Detached ova. + _c._ Egg-cells of frogs or _Ranina_.] + +_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 Stigmariæ 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] + +[Illustration: Fig. 526. Carboniferous rain-prints with worm-tracks (_a_, +_b_) on green shale, from Cape Breton, Nova Scotia.] + +[Illustration: Fig. 527. Casts of rain-prints on a portion of the +same slab, No. 526. seen on the under side of an incumbent layer +of arenaceous shale. + +The arrow represents the direction of the shower.] + +[Illustration: Fig. 528. Casts of carboniferous rain-prints and +shrinkage-cracks, (_a_) on the under side of a layer of sandstone, Cape +Breton, Nova Scotia.] + +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 Würtemberg 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. Jäger, of Stuttgart, for +having recently reminded us of it in his Memoir on the Fossil Mammalia of +Würtemberg.[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. Jäger's plate lxxi., fig. 15. + +[Illustration: Fig. 529. _Microlestes antiquus_, Plieninger. Molar tooth +magnified. Upper Trias, Diegerloch, near Stuttgart, Würtemberg. + + _a._ View of inner side? + _b._ same, outer side? + _c._ Same in profile. + _d._ Crown of same.] + +[Illustration: Fig. 530. _Microlestes antiquus_, Plien. + +View of same molar as No. 529. From a drawing by Herman von Meyer. + + _a._ View of inner side? + _b._ Crown of same.] + +[Illustration: Fig. 531. Molar of _Microlestes_? Plien. 4 times as large as +fig. 529. From the trias of Diegerloch, Stuttgart.] + +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 +lêstês+, 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 Würtemberg 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 Würtemberg 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 Cypræa +and Oliva, found at Faxoe; but M. A. d'Orbigny enumerates 24 species of +Gasteropoda from the white chalk (Terrain Sénonien) 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. Coniferæ and +Cycadeæ 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 Pläner-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 Coniferæ, 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 vertebræ 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] Würtembergisch. Naturwissen. Jahreshefte, 3 Jahr. Stuttgart, 1847. + +[xiii-B] Nov. Act. Acad. Cæsar. 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. + + + CHAPTER 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 + + + CHAPTER 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 + + + CHAPTER 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--Göppert'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 + + + CHAPTER 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 + + + CHAPTER 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 + + + CHAPTER 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 + + + CHAPTER 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 palæontological characters--Test of age by included + fragments--Frequent absence of strata of intervening periods--Principal + groups of strata in western Europe 96 + + + CHAPTER 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 + + + CHAPTER 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 + + + CHAPTER 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 + + + CHAPTER 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 + + + CHAPTER 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--Sewâlik Hills in India 161 + + + CHAPTER 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 + + + CHAPTER 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 + + + CHAPTER 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 + + + CHAPTER 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 + + + CHAPTER 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 + + + CHAPTER 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--Nerinæan 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 + + + CHAPTER 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 + + + CHAPTER 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 + + + CHAPTER 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--Palæoniscus 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 + + + CHAPTER 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, Sigillariæ, Stigmariæ,--Coniferæ--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 + + + CHAPTER 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 + + + CHAPTER 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 + + + CHAPTER XXVII. + + SILURIAN GROUP. + + Silurian strata formerly called transition--Term grauwacké--Subdivisions + of Upper and Lower Silurian--Ludlow formation and fossils--Wenlock + formation, corals and shells--Caradoc and Llandeilo + beds--Graptolites--Lingula--Trilobites--Cystideæ--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 + + + CHAPTER 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 + + + CHAPTER 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 + + + CHAPTER 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 + + + CHAPTER 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 + + + CHAPTER 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 Côme--Puy + de Pariou--Cones not denuded by general flood--Velay--Bones of + quadrupeds buried in scoriæ--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 + + + CHAPTER 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 + + + CHAPTER 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 + + + CHAPTER 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 + + + CHAPTER 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 + + + CHAPTER 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 + + + CHAPTER 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. + + + + +CHAPTER 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 +gê+, _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 Ardèche, +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 scoriæ, 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 scoriæ, 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 scoriæ 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 scoriæ, 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 +morphê+, +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 laminæ. + +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 laminæ, 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. + +[Illustration: Fig. 1. Cross section.] + +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.) + +[Illustration: Fig. 2. Section of strata of sandstone, grit, +and conglomerate.] + +[Illustration: Fig. 3. Section of sand at Sandy Hill, near Biggleswade, +Bedfordshire. Height 20 feet. (Greensand formation.)] + +_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 laminæ +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 laminæ 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. + +[Illustration: Fig. 4. Cross section.] + +[Illustration: Fig. 5. Cross section.] + +[Illustration: Fig. 6. Cliff between Mismer and Dunwich.] + +[Illustration: Fig. 7. Section from Monte Calvo to the sea by the valley of +Magnan, near Nice. + + A. Dolomite and sandstone. (Greensand formation?) + _a_, _b_, _d_. Beds of gravel and sand. + _c._ Fine marl and sand of St. Madeleine, with marine shells.] + +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°. 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. + +[Illustration: Fig. 8. Slab of ripple-marked (new red) sandstone +from Cheshire.] + +_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. + +[Illustration: Fig. 9. Sketch of ripples.] + +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. + + + + +CHAPTER 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. + +[Illustration: Fig. 10. Fossil _Gryphæa_, covered both on the outside and +inside with fossil serpulæ.] + +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 serpulæ, +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 serpulæ (_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 serpulæ have grown on the interior, one of them exactly on the place +where the adductor muscle of the _Gryphæa_ (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. + +[Illustration: Fig. 11. _Serpula_ attached to a fossil _Spatangus_ +from the chalk.] + +[Illustration: Fig. 12. Recent _Spatangus_ with the spines removed +from one side. + + _b._ Spine and tubercles, nat. size. + _a._ The same magnified.] + +[Illustration: Fig. 13. + + _a._ _Echinus_ from the chalk, with lower valve of the _Crania_ attached. + _b._ Upper valve of the _Crania_ 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 _Teredinæ_ 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 Diatomaceæ 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 Diatomaceæ, 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 spiculæ of _Spongilla_.] + +The remains of these Diatomaceæ 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 spiculæ 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 spiculæ, 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 Diatomaceæ and spiculæ of the _Spongilla_ are filled with, and +cemented together by, siliceous matter. It is supposed that the siliceous +remains of the most delicate Diatomaceæ 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 Diatomaceæ (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: _Cytheridæ_ and _Foraminifera_ from the chalk. + +Fig. 21. _Cythere_, Müll. + _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 Diatomaceæ 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 Cytheridæ. +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 Diatomaceæ 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 spiculæ 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] + +[Illustration: Fig. 25. _Cyclas obovata_; fossil. Hants.] + +[Illustration: Fig. 26. _Cyrena consobrina_; fossil. Grays, Essex.] + +[Illustration: Fig. 27. _Anodonta Cordierii_; fossil. Paris.] + +[Illustration: Fig. 28. _Anodonta latimarginatus_; recent. Bahia.] + +[Illustration: Fig. 29. _Unio littoralis_; recent. Auvergne.] + +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. + +[Illustration: Fig. 30. _Gryphæa incurva_, Sow. (_G. arcuata_, Lam.) upper +valve. Lias.] + +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. + +[Illustration: Fig. 31. _Planorbis euomphalus_; fossil. Isle of Wight.] + +[Illustration: Fig. 32. _Lymnea longiscata_; fossil. Hants.] + +[Illustration: Fig. 33. _Paludina lenta_; fossil. Hants.] + +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.) + +[Illustration: Fig. 34. _Succinea amphibia_; fossil. Loess, Rhine.] + +[Illustration: Fig. 35. _Ancylus elegans_; fossil. Hants.] + +[Illustration: Fig. 36. _Valvata_; fossil. Grays, Essex.] + +[Illustration: Fig. 37. _Physa hypnorum_; recent.] + +[Illustration: Fig. 38. _Auricula_; recent. Ava.] + +[Illustration: Fig. 39. _Melania inquinata._ Paris Basin.] + +[Illustration: Fig. 40. _Physa columnaris._ Paris Basin.] + +[Illustration: Fig. 41. _Melanopsis buccinoidea_; recent. Asia.] + +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] + +[Illustration: Fig. 42. _Neritina globulus._ Paris basin.] + +[Illustration: Fig. 43. _Nerita granulosa._ Paris basin.] + +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 _Neritæ_, the inner margin of the outer lip toothed or +crenulated. (See fig. 43.) + +[Illustration: Fig. 44. _Cerithium cinctum._ Paris basin.] + +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. + +[Illustration: Fig. 45. _Helix Turonensis._ Faluns, Touraine.] + +[Illustration: Fig. 46. _Cyclostoma elegans._ Loess.] + +[Illustration: Fig. 47. _Pupa tridens._ Loess.] + +[Illustration: Fig. 48. _Clausilia bidens._ Loess.] + +[Illustration: Fig. 49. _Bulimus lubricus._ Loess, Rhine.] + +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. + +[Illustration: Fig. 50. _Ampullaria glauca_, from the Jumna.] + +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.). + +[Illustration: Fig. 51. _Pleurotoma rotata._ Subap. hills, Italy.] + +[Illustration: Fig. 52. _Ancillaria subulata._ London clay.] + +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 _Charæ_ 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 _Charæ_, 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.) + +[Illustration: Fig. 53. _Chara medicaginula_; fossil. Isle of Wight. + + _a._ Seed-vessel. magnified 20 diameters. + _b._ Stem, magnified.] + +[Illustration: Fig. 54. _Chara elastica_; recent. Italy. + + _a._ Sessile seed vessel between the division of the leaves of the + female plant. + _b._ Transverse section of a branch, with five seed-vessels magnified, + seen from below upwards.] + +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 _Charæ_.[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. + + + + +CHAPTER 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--Göppert'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 +scoriæ 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 Ægean +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 laminæ; for these laminæ 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] + +[Illustration: Fig. 55. Calcareous nodules in Lias.] + +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 laminæ 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. + +[Illustration: Fig. 56. Spheroidal concretions in magnesian limestone.] + +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. + +[Illustration: Fig. 57. Block section.] + +_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 striæ, 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. + +[Illustration: Fig. 58. _Phasianella Heddingtonensis_, and cast of the +same. Coral Rag.] + +[Illustration: Fig. 59. _Trochus Anglicus_ and cast. Lias.] + +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. + +[Illustration: Fig. 60. Texture of a tree from the coal strata, magnified. +(Witham.) Transverse section.] + +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 +Göppert 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 Göppert 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 diatomaceæ, +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. + + + + +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. + + +_Land has been raised, not the sea lowered._--It has been already stated +that the aqueous rocks containing marine fossils extend over wide +continental tracts, and are seen in mountain chains rising to great heights +above the level of the sea. Hence it follows, that what is now dry land was +once under water. But if we admit this conclusion, we must imagine, either +that there has been a general lowering of the waters of the ocean, or that +the solid rocks, once covered by water, have been raised up bodily out of +the sea, and have thus become dry land. The earlier geologists, finding +themselves reduced to this alternative, embraced the former opinion, +assuming that the ocean was originally universal, and had gradually sunk +down to its actual level, so that the present islands and continents were +left dry. It seemed to them far easier to conceive that the water had gone +down, than that solid land had risen upwards into its present position. It +was, however, impossible to invent any satisfactory hypothesis to explain +the disappearance of so enormous a body of water throughout the globe, it +being necessary to infer that the ocean had once stood at whatever height +marine shells might be detected. It moreover appeared clear, as the science +of Geology advanced, that certain spaces on the globe had been alternately +sea, then land, then estuary, then sea again, and, lastly, once more +habitable land, having remained in each of these states for considerable +periods. In order to account for such phenomena, without admitting any +movement of the land itself, we are required to imagine several retreats +and returns of the ocean; and even then our theory applies merely to cases +where the marine strata composing the dry land are horizontal, leaving +unexplained those more common instances where strata are inclined, curved, +or placed on their edges, and evidently not in the position in which they +were first deposited. + +Geologists, therefore, were at last compelled to have recourse to the other +alternative, namely, the doctrine that the solid land has been repeatedly +moved upwards or downwards, so as permanently to change its position +relatively to the sea. There are several distinct grounds for preferring +this conclusion. First, it will account equally for the position of those +elevated masses of marine origin in which the stratification remains +horizontal, and for those in which the strata are disturbed, broken, +inclined, or vertical. Secondly, it is consistent with human experience +that land should rise gradually in some places and be depressed in others. +Such changes have actually occurred in our own days, and are now in +progress, having been accompanied in some cases by violent convulsions, +while in others they have proceeded so insensibly, as to have been +ascertainable only by the most careful scientific observations, made at +considerable intervals of time. On the other hand, there is no evidence +from human experience of a lowering of the sea's level in any region, and +the ocean cannot sink in one place without its level being depressed all +over the globe. + +These preliminary remarks will prepare the reader to understand the great +theoretical interest attached to all facts connected with the position of +strata, whether horizontal or inclined, curved or vertical. + +Now the first and most simple appearance is where strata of marine origin +occur above the level of the sea in horizontal position. Such are the +strata which we meet with in the south of Sicily, filled with shells for +the most part of the same species as those now living in the Mediterranean. +Some of these rocks rise to the height of more than 2000 feet above the +sea. Other mountain masses might be mentioned, composed of horizontal +strata of high antiquity, which contain fossil remains of animals wholly +dissimilar from any now known to exist. In the south of Sweden, for +example, near Lake Wener, the beds of one of the oldest of the +fossiliferous deposits, namely that formerly called Transition, and now +Silurian, by geologists, occur in as level a position as if they had +recently formed part of the delta of a great river, and been left dry on +the retiring of the annual floods. Aqueous rocks of about the same age +extend for hundreds of miles over the lake-district of North America, and +exhibit in like manner a stratification nearly undisturbed. The Table +Mountain at the Cape of Good Hope is another example of highly elevated yet +perfectly horizontal strata, no less than 3500 feet in thickness, and +consisting of sandstone of very ancient date. + +Instead of imagining that such fossiliferous rocks were always at their +present level, and that the sea was once high enough to cover them, we +suppose them to have constituted the ancient bed of the ocean, and that +they were gradually uplifted to their present height. This idea, however +startling it may at first appear, is quite in accordance, as before stated, +with the analogy of changes now going on in certain regions of the globe. +Thus, in parts of Sweden, and the shores and islands of the Gulf of +Bothnia, proofs have been obtained that the land is experiencing, and has +experienced for centuries, a slow upheaving movement. Playfair argued in +favour of this opinion in 1802; and in 1807, Von Buch, after his travels in +Scandinavia, announced his conviction that a rising of the land was in +progress. Celsius and other Swedish writers had, a century before, declared +their belief that a gradual change had, for ages, been taking place in the +relative level of land and sea. They attributed the change to a fall of the +waters both of the ocean and the Baltic. This theory, however, has now been +refuted by abundant evidence; for the alteration of relative level has +neither been universal nor every where uniform in quantity, but has +amounted, in some regions, to several feet in a century, in others to a few +inches; while in the southernmost part of Sweden, or the province of +Scania, there has been actually a loss instead of a gain of land, buildings +having gradually sunk below the level of the sea.[46-A] + +It appears, from the observations of Mr. Darwin and others, that very +extensive regions of the continent of South America have been undergoing +slow and gradual upheaval, by which the level plains of Patagonia, covered +with recent marine shells, and the Pampas of Buenos Ayres, have been raised +above the level of the sea.[46-B] On the other hand, the gradual sinking of +the west coast of Greenland, for the space of more than 600 miles from +north to south, during the last four centuries, has been established by the +observations of a Danish naturalist, Dr. Pingel. And while these proofs of +continental elevation and subsidence, by slow and insensible movements, +have been recently brought to light, the evidence has been daily +strengthened of continued changes of level effected by violent convulsions +in countries where earthquakes are frequent. There the rocks are rent from +time to time, and heaved up or thrown down several feet at once, and +disturbed in such a manner, that the original position of strata may, in +the course of centuries, be modified to any amount. + +It has also been shown by Mr. Darwin, that, in those seas where circular +coral islands and barrier reefs abound, there is a slow and continued +sinking of the submarine mountains on which the masses of coral are based; +while there are other areas of the South Sea, where the land is on the +rise, and where coral has been upheaved far above the sea-level. + +It would require a volume to explain to the reader the various facts which +establish the reality of these movements of land, whether of elevation or +depression, whether accompanied by earthquakes or accomplished slowly and +without local disturbance. Having treated fully of these subjects in the +Principles of Geology[46-C], I shall assume, in the present work, that such +changes are part of the actual course of nature; and when admitted, they +will be found to afford a key to the interpretation of a variety of +geological appearances, such as the elevation of horizontal, inclined, or +disturbed marine strata, and the superposition of freshwater to marine +deposits, afterwards to be described. It will also appear, in the sequel, +how much light the doctrine of a continued subsidence of land may throw on +the manner in which a series of strata, formed in shallow water, may have +accumulated to a great thickness. The excavation of valleys also, and other +effects of _denudation_, of which I shall presently treat, can alone be +understood when we duly appreciate the proofs, now on record, of the +prolonged rising and sinking of land, throughout wide areas. + +To conclude this subject, I may remind the reader, that were we to embrace +the doctrine which ascribes the elevated position of marine formations, and +the depression of certain freshwater strata, to oscillations in the level +of the waters instead of the land, we should be compelled to admit that the +ocean has been sometimes every where much shallower than at present, and at +others more than three miles deeper. + +[Illustration: Fig. 61. Vertical conglomerate and sandstone.] + +_Inclined stratification._--The most unequivocal evidence of a change in +the original position of strata is afforded by their standing up +perpendicularly on their edges, which is by no means a rare phenomenon, +especially in mountainous countries. Thus we find in Scotland, on the +southern skirts of the Grampians, beds of pudding-stone alternating with +thin layers of fine sand, all placed vertically to the horizon. When +Saussure first observed certain conglomerates in a similar position in the +Swiss Alps, he remarked that the pebbles, being for the most part of an +oval shape, had their longer axes parallel to the planes of stratification +(See fig. 61.). From this he inferred, that such strata must, at first, +have been horizontal, each oval pebble having originally settled at the +bottom of the water, with its flatter side parallel to the horizon, for the +same reason that an egg will not stand on either end if unsupported. Some +few, indeed, of the rounded stones in a conglomerate occasionally afford an +exception to the above rule, for the same reason that we see on a shingle +beach some oval or flat-sided pebbles resting on their ends or edges; these +having been forced along the bottom and against each other by a wave or +current so as to settle in this position. + +Vertical strata, when they can be traced continuously upwards or downwards +for some depth, are almost invariably seen to be parts of great curves, +which may have a diameter of a few yards, or of several miles. I shall +first describe two curves of considerable regularity, which occur in +Forfarshire, extending over a country twenty miles in breadth, from the +foot of the Grampians to the sea near Arbroath. + +The mass of strata here shown may be nearly 2000 feet in thickness, +consisting of red and white sandstone, and various coloured shales, the +beds being distinguishable into four principal groups, namely, No. 1. red +marl or shale; No. 2. red sandstone, used for building; No. 3. +conglomerate; and No. 4. grey paving-stone, and tile-stone, with green and +reddish shale, containing peculiar organic remains. A glance at the section +will show that each of the formations 2, 3, 4, are repeated thrice at the +surface, twice with a southerly, and once with a northerly inclination or +_dip_, and the beds in No. 1., which are nearly horizontal, are still +brought up twice by a slight curvature to the surface, once on each side of +A. Beginning at the north-west extremity, the tile-stones and conglomerates +No. 4. and No. 3. are vertical, and they generally form a ridge parallel to +the southern skirts of the Grampians. The superior strata Nos. 2. and 1. +become less and less inclined on descending to the valley of Strathmore, +where the strata, having a concave bend, are said by geologists to lie in a +"trough" or "basin." Through the centre of this valley runs an imaginary +line A, called technically a "synclinal line," where the beds, which are +tilted in opposite directions, may be supposed to meet. It is most +important for the observer to mark such lines, for he will perceive by the +diagram, that in travelling from the north to the centre of the basin, he +is always passing from older to newer beds; whereas, after crossing the +line A, and pursuing his course in the same southerly direction, he is +continually leaving the newer, and advancing upon older strata. All the +deposits which he had before examined begin then to recur in reversed +order, until he arrives at the central axis of the Sidlaw hills, where the +strata are seen to form an arch or _saddle_, having an _anticlinal_ line B, +in the centre. On passing this line, and continuing towards the S.E., the +formations 4, 3, and 2, are again repeated, in the same relative order of +superposition, but with a northerly dip. At Whiteness (see diagram) it will +be seen that the inclined strata are covered by a newer deposit, _a_, in +horizontal beds. These are composed of red conglomerate and sand, and are +newer than any of the groups, 1, 2, 3, 4, before described, and rest +_unconformably_ upon strata of the sandstone group, No. 2. + +[Illustration: Fig. 62. Section of Forfarshire, from N.W. to S.E., from +foot of the Grampians to the sea at Arbroath (volcanic or trap rocks +omitted). Length of section twenty miles.] + +An example of curved strata, in which the bends or convolutions of the rock +are sharper and far more numerous within an equal space, has been well +described by Sir James Hall.[48-A] It occurs near St. Abb's Head, on the +east coast of Scotland, where the rocks consist principally of a bluish +slate, having frequently a ripple-marked surface. The undulations of the +beds reach from the top to the bottom of cliffs from 200 to 300 feet in +height, and there are sixteen distinct bendings in the course of about six +miles, the curvatures being alternately concave and convex upwards. + +[Illustration: Fig. 63. Curved strata of slate near St. Abb's Head, +Berwickshire. (Sir J. Hall.)] + +[Illustration: Fig. 64. Block section.] + +An experiment was made by Sir James Hall, with a view of illustrating the +manner in which such strata, assuming them to have been originally +horizontal, may have been forced into their present position. A set of +layers of clay were placed under a weight, and their opposite ends pressed +towards each other with such force as to cause them to approach more nearly +together. On the removal of the weight, the layers of clay were found to be +curved and folded, so as to bear a miniature resemblance to the strata in +the cliffs. We must, however, bear in mind, that in the natural section or +sea-cliff we only see the foldings imperfectly, one part being invisible +beneath the sea, and the other, or upper portion, being supposed to have +been carried away by _denudation_, or that action of water which will be +explained in the next chapter. The dark lines in the accompanying plan +(fig. 64.) represent what is actually seen of the strata in part of the +line of cliff alluded to; the fainter lines, that portion which is +concealed beneath the sea level, as also that which is supposed to have +once existed above the present surface. + +[Illustration: Fig. 65. Experimental set-up.] + +We may still more easily illustrate the effects which a lateral thrust +might produce on flexible strata, by placing several pieces of differently +coloured cloths upon a table, and when they are spread out horizontally, +cover them with a book. Then apply other books to each end, and force them +towards each other. The folding of the cloths will exactly imitate those of +the bent strata. (See fig. 65.) + +Whether the analogous flexures in stratified rocks have really been due +to similar sideway movements is a question of considerable difficulty. +It will appear when the volcanic and granitic rocks are described, that +some of them have, when melted, been injected forcibly into fissures, +while others, already in a solid state, have been protruded upwards +through the incumbent crust of the earth, by which a great displacement +of flexible strata must have been caused. + +But we also know by the study of regions liable to earthquakes, that there +are causes at work in the interior of the earth capable of producing a +sinking in of the ground, sometimes very local, but sometimes extending +over a wide area. The frequent repetition, or continuance throughout long +periods, of such downward movements seems to imply the formation and +renewal of cavities at a certain depth below the surface, whether by the +removal of matter by volcanos and hot springs, or by the contraction of +argillaceous rocks by heat and pressure, or any other combination of +circumstances. Whatever conjectures we may indulge respecting the causes, +it is certain that pliable beds may, in consequence of unequal degrees of +subsidence, become folded to any amount, and have all the appearance of +having been compressed suddenly by a lateral thrust. + +The "Creeps," as they are called in coal-mines, afford an excellent +illustration of this fact.--First, it may be stated generally, that the +excavation of coal at a considerable depth causes the mass of overlying +strata to sink down bodily, even when props are left to support the roof of +the mine. "In Yorkshire," says Mr. Buddle, "three distinct subsidences were +perceptible at the surface, after the clearing out of three seams of coal +below, and innumerable vertical cracks were caused in the incumbent mass of +sandstone and shale, which thus settled down."[50-A] The exact amount of +depression in these cases can only be accurately measured where water +accumulates on the surface, or a railway traverses a coal-field. + +[Illustration: Fig. 66. Section of carboniferous strata, at Wallsend, +Newcastle, showing "Creeps." (J. Buddle, Esq.) Horizontal length of +section 174 feet. The upper seam, or main coal, here worked out, was +630 feet below the surface.] + +When a bed of coal is worked out, pillars or rectangular masses of coal are +left at intervals as props to support the roof, and protect the colliers. +Thus in fig. 66., representing a section at Wallsend, Newcastle, the +galleries which have been excavated are represented by the white spaces _a +b_, while the adjoining dark portions are parts of the original coal-seam +left as props, beds of sandy clay or shale constituting the floor of the +mine. When the props have been reduced in size, they are pressed down by +the weight of overlying rocks (no less than 630 feet thick) upon the shale +below, which is thereby squeezed and forced up into the open spaces. + +Now it might have been expected, that instead of the floor rising up, the +ceiling would sink down, and this effect, called a "Thrust," does, in fact, +take place where the pavement is more solid than the roof. But it usually +happens, in coal-mines, that the roof is composed of hard shale, or +occasionally of sandstone, more unyielding than the foundation, which often +consists of clay. Even where the argillaceous substrata are hard at first, +they soon become softened and reduced to a plastic state when exposed to +the contact of air and water in the floor of a mine. + +The first symptom of a "creep," says Mr. Buddle, is a slight curvature +at the bottom of each gallery, as at _a_, fig. 66.: then the pavement +continuing to rise, begins to open with a longitudinal crack, as at _b_: +then the points of the fractured ridge reach the roof, as at _c_; and, +lastly, the upraised beds close up the whole gallery, and the broken +portions of the ridge are re-united and flattened at the top, exhibiting +the flexure seen at _d_. Meanwhile the coal in the props has become +crushed and cracked by pressure. It is also found, that below the creeps +_a_, _b_, _c_, _d_, an inferior stratum, called the "metal coal," which +is 3 feet thick, has been fractured at the points _e_, _f_, _g_, _h_, +and has risen, so as to prove that the upward movement, caused by the +working out of the "main coal," has been propagated through a thickness +of 54 feet of argillaceous beds, which intervene between the two coal +seams. This same displacement has also been traced downwards more than +150 feet below the metal coal, but it grows continually less and less +until it becomes imperceptible. + +No part of the process above described is more deserving of our notice than +the slowness with which the change in the arrangement of the beds is +brought about. Days, months, or even years, will sometimes elapse between +the first bending of the pavement and the time of its reaching the roof. +Where the movement has been most rapid, the curvature of the beds is most +regular, and the reunion of the fractured ends most complete; whereas the +signs of displacement or violence are greatest in those creeps which have +required months or years for their entire accomplishment. Hence we may +conclude that similar changes may have been wrought on a larger scale in +the earth's crust by partial and gradual subsidences, especially where the +ground has been undermined throughout long periods of time; and we must be +on our guard against inferring sudden violence, simply because the +distortion of the beds is excessive. + +Between the layers of shale, accompanying coal, we sometimes see the +leaves of fossil ferns spread out as regularly as dried plants between +sheets of paper in the herbarium of a botanist. These fern-leaves, or +fronds, must have rested horizontally on soft mud, when first deposited. +If, therefore, they and the layers of shale are now inclined, or +standing on end, it is obviously the effect of subsequent derangement. +The proof becomes, if possible, still more striking when these strata, +including vegetable remains, are curved again and again, and even folded +into the form of the letter Z, so that the same continuous layer of coal +is cut through several times in the same perpendicular shaft. Thus, in +the coal-field near Mons, in Belgium, these zigzag bendings are repeated +four or five times, in the manner represented in fig. 67., the black +lines representing seams of coal.[53-A] + +[Illustration: Fig. 67. Zigzag flexures of coal near Mons.] + +_Dip and Strike._--In the above remarks, several technical terms have been +used, such as _dip_, the _unconformable position_ of strata, and the +_anticlinal_ and _synclinal_ lines, which, as well as the _strike_ of the +beds, I shall now explain. If a stratum or bed of rock, instead of being +quite level, be inclined to one side, it is said to _dip_; the point of the +compass to which it is inclined is called the _point of dip_, and the +degree of deviation from a level or horizontal line is called _the amount +of dip_, or _the angle of dip_. Thus, in the annexed diagram (fig. 68.), a +series of strata are inclined, and they dip to the north at an angle of +forty-five degrees. The _strike_, or _line of bearing_, is the prolongation +or extension of the strata in a direction _at right angles_ to the dip; and +hence it is sometimes called the _direction_ of the strata. Thus, in the +above instance of strata dipping to the north, their strike must +necessarily be east and west. We have borrowed the word from the German +geologists, _streichen_ signifying to extend, to have a certain direction. +Dip and strike may be aptly illustrated by a row of houses running east and +west, the long ridge of the roof representing the strike of the stratum of +slates, which dip on one side to the north, and on the other to the south. + +[Illustration: Fig. 68. Diagram.] + +A stratum which is horizontal, or quite level in all directions, has +neither dip nor strike. + +It is always important for the geologist, who is endeavouring to comprehend +the structure of a country, to learn how the beds dip in every part of the +district; but it requires some practice to avoid being occasionally +deceived, both as to the point of dip and the amount of it. + +[Illustration: Fig. 69. Apparent horizontality of inclined strata.] + +If the upper surface of a hard stony stratum be uncovered, whether +artificially in a quarry, or by the waves at the foot of a cliff, it is +easy to determine towards what point of the compass the slope is steepest, +or in what direction water would flow, if poured upon it. This is the true +dip. But the edges of highly inclined strata may give rise to perfectly +horizontal lines in the face of a vertical cliff, if the observer see the +strata in the line of their strike, the dip being inwards from the face of +the cliff. If, however, we come to a break in the cliff, which exhibits a +section exactly at right angles to the line of the strike, we are then able +to ascertain the true dip. In the annexed drawing (fig. 69.), we may +suppose a headland, one side of which faces to the north, where the beds +would appear perfectly horizontal to a person in the boat; while in the +other side facing the west, the true dip would be seen by the person on +shore to be at an angle of 40°. If, therefore, our observations are +confined to a vertical precipice facing in one direction, we must endeavour +to find a ledge or portion of the plane of one of the beds projecting +beyond the others, in order to ascertain the true dip. + +[Illustration: Fig. 70. Explanatory sketch.] + +It is rarely important to determine the angle of inclination with such +minuteness as to require the aid of the instrument called a clinometer. We +may measure the angle within a few degrees by standing exactly opposite to +a cliff where the true dip is exhibited, holding the hands immediately +before the eyes, and placing the fingers of one in a perpendicular, and of +the other in a horizontal position, as in fig. 70. It is thus easy to +discover whether the lines of the inclined beds bisect the angle of 90°, +formed by the meeting of the hands, so as to give an angle of 45°, or +whether it would divide the space into two equal or unequal portions. The +upper dotted line may express a stratum dipping to the north; but should +the beds dip precisely to the opposite point of the compass as in the +lower dotted line, it will be seen that the amount of inclination may still +be measured by the hands with equal facility. + +[Illustration: Fig. 71. Section illustrating the structure of +the Swiss Jura.] + +[Illustration: Fig. 72. Ground plan of the denuded ridge, fig. 71.] + +[Illustration: Fig. 73. Transverse section.] + +It has been already seen, in describing the curved strata on the east coast +of Scotland, in Forfarshire and Berwickshire, that a series of concave and +convex bendings are occasionally repeated several times. These usually form +part of a series of parallel waves of strata, which are prolonged in the +same direction throughout a considerable extent of country. Thus, for +example, in the Swiss Jura, that lofty chain of mountains has been proved +to consist of many parallel ridges, with intervening longitudinal valleys, +as in fig. 71., the ridges being formed by curved fossiliferous strata, of +which the nature and dip are occasionally displayed in deep transverse +gorges, called "cluses," caused by fractures at right angles to the +direction of the chain.[55-A] Now let us suppose these ridges and parallel +valleys to run north and south, we should then say that the _strike_ of the +beds is north and south, and the _dip_ east and west. Lines drawn along the +summits of the ridges, A, B, would be anticlinal lines, and one following +the bottom of the adjoining valleys a synclinal line. It will be observed +that some of these ridges, A, B, are unbroken on the summit, whereas one of +them, C, has been fractured along the line of strike, and a portion of it +carried away by denudation, so that the ridges of the beds in the +formations _a_, _b_, _c_, come out to the day, or, as the miners say, _crop +out_, on the sides of a valley. The ground plan of such a denuded ridge as +C, as given in a geological map, may be expressed by the diagram fig. 72., +and the cross section of the same by fig. 73. The line D E, fig. 72., is +the anticlinal line, on each side of which the dip is in opposite +directions, as expressed by the arrows. The emergence of strata at the +surface is called by miners their _outcrop_ or _basset_. + +If, instead of being folded into parallel ridges, the beds form a boss or +dome-shaped protuberance, and if we suppose the summit of the dome carried +off, the ground plan would exhibit the edges of the strata forming a +succession of circles, or ellipses, round a common centre. These circles +are the lines of strike, and the dip being always at right angles is +inclined in the course of the circuit to every point of the compass, +constituting what is termed a qua-quaversal dip--that is, turning each way. + +There are endless variations in the figures described by the basset-edges +of the strata, according to the different inclination of the beds, and the +mode in which they happen to have been denuded. One of the simplest rules +with which every geologist should be acquainted, relates to the V-like form +of the beds as they crop out in an ordinary valley. First, if the strata be +horizontal, the V-like form will be also on a level, and the newest strata +will appear at the greatest heights. + +Secondly, if the beds be inclined and intersected by a valley sloping in +the same direction, and the dip of the beds be less steep than the slope +of the valley, then the V's, as they are often termed by miners, will +point upwards (see fig. 74.), those formed by the newer beds appearing +in a superior position, and extending highest up the valley, as A +is seen above B. + +[Illustration: Fig. 74. Slope of valley 40°, dip of strata 20°.] + +Thirdly, if the dip of the beds be steeper than the slope of the valley, +then the V's will point downwards (see fig. 75.), and those formed of the +older beds will now appear uppermost, as B appears above A. + +[Illustration: Fig. 75. Slope of valley 20°, dip of strata 50°.] + +Fourthly, in every case where the strata dip in a contrary direction to +the slope of the valley, whatever be the angle of inclination, the newer +beds will appear the highest, as in the first and second cases. This is +shown by the drawing (fig. 76.), which exhibits strata rising at an +angle of 20°, and crossed by a valley, which declines in an opposite +direction at 20°.[57-A] + +[Illustration: Fig. 76. Slope of valley 20°, dip of strata 20°, +in opposite directions.] + +These rules may often be of great practical utility; for the different +degrees of dip occurring in the two cases represented in figures 74 and 75. +may occasionally be encountered in following the same line of flexure at +points a few miles distant from each other. A miner unacquainted with the +rule, who had first explored the valley (fig. 74.), may have sunk a +vertical shaft below the coal seam A, until he reached the inferior bed B. +He might then pass to the valley fig. 75., and discovering there also the +outcrop of two coal seams, might begin his workings in the uppermost in the +expectation of coming down to the other bed A, which would be observed +cropping out lower down the valley. But a glance at the section will +demonstrate the futility of such hopes. + +In the majority of cases, an anticlinal axis forms a ridge, and a synclinal +axis a valley, as in A, B, fig. 62. p. 48.; but there are exceptions to +this rule, the beds sometimes sloping inwards from either side of a +mountain, as in fig. 77. + +[Illustration: Fig. 77. Cross section.] + +On following one of the anticlinal ridges of the Jura, before mentioned, A, +B, C, fig. 71., we often discover longitudinal cracks and sometimes large +fissures along the line where the flexure was greatest. Some of these, as +above stated, have been enlarged by denudation into valleys of considerable +width, as at C, fig. 71., which follow the line of strike, and which we may +suppose to have been hollowed out at the time when these rocks were still +beneath the level of the sea, or perhaps at the period of their gradual +emergence from beneath the waters. The existence of such cracks at the +point of the sharpest bending of solid strata of limestone is precisely +what we should have expected; but the occasional want of all similar signs +of fracture, even where the strain has been greatest, as at _a_, fig. 71., +is not always easy to explain. We must imagine that many strata of +limestone, chert, and other rocks which are now brittle, were pliant when +bent into their present position. They may have owed their flexibility in +part to the fluid matter which they contained in their minute pores, as +before described (p. 35.), and in part to the permeation of sea-water while +they were yet submerged. + +[Illustration: Fig. 78. Strata of chert, grit, and marl, near St. +Jean de Luz.] + +At the western extremity of the Pyrenees, great curvatures of the strata +are seen in the sea cliffs, where the rocks consist of marl, grit, and +chert. At certain points, as at _a_, fig. 78., some of the bendings of the +flinty chert are so sharp, that specimens might be broken off, well fitted +to serve as ridge-tiles on the roof of a house. Although this chert could +not have been brittle as now, when first folded into this shape, it +presents, nevertheless, here and there at the points of greatest flexure +small cracks, which show that it was solid, and not wholly incapable of +breaking at the period of its displacement. The numerous rents alluded to +are not empty, but filled with calcedony and quartz. + +[Illustration: Fig. 79. Cross section. + + _g._ gypsum. + _m._ marl.] + +Between San Caterina and Castrogiovanni, in Sicily, bent and undulating +gypseous marls occur, with here and there thin beds of solid gypsum +interstratified. Sometimes these solid layers have been broken into +detached fragments, still preserving their sharp edges (_g g_, fig. +79.), while the continuity of the more pliable and ductile marls, _m m_, +has not been interrupted. + +[Illustration: Fig. 80. Cross section.] + +I shall conclude my remarks on bent strata by stating, that, in mountainous +regions like the Alps, it is often difficult for an experienced geologist +to determine correctly the relative age of beds by superposition, so often +have the strata been folded back upon themselves, the upper parts of the +curve having been removed by denudation. Thus, if we met with the strata +seen in the section fig. 80., we should naturally suppose that there were +twelve distinct beds, or sets of beds, No. 1. being the newest, and No. 12. +the oldest of the series. But this section may, perhaps, exhibit merely six +beds, which have been folded in the manner seen in fig. 81., so that each +of them is twice repeated, the position of one half being reversed, and +part of No. 1., originally the uppermost, having now become the lowest of +the series. These phenomena are often observable on a magnificent scale in +certain regions in Switzerland in precipices from 2000 to 3000 feet in +perpendicular height. In the Iselten Alp, in the valley of the Lutschine, +between Unterseen and Grindelwald, curves of calcareous shale are seen from +1000 to 1500 feet in height, in which the beds sometimes plunge down +vertically for a depth of 1000 feet and more, before they bend round again. +There are many flexures not inferior in dimensions in the Pyrenees, as +those near Gavarnie, at the base of Mont Perdu. + +[Illustration: Fig. 81. Cross section.] + +[Illustration: Fig. 82. Curved strata of the Iselten Alp.] + +[Illustration: Fig. 83. Unconformable junction of old red sandstone and +Silurian schist at the Siccar Point, near St. Abb's Head, Berwickshire. See +also Frontispiece.] + +_Unconformable stratification._--Strata are said to be unconformable, +when one series is so placed over another, that the planes of the +superior repose on the edges of the inferior (see fig. 83.). In this +case it is evident that a period had elapsed between the production of +the two sets of strata, and that, during this interval, the older +series had been tilted and disturbed. Afterwards the upper series was +thrown down in horizontal strata upon it. If these superior beds, as +_d_, _d_, fig. 83., are also inclined, it is plain that the lower +strata, _a_, _a_, have been twice displaced; first, before the +deposition of the newer beds, _d_, _d_, and a second time when these +same strata were thrown out of the horizontal position. + +Playfair has remarked[60-A] that this kind of junction which we now call +unconformable had been described before the time of Hutton, but that he was +the first geologist who appreciated its importance, as illustrating the +high antiquity and great revolutions of the globe. He had observed that +where such contacts occur, the lowest beds of the newer series very +generally consist of a breccia or conglomerate consisting of angular and +rounded fragments, derived from the breaking up of the more ancient rocks. +On one occasion the Scotch geologist took his two distinguished pupils, +Playfair and Sir James Hall, to the cliffs on the east coast of Scotland, +near the village of Eyemouth, not far from St. Abb's Head, where the +schists of the Lammermuir range are undermined and dissected by the sea. +Here the curved and vertical strata, now known to be of Silurian age, and +which often exhibit a ripple-marked surface[60-B], are well exposed at the +headland called the Siccar Point, penetrating with their edges into the +incumbent beds of slightly inclined sandstone, in which large pieces of the +schist, some round and others angular, are united by an arenaceous cement. +"What clearer evidence," exclaims Playfair, "could we have had of the +different formation of these rocks, and of the long interval which +separated their formation, had we actually seen them emerging from the +bosom of the deep? We felt ourselves necessarily carried back to the time +when the schistus on which we stood was yet at the bottom of the sea, and +when the sandstone before us was only beginning to be deposited in the +shape of sand or mud, from the waters of a superincumbent ocean. An epoch +still more remote presented itself, when even the most ancient of these +rocks, instead of standing upright in vertical beds, lay in horizontal +planes at the bottom of the sea, and was not yet disturbed by that +immeasurable force which has burst asunder the solid pavement of the globe. +Revolutions still more remote appeared in the distance of this +extraordinary perspective. The mind seemed to grow giddy by looking so far +into the abyss of time; and while we listened with earnestness and +admiration to the philosopher who was now unfolding to us the order and +series of these wonderful events, we became sensible how much farther +reason may sometimes go than imagination can venture to follow."[60-C] + +In the frontispiece of this volume the reader will see a view of this +classical spot, reduced from a large picture, faithfully sketched and +coloured from nature by the youngest son of the late Sir James Hall. It was +impossible, however, to do justice to the original sketch, in an +engraving, as the contrast of the red sandstone and the light fawn-coloured +vertical schists could not be expressed. From the point of view here +selected, the underlying beds of the perpendicular schist, _a_, are visible +at _b_ through a small opening in the fractured beds of the covering of red +sandstone, _d d_, while on the vertical face of the old schist at _a' a"_ a +conspicuous ripple-mark is displayed. + +[Illustration: Fig. 84. Junction of unconformable strata near +Mons, in Belgium.] + +It often happens that in the interval between the deposition of two sets of +unconformable strata, the inferior rock has not only been denuded, but +drilled by perforating shells. Thus, for example, at Autreppe and Gusigny, +near Mons, beds of an ancient (paleozoic) limestone, highly inclined, and +often bent, are covered with horizontal strata of greenish and whitish +marls of the Cretaceous formation. The lowest and therefore the oldest bed +of the horizontal series is usually the sand and conglomerate, _a_, in +which are rounded fragments of stone, from an inch to two feet in diameter. +These fragments have often adhering shells attached to them, and have been +bored by perforating mollusca. The solid surface of the inferior limestone +has also been bored, so as to exhibit cylindrical and pear-shaped cavities, +as at _c_, the work of saxicavous mollusca; and many rents, as at _b_, +which descend several feet or yards into the limestone, have been filled +with sand and shells, similar to those in the stratum _a_. + +_Fractures of the strata and faults._--Numerous rents may often be seen in +rocks which appear to have been simply broken, the separated parts +remaining in the same places; but we often find a fissure, several inches +or yards wide, intervening between the disunited portions. These fissures +are usually filled with fine earth and sand, or with angular fragments of +stone, evidently derived from the fracture of the contiguous rocks. + +The face of each wall of the fissure is often beautifully polished, as if +glazed, and not unfrequently striated or scored with parallel furrows and +ridges, such as would be produced by the continued rubbing together of +surfaces of unequal hardness. These polished surfaces are called by miners +"slickensides." It is supposed that the lines of the striæ indicate the +direction in which the rocks were moved. During one of the minor +earthquakes in Chili, which happened about the year 1840, and was described +to me by an eye-witness, the brick walls of a building were rent vertically +in several places, and made to vibrate for several minutes during each +shock, after which they remained uninjured, and without any opening, +although the line of each crack was still visible. When all movement had +ceased, there were seen on the floor of the house, at the bottom of each +rent, small heaps of fine brickdust, evidently produced by trituration. + +[Illustration: Fig. 85. Faults. A B perpendicular, C D oblique +to the horizon.] + +It is not uncommon to find the mass of rock, on one side of a fissure, +thrown up above or down below the mass with which it was once in contact on +the other side. This mode of displacement is called a shift, slip, or +fault. "The miner," says Playfair, describing a fault, "is often perplexed, +in his subterraneous journey, by a derangement in the strata, which changes +at once all those lines and bearings which had hitherto directed his +course. When his mine reaches a certain plane, which is sometimes +perpendicular, as in A B, fig. 85., sometimes oblique to the horizon (as in +C D, ibid.), he finds the beds of rock broken asunder, those on the one +side of the plane having changed their place, by sliding in a particular +direction along the face of the others. In this motion they have sometimes +preserved their parallelism, as in fig. 85., so that the strata on each +side of the faults A B, C D, continue parallel to one another; in other +cases, the strata on each side are inclined, as in _a_, _b_, _c_, _d_ (fig. +86.), though their identity is still to be recognized by their possessing +the same thickness, and the same internal characters."[62-A] + +[Illustration: Fig. 86. E F, fault or fissure filled with rubbish, on each +side of which the shifted strata are not parallel.] + +In Coalbrook Dale, says Mr. Prestwich[62-B], deposits of sandstone, shale, +and coal, several thousand feet thick, and occupying an area of many miles, +have been shivered into fragments, and the broken remnants have been placed +in very discordant positions, often at levels differing several hundred +feet from each other. The sides of the faults, when perpendicular, are +commonly separated several yards, but are sometimes as much as 50 yards +asunder, the interval being filled with broken _débris_ of the strata. In +following the course of the same fault it is sometimes found to produce in +different places very unequal changes of level, the amount of shift being +in one place 300, and in another 700 feet, which arises, in some cases, +from the union of two or more faults. In other words, the disjointed strata +have in certain districts been subjected to renewed movements, which they +have not suffered elsewhere. + +We may occasionally see exact counterparts of these slips, on a small +scale, in pits of fine loose sand and gravel, many of which have doubtless +been caused by the drying and shrinking of argillaceous and other beds, +slight subsidences having taken place from failure of support. Sometimes, +however, even these small slips may have been produced during earthquakes; +for land has been moved, and its level, relatively to the sea, considerably +altered, within the period when much of the alluvial sand and gravel now +covering the surface of continents was deposited. + +I have already stated that a geologist must be on his guard, in a region of +disturbed strata, against inferring repeated alternations of rocks, when, +in fact, the same strata, once continuous, have been bent round so as to +recur in the same section, and with the same dip. A similar mistake has +often been occasioned by a series of faults. + +[Illustration: Fig. 87. Apparent alternations of strata caused +by vertical faults.] + +If, for example, the dark line A H (fig. 87.) represent the surface of a +country on which the strata _a b c_ frequently crop out, an observer, +who is proceeding from H to A, might at first imagine that at every step +he was approaching new strata, whereas the repetition of the same beds +has been caused by vertical faults, or downthrows. Thus, suppose the +original mass, A, B, C, D, to have been a set of uniformly inclined +strata, and that the different masses under E F, F G, and G D, sank down +successively, so as to leave vacant the spaces marked in the diagram by +dotted lines, and to occupy those marked by the continuous lines, then +let denudation take place along the line A H, so that the protruding +masses indicated by the fainter lines are swept away,--a miner, who has +not discovered the faults, finding the mass _a_, which we will suppose +to be a bed of coal four times repeated, might hope to find four beds, +workable to an indefinite depth, but first on arriving at the fault G he +is stopped suddenly in his workings, upon reaching the strata of +sandstone _c_, or on arriving at the line of fault F he comes partly +upon the shale _b_, and partly on the sandstone _c_, and on reaching E +he is again stopped by a wall composed of the rock _d_. + +[Illustration: Fig. 88. Cross section.] + +The very different levels at which the separated parts of the same strata +are found on the different sides of the fissure, in some faults, is truly +astonishing. One of the most celebrated in England is that called the +"ninety-fathom dike," in the coal-field of Newcastle. This name has been +given to it, because the same beds are ninety fathoms lower on the northern +than they are on the southern side. The fissure has been filled by a body +of sand, which is now in the state of sandstone, and is called the dike, +which is sometimes very narrow, but in other places more than twenty yards +wide.[64-A] The walls of the fissure are scored by grooves, such as would +have been produced if the broken ends of the rock had been rubbed along the +plane of the fault.[64-B] In the Tynedale and Craven faults, in the north +of England, the vertical displacement is still greater, and has extended in +a horizontal direction for a distance of thirty miles or more. Some +geologists consider it necessary to imagine that the upward or downward +movement in these cases was accomplished at a single stroke, and not by a +series of sudden but interrupted movements. This idea appears to have been +derived from a notion that the grooved walls have merely been rubbed in one +direction. But this is so far from being a constant phenomenon in faults, +that it has often been objected to the received theory respecting those +polished surfaces called "slickensides" (see above, p. 61.), that the striæ +are not always parallel, but often curved and irregular. It has, moreover, +been remarked, that not only the walls of the fissure or fault, but its +earthy contents, sometimes present the same polished and striated faces. +Now these facts seem to indicate partial changes in the direction of the +movement, and some slidings subsequent to the first filling up of the +fissure. Suppose the mass of rock A, B, C, to overlie an extensive chasm _d +e_, formed at the depth of several miles, whether by the gradual +contraction in bulk of a melted mass passing into a solid or crystalline +state, or the shrinking of argillaceous strata, baked by a moderate heat, +or by the subtraction of matter by volcanic action, or any other cause. +Now, if this region be convulsed by earthquakes, the fissures _f g_, and +others at right angles to them, may sever the mass B from A and from C, so +that it may move freely, and begin to sink into the chasm. A fracture may +be conceived so clean and perfect as to allow it to subside at once to the +bottom of the subterranean cavity; but it is far more probable that the +sinking will be effected at successive periods during different +earthquakes, the mass always continuing to slide in the same direction +along the planes of the fissures _f g_, and the edges of the falling mass +being continually more broken and triturated at each convulsion. If, as is +not improbable, the circumstances which have caused the failure of support +continue in operation, it may happen that when the mass B has filled the +cavity first formed, its foundations will again give way under it, so that +it will fall again in the same direction. But, if the direction should +change, the fact could not be discovered by observing the slickensides, +because the last scoring would efface the lines of previous friction. In +the present state of our ignorance of the causes of subsidence, an +hypothesis which can explain the great amount of displacement in some +faults, on sound mechanical principles, by a succession of movements, is +far preferable to any theory which assumes each fault to have been +accomplished by a single upcast or downthrow of several thousand feet. For +we know that there are operations now in progress, at great depths in the +interior of the earth, by which both large and small tracts of ground are +made to rise above and sink below their former level, some slowly and +insensibly, others suddenly and by starts, a few feet or yards at a time; +whereas there are no grounds for believing that, during the last 3000 years +at least, any regions have been either upheaved or depressed, at a single +stroke, to the amount of several hundred, much less several thousand feet. +When some of the ancient marine formations are described in the sequel, it +will appear that their structure and organic contents point to the +conclusion, that the floor of the ocean was slowly sinking at the time of +their origin. The downward movement was very gradual, and in Wales and the +contiguous parts of England a maximum thickness of 32,000 feet (more than +six miles) of Carboniferous, Devonian, and Silurian rock was formed, whilst +the bed of the sea was all the time continuously and tranquilly +subsiding.[65-A] Whatever may have been the changes which the solid +foundation underwent, whether accompanied by the melting, consolidation, +crystallization, or desiccation of subjacent mineral matter, it is clear +from the fact of the sea having remained shallow all the while that the +bottom never sank down suddenly to the depth of many hundred feet at once. + +It is by assuming such reiterated variations of level, each separately of +small vertical amount, but multiplied by time till they acquire importance +in the aggregate, that we are able to explain the phenomena of denudation, +which will be treated of in the next chapter. By such movements every +portion of the surface of the land becomes in its turn a line of coast, and +is exposed to the action of the waves and tides. A country which is +undergoing such movement is never allowed to settle into a state of +equilibrium, therefore the force of rivers and torrents to remove or +excavate soil and rocky masses is sustained in undiminished energy. + + +FOOTNOTES: + +[46-A] In the first three editions of my Principles of Geology, I expressed +many doubts as to the validity of the alleged proofs of a gradual rise of +land in Sweden; but after visiting that country, in 1834, I retracted these +objections, and published a detailed statement of the observations which +led me to alter my opinion in the Phil. Trans. 1835, Part I. See also the +Principles, 4th and subsequent editions. + +[46-B] See his Journal of a Naturalist in Voyage of the Beagle, and his +work on Coral Reefs. + +[46-C] See chapters xxviii. to xxxi. inclusive. + +[48-A] Edin. Trans. vol. vii. pl. 3. + +[50-A] Proceedings of Geol. Soc. vol. iii. p. 148. + +[53-A] See plan by M. Chevalier, Burat's D'Aubuisson, tom. ii. p. 334. + +[55-A] See M. Thurmann's work, "Essai sur les Soulèvemens Jurassiques +du Porrentruy, Paris, 1832," with whom I examined part of these +mountains in 1835. + +[57-A] I am indebted to the kindness of T. Sopwith, Esq., for three +models which I have copied in the above diagrams; but the beginner may +find it by no means easy to understand such copies, although, if he were +to examine and handle the originals, turning them about in different +ways, he would at once comprehend their meaning as well as the import of +others far more complicated, which the same engineer has constructed to +illustrate _faults_. + +[60-A] Biographical account of Dr. Hutton. + +[60-B] See above, p. 49. and section. + +[60-C] Playfair, ibid.; see his Works, Edin. 1822, vol. iv. p. 81. + +[62-A] Playfair, Illust. of Hutt. Theory, § 42. + +[62-B] Geol. Trans. second series, vol. v. p. 452. + +[64-A] Conybeare and Phillips, Outlines, &c. p. 376. + +[64-B] Phillips, Geology, Lardner's Cyclop. p. 41. + +[65-A] See the results of the "Geological Survey of Great Britain;" +Memoirs, vols. i. and ii., by Sir H. De la Beche, Mr. A. C. Ramsay, +and Mr. John Phillips. + + + + +CHAPTER 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. + + +Denudation, which has been occasionally spoken of in the preceding +chapters, is the removal of solid matter by water in motion, whether of +rivers or of the waves and currents of the sea, and the consequent laying +bare of some inferior rock. Geologists have perhaps been seldom in the +habit of reflecting that this operation has exerted an influence on the +structure of the earth's crust as universal and important as sedimentary +deposition itself; for denudation is the inseparable accompaniment of the +production of all new strata of mechanical origin. The formation of every +new deposit by the transport of sediment and pebbles necessarily implies +that there has been, somewhere else, a grinding down of rock into rounded +fragments, sand, or mud, equal in quantity to the new strata. All +deposition, therefore, except in the case of a shower of volcanic ashes, is +the sign of superficial waste going on contemporaneously, and to an equal +amount elsewhere. The gain at one point is no more than sufficient to +balance the loss at some other. Here a lake has grown shallower, there a +ravine has been deepened. The bed of the sea has in one region been raised +by the accumulation of new matter, in another its depth has been augmented +by the abstraction of an equal quantity. + +When we see a stone building, we know that somewhere, far or near, a quarry +has been opened. The courses of stone in the building may be compared to +successive strata, the quarry to a ravine or valley which has suffered +denudation. As the strata, like the courses of hewn stone, have been laid +one upon another gradually, so the excavation both of the valley and quarry +have been gradual. To pursue the comparison still farther, the superficial +heaps of mud, sand, and gravel, usually called alluvium, may be likened to +the rubbish of a quarry which has been rejected as useless by the workmen, +or has fallen upon the road between the quarry and the building, so as to +lie scattered at random over the ground. + +If, then, the entire mass of stratified deposits in the earth's crust is at +once the monument and measure of the denudation which has taken place, on +how stupendous a scale ought we to find the signs of this removal of +transported materials in past ages! Accordingly, there are different +classes of phenomena, which attest in a most striking manner the vast +spaces left vacant by the erosive power of water. I may allude, first, to +those valleys on both sides of which the same strata are seen following +each other in the same order, and having the same mineral composition and +fossil contents. We may observe, for example, several formations, as Nos. +1, 2, 3, 4, in the accompanying diagram (fig. 89.); No. 1. conglomerate, +No. 2. clay, No. 3. grit, and No. 4. limestone, each repeated in a series +of hills separated by valleys varying in depth. When we examine the +subordinate parts of these four formations, we find, in like manner, +distinct beds in each, corresponding, on the opposite sides of the valleys, +both in composition and order of position. No one can doubt that the strata +were originally continuous, and that some cause has swept away the portions +which once connected the whole series. A torrent on the side of a mountain +produces similar interruptions; and when we make artificial cuts in +lowering roads, we expose, in like manner, corresponding beds on either +side. But in nature, these appearances occur in mountains several thousand +feet high, and separated by intervals of many miles or leagues in extent, +of which a grand exemplification is described by Dr. MacCulloch, on the +north-western coast of Ross-shire, in Scotland.[67-A] The fundamental rock +of that country is gneiss, in disturbed strata, on which beds of nearly +horizontal red sandstone rest unconformably. The latter are often very +thin, forming mere flags, with their surfaces, distinctly ripple-marked. +They end abruptly on the declivities of many insulated mountains, which +rise up at once to the height of about 2000 feet above the gneiss of the +surrounding plain or table land, and to an average elevation of about 3000 +feet above the sea, which all their summits generally attain. The base of +gneiss varies in height, so that the lower portions of the sandstone occupy +different levels, and the thickness of the mass is various, sometimes +exceeding 3000 feet. It is impossible to compare these scattered and +detached portions without imagining that the whole country has once been +covered with a great body of sandstone, and that masses from 1000 to more +than 3000 feet in thickness have been removed. + +[Illustration: Fig. 89. Valleys of denudation. _a._ alluvium.] + +[Illustration: Fig. 90. Denudation of red sandstone on north-west coast of +Ross-shire. (MacCulloch.)] + +In the "Survey of Great Britain" (vol. i.), Professor Ramsay has shown +that the missing beds, removed from the summit of the Mendips, must have +been nearly a mile in thickness; and he has pointed out considerable areas +in South Wales and some of the adjacent counties of England, where a series +of palæozoic strata, not less than 11,000 feet in thickness, have been +stripped off. All these materials have of course been transported to new +regions, and have entered into the composition of more modern formations. +On the other hand, it is shown by observations in the same "Survey," that +the palæozoic strata are from 20,000 to 30,000 feet thick. It is clear that +such rocks, formed of mud and sand, now for the most part consolidated, are +the monuments of denuding operations, which took place on a grand scale at +a very remote period in the earth's history. For, whatever has been given +to one area must always have been borrowed from another; a truth which, +obvious as it may seem when thus stated, must be repeatedly impressed on +the student's mind, because in many geological speculations it is taken for +granted that the external crust of the earth has been always growing +thicker, in consequence of the accumulation, period after period, of +sedimentary matter, as if the new strata were not always produced at the +expense of pre-existing rocks, stratified or unstratified. By duly +reflecting on the fact, that all deposits of mechanical origin imply the +transportation from some other region, whether contiguous or remote, of an +equal amount of solid matter, we perceive that the stony exterior of the +planet must always have grown thinner in one place whenever, by accessions +of new strata, it was acquiring density in another. No doubt the vacant +space left by the missing rocks, after extensive denudation, is less +imposing to the imagination than a vast thickness of conglomerate or +sandstone, or the bodily presence as it were of a mountain-chain, with all +its inclined and curved strata. But the denuded tracts speak a clear and +emphatic language to our reason, and, like repeated layers of fossil +nummulites, corals or shells, or like numerous seams of coal, each based on +its under clay full of the roots of trees, still remaining in their natural +position, demand an indefinite lapse of time for their elaboration. + +No one will maintain that the fossils entombed in these rocks did not +belong to many successive generations of plants and animals. In like +manner, each sedimentary deposit attests a slow and gradual action, and the +strata not only serve as a measure of the amount of denudation +simultaneously effected elsewhere, but are also a correct indication of the +rate at which the denuding operation was carried on. + +Perhaps the most convincing evidence of denudation on a magnificent scale +is derived from the levelled surfaces of districts where large faults +occur. I have shown, in fig. 87. p. 63., and in fig. 91., how angular and +protruding masses of rock might naturally have been looked for on the +surface immediately above great faults, although in fact they rarely exist. +This phenomenon may be well studied in those districts where coal has been +extensively worked, for there the former relation of the beds which have +shifted their position may be determined with great accuracy. Thus in the +coal field of Ashby de la Zouch, in Leicestershire (see fig. 91.), a fault +occurs, on one side of which the coal beds _a b c d_ rise to the height of +500 feet above the corresponding beds on the other side. But the uplifted +strata do not stand up 500 feet above the general surface; on the contrary, +the outline of the country, as expressed by the line _z z_, is uniform and +unbroken, and the mass indicated by the dotted outline must have been +washed away.[69-A] There are proofs of this kind in some level countries, +where dense masses of strata have been cleared away from areas several +hundred square miles in extent. + +[Illustration: Fig. 91. Faults and denuded coal strata, Ashby de +la Zouch. (Mammat.)] + +In the Newcastle coal district it is ascertained that faults occur in +which the upward or downward movement could not have been less than 140 +fathoms, which, had they affected equally the configuration of the +surface to that amount, would produce mountains with precipitous +escarpments nearly 1000 feet high, or chasms of the like depth; yet is +the actual level of the country absolutely uniform--affording no trace +whatever of subterranean movements.[69-B] + +The ground from which these materials have been removed is usually +overspread with heaps of sand and gravel, formed out of the ruins of the +very rocks which have disappeared. Thus, in the districts above referred +to, they consist of rounded and angular fragments of hard sandstone, +limestone, and ironstone, with a small quantity of the more destructible +shale, and even rounded pieces of coal. + +Allusion has been already made to the shattered state and discordant +position of the carboniferous strata in Coalbrook Dale (p. 62.). The +collier cannot proceed three or four yards without meeting with small +slips, and from time to time he encounters faults of considerable +magnitude, which have thrown the rocks up or down several hundred feet. Yet +the superficial inequalities to which these dislocated masses originally +gave rise are no longer discernible, and the comparative flatness of the +existing surface can only be explained, as Mr. Prestwich has observed, by +supposing the fractured portions to have been removed by water. It is also +clear that strata of red sandstone, more than 1000 feet thick, which once +covered the coal, in the same region, have been carried away from large +areas. That water has, in this case, been the denuding agent, we may infer +from the fact that the rocks have yielded according to their different +degrees of hardness; the hard trap of the Wrekin, for example, and other +hills, having resisted more than the softer shale and sandstone, so as now +to stand out in bold relief.[70-A] + +_Origin of valleys._--Many of the earlier geologists, and Dr. Hutton among +them, taught that "rivers have in general hollowed out their valleys." This +is true only of rivulets and torrents which are the feeders of the larger +streams, and which, descending over rapid slopes, are most subject to +temporary increase and diminution in the volume of their waters. The +quantity of mud, sand, and pebbles constituting many a modern delta proves +indisputably that no small part of the inequalities now existing on the +earth's surface are due to fluviatile action; but the principal valleys in +almost every great hydrographical basin in the world, are of a shape and +magnitude which imply that they have been due to other causes besides the +mere excavating power of rivers. + +Some geologists have imagined that a deluge, or succession of deluges, may +have been the chief denuding agency, and they have speculated on a series +of enormous waves raised by the instantaneous upthrow of continents or +mountain chains out of the sea. But even were we disposed to grant such +sudden upheavals of the floor of the ocean, and to assume that great waves +would be the consequence of each convulsion, it is not easy to explain the +observed phenomena by the aid of so gratuitous an hypothesis. + +On the other hand, a machinery of a totally different kind seems capable of +giving rise to effects of the required magnitude. It has now been +ascertained that the rising and sinking of extensive portions of the +earth's crust, whether insensibly or by a repetition of sudden shocks, is +part of the actual course of nature, and we may easily comprehend how the +land may have been exposed during these movements to abrasion by the waves +of the sea. In the same manner as a mountain mass may, in the course of +ages, be formed by sedimentary deposition, layer after layer, so masses +equally voluminous may in time waste away by inches; as, for example, if +beds of incoherent materials are raised slowly in an open sea where a +strong current prevails. It is well known that some of these oceanic +currents have a breadth of 200 miles, and that they sometimes run for a +thousand miles or more in one direction, retaining a considerable velocity +even at the depth of several hundred feet. Under these circumstances, the +flowing waters may have power to clear away each stratum of incoherent +materials as it rises and approaches the surface, where the waves exert the +greatest force; and in this manner a voluminous deposit may be entirely +swept away, so that, in the absence of faults, no evidence may remain of +the denuding operation. It may indeed be affirmed that the signs of waste +will usually be least obvious where the destruction has been most +complete; for the annihilation may have proceeded so far, that no ruins are +left of the dilapidated rocks. + +Although denudation has had a levelling influence on some countries of +shattered and disturbed strata (see fig. 87. p. 63. and fig. 91. p. 69.), +it has more commonly been the cause of superficial inequalities, especially +in regions of horizontal stratification. The general outline of these +regions is that of flat and level platforms, interrupted by valleys often +of considerable depth, and ramifying in various directions. These hollows +may once have formed bays and channels between islands, and the steepest +slope on the sides of each valley may have been a sea-cliff, which was +undermined for ages, as the land emerged gradually from the deep. We may +suppose the position and course of each valley to have been originally +determined by differences in the hardness of the rocks, and by rents and +joints which usually occur even in horizontal strata. In mountain chains, +such as the Jura before described (see fig. 71. p. 55.), we perceive at +once that the principal valleys have not been due to aqueous excavation, +but to those mechanical movements which have bent the rocks into their +present form. Yet even in the Jura there are many valleys, such as C (fig. +71.), which have been hollowed out by water; and it may be stated that in +every part of the globe the unevenness of the surface of the land has been +due to the combined influence of subterranean movements and denudation. + +I may now recapitulate a few of the conclusions to which we have +arrived: first, all the mechanical strata have been accumulated +gradually, and the concomitant denudation has been no less gradual: +secondly, the dry land consists in great part of strata formed +originally at the bottom of the sea, and has been made to emerge and +attain its present height by a force acting from beneath: thirdly, no +combination of causes has yet been conceived so capable of producing +extensive and gradual denudation, as the action of the waves and +currents of the ocean upon land slowly rising out of the deep. + +Now, if we adopt these conclusions, we shall naturally be led to look +everywhere for marks of the former residence of the sea upon the land, +especially near the coasts from which the last retreat of the waters took +place, and it will be found that such signs are not wanting. + +I shall have occasion to speak of ancient sea-cliffs, now far inland, in +the south-east of England, when treating in Chapter XIX. of the denudation +of the chalk in Surrey, Kent, and Sussex. Lines of upraised sea-beaches of +more modern date are traced, at various levels from 20 to 100 feet and +upwards above the present sea-level, for great distances on the east and +west coasts of Scotland, as well as in Devonshire, and other counties in +England. These ancient beach-lines often form terraces of sand and gravel, +including littoral shells, some broken, others entire, and corresponding +with species now living on the adjoining coast. But it would be +unreasonable to expect to meet everywhere with the signs of ancient shores, +since no geologist can have failed to observe how soon all recent marks of +the kind above alluded to are obscured or entirely effaced, wherever, in +consequence of the altered state of the tides and currents, the sea has +receded for a few centuries. We see the cliffs crumble down in a few years +if composed of sand or clay, and soon reduced to a gentle slope. If there +were shells on the beach they decompose, and their materials are washed +away, after which the sand and shingle may resemble any other alluviums +scattered over the interior. + +[Illustration: Fig. 92. Section of inland cliff at Abesse, near Dax. + + _a._ Sand of the Landes. + _b._ Limestone. + _c._ Clay.] + +The features of an ancient shore may sometimes be concealed by the +growth of trees and shrubs, or by a covering of blown sand, a good +example of which occurs a few miles west from Dax, near Bordeaux, in the +south of France. About twelve miles inland, a steep bank may be traced +running in a direction nearly north-east and south-west, or parallel to +the contiguous coast. This sudden fall of about 50 feet conducts us from +the higher platform of the Landes to a lower plain which extends to the +sea. The outline of the ground suggested to me, as it would do to every +geologist, the opinion that the bank in question was once a sea-cliff, +when the whole country stood at a lower level. But this is no longer +matter of conjecture, for, in making excavations in 1830 for the +foundation of a building at Abesse, a quantity of loose sand, which +formed the slope _d e_, was removed; and a perpendicular cliff, about 50 +feet in height, which had hitherto been protected from the agency of the +elements, was exposed. At the bottom appeared the limestone _b_, +containing tertiary shells and corals, immediately below it the clay +_c_, and above it the usual tertiary sand _a_, of the department of the +Landes. At the base of the precipice were seen large partially rounded +masses of rock, evidently detached from the stratum _b_. The face of the +limestone was hollowed out and weathered into such forms as are seen in +the calcareous cliffs of the adjoining coast, especially at Biaritz, +near Bayonne. It is evident that, when the country was at a somewhat +lower level, the sea advanced along the surface of the argillaceous +stratum _c_, which, from its yielding nature, favoured the waste by +allowing the more solid superincumbent stone _b_ to be readily +undermined. Afterwards, when the country had been elevated, part of the +sand, _a_, fell down, or was drifted by the winds, so as to form the +talus, _d e_, which masked the inland cliff until it was artificially +laid open to view. + +When we are considering the various causes which, in the course of ages, +may efface the characters of an ancient sea-coast, earthquakes must not be +forgotten. During violent shocks, steep and overhanging cliffs are often +thrown down and become a heap of ruins. Sometimes unequal movements of +upheaval or depression entirely destroy that horizontality of the base-line +which constitutes the chief peculiarity of an ancient sea-cliff. + +It is, however, in countries where hard limestone rocks abound, that inland +cliffs retain faithfully the characters which they acquired when they +constituted the boundary of land and sea. Thus, in the Morea, no less than +three, or even four, ranges of what were once sea-cliffs are well +preserved. These have been described, by MM. Boblaye and Virlet, as rising +one above the other at different distances from the actual shore, the +summit of the highest and oldest occasionally exceeding 1000 feet in +elevation. At the base of each there is usually a terrace, which is in some +places a few yards, in others above 300 yards wide, so that we are +conducted from the high land of the interior to the sea by a succession of +great steps. These inland cliffs are most perfect, and most exactly +resemble those now washed by the waves of the Mediterranean, where they are +formed of calcareous rock, especially if the rock be a hard crystalline +marble. The following are the points of correspondence observed between the +ancient coast lines and the borders of the present sea:--1. A range of +vertical precipices, with a terrace at their base. 2. A weathered state of +the surface of the naked rock, such as the spray of the sea produces. 3. A +line of littoral caverns at the foot of the cliffs. 4. A consolidated beach +or breccia with occasional marine shells, found at the base of the cliffs, +or in the caves. 5. Lithodomous perforations. + +In regard to the first of these, it would be superfluous to dwell on the +evidence afforded of the undermining power of waves and currents by +perpendicular precipices. The littoral caves, also, will be familiar to +those who have had opportunities of observing the manner in which the waves +of the sea, when they beat against rocks, have power to scoop out caverns. +As to the breccia, it is composed of pieces of limestone and rolled +fragments of thick solid shell, such as _Strombus_ and _Spondylus_, all +bound together by a crystalline calcareous cement. Similar aggregations are +now forming on the modern beaches of Greece, and in caverns on the +sea-side; and they are only distinguishable in character from those of more +ancient date, by including many pieces of pottery. In regard to the +_lithodomi_ above alluded to, these bivalve mollusks are well known to have +the power of excavating holes in the hardest limestones, the size of the +cavity keeping pace with the growth of the shell. When living they require +to be always covered by salt water, but similar pear-shaped hollows, +containing the dead shells of these creatures, are found at different +heights on the face of the inland cliffs above mentioned. Thus, for +example, they have been observed near Modon and Navarino on cliffs in the +interior 125 feet high above the Mediterranean. As to the weathered surface +of the calcareous rocks, all limestones are known to suffer chemical +decomposition when moistened by the spray of the salt water, and are +corroded still more deeply at points lower down where they are just reached +by the breakers. By this action the stone acquires a wrinkled and furrowed +outline, and very near the sea it becomes rough and branching, as if +covered with corals. Such effects are traced not only on the present shore, +but at the base of the ancient cliffs far in the interior. Lastly, it +remains only to speak of the terraces, which extend with a gentle slope +from the base of almost all the inland cliffs, and are for the most part +narrow where the rock is hard, but sometimes half a mile or more in breadth +where it is soft. They are the effects of the encroachment of the ancient +sea upon the shore at those levels at which the land remained for a long +time stationary. The justness of this view is apparent on examining the +shape of the modern shore wherever the sea is advancing upon the land, and +removing annually small portions of undermined rock. By this agency a +submarine platform is produced on which we may walk for some distance from +the beach in shallow water, the increase of depth being very gradual, until +we reach a point where the bottom plunges down suddenly. This platform is +widened with more or less rapidity according to the hardness of the rocks, +and when upraised it constitutes an inland terrace. + +But the four principal lines of cliff observed in the Morea do not imply, +as some have imagined, four great eras of sudden upheaval; they simply +indicate the intermittence of the upheaving force. Had the rise of the land +been continuous and uninterrupted, there would have been no one prominent +line of cliff; for every portion of the surface having been, in its turn, +and for an equal period of time, a sea-shore, would have presented a nearly +similar aspect. But if pauses occur in the process of upheaval, the waves +and currents have time to sap, throw down, and clear away considerable +masses of rock, and to shape out at certain levels lofty ranges of cliffs +with broad terraces at their base. + +There are some levelled spaces, however, both ancient and modern, in the +Morea, which are not due to denudation, although resembling in outline +the terraces above described. They may be called Terraces of Deposition, +since they have resulted from the gain of land upon the sea where rivers +and torrents have produced deltas. If the sedimentary matter has filled +up a bay or gulf surrounded by steep mountains, a flat plain is formed +skirting the inland precipices; and if these deposits are upraised, +they form a feature in the landscape very similar to the areas of +denudation before described. + +In the island of Sicily I have examined many inland cliffs like those of +the Morea; as, for example, near Palermo, where a precipice is seen +consisting of limestone at the base of which are numerous caves. One of +these called San Ciro, about 2 miles distant from Palermo, is about 20 feet +high, 10 wide, and 180 above the sea. Within it is found an ancient beach +(_b_, fig. 93.), formed of pebbles of various rocks, many of which must +have come from places far remote. Broken pieces of coral and shell, +especially of oysters and pectens, are seen intermingled with the pebbles. +Immediately above the level of this beach, _serpulæ_ are still found +adhering to the face of the rock, and the limestone is perforated by +_lithodomi_. Within the grotto, also, at the same level, similar +perforations occur; and so numerous are the holes, that the rock is +compared by Hoffmann to a target pierced by musket balls. But in order to +expose to view these marks of boring-shells in the interior of the cave, it +was necessary first to remove a mass of breccia, which consisted of +numerous fragments of rock and an immense quantity of bones of the mammoth, +hippopotamus, and other quadrupeds, imbedded in a dark brown calcareous +marl. Many of the bones were rolled as if partially subjected to the action +of the waves. Below this breccia, which is about 20 feet thick, was found a +bed of sand filled with sea-shells of recent species; and underneath the +sand, again, is the secondary limestone of Monte Grifone. The state of the +surface of the limestone in the cave above the level of the marine sand is +very different from that below it. _Above_, the rock is jagged and uneven, +as is usual in the roofs and sides of limestone caverns; _below_, the +surface is smooth and polished, as if by the attrition of the waves. + +[Illustration: Fig. 93. Cross section. + + _a._ Monte Grifone. + _b._ Cave of San Ciro.[75-A] + _c._ Plain of Palermo, in which are Newer Pliocene strata of + limestone and sand. + _d._ Bay of Palermo.] + +The platform indicated at _c_, fig. 93., is formed by a tertiary deposit +containing marine shells almost all of living species, and it affords an +illustration of the terrace of deposition, or the last of the two kinds +before mentioned (p. 74.). + +There are also numerous instances in Sicily of terraces of denudation. One +of these occurs on the east coast to the north of Syracuse, and the same is +resumed to the south beyond the town of Noto, where it may be traced +forming a continuous and lofty precipice, _a b_, fig. 94., facing towards +the sea, and constituting the abrupt termination of a calcareous formation, +which extends in horizontal strata far inland. This precipice varies in +height from 500 to 700 feet, and between its base and the sea is an +inferior platform, _c b_, consisting of similar white limestone. All the +beds dip towards the sea, but are usually inclined at a very slight angle: +they are seen to extend uninterruptedly from the base of the escarpment +into the platform, showing distinctly that the lofty cliff was not produced +by a fault or vertical shift of the beds, but by the removal of a +considerable mass of rock. Hence we may conclude that the sea, which is now +undermining the cliffs of the Sicilian coast, reached at some former +period the base of the precipice _a b_, at which time the surface of the +terrace _c b_ must have been covered by the Mediterranean. There was a +pause, therefore, in the upward movement, when the waves of the sea had +time to carve out the platform _c b_; but there may have been many other +stationary periods of minor duration. Suppose, for example, that a series +of escarpments _e_, _f_, _g_, _h_, once existed, and that the sea, during a +long interval free from subterranean movements, advances along the line _c +b_, all preceding cliffs must have been swept away one after the other, and +reduced to the single precipice _a b_. + +[Illustration: Fig. 94. Cross section.] + +[Illustration: Fig. 95. Valley called Gozzo degli Martiri, below +Melilli, Val di Noto.] + +That such a series of smaller cliffs, as those represented at _e_, _f_, +_g_, _h_, fig. 94., did really once exist at intermediate heights in place +of the single precipice _a b_, is rendered highly probable by the fact, +that in certain bays and inland valleys opening towards the east coast of +Sicily, and not far from the section given in fig. 94., the solid limestone +is shaped out into a great succession of ledges, separated from each other +by small vertical cliffs. These are sometimes so numerous, one above the +other, that where there is a bend at the head of a valley, they produce an +effect singularly resembling the seats of a Roman amphitheatre. A good +example of this configuration occurs near the town of Melilli, as seen in +the annexed view (fig. 95.). In the south of the island, near Spaccaforno, +Scicli, and Modica, precipitous rocks of white limestone, ascending to the +height of 500 feet, have been carved out into similar forms. + +[Illustration: Fig. 96. Cross section.] + +This appearance of a range of marble seats circling round the head of a +valley, or of great flights of steps descending from the top to the bottom, +on the opposite sides of a gorge, may be accounted for, as already hinted, +by supposing the sea to have stood successively at many different levels, +as at _a a_, _b b_, _c c_, in the accompanying fig. 96. But the causes of +the gradual contraction of the valley from above downwards may still be +matter of speculation. Such contraction may be due to the greater force +exerted by the waves when the land at its first emergence was smaller in +quantity, and more exposed to denudation in an open sea; whereas the wear +and tear of the rocks might diminish in proportion as this action became +confined within bays or channels closed in on two or three sides. Or, +secondly, the separate movements of elevation may have followed each other +more rapidly as the land continued to rise, so that the times of those +pauses, during which the greatest denudation was accomplished at certain +levels, were always growing shorter. It should be remarked, that the cliffs +and small terraces are rarely found on the opposite sides of the Sicilian +valleys at heights so precisely answering to each other as those given in +fig. 96., and this might have been expected, to whichever of the two +hypotheses above explained we incline; for, according to the direction of +the prevailing winds and currents, the waves may beat with unequal force on +different parts of the shore, so that while no impression is made on one +side of a bay, the sea may encroach so far on the other as to unite several +smaller cliffs into one. + +Before quitting the subject of ancient sea-cliffs, carved out of +limestone, I shall mention the range of precipitous rocks, composed of a +white marble of the Oolitic period, which I have seen near the northern +gate of St. Mihiel in France. They are situated on the right bank of the +Meuse, at a distance of 200 miles from the nearest sea, and they present +on the precipice facing the river three or four horizontal grooves, one +above the other, precisely resembling those which are scooped out by the +undermining waves. The summits of several of these masses are detached +from the adjoining hill, in which case the grooves pass all round them, +facing towards all points of the compass, as if they had once formed +rocky islets near the shore.[78-A] + +Captain Bayfield, in his survey of the Gulf of St. Lawrence, discovered in +several places, especially in the Mingan islands, a counterpart of the +inland cliffs of St. Mihiel, and traced a succession of shingle beaches, +one above the other, which agreed in their level with some of the principal +grooves scooped out of the limestone pillars. These beaches consisted of +calcareous shingle, with shells of recent species, the farthest from the +shore being 60 feet above the level of the highest tides. In addition to +the drawings of the pillars called the flower-pots, which he has +published[78-B], I have been favoured with other views of rocks on the same +coast, drawn by Lieut. A. Bowen, R. N. (See fig. 97.) + +[Illustration: Fig. 97. Limestone columns in Niapisca Island, in the Gulf +of St. Lawrence. Height of the second column on the left, 60 feet.] + +In the North-American beaches above mentioned rounded fragments of +limestone have been found perforated by _lithodomi_; and holes drilled +by the same mollusks have been detected in the columnar rocks or +"flower-pots," showing that there has been no great amount of +atmospheric decomposition on the surface, or the cavities alluded +to would have disappeared. + +[Illustration: Fig. 98. The North Rocks, Bermuda, lying outside the +great coral reef. A. 16 feet high, and B. 12 feet. _c._ _c._ Hollows +worn by the sea.] + +We have an opportunity of seeing in the Bermuda islands the manner in +which the waves of the Atlantic have worn, and are now wearing out, deep +smooth hollows on every side of projecting masses of hard limestone. In the +annexed drawing, communicated to me by Lieut. Nelson, the excavations _c_, +_c_, _c_, have been scooped out by the waves in a stone of very modern +date, which, although extremely hard, is full of recent corals and shells, +some of which retain their colour. + +When the forms of these horizontal grooves, of which the surface is +sometimes smooth and almost polished, and the roofs of which often overhang +to the extent of 5 feet or more, have been carefully studied by geologists, +they will serve to testify the former action of the waves at innumerable +points far in the interior of the continents. But we must learn to +distinguish the indentations due to the original action of the sea, and +those caused by subsequent chemical decomposition of calcareous rocks, to +which they are liable in the atmosphere. + +Notwithstanding the enduring nature of the marks left by littoral action on +calcareous rocks, we can by no means detect sea-beaches and inland cliffs +everywhere, even in Sicily and the Morea. On the contrary, they are, upon +the whole, extremely partial, and are often entirely wanting in districts +composed of argillaceous and sandy formations, which must, nevertheless, +have been upheaved at the same time, and by the same intermittent +movements, as the adjoining calcareous rocks. + + +FOOTNOTES: + +[67-A] Western Islands, vol. ii. p. 93. pl. 31. fig. 4. + +[69-A] See Mammat's Geological Facts, &c. p. 90. and plate. + +[69-B] Conybeare's Report to Brit. Assoc. 1842, p. 381. + +[70-A] Prestwich, Geol. Trans. second series, vol. v. pp. 452. 473. + +[75-A] Section given by Dr. Christie, Edin. New Phil. Journ. No. +xxiii., called by mistake the Cave of Mardolce, by the late M. Hoffmann. +See account by Mr. S. P. Pratt, F. G. S. Proceedings of Geol. Soc. +No. 32. 1833. + +[78-A] I was directed by M. Deshayes to this spot, which I visited +in June, 1833. + +[78-B] See Trans. of Geol. Soc., second series, vol. v. plate v. + + + + +CHAPTER 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. + + +Between the superficial covering of vegetable mould and the subjacent rock +there usually intervenes in every district a deposit of loose gravel, sand, +and mud, to which the name of alluvium has been applied. The term is +derived from _alluvio_, an inundation, or _alluo_, to wash, because the +pebbles and sand commonly resemble those of a river's bed or the mud and +gravel spread over low lands by a flood. + +A partial covering of such alluvium is found alike in all climates, from +the equatorial to the polar regions; but in the higher latitudes of Europe +and North America it assumes a distinct character, being very frequently +devoid of stratification, and containing huge fragments of rock, some +angular and others rounded, which have been transported to great distances +from their parent mountains. When it presents itself in this form, it has +been called "diluvium," "drift," or the "boulder formation;" and its +probable connexion with the agency of floating ice and glaciers will be +treated of more particularly in the eleventh and twelfth chapters. + +[Illustration: Fig. 99. Lavas of Auvergne resting on alluviums of +different ages.] + +The student will be prepared, by what I have said in the last chapter on +denudation, to hear that loose gravel and sand are often met with, not +only on the low grounds bordering rivers, but also at various points on +the sides or even summits of mountains. For, in the course of those +changes in physical geography which may take place during the gradual +emergence of the bottom of the sea and its conversion into dry land, any +spot may either have been a sunken reef, or a bay, or estuary, or +sea-shore, or the bed of a river. For this reason it would be +unreasonable to hope that we should ever be able to account for all the +alluvial phenomena of each particular country, seeing that the causes of +their origin are so complicated. Moreover, the last operations of water +have a tendency to disturb and confound together all pre-existing +alluviums. Hence we are always in danger of regarding as the work of a +single era, and the effect of one cause, what has in reality been the +result of a variety of distinct agents, during a long succession of +geological epochs. Much useful instruction may therefore be gained from +the exploration of a country like Auvergne, where the superficial gravel +of very different eras happens to have been preserved by sheets of lava, +which were poured out one after the other at periods when the +denudation, and probably the upheaval, of rocks were in progress. That +region had already acquired in some degree its present configuration +before any volcanos were in activity, and before any igneous matter was +superimposed upon the granitic and fossiliferous formations. The pebbles +therefore in the older gravels are exclusively constituted of granite +and other aboriginal rocks; and afterwards, when volcanic vents burst +forth into eruption, those earlier alluviums were covered by streams of +lava, which protected them from intermixture with gravel of subsequent +date. In the course of ages, a new system of valleys was excavated, so +that the rivers ran at lower levels than those at which the first +alluviums and sheets of lava were formed. When, therefore, fresh +eruptions gave rise to new lava, the melted matter was poured out over +lower grounds; and the gravel of these plains differed from the first +or upland alluvium, by containing in it rounded fragments of various +volcanic rocks, and often bones belonging to distinct groups of land +animals which flourished in the country in succession. + +The annexed drawing will explain the different heights at which beds of +lava and gravel, each distinct from the other in composition and age, are +observed, some on the flat tops of hills, 700 or 800 feet high, others on +the slope of the same hills, and the newest of all in the channel of the +existing river where there is usually gravel alone, but in some cases a +narrow stripe of solid lava sharing the bottom of the valley with the +river. In all these accumulations of transported matter of different ages +the bones of extinct quadrupeds have been found belonging to assemblages of +land mammalia which flourished in the country in succession, and which vary +specifically, the one from the other, in a greater or less degree, in +proportion as the time which separated their entombment has been more or +less protracted. The streams in the same district are still undermining +their banks and grinding down into pebbles or sand, columns of basalt and +fragments of granite and gneiss; but the older alluviums, with the fossil +remains belonging to them, are prevented from being mingled with the gravel +of recent date by the cappings of lava before mentioned. But for the +accidental interference, therefore, of this peculiar cause, all the +alluviums might have passed so insensibly the one into the other, that +those formed at the remotest era might have appeared of the same date as +the newest, and the whole formation might have been regarded by some +geologists as the result of one sudden and violent catastrophe. + +In almost every country, the alluvium consists in its upper part of +transported materials, but it often passes downwards into a mass of +broken and angular fragments derived from the subjacent rock. To this +mass the provincial name of "rubble," or "brash," is given in many +parts of England. It may be referred to the weathering or disintegration +of stone on the spot, the effects of air and water, sun and frost, +and chemical decomposition. + +[Illustration: Fig. 100. Cross section. + + _a._ Vegetable soil. + _b._ Alluvium. + _c._ Mass of same, apparently detached.] + +The inferior surface of alluvial deposits is often very irregular, +conforming to all the inequalities of the fundamental rocks (fig. 100.). +Occasionally, a small mass, as at _c_, appears detached, and as if included +in the subjacent formation. Such isolated portions are usually sections of +winding subterranean hollows filled up with alluvium. They may have been +the courses of springs or subterranean streamlets, which have flowed +through and enlarged natural rents; or, when on a small scale and in soft +strata, they may be spaces which the roots of large trees have once +occupied, gravel and sand having been introduced after their decay. + +[Illustration: Fig. 101. Sand-pipes in the chalk at Eaton, near Norwich.] + +But there are other deep hollows of a cylindrical form found in England, +France, and elsewhere, penetrating the white chalk, and filled with sand +and gravel, which are not so readily explained. They are sometimes called +"sand-pipes," or "sand-galls," and "puits naturels," in France. Those +represented in the annexed cut were observed by me in 1839, laid open in a +large chalk-pit near Norwich. They were of very symmetrical form, the +largest more than 12 feet in diameter, and some of them had been traced, by +boring, to the depth of more than 60 feet. The smaller ones varied from a +few inches to a foot in diameter, and seldom descended more than 12 feet +below the surface. Even where three of them occurred, as at _a_, fig. 101., +very close together, the parting walls of soft white chalk were not broken +through. They all taper downwards and end in a point. As a general rule, +sand and pebbles occupy the central parts of each pipe, while the sides and +bottom are lined with clay. + +Mr. Trimmer, in speaking of appearances of the same kind in the Kentish +chalk, attributes the origin of such "sand-galls" to the action of the sea +on a beach or shoal, where the waves, charged with shingle and sand, not +only wear out longitudinal furrows, such as may be observed on the surface +of the chalk near Norwich when the incumbent gravel is removed, but also +drill deep circular hollows by the rotatory motion imparted to sand and +pebbles. Such furrows, as well as vertical cavities, are now formed, he +observes, on the coast where the shores are composed of chalk.[82-A] + +That the commencement of many of the tubular cavities now under +consideration has been due to the cause here assigned, I have little doubt. +But such mechanical action could not have hollowed out the whole of the +sand-pipes _c_ and _d_, fig. 101., because several large chalk-flints seen +protruding from the walls of the pipes have not been eroded, while sand and +gravel have penetrated many feet below them. In other cases, as at _b b_, +similar unrounded nodules of flint, still preserving their irregular form +and white coating, are found at various depths in the midst of the loose +materials filling the pipe. These have evidently been detached from regular +layers of flints occurring above. It is also to be remarked that the course +of the same sand-pipe, _b b_, is traceable above the level of the chalk for +some distance upwards, through the incumbent gravel and sand, by the +obliteration of all signs of stratification. Occasionally, also, as in the +pipe _d_, the overlying beds of gravel bend downwards into the mouth of the +pipe, so as to become in part vertical, as would happen if horizontal +layers had sunk gradually in consequence of a failure of support. All these +phenomena may be accounted for by attributing the enlargement and deepening +of the sand-pipes to the chemical action of water charged with carbonic +acid, derived from the vegetable soil and the decaying roots of trees. Such +acid might corrode the chalk, and deepen indefinitely any previously +existing hollow, but could not dissolve the flints. The water, after it had +become saturated with carbonate of lime, might freely percolate the +surrounding porous walls of chalk, and escape through them and from the +bottom of the tube, so as to carry away in the course of time large masses +of dissolved calcareous rock[83-A], and leave behind it on the edges of +each tubular hollow a coating of fine clay, which the white chalk contains. + +I have seen tubes precisely similar and from 1 to 5 feet in diameter +traversing vertically the upper half of the soft calcareous building +stone, or chalk without flints, constituting St. Peter's Mount, +Maestricht. These hollows are filled with pebbles and clay, derived from +overlying beds of gravel, and all terminate downwards like those of +Norfolk. I was informed that, 6 miles from Maestricht, one of these +pipes, 2 feet in diameter, was traced downwards to a bed of flattened +flints, forming an almost continuous layer in the chalk. Here it +terminated abruptly, but a few small root-like prolongations of it were +detected immediately below, probably where the dissolving substance had +penetrated at some points through openings in the siliceous mass. + +It is not so easy as may at first appear to draw a clear line of +distinction between the _fixed_ rocks, or regular strata (rocks _in +situ_ or _in place_), and _alluvium_. If the bed of a torrent or river +be dried up, we call the gravel, sand, and mud left in their channels, +or whatever, during floods, they may have scattered over the +neighbouring plains, alluvium. The very same materials carried into a +lake, where they become sorted by water and arranged in more distinct +layers, especially if they inclose the remains of plants, shells, or +other fossils, are termed regular strata. + +In like manner we may sometimes compare the gravel, sand, and broken +shells, strewed along the path of a rapid marine current, with a deposit +formed contemporaneously by the discharge of similar materials, year after +year, into a deeper and more tranquil part of the sea. In such cases, when +we detect marine shells or other organic remains entombed in the strata, +which enable us to determine their age and mode of origin, we regard them +as part of the regular series of fossiliferous formations, whereas, if +there are no fossils, we have frequently no power of separating them from +the general mass of superficial alluvium. + +The usual rarity of organic remains in beds of loose gravel and sand is +partly owing to the rapid and turbid water in which they were formed having +been in a condition unfavourable to the habitation of aquatic beings, and +partly to their porous nature, which, by allowing the free percolation of +rain-water, has promoted the decomposition and removal of organic matter. + +It has long been a matter of common observation that most rivers are now +cutting their channels through alluvial deposits of greater depth and +extent than could ever have been formed by the present streams. From this +fact a rash inference has sometimes been drawn, that rivers in general have +grown smaller, or become less liable to be flooded than formerly. But such +phenomena would be a natural result of any considerable oscillations in the +level of the land experienced since the existing valleys originated. + +Suppose part of a continent, comprising within it a large hydrographical +basin like that of the Mississippi, to subside several inches or feet in +a century, as the west coast of Greenland, extending 600 miles north and +south, has been sinking for three or four centuries, between the +latitudes 60° and 69° N.[84-A] There might be no encroachment of the sea +at the river's mouth in consequence of this change of level, but the +fall of the waters flowing from the interior being lessened, the main +river and its tributaries would have less power to carry down to its +delta, and to discharge into the ocean, the sedimentary matter with +which they are annually loaded. They would all begin to raise their +channels and alluvial plains by depositing in them the heavier sand and +pebbles washed down from the upland country, and this operation would +take place most effectively if the amount of subsidence in the interior +was unequal, and especially if, on the whole, it exceeded that of the +region near the sea. If then the same area of land be again upheaved to +its former height, the fall, and consequently the velocity, of every +river would begin to augment. Each of them would be less given to +overflow its alluvial plain; and their power of carrying earthy matter +seaward, and of scouring out and deepening their channels, would +continue till, after a lapse of many thousand years, each of them would +have eroded a new channel or valley through a fluviatile formation of +modern date. The surface of what was once the river-plain at the period +of greatest depression, would remain fringing the valley sides in the +form of a terrace apparently flat, but in reality sloping down with the +general inclination of the river. Everywhere this terrace would present +cliffs of gravel and sand, facing the river. That such a series of +movements has actually taken place in the main valley of the Mississippi +and in its tributary valleys during oscillations of level, I have +endeavoured to show in my description of that country[85-A]; and the +freshwater shells of existing species and bones of land quadrupeds, +partly of extinct races preserved in the terraces of fluviatile origin, +attest the exclusion of the sea during the whole process of filling up +and partial re-excavation. + +In many cases, the alluvium in which rivers are now cutting their channels, +originated when the land first rose out of the sea. If, for example, the +emergence was caused by a gradual and uniform motion, every bay and +estuary, or the straits between islands, would dry up slowly, and during +their conversion into valleys, every part of the upheaved area would in its +turn be a sea-shore, and might be strewed over with littoral sand and +pebbles, or each spot might be the point where a delta accumulated during +the retreat and exclusion of the sea. Materials so accumulated would +conform to the general slope of a valley from its head to the sea-coast. + +_River terraces._--We often observe at a short distance from the present +bed of a river a steep cliff a few feet or yards high, and on a level with +the top of it a flat terrace corresponding in appearance to the alluvial +plain which immediately borders the river. This terrace is again bounded by +another cliff, above which a second terrace sometimes occurs: and in this +manner two or three ranges of cliffs and terraces are occasionally seen on +one or both sides of the stream, the number varying, but those on the +opposite sides often corresponding in height. + +[Illustration: Fig. 102. River Terraces and Parallel Roads.] + +These terraces are seldom continuous for great distances, and their +surface slopes downwards, with an inclination similar to that of the +river. They are readily explained if we adopt the hypothesis before +suggested, of a gradual rise of the land; especially if, while rivers +are shaping out their beds, the upheaving movement be intermittent, so +that long pauses shall occur, during which the stream will have time to +encroach upon one of its banks, so as to clear away and flatten a large +space. This operation being afterwards repeated at lower levels, there +will be several successive cliffs and terraces. + +_Parallel roads._--The parallel shelves, or roads, as they have been +called, of Lochaber or Glen Roy and other contiguous valleys in Scotland, +are distinct both in character and origin from the terraces above +described; for they have no slope towards the sea like the channel of a +river, nor are they the effect of denudation. Glen Roy is situated in the +western Highlands, about ten miles north of Fort William, near the western +end of the great glen of Scotland, or Caledonian Canal, and near the foot +of the highest of the Grampians, Ben Nevis. Throughout its whole length, a +distance of more than ten miles, two, and in its lower part three, parallel +roads or shelves are traced along the steep sides of the mountains, as +represented in the annexed figure, fig. 102., each maintaining a perfect +horizontality, and continuing at exactly the same level on the opposite +sides of the glen. Seen at a distance, they appear like ledges or roads, +cut artificially out of the sides of the hills; but when we are upon them +we can scarcely recognize their existence, so uneven is their surface, and +so covered with boulders. They are from 10 to 60 feet broad, and merely +differ from the side of the mountain by being somewhat less steep. + +On closer inspection, we find that these terraces are stratified in the +ordinary manner of alluvial or littoral deposits, as may be seen at those +points where ravines have been excavated by torrents. The parallel shelves, +therefore, have not been caused by denudation, but by the deposition of +detritus, precisely similar to that which is dispersed in smaller +quantities over the declivities of the hills above. These hills consist of +clay-slate, mica-schist, and granite, which rocks have been worn away and +laid bare at a few points only, in a line just above the parallel roads. +The highest of these roads is about 1250 feet above the level of the sea, +the next about 200 feet lower than the uppermost, and the third still lower +by about 50 feet. It is only this last, or the lowest of the three, which +is continued throughout Glen Spean, a large valley with which Glen Roy +unites. As the shelves are always at the same height above the sea, they +become continually more elevated above the river in proportion as we +descend each valley; and they at length terminate very abruptly, without +any obvious cause, either in the shape of the ground, or any change in the +composition or hardness of the rocks. I should exceed the limits of this +work, were I to attempt to give a full description of all the geographical +circumstances attending these singular terraces, or to discuss the +ingenious theories which have been severally proposed to account for them +by Dr. MacCulloch, Sir T. D. Lauder, and Messrs. Darwin, Agassiz, Milne, +and Chambers. There is one point, however, on which all are agreed, namely, +that these shelves are ancient beaches, or littoral formations accumulated +round the edges of one or more sheets of water which once stood at the +level, first of the highest shelf, and successively at the height of the +two others. It is well known, that wherever a lake or marine fiord exists +surrounded by steep mountains subject to disintegration by frost or the +action of torrents, some loose matter is washed down annually, especially +during the melting of snow, and a check is given to the descent of this +detritus at the point where it reaches the waters of the lake. The waves +then spread out the materials along the shore, and throw some of them upon +the beach; their dispersing power being aided by the ice, which often +adheres to pebbles during the winter months, and gives buoyancy to them. +The annexed diagram illustrates the manner in which Dr. MacCulloch and Mr. +Darwin suppose "the roads" to constitute mere indentations in a superficial +alluvial coating which rests upon the hillside, and consists chiefly of +clay and sharp unrounded stones. + +[Illustration: Fig. 103. Cross section. + +A B. Supposed original surface of rock. + +C D. Roads or shelves in the outer alluvial covering of the hill.] + +Among other proofs that the parallel roads have really been formed along +the margin of a sheet of water, it may be mentioned, that wherever an +isolated hill rises in the middle of the glen above the level of any +particular shelf, a corresponding shelf is seen at the same level passing +round the hill, as would have happened if it had once formed an island in a +lake or fiord. Another very remarkable peculiarity in these terraces is +this; each of them comes in some portion of its course to a _col_, or +passage between the heads of glens, the explanation of which will be +considered in the sequel. + +Those writers who first advocated the doctrine that the roads were the +ancient beaches of freshwater lakes, were unable to offer any probable +hypothesis respecting the formation and subsequent removal of barriers of +sufficient height and solidity to dam up the water. To introduce any +violent convulsion for their removal was inconsistent with the +uninterrupted horizontality of the roads, and with the undisturbed aspect +of those parts of the glens where the shelves come suddenly to an end. Mr. +Agassiz and Dr. Buckland, desirous, like the defenders of the lake theory, +to account for the limitation of the shelves to certain glens, and their +absence in contiguous glens, where the rocks are of the same composition, +and the slope and inclination of the ground very similar, started the +conjecture that these valleys were once blocked up by enormous glaciers +descending from Ben Nevis, giving rise to what are called in Switzerland +and in the Tyrol, glacier-lakes. After a time the icy barrier was broken +down, or melted, first, to the level of the second, and afterwards to that +of the third road or shelf. + +In corroboration of this view, they contended that the alluvium of Glen +Roy, as well as of other parts of Scotland, agrees in character with the +moraines of glaciers seen in the Alpine valleys of Switzerland. Allusion +will be made in the eleventh chapter to the former existence of glaciers in +the Grampians: in the mean time it will readily be conceded that this +hypothesis is preferable to any previous lacustrine theory, by accounting +more easily for the temporary existence and entire disappearance of lofty +transverse barriers, although the height required for the imaginary dams +of ice may be startling. + +Before the idea last alluded to had been entertained, Mr. Darwin examined +Glen Roy, and came to the opinion that the shelves were formed when the +glens were still arms of the sea, and, consequently, that there never were +any barriers. According to him, the land emerged during a slow and uniform +upward movement, like that now experienced throughout a large part of +Sweden and Finland; but there were certain pauses in the upheaving process, +at which times the waters of the sea remained stationary for so many +centuries as to allow of the accumulation of an extraordinary quantity of +detrital matter, and the excavation, at points immediately above, of many +deep notches and bare cliffs in the hard and solid rock. + +The phenomena which are most difficult to reconcile with this theory +are, first, the abrupt cessation of the roads at certain points in the +different glens; secondly, their unequal number in different valleys +connecting with each other, there being three, for example, in Glen Roy +and only one in Glen Spean; thirdly, the precise horizontality of level +maintained by the same shelf over a space many leagues in length +requiring us to assume, that during a rise of 1250 feet no one portion +of the land was raised even a few yards above another; fourthly, the +coincidence of level already alluded to of each shelf with a _col_, or +the point forming the head of two glens, from which the rain-waters flow +in opposite directions. This last-mentioned feature in the physical +geography of Lochaber seems to have been explained in a satisfactory +manner by Mr. Darwin. He calls these _cols_ "landstraits," and regards +them as having been anciently sounds or channels between islands. He +points out that there is a tendency in such sounds to be silted up, and +always the more so in proportion to their narrowness. In a chart of the +Falkland Islands by Capt. Sullivan, R.N., it appears that there are +several examples there of straits where the soundings diminish regularly +towards the narrowest part. One is so nearly dry that it can be walked +over at low water, and another, no longer covered by the sea, is +supposed to have recently dried up in consequence of a small shift in +the relative level of sea and land. "Similar straits," observes Mr. +Chambers, "hovering, in character, between sea and land, and which may +be called fords, are met with in the Hebrides. Such, for example, is the +passage dividing the islands of Lewis and Harris, and that between North +Uist and Benbecula, both of which would undoubtedly appear as _cols_, +coinciding with a terrace or raised beach, all round the islands, if the +sea were to subside."[88-A] + +The precise horizontality of level maintained by the roads or shelves of +Lochaber over an area many leagues in length and breadth, is a +difficulty common in some degree to all the rival hypotheses, whether of +lakes, or glaciers, or of the simple upheaval of the land above the sea. +For we cannot suppose the roads to be more ancient than the glacial +period, or the era of the boulder formation of Scotland, of which I +shall speak in the eleventh and twelfth chapters. Strata of that era of +marine origin containing northern shells of existing species have been +found at various heights in Scotland, some on the east, and others on +the west coast, from 20 to 400 feet high; and in one region in +Lanarkshire not less than 524 feet above high-water mark. It seems, +therefore, in the highest degree improbable that Glen Roy should have +escaped entirely the upward movement experienced in so many surrounding +regions,--a movement implied by the position of these marine deposits, +in which the shells are almost all of known recent species. But if the +motion has really extended to Glen Roy and the contiguous glens, it must +have uplifted them bodily, without in the slightest degree affecting +their horizontality; and this being admitted, the principal objection to +the theory of marine beaches, founded on the uniformity of upheaval, is +removed, or is at least common to every theory hitherto proposed. + +To assume that the ocean has gone down from the level of the uppermost +shelf, or 1250 feet, simultaneously all over the globe, while the land +remained unmoved, is a view which will find favour with very few +geologists, for the reasons explained in the fifth chapter. + +The student will perceive, from the above sketch of the controversy +respecting the formation of these curious shelves, that this problem, like +many others in geology, is as yet only solved in part; and that a larger +number of facts must be collected and reasoned upon before the question +can be finally settled. + + +FOOTNOTES: + +[82-A] Trimmer, Proceedings of Geol. Soc. vol. iv. p. 7. 1842. + +[83-A] See Lyell on Sand-pipes, &c., Phil. Mag., third series, vol. +xv. p. 257., Oct. 1839. + +[84-A] Principles of Geology, 7th ed. p. 506., 8th ed. 509. + +[85-A] Second Visit to the U. S. vol. ii. chap. 34. + +[88-A] "Ancient Sea Margins," p. 114., by R. Chambers. + + + + +CHAPTER 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. + + +In the first chapter it was stated that the four great classes of rocks, +the aqueous, the volcanic, the plutonic, and the metamorphic, would each +be considered not only in reference to their mineral characters, and +mode of origin, but also to their relative age. The preservation of the +shelves may have required, says Darwin, the quick growth of green turf +on a good soil; their abrupt cessation may mark the place where the soil +was barren, and when a green sward formed slowly. In regard to the +aqueous rocks, we have already seen that they are stratified, that some +are calcareous, others argillaceous or siliceous, some made up of sand, +others of pebbles; that some contain freshwater, others marine fossils, +and so forth; but the student has still to learn which rocks, exhibiting +some or all of these characters, have originated at one period of the +earth's history, and which at another. + +To determine this point in reference to the fossiliferous formations is +more easy than in any other class, and it is therefore the most +convenient and natural method to begin by establishing a chronology for +these fossiliferous strata, and then to endeavour to refer to the same +divisions, the several groups of plutonic, volcanic, and metamorphic +rocks. This system of classification is not only recommended by its +greater clearness and facility of application, but is also best fitted +to strike the imagination by bringing into one view the past changes of +the inorganic world, and the contemporaneous revolutions of the organic +creation. For the sedimentary formations of successive periods are most +readily distinguished by the different species of fossil animals and +plants which they inclose, and of which one race after another has +flourished and then disappeared from the earth. + +But before entering specially on the subdivisions of the aqueous rocks +arranged according to the order of time, it will be desirable to say a few +words on the chronology of rocks in general, although in doing so we shall +be unavoidably led to allude to some classes of phenomena which the +beginner must not yet expect fully to comprehend. + +It was for many years a received opinion, that the formation of entire +families of rocks, such as the plutonic and those crystalline schists +spoken of in the first chapter as metamorphic, began and ended before any +members of the aqueous and volcanic orders were produced; and although this +idea has long been modified, and is nearly exploded, it will be necessary +to give some account of the ancient doctrine, in order that beginners may +understand whence many prevailing opinions, and some part of the +nomenclature of geology, still partially in use, was derived. + +About the middle of the last century, Lehman, a German miner, proposed to +divide rocks into three classes, the first and oldest to be called +primitive, comprising the hypogene, or plutonic and metamorphic rocks; the +next to be termed secondary, comprehending the aqueous or fossiliferous +strata; and the remainder, or third class, corresponding to our alluvium, +ancient and modern, which he referred to "local floods, and the deluge of +Noah." In the primitive class, he said, such as granite and gneiss, there +are no organic remains, nor any signs of materials derived from the ruins +of pre-existing rocks. Their origin, therefore, may have been purely +chemical, antecedent to the creation of living beings, and probably coeval +with the birth of the world itself. The secondary formations, on the +contrary, which often contain sand, pebbles, and organic remains, must have +been mechanical deposits, produced after the planet had become the +habitation of animals and plants. This bold generalization, although +anticipated in some measure by Steno, a century before, in Italy, formed at +the time an important step in the progress of geology, and sketched out +correctly some of the leading divisions into which rocks may be separated. +About half a century later, Werner, so justly celebrated for his improved +methods of discriminating the mineralogical characters of rocks, attempted +to improve Lehman's classification, and with this view intercalated a +class, called by him "the transition formations," between the primitive and +secondary. Between these last he had discovered, in northern Germany, a +series of strata, which in their mineral peculiarities were of an +intermediate character, partaking in some degree of the crystalline nature +of micaceous schist and clay-slate, and yet exhibiting here and there signs +of a mechanical origin and organic remains. For this group, therefore, +forming a passage between Lehman's primitive and secondary rocks, the name +of _übergang_ or transition was proposed. They consisted principally of +clay-slate and an argillaceous sandstone, called grauwacke, and partly of +calcareous beds. It happened in the district which Werner first +investigated, that both the primitive and transition strata were highly +inclined, while the beds of the newer fossiliferous rocks, the secondary of +Lehman, were horizontal. To these latter, therefore, he gave the name +_flötz_, or "a level floor;" and every deposit more modern than the chalk, +which was classed as the uppermost of the flötz series, was designated "the +overflowed land," an expression which may be regarded as equivalent to +alluvium, although under this appellation were confounded all the strata +afterwards called tertiary, of which Werner had scarcely any knowledge. As +the followers of Werner soon discovered that the inclined position of the +"transition beds," and the horizontality of the flötz, or newer +fossiliferous strata, were mere local accidents, they soon abandoned the +term flötz; and the four divisions of the Wernerian school were then named +primitive, transition, secondary, and alluvial. + +As to the trappean rocks, although their igneous origin had been already +demonstrated by Arduino, Fortis, Faujas, and others, and especially by +Desmarest, they were all regarded by Werner as aqueous, and as mere +subordinate members of the secondary series.[91-A] + +This theory of Werner's was called the "Neptunian," and for many years +enjoyed much popularity. It assumed that the globe had been at first +invested by an universal chaotic ocean, holding the materials of all rocks +in solution. From the waters of this ocean, granite, gneiss, and other +crystalline formations, were first precipitated; and afterwards, when the +waters were purged of these ingredients, and more nearly resembled those of +our actual seas, the transition strata were deposited. These were of a +mixed character, not purely chemical, because the waves and currents had +already begun to wear down solid land, and to give rise to pebbles, sand, +and mud; nor entirely without fossils, because a few of the first marine +animals had begun to exist. After this period, the secondary formations +were accumulated in waters resembling those of the present ocean, except at +certain intervals, when, from causes wholly unexplained, a partial +recurrence of the "chaotic fluid" took place, during which various trap +rocks, some highly crystalline, were formed. This arbitrary hypothesis +rejected all intervention of igneous agency, volcanos being regarded as +modern, partial, and superficial accidents, of trifling account among the +great causes which have modified the external structure of the globe. + +Meanwhile Hutton, a contemporary of Werner, began to teach, in Scotland, +that granite as well as trap was of igneous origin, and had at various +periods intruded itself in a fluid state into different parts of the +earth's crust. He recognized and faithfully described many of the +phenomena of granitic veins, and the alterations produced by them on the +invaded strata, which will be treated of in the thirty-second chapter. +He, moreover, advanced the opinion, that the crystalline strata called +primitive had not been precipitated from a primæval ocean, but were +sedimentary strata altered by heat. In his writings, therefore, and in +those of his illustrator, Playfair, we find the germ of that metamorphic +theory which has been already hinted at in the first chapter, and +which will be more fully expounded in the thirty-fourth and +thirty-fifth chapters. + +At length, after much controversy, the doctrine of the igneous origin of +trap and granite made its way into general favour; but although it was, in +consequence, admitted that both granite and trap had been produced at many +successive periods, the term primitive or primary still continued to be +applied to the crystalline formations in general, whether stratified, like +gneiss, or unstratified, like granite. The pupil was told that granite was +a primary rock, but that some granites were newer than certain secondary +formations; and in conformity with the spirit of the ancient language, to +which the teacher was still determined to adhere, a desire was naturally +engendered of extenuating the importance of those more modern granites, the +true dates of which new observations were continually bringing to light. + +A no less decided inclination was shown to persist in the use of the +term "transition," after it had been proved to be almost as faulty in +its original application as that of flötz. The name of transition, as +already stated, was first given by Werner, to designate a mineral +character, intermediate between the highly crystalline or metamorphic +state and that of an ordinary fossiliferous rock. But the term acquired +also from the first a chronological import, because it had been +appropriated to sedimentary formations, which, in the Hartz and other +parts of Germany, were more ancient than the oldest of the secondary +series, and were characterized by peculiar fossil zoophytes and shells. +When, therefore, geologists found in other districts stratified rocks +occupying the same position, and inclosing similar fossils, they gave to +them also the name of _transition_, according to rules which will be +explained in the next chapter; yet, in many cases, such rocks were found +not to exhibit the same mineral texture which Werner had called +transition. On the contrary, many of them were not more crystalline than +different members of the secondary class; while, on the other hand, +these last were sometimes found to assume a semi-crystalline and almost +metamorphic aspect, and thus, on lithological grounds, to deserve +equally the name of transition. So remarkably was this the case in the +Swiss Alps, that certain rocks, which had for years been regarded by +some of the most skilful disciples of Werner to be transition, were at +last acknowledged, when their relative position and fossils were better +understood, to belong to the newest of the secondary groups; nay, some +of them have actually been discovered to be members of the lower +tertiary series! If, under such circumstances, the name of transition +was retained, it is clear that it ought to have been applied without +reference to the age of strata, and simply as expressive of a mineral +peculiarity. The continued appropriation of the term to formations of a +given date, induced geologists to go on believing that the ancient +strata so designated bore a less resemblance to the secondary than is +really the case, and to imagine that these last never pass, as they +frequently do, into metamorphic rocks. + +The poet Waller, when lamenting over the antiquated style of Chaucer, +complains that-- + + We write in sand, our language grows, + And, like the tide, our work o'erflows. + +But the reverse is true in geology; for here it is our work which +continually outgrows the language. The tide of observation advances with +such speed that improvements in theory outrun the changes of nomenclature; +and the attempt to inculcate new truths by words invented to express a +different or opposite opinion, tends constantly, by the force of +association, to perpetuate error; so that dogmas renounced by the reason +still retain a strong hold upon the imagination. + +In order to reconcile the old chronological views with the new doctrine of +the igneous origin of granite, the following hypothesis was substituted for +that of the Neptunists. Instead of beginning with an aqueous menstruum or +chaotic fluid, the materials of the present crust of the earth were +supposed to have been at first in a state of igneous fusion, until part of +the heat having been diffused into surrounding space, the surface of the +fluid consolidated, and formed a crust of granite. This covering of +crystalline stone, which afterwards grew thicker and thicker as it cooled, +was so hot, at first, that no water could exist upon it; but as the +refrigeration proceeded, the aqueous vapour in the atmosphere was +condensed, and, falling in rain, gave rise to the first _thermal ocean_. So +high was the temperature of this boiling sea, that no aquatic beings could +inhabit its waters, and its deposits were not only devoid of fossils, but, +like those of some hot springs, were highly crystalline. Hence the origin +of the primary or crystalline strata,--gneiss, mica-schist, and the rest. + +Afterwards, when the granitic crust had been partially broken up, land +and mountains began to rise above the waters, and rains and torrents +ground down rock, so that sediment was spread over the bottom of the +seas. Yet the heat still remaining in the solid supporting substances +was sufficient to increase the chemical action exerted by the water, +although not so intense as to prevent the introduction and increase of +some living beings. During this state of things some of the residuary +mineral ingredients of the primæval ocean were precipitated, and formed +deposits (the transition strata of Werner), half chemical and half +mechanical, and containing a few fossils. + +By this new theory, which was in part a revival of the doctrine of +Leibnitz, published in 1680, on the igneous origin of the planet, the old +ideas respecting the priority of all crystalline rocks to the creation of +organic beings, were still preserved; and the mistaken notion that all the +semi-crystalline and partially fossiliferous rocks belonged to one period, +while all the earthy and uncrystalline formations originated at a +subsequent epoch, was also perpetuated. + +It may or may not be true, as the great Leibnitz imagined, that the whole +planet was once in a state of liquefaction by heat; but there are certainly +no geological proofs that the granite which constitutes the foundation of +so much of the earth's crust was ever at once in a state of universal +fusion. On the contrary, all our evidence tends to show that the formation +of granite, like the deposition of the stratified rocks, has been +successive, and that different portions of granite have been in a melted +state at distinct and often distant periods. One mass was solid, and had +been fractured, before another body of granitic matter was injected into +it, or through it, in the form of veins. Some granites are more ancient +than any known fossiliferous rocks; others are of secondary; and some, such +as that of Mont Blanc and part of the central axis of the Alps, of tertiary +origin. In short, the universal fluidity of the crystalline foundations of +the earth's crust, can only be understood in the same sense as the +universality of the ancient ocean. All the land has been under water, but +not all at one time; so all the subterranean unstratified rocks to which +man can obtain access have been melted, but not simultaneously. + +In the present work the four great classes of rocks, the aqueous, plutonic, +volcanic, and metamorphic, will form four parallel, or nearly parallel, +columns in one chronological table. They will be considered as four sets of +monuments relating to four contemporaneous, or nearly contemporaneous, +series of events. I shall endeavour, in a subsequent chapter on the +plutonic rocks, to explain the manner in which certain masses belonging to +each of the four classes of rocks may have originated simultaneously at +every geological period, and how the earth's crust may have been +continually remodelled, above and below, by aqueous and igneous causes, +from times indefinitely remote. In the same manner as aqueous and +fossiliferous strata are now formed in certain seas or lakes, while in +other places volcanic rocks break out at the surface, and are connected +with reservoirs of melted matter at vast depths in the bowels of the +earth,--so, at every era of the past, fossiliferous deposits and +superficial igneous rocks were in progress contemporaneously with others of +subterranean and plutonic origin, and some sedimentary strata were exposed +to heat and made to assume a crystalline or metamorphic structure. + +It can by no means be taken for granted, that during all these changes the +solid crust of the earth has been increasing in thickness. It has been +shown, that so far as aqueous action is concerned, the gain by fresh +deposits, and the loss by denudation, must at each period have been equal +(see above, p. 68.); and in like manner, in the inferior portion of the +earth's crust, the acquisition of new crystalline rocks, at each successive +era, may merely have counter-balanced the loss sustained by the melting of +materials previously consolidated. As to the relative antiquity of the +crystalline foundations of the earth's crust, when compared to the +fossiliferous and volcanic rocks which they support, I have already stated, +in the first chapter, that to pronounce an opinion on this matter is as +difficult as at once to decide which of the two, whether the foundations or +superstructure of an ancient city built on wooden piles, may be the oldest. +We have seen that, to answer this question, we must first be prepared to +say whether the work of decay and restoration had gone on most rapidly +above or below, whether the average duration of the piles has exceeded that +of the stone buildings, or the contrary. So also in regard to the relative +age of the superior and inferior portions of the earth's crust; we cannot +hazard even a conjecture on this point, until we know whether, upon an +average, the power of water above, or that of heat below, is most +efficacious in giving new forms to solid matter. + +After the observations which have now been made, the reader will +perceive that the term primary must either be entirely renounced, or, if +retained, must be differently defined, and not made to designate a set +of crystalline rocks, some of which are already ascertained to be newer +than all the secondary formations. In this work I shall follow most +nearly the method proposed by Mr. Boué, who has called all +_fossiliferous_ rocks older than the secondary by the name of primary. +To prevent confusion, however, I shall always speak of these, when they +are of the aqueous class, as the _primary fossiliferous_ formations, +because the word primary has hitherto been almost inseparably connected +with the idea of a non-fossiliferous rock. + +If we can prove any plutonic, volcanic, or metamorphic rocks to be older +than the secondary formations, such rocks will also be primary, according +to this system. Mr. Boué having with great propriety excluded the +metamorphic rocks, _as a class_, from the primary formations, proposed to +call them all "crystalline schists." + +As there are secondary fossiliferous strata, so we shall find that there +are plutonic, volcanic, and metamorphic rocks of contemporaneous origin, +which I shall also term secondary. + +In the next chapter it will be shown that the strata above the chalk have +been called tertiary. If, therefore, we discover any volcanic, plutonic, or +metamorphic rocks, which have originated since the deposition of the chalk, +these also will rank as tertiary formations. + +It may perhaps be suggested that some metamorphic strata, and some +granites, may be anterior in date to the oldest of the primary +fossiliferous rocks. This opinion is doubtless true, and will be discussed +in future chapters; but I may here observe, that when we arrange the four +classes of rocks in four parallel columns in one table of chronology, it is +by no means assumed that these columns are all of equal length; one may +begin at an earlier period than the rest, and another may come down to a +later point of time. In the small part of the globe hitherto examined, it +is hardly to be expected that we should have discovered either the oldest +or the newest members of each of the four classes of rocks. Thus, if there +be primary, secondary, and tertiary rocks of the aqueous or fossiliferous +class, and in like manner primary, secondary, and tertiary hypogene +formations, we may not be yet acquainted with the most ancient of the +primary fossiliferous beds, or with the newest of the hypogene. + + +FOOTNOTES: + +[91-A] See Principles, vol. i. chap. iv. + + + + +CHAPTER 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 palæontological characters--Test of age by included + fragments--Frequent absence of strata of intervening + periods--Principal groups of strata in western Europe. + + +In the last chapter I spoke generally of the chronological relations of the +four great classes of rocks, and I shall now treat of the aqueous rocks in +particular, or of the successive periods at which the different +fossiliferous formations have been deposited. + +There are three principal tests by which we determine the age of a given +set of strata; first, superposition; secondly, mineral character; and, +thirdly, organic remains. Some aid can occasionally be derived from a +fourth kind of proof, namely, the fact of one deposit including in it +fragments of a pre-existing rock, by which the relative ages of the two +may, even in the absence of all other evidence, be determined. + +_Superposition._--The first and principal test of the age of one aqueous +deposit, as compared to another, is relative position. It has been already +stated, that where strata are horizontal, the bed which lies uppermost is +the newest of the whole, and that which lies at the bottom the most +ancient. So, of a series of sedimentary formations, they are like volumes +of history, in which each writer has recorded the annals of his own times, +and then laid down the book, with the last written page uppermost, upon the +volume in which the events of the era immediately preceding were +commemorated. In this manner a lofty pile of chronicles is at length +accumulated; and they are so arranged as to indicate, by their position +alone, the order in which the events recorded in them have occurred. + +In regard to the crust of the earth, however, there are some regions where, +as the student has already been informed, the beds have been disturbed, and +sometimes extensively thrown over and turned upside down. (See pp. 58, 59.) +But an experienced geologist can rarely be deceived by these exceptional +cases. When he finds that the strata are fractured, curved, inclined, or +vertical, he knows that the original order of superposition must be +doubtful, and he then endeavours to find sections in some neighbouring +district where the strata are horizontal, or only slightly inclined. Here +the true order of sequence of the entire series of deposits being +ascertained, a key is furnished for settling the chronology of those strata +where the displacement is extreme. + +_Mineral character._--The same rocks may often be observed to retain for +miles, or even hundreds of miles, the same mineral peculiarities, if we +follow the planes of stratification, or trace the beds, if they be +undisturbed, in a horizontal direction. But if we pursue them vertically, +or in any direction transverse to the planes of stratification, this +uniformity ceases almost immediately. In that case we can scarcely ever +penetrate a stratified mass for a few hundred yards without beholding a +succession of extremely dissimilar, calcareous, argillaceous, and siliceous +rocks. These phenomena lead to the conclusion, that rivers and currents +have dispersed the same sediment over wide areas at one period, but at +successive periods have been charged, in the same region, with very +different kinds of matter. The first observers were so astonished at the +vast spaces over which they were able to follow the same homogeneous rocks +in a horizontal direction, that they came hastily to the opinion, that the +whole globe had been environed by a succession of distinct aqueous +formations, disposed round the nucleus of the planet, like the concentric +coats of an onion. But although, in fact, some formations may be continuous +over districts as large as half of Europe, or even more, yet most of them +either terminate wholly within narrower limits, or soon change their +lithological character. Sometimes they thin out gradually, as if the supply +of sediment had failed in that direction, or they come abruptly to an end, +as if we had arrived at the borders of the ancient sea or lake which served +as their receptacle. It no less frequently happens that they vary in +mineral aspect and composition, as we pursue them horizontally. For +example, we trace a limestone for a hundred miles, until it becomes more +arenaceous, and finally passes into sand, or sandstone. We may then follow +this sandstone, already proved by its continuity to be of the same age, +throughout another district a hundred miles or more in length. + +_Organic remains._--This character must be used as a criterion of the +age of a formation, or of the contemporaneous origin of two deposits +in distant places, under very much the same restrictions as the test +of mineral composition. + +First, the same fossils may be traced over wide regions, if we examine +strata in the direction of their planes, although by no means for +indefinite distances. + +Secondly, while the same fossils prevail in a particular set of strata for +hundreds of miles in a horizontal direction, we seldom meet with the same +remains for many fathoms, and very rarely for several hundred yards, in a +vertical line, or a line transverse to the strata. This fact has now been +verified in almost all parts of the globe, and has led to a conviction, +that at successive periods of the past, the same area of land and water has +been inhabited by species of animals and plants even more distinct than +those which now people the antipodes, or which now co-exist in the arctic, +temperate, and tropical zones. It appears, that from the remotest periods +there has been ever a coming in of new organic forms, and an extinction of +those which pre-existed on the earth; some species having endured for a +longer, others for a shorter, time; while none have ever reappeared after +once dying out. The law which has governed the creation and extinction of +species seems to be expressed in the verse of the poet,-- + + Natura il fece, e poi ruppe la stampa. ARIOSTO. + Nature made him, and then broke the die. + +And this circumstance it is, which confers on fossils their highest value +as chronological tests, giving to each of them, in the eyes of the +geologist, that authority which belongs to contemporary medals in history. + +The same cannot be said of each peculiar variety of rock; for some of +these, as red marl and red sandstone, for example, may occur at once at the +top, bottom, and middle of the entire sedimentary series; exhibiting in +each position so perfect an identity of mineral aspect as to be +undistinguishable. Such exact repetitions, however, of the same mixtures of +sediment have not often been produced, at distant periods, in precisely the +same parts of the globe; and even where this has happened, we are seldom in +any danger of confounding together the monuments of remote eras, when we +have studied their imbedded fossils and relative position. + +It was remarked that the same species of organic remains cannot be traced +horizontally, or in the direction of the planes of stratification for +indefinite distances. This might have been expected from analogy; for when +we inquire into the present distribution of living beings, we find that the +habitable surface of the sea and land may be divided into a considerable +number of distinct provinces, each peopled by a peculiar assemblage of +animals and plants. In the Principles of Geology, I have endeavoured to +point out the extent and probable origin of these separate divisions; and +it was shown that climate is only one of many causes on which they depend, +and that difference of longitude as well as latitude is generally +accompanied by a dissimilarity of indigenous species. + +As different seas, therefore, and lakes are inhabited at the same period, +by different aquatic animals and plants, and as the lands adjoining these +may be peopled by distinct terrestrial species, it follows that distinct +fossils will be imbedded in contemporaneous deposits. If it were +otherwise--if the same species abounded in every climate, or in every part +of the globe where, so far as we can discover, a corresponding temperature +and other conditions favourable to their existence are found--the +identification of mineral masses of the same age, by means of their +included organic contents, would be a matter of still greater certainty. + +Nevertheless, the extent of some single zoological provinces, especially +those of marine animals, is very great; and our geological researches have +proved that the same laws prevailed at remote periods; for the fossils are +often identical throughout wide spaces, and in a great number of detached +deposits, in which the mineral nature of the rocks is variable. + +The doctrine here laid down will be more readily understood, if we +reflect on what is now going on in the Mediterranean. That entire sea +may be considered as one zoological province; for, although certain +species of testacea and zoophytes may be very local, and each region has +probably some species peculiar to it, still a considerable number are +common to the whole Mediterranean. If, therefore, at some future period, +the bed of this inland sea should be converted into land, the geologist +might be enabled, by reference to organic remains, to prove the +contemporaneous origin of various mineral masses scattered over a space +equal in area to the half of Europe. + +Deposits, for example, are well known to be now in progress in this sea in +the deltas of the Po, Rhone, Nile, and other rivers, which differ as +greatly from each other in the nature of their sediment as does the +composition of the mountains which they drain. There are also other +quarters of the Mediterranean, as off the coast of Campania, or near the +base of Etna, in Sicily, or in the Grecian Archipelago, where another class +of rocks is now forming; where showers of volcanic ashes occasionally fall +into the sea, and streams of lava overflow its bottom; and where, in the +intervals between volcanic eruptions, beds of sand and clay are frequently +derived from the waste of cliffs, or the turbid waters of rivers. +Limestones, moreover, such as the Italian travertins, are here and there +precipitated from the waters of mineral springs, some of which rise up from +the bottom of the sea. In all these detached formations, so diversified in +their lithological characters, the remains of the same shells, corals, +crustacea, and fish are becoming inclosed; or, at least, a sufficient +number must be common to the different localities to enable the zoologist +to refer them all to one contemporaneous assemblage of species. + +There are, however, certain combinations of geographical circumstances +which cause distinct provinces of animals and plants to be separated +from each other by very narrow limits; and hence it must happen, that +strata will be sometimes formed in contiguous regions, differing widely +both in mineral contents and organic remains. Thus, for example, the +testacea, zoophytes, and fish of the Red Sea are, as a group, extremely +distinct from those inhabiting the adjoining parts of the Mediterranean, +although the two seas are separated only by the narrow isthmus of Suez. +Of the bivalve shells, according to Philippi, not more than a fifth are +common to the Red Sea and the sea around Sicily, while the proportion of +univalves (or Gasteropoda) is still smaller, not exceeding eighteen in a +hundred. Calcareous formations have accumulated on a great scale in the +Red Sea in modern times, and fossil shells of existing species are well +preserved therein; and we know that at the mouth of the Nile large +deposits of mud are amassed, including the remains of Mediterranean +species. It follows, therefore, that if at some future period the bed of +the Red Sea should be laid dry, the geologist might experience great +difficulties in endeavouring to ascertain the relative age of these +formations, which, although dissimilar both in organic and mineral +characters, were of synchronous origin. + +But, on the other hand, we must not forget that the north-western shores of +the Arabian Gulf, the plains of Egypt, and the isthmus of Suez, are all +parts of one province of _terrestrial_ species. Small streams, therefore, +occasional land-floods, and those winds which drift clouds of sand along +the deserts, might carry down into the Red Sea the same shells of +fluviatile and land testacea which the Nile is sweeping into its delta, +together with some remains of terrestrial plants and the bones of +quadrupeds, whereby the groups of strata, before alluded to, might, +notwithstanding the discrepancy of their mineral composition and _marine_ +organic fossils, be shown to have belonged to the same epoch. + +Yet while rivers may thus carry down the same fluviatile and terrestrial +spoils into two or more seas inhabited by different marine species, it +will much more frequently happen, that the co-existence of terrestrial +species of distinct zoological and botanical provinces will be proved by +the identity of the marine beings which inhabited the intervening space. +Thus, for example, the land quadrupeds and shells of the south of +Europe, north of Africa, and north-west of Asia, are different, yet +their remains are all washed down by rivers flowing from these three +countries into the Mediterranean. + +In some parts of the globe, at the present period, the line of demarcation +between distinct provinces of animals and plants is not very strongly +marked, especially where the change is determined by temperature, as in +seas extending from the temperate to the tropical zone, or from the +temperate to the arctic regions. Here a gradual passage takes place from +one set of species to another. In like manner the geologist, in studying +particular formations of remote periods, has sometimes been able to trace +the gradation from one ancient province to another, by observing carefully +the fossils of all the intermediate places. His success in thus acquiring a +knowledge of the zoological or botanical geography of very distant eras +has been mainly owing to this circumstance, that the mineral character has +no tendency to be affected by climate. A large river may convey yellow or +red mud into some part of the ocean, where it may be dispersed by a current +over an area several hundred leagues in length, so as to pass from the +tropics into the temperate zone. If the bottom of the sea be afterwards +upraised, the organic remains imbedded in such yellow or red strata may +indicate the different animals or plants which once inhabited at the same +time the temperate and equatorial regions. + +It may be true, as a general rule, that groups of the same species of +animals and plants may extend over wider areas than deposits of homogeneous +composition; and if so, palæontological characters will be of more +importance in geological classification than mineral composition; but it is +idle to discuss the relative value of these tests, as the aid of both is +indispensable, and it fortunately happens, that where the one criterion +fails, we can often avail ourselves of the other. + +_Test by included fragments of older rocks._--It was stated, that +independent proof may sometimes be obtained of the relative date of two +formations, by fragments of an older rock being included in a newer one. +This evidence may sometimes be of great use, where a geologist is at a +loss to determine the relative age of two formations from want of clear +sections exhibiting their true order of position, or because the strata +of each group are vertical. In such cases we sometimes discover that the +more modern rock has been in part derived from the degradation of the +older. Thus, for example, we may find in one part of a country chalk +with flints; and, in another, a distinct formation, consisting of +alternations of clay, sand, and pebbles. If some of these pebbles +consist of similar flint and fossil shells, sponges, and foraminiferæ, +of the same species as those in the chalk, we may confidently infer that +the chalk is the oldest of the two formations. + +_Chronological groups._--The number of groups into which the +fossiliferous strata may be separated are more or less numerous, +according to the views of classification which different geologists +entertain; but when we have adopted a certain system of arrangement, we +immediately find that a few only of the entire series of groups occur +one upon the other in any single section or district. + +[Illustration: Fig. 104. Block section.] + +The thinning out of individual strata was before described (p. 16.). But +let the annexed diagram represent seven fossiliferous groups, instead of +as many strata. It will then be seen that in the middle all the +superimposed formations are present; but in consequence of some of them +thinning out, No. 2. and No. 5. are absent at one extremity of the +section, and No. 4. at the other. + +[Illustration: Fig. 105. Section South of Bristol. A. C. Ramsay. + +Length of section 4 miles. _a_, _b_. Level of the sea. + 1. Inferior oolite. + 2. Lias. + 3. New red sandstone. + 4. Magnesian conglomerate. + 5. Coal measure. + 6. Carboniferous limestone. + 7. Old red sandstone.] + +In the annexed diagram, fig. 105., a real section of the geological +formations in the neighbourhood of Bristol and the Mendip Hills, is +presented to the reader as laid down on a true scale by Professor +Ramsay, where the newer groups 1, 2, 3, 4. rest unconformably on the +formations 5 and 6. Here at the southern end of the line of section we +meet with the beds No. 3. (the New Red Sandstone) resting immediately on +No. 6., while farther north, as at Dundry Hill, we behold six groups +superimposed one upon the other, comprising all the strata from the +inferior oolite to the coal and carboniferous limestone. The limited +extension of the groups 1 and 2. is owing to denudation, as these +formations end abruptly, and have left outlying patches to attest the +fact of their having originally covered a much wider area. + +In many instances, however, the entire absence of one or more formations of +intervening periods between two groups, such as 3. and 5. in the same +section, arises, not from the destruction of what once existed, but because +no strata of an intermediate age were ever deposited on the inferior rock. +They were not formed at that place, either because the region was dry land +during the interval, or because it was part of a sea or lake to which no +sediment was carried. + +In order, therefore, to establish a chronological succession of +fossiliferous groups, a geologist must begin with a single section, in +which several sets of strata lie one upon the other. He must then trace +these formations, by attention to their mineral character and fossils, +continuously, as far as possible, from the starting point. As often as he +meets with new groups, he must ascertain by superposition their age +relatively to those first examined, and thus learn how to intercalate them +in a tabular arrangement of the whole. + +By this means the German, French, and English geologists have determined +the succession of strata throughout a great part of Europe, and have +adopted pretty generally the following groups, almost all of which have +their representatives in the British Islands. + +_Groups of Fossiliferous Strata observed in Western Europe, arranged in +what is termed a descending Series, or beginning with the newest._ (_See a +more detailed Tabular view_, pp. 360. 365.) + + 1. Post-Pliocene, including those of the + Recent, or human period. + + 2. Newer Pliocene, or Pleistocene. } + 3. Older Pliocene. } Tertiary, Supracretaceous[103-A], + 4. Miocene. } or Cainozoic.[103-B] + 5. Eocene. } + + 6. Chalk. } + 7. Greensand. } + 8. Wealden. } + 9. Upper Oolite. } Secondary, or Mesozoic.[103-B] + 10. Middle Oolite. } + 11. Lower Oolite. } + 12. Lias. } + 13. Trias. } + + 14. Permian. } + 15. Coal. } + 16. Old Red sandstone, or Devonian. } Primary fossiliferous, + 17. Upper Silurian. } or paleozoic.[103-B] + 18. Lower Silurian. } + 19. Cambrian and older fossiliferous strata. } + +It is not pretended that the three principal sections in the above table, +called primary, secondary, and tertiary, are of equivalent importance, or +that the eighteen subordinate groups comprise monuments relating to equal +portions of past time, or of the earth's history. But we can assert that +they each relate to successive periods, during which certain animals and +plants, for the most part peculiar to their respective eras, have +flourished, and during which different kinds of sediment were deposited in +the space now occupied by Europe. + +If we were disposed, on palæontological grounds[103-C], to divide the +entire fossiliferous series into a few groups less numerous than those in +the above table, and more nearly co-ordinate in value than the sections +called primary, secondary, and tertiary, we might, perhaps, adopt the six +groups or periods given in the next table (p. 104.). + +At the same time, I may observe, that, in the present state of the +science, when we have not yet compared the evidence derivable from all +classes of fossils, not even those most generally distributed, such as +shells, corals, and fish, such generalizations are premature, and can +only be regarded as conjectural or provisional schemes for the founding +of large natural groups. + +_Fossiliferous Strata of Western Europe divided into Six Groups._ + + 1. Post Pliocene and } from the Post-Pliocene to the + Tertiary } Eocene inclusive. + + 2. Cretaceous { from the Maestricht Chalk to the Lower + { Greensand inclusive. + + 3. Oolitic from the Wealden to the Lias inclusive. + + 4. Triassic { including the Keuper, Muschelkalk, and + { Bunter Sandstein of the Germans. + + 5. Permian, Carboniferous, } including Magnesian Limestone (Zechstein), + and Devonian } Coal, Mountain Limestone, and + } Old Red sandstone. + + 6. Silurian and Cambrian } from the Upper Silurian to the oldest + } fossiliferous rocks inclusive. + + +FOOTNOTES: + +[103-A] For tertiary, Sir H. De la Beche has used the term +"supracretaceous," a name implying that the strata so called are +superior in position to the chalk. + +[103-B] Professor Phillips has adopted these terms: Cainozoic, from ++kainos+, _cainos_, recent, and +zôon+, _zoon_, animal; Mesozoic, +from +mesos+, _mesos_, middle, &c.; Paleozoic, from +palaios+, +_palaios_, ancient, &c. + +[103-C] Palæontology is the science which treats of fossil remains, both +animal and vegetable. Etym. +palaios+, _palaios_, ancient, +onta+, _onta_, +beings, and +logos+, _logos_, a discourse. + + + + +CHAPTER 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. + + +Before describing the most modern of the sets of strata enumerated in +the tables given at the end of the last chapter, it will be necessary +to say something generally of the mode of classifying the formations +called tertiary. + +The name of tertiary has been given to them, because they are all posterior +in date to the rocks termed "secondary," of which the chalk constitutes the +newest group. These tertiary strata were at first confounded, as before +stated, p. 91., with the superficial alluviums of Europe; and it was long +before their real extent and thickness, and the various ages to which they +belong, were fully recognized. They were observed to occur in patches, some +of freshwater, others of marine origin, their geographical area being +usually small as compared to the secondary formations, and their position +often suggesting the idea of their having been deposited in different bays, +lakes, estuaries, or inland seas, after a large portion of the space now +occupied by Europe had already been converted into dry land. + +The first deposits of this class, of which the characters were accurately +determined, were those occurring in the neighbourhood of Paris, described +in 1810 by MM. Cuvier and Brongniart. They were ascertained to consist of +successive sets of strata, some of marine, others of freshwater origin, +lying one upon the other. The fossil shells and corals were perceived to be +almost all of unknown species, and to have in general a near affinity to +those now inhabiting warmer seas. The bones and skeletons of land animals, +some of them of large size, and belonging to more than forty distinct +species, were examined by Cuvier, and declared by him not to agree +specifically and for the most part not even generically, with any hitherto +observed in the living creation. + +Strata were soon afterwards brought to light in the vicinity of London, and +in Hampshire, which, although dissimilar in mineral composition, were +justly inferred by Mr. T. Webster to be of the same age as those of Paris, +because the greater number of the fossil shells were specifically +identical. For the same reason rocks found on the Gironde, in the South of +France, and at certain points in the North of Italy, were suspected to be +of contemporaneous origin. + +A variety of deposits were afterwards found in other parts of Europe, all +reposing immediately on rocks as old or older than the chalk, and which +exhibited certain general characters of resemblance in their organic +remains to those previously observed near Paris and London. An attempt was +therefore made at first to refer the whole to one period; and when at +length this seemed impracticable, it was contended that as in the Parisian +series there were many subordinate formations of considerable thickness +which must have accumulated one after the other, during a great lapse of +time, so the various patches of tertiary strata scattered over Europe might +correspond in age, some of them to the older, and others to the newer, +subdivisions of the Parisian series. + +This error, although most unavoidable on the part of those who made the +first generalizations in this branch of geology, retarded seriously for +some years the progress of classification. A more scrupulous attention to +specific distinctions, aided by a careful regard to the relative position +of the strata containing them, led at length to the conviction that there +were formations both marine and freshwater of various ages, and all newer +than the strata of the neighbourhood of Paris and London. + +One of the first steps in this chronological reform was made in 1811, by +an English naturalist, Mr. Parkinson, who pointed out the fact that +certain shelly strata, provincially termed "Crag" in Suffolk, lay +decidedly over a deposit which was the continuation of the blue clay of +London. At the same time he remarked that the fossil testacea in these +newer beds were distinct from those of the blue clay, and that while +some of them were of unknown species, others were identical with species +now inhabiting the British seas. + +Another important discovery was soon afterwards made by Brocchi in Italy, +who investigated the argillaceous and sandy deposits replete with shells +which form a low range of hills, flanking the Apennines on both sides, from +the plains of the Po to Calabria. These lower hills were called by him the +Subapennines, and were formed of strata of different ages, all newer than +those of Paris and London. + +Another tertiary group occurring in the neighbourhood of Bordeaux and Dax, +in the south of France, was examined by M. de Basterot in 1825, who +described and figured several hundred species of shells, which differed for +the most part both from the Parisian series and those of the Subapennine +hills. It was soon, therefore, suspected that this fauna might belong to a +period intermediate between that of the Parisian and Subapennine strata, +and it was not long before the evidence of superposition was brought to +bear in support of this opinion; for other strata, contemporaneous with +those of Bordeaux, were observed in one district (the Valley of the Loire), +to overlie the Parisian formation, and in another (in Piedmont) to underlie +the Subapennine beds. The first example of these was pointed out in 1829 by +M. Desnoyers, who ascertained that the sand and marl of marine origin +called Faluns, near Tours, in the basin of the Loire, full of sea-shells +and corals, rested upon a lacustrine formation, which constitutes the +uppermost subdivision of the Parisian group, extending continuously +throughout a great table-land intervening between the basin of the Seine +and that of the Loire. The other example occurs in Italy, where strata, +containing many fossils similar to those of Bordeaux, were observed by +Bonelli and others in the environs of Turin, subjacent to strata belonging +to the Subapennine group of Brocchi. + +Without pretending to give a complete sketch of the progress of +discovery, I may refer to the facts above enumerated, as illustrating +the course usually pursued by geologists when they attempt to found new +chronological divisions. The method bears some analogy to that pursued +by the naturalist in the construction of genera, when he selects a +typical species, and then classes as congeners all other species of +animals and plants which agree with this standard within certain limits. +The genera A. and C. having been founded on these principles, a new +species is afterwards met with, departing widely both from A. and C., +but in many respects of an intermediate character. For this new type it +becomes necessary to institute the new genus B., in which are included +all species afterwards brought to light, which agree more nearly with B. +than with the types of A. or C. In like manner a new formation is met +with in geology, and the characters of its fossil fauna and flora +investigated. From that moment it is considered as a record of a certain +period of the earth's history, and a standard to which other deposits +may be compared. If any are found containing the same or nearly the same +organic remains, and occupying the same relative position, they are +regarded in the light of contemporary annals. All such monuments are +said to relate to one period, during which certain events occurred, such +as the formation of particular rocks by aqueous or volcanic agency, or +the continued existence and fossilization of certain tribes of animals +and plants. When several of these periods have had their true places +assigned to them in a chronological series, others are discovered +which it becomes necessary to intercalate between those first known; +and the difficulty of assigning clear lines of separation must +unavoidably increase in proportion as chasms in the past history of +the globe are filled up. + +Every zoologist and botanist is aware that it is a comparatively easy +task to establish genera in departments which have been enriched with +only a small number of species, and where there is as yet no tendency in +one set of characters to pass almost insensibly, by a multitude of +connecting links, into another. They also know that the difficulty of +classification augments, and that the artificial nature of their +divisions becomes more apparent, in proportion to the increased number +of objects brought to light. But in separating families and genera, they +have no other alternative than to avail themselves of such breaks as +still remain, or of every hiatus in the chain of animated beings which +is not yet filled up. So in geology, we may be eventually compelled to +resort to sections of time as arbitrary, and as purely conventional, as +those which divide the history of human events into centuries. But in +the present state of our knowledge, it is more convenient to use the +interruptions which still occur in the regular sequence of geological +monuments, as boundary lines between our principal groups or periods, +even though the groups thus established are of very unequal value. + +The isolated position of distinct tertiary deposits in different parts +of Europe has been already alluded to. In addition to the difficulty +presented by this want of continuity when we endeavour to settle the +chronological relations of these deposits, another arises from the +frequent dissimilarity in mineral character of strata of contemporaneous +date, such, for example, as those of London and Paris before mentioned. +The identity or non-identity of species is also a criterion which often +fails us. For this we might have been prepared, for we have already +seen, that the Mediterranean and Red Sea, although within 70 miles of +each other, on each side of the Isthmus of Suez, have each their +peculiar fauna; and a marked difference is found in the four groups of +testacea now living in the Baltic, English Channel, Black Sea, and +Mediterranean, although all these seas have many species in common. In +like manner a considerable diversity in the fossils of different +tertiary formations, which have been thrown down in distinct seas, +estuaries, bays, and lakes, does not always imply a distinctness in the +times when they were produced, but may have arisen from climate and +conditions of physical geography wholly independent of time. On the +other hand, it is now abundantly clear, as the result of geological +investigation, that different sets of tertiary strata, immediately +superimposed upon each other, contain distinct imbedded species of +fossils, in consequence of fluctuations which have been going on in the +animate creation, and by which in the course of ages one state of things +in the organic world has been substituted for another wholly dissimilar. +It has also been shown that in proportion as the age of a tertiary +deposit is more modern, so is its fauna more analogous to that now in +being in the neighbouring seas. It is this law of a nearer agreement of +the fossil testacea with the species now living, which may often furnish +us with a clue for the chronological arrangement of scattered deposits, +where we cannot avail ourselves of any one of the three ordinary +chronological tests; namely, superposition, mineral character, and the +specific identity of the fossils. + +Thus, for example, on the African border of the Red Sea, at the height +of 40 feet, and sometimes more, above its level, a white calcareous +formation has been observed, containing several hundred species of +shells differing from those found in the clay and volcanic tuff of the +country round Naples, and of the contiguous island of Ischia. Another +deposit has been found at Uddevalla, in Sweden, in which the shells do +not agree with those found near Naples. But although in these three +cases there may be scarcely a single shell common to the three different +deposits, we do not hesitate to refer them all to one period (the +Post-Pliocene), because of the very close agreement of the fossil +species in every instance with those now living in the contiguous seas. + +To take another example, where the fossil fauna recedes a few steps farther +back from our own times. We may compare, first, the beds of loam and clay +bordering the Clyde in Scotland (called glacial by some geologists), +secondly, others of fluvio-marine origin near Norwich, and, lastly, a third +set often rising to considerable heights in Sicily, and we discover that in +every case more than three-fourths of the shells agree with species still +living, while the remainder are extinct. Hence we may conclude that all +these, greatly diversified as are their organic remains, belong to one and +the same era, or to a period immediately antecedent to the Post-Pliocene, +because there has been time in each of the areas alluded to for an equal or +nearly equal amount of change in the marine testaceous fauna. +Contemporaneousness of origin is inferred in these cases, in spite of the +most marked differences of mineral character or organic contents, from a +similar degree of divergence in the shells from those now living in the +adjoining seas. The advantage of such a test consists in supplying us with +a common point of departure in all countries, however remote. + +But the farther we recede from the present times, and the smaller the +relative number of recent as compared with extinct species in the +tertiary deposits, the less confidence can we place in the exact value +of such a test, especially when comparing the strata of very distant +regions; for we cannot presume that the rate of former alterations in +the animate world, or the continual going out and coming in of species, +has been every where exactly equal in equal quantities of time. The +form of the land and sea, and the climate, may have changed more in +one region than in another; and consequently there may have been a +more rapid destruction and renovation of species in one part of the +globe than elsewhere. Considerations of this kind should undoubtedly +put us on our guard against relying too implicitly on the accuracy of +this test; yet it can never fail to throw great light on the +chronological relations of tertiary groups with each other, and with +the Post-Pliocene period. + +We may derive a conviction of this truth not only from a study of +geological monuments of all ages, but also by reflecting on the tendency +which prevails in the present state of nature to a uniform rate of +simultaneous fluctuation in the flora and fauna of the whole globe. The +grounds of such a doctrine cannot be discussed here, and I have explained +them at some length in the third Book of the Principles of Geology, where +the causes of the successive extinction of species are considered. It will +be there seen that each local change in climate and physical geography is +attended with the immediate increase of certain species, and the limitation +of the range of others. A revolution thus effected is rarely, if ever, +confined to a limited space, or to one geographical province of animals or +plants, but affects several other surrounding and contiguous provinces. In +each of these, moreover, analogous alterations of the stations and +habitations of species are simultaneously in progress, reacting in the +manner already alluded to on the first province. Hence, long before the +geography of any particular district can be essentially altered, the flora +and fauna throughout the world will have been materially modified by +countless disturbances in the mutual relation of the various members of the +organic creation to each other. To assume that in one large area inhabited +exclusively by a single assemblage of species any important revolution in +physical geography can be brought about, while other areas remain +stationary in regard to the position of land and sea, the height of +mountains, and so forth, is a most improbable hypothesis, wholly opposed to +what we know of the laws now governing the aqueous and igneous causes. On +the other hand, even were this conceivable, the communication of heat and +cold between different parts of the atmosphere and ocean is so free and +rapid, that the temperature of certain zones cannot be materially raised or +lowered without others being immediately affected; and the elevation or +diminution in height of an important chain of mountains or the submergence +of a wide tract of land would modify the climate even of the antipodes. + +It will be observed that in the foregoing allusions to organic remains, the +testacea or the shell-bearing mollusca are selected as the most useful and +convenient class for the purposes of general classification. In the first +place, they are more universally distributed through strata of every age +than any other organic bodies. Those families of fossils which are of rare +and casual occurrence are absolutely of no avail in establishing a +chronological arrangement. If we have plants alone in one group of strata +and the bones of mammalia in another, we can draw no conclusion respecting +the affinity or discordance of the organic beings of the two epochs +compared; and the same may be said if we have plants and vertebrated +animals in one series and only shells in another. Although corals are more +abundant, in a fossil state, than plants, reptiles, or fish, they are still +rare when contrasted with shells, especially in the European tertiary +formations. The utility of the testacea is, moreover, enhanced by the +circumstance that some forms are proper to the sea, others to the land, and +others to freshwater. Rivers scarcely ever fail to carry down into their +deltas some land shells, together with species which are at once fluviatile +and lacustrine. By this means we learn what terrestrial, freshwater, and +marine species co-existed at particular eras of the past; and having thus +identified strata formed in seas with others which originated +contemporaneously in inland lakes, we are then enabled to advance a step +farther, and show that certain quadrupeds or aquatic plants, found fossil +in lacustrine formations, inhabited the globe at the same period when +certain fish, reptiles, and zoophytes lived in the ocean. + +Among other characters of the molluscous animals, which render them +extremely valuable in settling chronological questions in geology, may be +mentioned, first, the wide geographical range of many species; and, +secondly, what is probably a consequence of the former, the great duration +of species in this class, for they appear to have surpassed in longevity +the greater number of the mammalia and fish. Had each species inhabited a +very limited space, it could never, when imbedded in strata, have enabled +the geologist to identify deposits at distant points; or had they each +lasted but for a brief period, they could have thrown no light on the +connection of rocks placed far from each other in the chronological, or, as +it is often termed, vertical series. + +Many authors have divided the European tertiary strata into three +groups--lower, middle, and upper; the lower comprising the oldest +formations of Paris and London before-mentioned; the middle those of +Bordeaux and Touraine; and the upper all those newer than the middle group. + +When engaged in 1828 in preparing my work on the Principles of Geology, I +conceived the idea of classing the whole series of tertiary strata in four +groups, and endeavouring to find characters for each, expressive of their +different degrees of affinity to the living fauna. With this view, I +obtained information respecting the specific identity of many tertiary and +recent shells from several Italian naturalists, and among others from +Professors Bonelli, Guidotti, and Costa. Having in 1829 become acquainted +with M. Deshayes, of Paris, already well known by his conchological works, +I learnt from him that he had arrived, by independent researches, and by +the study of a large collection of fossil and recent shells, at very +similar views respecting the arrangement of tertiary formations. At my +request he drew up, in a tabular form, lists of all the shells known to him +to occur both in some tertiary formation and in a living state, for the +express purpose of ascertaining the proportional number of fossil species +identical with the recent which characterized successive groups; and this +table, planned by us in common, was published by me in 1833.[110-A] The +number of tertiary fossil shells examined by M. Deshayes was about 3000; +and the recent species with which they had been compared about 5000. The +result then arrived at was, that in the lower tertiary strata, or those of +London and Paris, there were about 3-1/2 per cent. of species identical +with recent; in the middle tertiary of the Loire and Gironde about 17 per +cent.; and in the upper tertiary or Subapennine beds, from 35 to 50 per +cent. In formations still more modern, some of which I had particularly +studied in Sicily, where they attain a vast thickness and elevation above +the sea, the number of species identical with those now living was believed +to be from 90 to 95 per cent. For the sake of clearness and brevity, I +proposed to give short technical names to these four groups, or the periods +to which they respectively belonged. I called the first or oldest of them +Eocene, the second Miocene, the third Older Pliocene, and the last or +fourth Newer Pliocene. The first of the above terms, Eocene, is derived +from +êôs+, eos, _dawn_, and +kainos+, cainos, _recent_, because the fossil +shells of this period contain an extremely small proportion of living +species, which may be looked upon as indicating the dawn of the existing +state of the testaceous fauna, no recent species having been detected in +the older or secondary rocks. + +The term Miocene (from +meion+, meion, _less_, and +kainos+, cainos, +_recent_) is intended to express a minor proportion of recent species (of +testacea), the term Pliocene (from +pleion+, pleion, _more_, and +kainos+, +cainos, _recent_) a comparative plurality of the same. It may assist the +memory of students to remind them, that the _Mi_ocene contain a _mi_nor +proportion, and _Pl_iocene a comparative _pl_urality of recent species; and +that the greater number of recent species always implies the more modern +origin of the strata. + +It has sometimes been objected to this nomenclature that certain species of +infusoria found in the chalk are still existing, and, on the other hand, +the Miocene and Older Pliocene deposits often contain the remains of +mammalia, reptiles, and fish, exclusively of extinct species. But the +reader must bear in mind that the terms Eocene, Miocene, and Pliocene were +originally invented with reference purely to conchological data, and in +that sense have always been and are still used by me. + +The distribution of the fossil species from which the results before +mentioned were obtained in 1830 by M. Deshayes was as follows:-- + + In the formations of the Pliocene periods, older and newer 777 + In the Miocene 1021 + In the Eocene 1238 + ---- + 3036 + ---- + +Since the year 1830 the progress of conchological science has been most +rapid, and the number of living species obtained from different parts of +the globe has been raised from about 5000 to more than 10,000. New +fossil species have also been added to our collections in great +abundance; and at the same time a more copious supply of individuals +both of fossil and recent species, some of which were previously very +rare, have been procured, affording more ample data for determining the +specific character. Besides the reforms introduced in consequence of +these new zoological facilities, other errors of a geological nature +have been in many instances removed. + + +POST-PLIOCENE FORMATIONS. + +I have adopted the term Post-Pliocene for those strata which are sometimes +called post-tertiary or modern, and which are characterized by having all +the imbedded fossil shells identical with species now living, whereas even +the Newer Pliocene, or newest of the tertiary deposits above alluded to, +contain always some small proportion of shells of extinct species. + +These modern formations, thus defined, comprehend not only those strata +which can be shown to have originated since the earth was inhabited by man, +but also deposits of far greater extent and thickness, in which no signs of +man or his works can be detected. In some of these, of a date long anterior +to the times of history and tradition, the bones of extinct quadrupeds have +been met with of species which probably never co-existed with the human +race, as, for example, the mammoth, mastodon, megatherium, and others, and +yet the shells are the same as those now living. + +That portion of the post-pliocene group which belongs to the human epoch, +and which is sometimes called _Recent_, forms a very unimportant feature in +the geological structure of the earth's crust. I have shown, however, in +"The Principles," where the recent changes of the earth illustrative of +geology are described at length, that the deposits accumulated at the +bottom of lakes and seas within the last 4000 or 5000 years can neither be +insignificant in volume or extent. They lie hidden, for the most part, from +our sight; but we have opportunities of examining them at certain points +where newly-gained land in the deltas of rivers has been cut through during +floods, or where coral reefs are growing rapidly, or where the bed of a sea +or lake has been heaved up by subterranean movements and laid dry. Their +age may be recognized either by our finding in them the bones of man in a +fossil state, that is to say, imbedded in them by natural causes, or by +their containing articles fabricated by the hands of man. + +Thus at Puzzuoli, near Naples, marine strata are seen containing +fragments of sculpture, pottery, and the remains of buildings, together +with innumerable shells retaining in part their colour, and of the same +species as those now inhabiting the Bay of Baiæ. The uppermost of these +beds is about 20 feet above the level of the sea. Their emergence can be +proved to have taken place since the beginning of the sixteenth +century.[112-A] Now here, as in almost every instance where any +alterations of level have been going on in historical periods, it is +found that rocks containing shells, all, or nearly all, of which still +inhabit the neighbouring sea, may be traced for some distance into the +interior, and often to a considerable elevation above the level of the +sea. Thus, in the country round Naples, the post-pliocene strata, +consisting of clay and horizontal beds of volcanic tuff, rise at certain +points to the height of 1500 feet. Although the marine shells are +exclusively of living species, they are not accompanied like those on +the coast at Puzzuoli by any traces of man or his works. Had any such +been discovered, it would have afforded to the antiquary and geologist +matter of great surprise, since it would have shown that man was an +inhabitant of that part of the globe, while the materials composing +the present hills and plains of Campania were still in the progress +of deposition at the bottom of the sea; whereas we know that for +nearly 3000 years, or from the times of the earliest Greek colonists, +no material revolution in the physical geography of that part of +Italy has occurred. + +In Ischia, a small island near Naples, composed in like manner of marine +and volcanic formations, Dr. Philippi collected in the stratified tuff and +clay ninety-two species of shells of existing species. In the centre of +Ischia, the lofty hill called Epomeo, or San Nicola, is composed of +greenish indurated tuff, of a prodigious thickness, interstratified in some +parts with marl, and here and there with great beds of solid lava. Visconti +ascertained by trigonometrical measurement that this mountain was 2605 feet +above the level of the sea. Not far from its summit, at the height of about +2000 feet, as also near Moropano, a village only 100 feet lower, on the +southern declivity of the mountain, I collected, in 1828, many shells of +species now inhabiting the neighbouring gulf. It is clear, therefore, that +the great mass of Epomeo was not only raised to its present height, but was +also _formed_ beneath the waters, within the post-pliocene period. + +It is a fact, however, of no small interest, that the fossil shells from +these modern tuffs of the volcanic region surrounding the Bay of Baiæ, +although none of them extinct, indicate a slight want of correspondence +between the ancient fauna and that now inhabiting the Mediterranean. +Philippi informs us that when he and M. Scacchi had collected ninety-nine +species of them, he found that only one, _Pecten medius_, now living in the +Red Sea, was absent from the Mediterranean. Notwithstanding this, he adds, +"the condition of the sea when the tufaceous beds were deposited must have +been considerably different from its present state; for _Tellina striata_ +was then common, and is now rare; _Lucina spinosa_ was both more abundant +and grew to a larger size; _Lucina fragilis_, now rare, and hardly +measuring 6 lines, then attained the enormous dimensions of 14 lines, and +was extremely abundant; and _Ostrea lamellosa_, Broc., no longer met with +near Naples, existed at that time, and attained a size so large that one +lower valve has been known to measure 5 inches 9 lines in length, 4 inches +in breadth, 1-1/2 inch in thickness, and weighed 26-1/2 ounces."[113-A] + +There are other parts of Europe where no volcanic action manifests itself +at the surface, as at Naples, whether by the eruption of lava or by +earthquakes, and yet where the land and bed of the adjoining sea are +undergoing upheaval. The motion is so gradual as to be insensible to the +inhabitants, being only ascertainable by careful scientific measurements +compared after long intervals. Such an upward movement has been proved to +be in progress in Norway and Sweden throughout an area about 1000 miles N. +and S., and for an unknown distance E. and W., the amount of elevation +always increasing as we proceed towards the North Cape, where it may equal +5 feet in a century. If we could assume that there had been an average rise +of 2-1/2 feet in each hundred years for the last fifty centuries, this +would give an elevation of 125 feet in that period. In other words, it +would follow that the shores, and a considerable area of the former bed of +the Baltic and North Sea, had been uplifted vertically to that amount, and +converted into land in the course of the last 5000 years. Accordingly, we +find near Stockholm, in Sweden, horizontal beds of sand, loam, and marl +containing the same peculiar assemblage of testacea which now live in the +brackish waters of the Baltic. Mingled with these, at different depths, +have been detected various works of art implying a rude state of +civilization, and some vessels built before the introduction of iron, the +whole marine formation having been upraised, so that the upper beds are now +60 feet higher than the surface of the Baltic. In the neighbourhood of +these recent strata, both to the north-west and south of Stockholm, other +deposits similar in mineral composition occur, which ascend to greater +heights, in which precisely the same assemblage of fossil shells is met +with, but without any intermixture of human bones or fabricated articles. + +On the opposite or western coast of Sweden, at Uddevalla, post-pliocene +strata, containing recent shells, not of that brackish water character +peculiar to the Baltic, but such as now live in the northern ocean, ascend +to the height of 200 feet; and beds of clay and sand of the same age attain +elevations of 300 and even 700 feet in Norway, where they have been usually +described as "raised beaches." They are, however, thick deposits of +submarine origin, spreading far and wide, and filling valleys in the +granite and gneiss, just as the tertiary formations, in different parts of +Europe, cover or fill depressions in the older rocks. + +It is worthy of remark, that although the fossil fauna characterizing these +upraised sands and clays consists exclusively of existing northern species +of testacea, yet, according to Lovén (an able living naturalist of Norway), +the species do not constitute such an assemblage as now inhabits +corresponding latitudes in the German Ocean. On the contrary, they +decidedly represent a more arctic fauna.[114-A] In order to find the same +species flourishing in equal abundance, or in many cases to find them at +all, we must go northwards to higher latitudes than Uddevalla in Sweden, or +even nearer the pole than Central Norway. + +Judging by the uniformity of climate now prevailing from century to +century, and the insensible rate of variation in the organic world in our +own times, we may presume that an extremely lengthened period was required +even for so slight a modification of the molluscous fauna, as that of which +the evidence is here brought to light. On the other hand, we have every +reason for inferring on independent grounds (namely, the rate of upheaval +of land in modern times) that the antiquity of the deposits in question +must be very great. For if we assume, as before suggested, that the mean +rate of continuous vertical elevation has amounted to 2-1/2 feet in a +century (and this is probably a high average), it would require 27,500 +years for the sea-coast to attain the height of 700 feet, without making +allowance for any pauses such as are now experienced in a large part of +Norway, or for any oscillations of level. + +In England, buried ships have been found in the ancient and now deserted +channels of the Rother in Sussex, of the Mersey in Kent, and the Thames +near London. Canoes and stone hatchets have been dug up, in almost all +parts of the kingdom, from peat and shell-marl; but there is no evidence, +as in Sweden, Italy, and many other parts of the world, of the bed of the +sea, and the adjoining coast, having been uplifted bodily to considerable +heights within the human period. Recent strata have been traced along the +coasts of Peru and Chili, inclosing shells in abundance, all agreeing +specifically with those now swarming in the Pacific. In one bed of this +kind, in the island of San Lorenzo, near Lima, Mr. Darwin found, at the +altitude of 85 feet above the sea, pieces of cotton-thread, plaited rush, +and the head of a stalk of Indian corn, the whole of which had evidently +been imbedded with the shells. At the same height on the neighbouring +mainland, he found other signs corroborating the opinion that the ancient +bed of the sea had there also been uplifted 85 feet, since the region was +first peopled by the Peruvian race.[115-A] But similar shelly masses are +also met with at much higher elevations, at innumerable points between the +Chilian and Peruvian Andes and the sea-coast, in which no human remains +were ever, or in all probability ever will be, discovered. + +In the West Indies, also, in the island of Guadaloupe, a solid limestone +occurs, at the level of the sea-beach, enveloping human skeletons. The +stone is extremely hard, and chiefly composed of comminuted shell and +coral, with here and there some entire corals and shells, of species now +living in the adjacent ocean. With them are included arrow-heads, fragments +of pottery, and other articles of human workmanship. A limestone with +similar contents has been formed, and is still forming, in St. Domingo. But +there are also more ancient rocks in the West Indian Archipelago, as in +Cuba, near the Havanna, and in other islands, in which are shells identical +with those now living in corresponding latitudes; some well-preserved, +others in the state of casts, all referable to the post-pliocene period. + +I have already described in the seventh chapter, p. 84., what would be +the effects of oscillations and changes of level in any region drained +by a great river and its tributaries, supposing the area to be first +depressed several hundred feet, and then re-elevated. I believe that +such changes in the relative level of land and sea have actually +occurred in the post-pliocene era in the hydrographical basin of the +Mississippi and in that of the Rhine. The accumulation of fluviatile +matter in a delta during a slow subsidence may raise the newly gained +land superficially at the same rate at which its foundations sink, so +that these may go down hundreds or thousands of feet perpendicularly, +and yet the sea bordering the delta may always be excluded, the whole +deposit continuing to be terrestrial or freshwater in character. This +appears to have happened in the deltas both of the Po and Ganges, for +recent artesian borings, penetrating to the depth of 400 feet, have +there shown that fluviatile strata, with shells of recent species, +together with ancient surfaces of land supporting turf and forests, are +depressed hundreds of feet below the sea level.[116-A] Should these +countries be once more slowly upraised, the rivers would carve out +valleys through the horizontal and unconsolidated strata as they rose, +sweeping away the greater portion of them, and leaving mere fragments in +the shape of terraces skirting newly-formed alluvial plains, as +monuments of the former levels at which the rivers ran. Of this nature +are "the bluffs," or river cliffs, now bounding the valley of the +Mississippi throughout a large portion of its course. Thus let _a b_, +fig. 106., represent the alluvial plain of the Mississippi, a plain +which, at the point alluded to, is more than 30 miles broad, and is +truly a prolongation of the modern delta of that river. It is bounded by +bluffs, the upper portions of which consist, both on the east and west +side, of shelly loam, No. 2. rising from 100 to 200 feet above the level +of the plain, and containing land and freshwater shells of the genera +_Helix_, _Pupa_, _Succinea_, and _Lymnea_, of the same species as those +now inhabiting the neighbouring forests and swamps. In the same loam +also, No. 2., are found the bones of the Mastodon, Elephant, Megalonyx, +and other extinct quadrupeds. + +[Illustration: Fig. 106. Valley of the Mississippi. + + 1. Alluvium. + 2. Loess. + 3. _f_. Eocene. + 4. Cretaceous.] + +I have endeavoured to show that the deposits forming the delta and alluvial +plain of the Mississippi consist of sedimentary matter, extending over an +area of 30,000 square miles, and known in some parts to be several hundred +feet deep. Although we cannot estimate correctly how many years it may have +required for the river to bring down from the upper country so large a +quantity of earthy matter--the data for such a computation being as yet +incomplete--we may still approximate to a minimum of the time which such an +operation must have taken, by ascertaining experimentally the annual +discharge of water by the Mississippi, and the mean annual amount of solid +matter contained in its waters. The lowest estimate of the time required +would lead us to assign a high antiquity, amounting to many tens of +thousands of years to the existing delta, the origin of which is +nevertheless an event of yesterday when contrasted with those terraces, +_c_, and _d e_, fig. 106., formed of the loam No. 2. above mentioned. These +materials of the bluffs _a_ and _d_ were produced, the reader will observe, +during the first part of that great oscillation of level which depressed to +a depth of 200 feet a larger area than the modern delta and plain of the +Mississippi, and then restored the region to its former position.[117-A] + +_Loess of the Valley of the Rhine._--A similar succession of geographical +changes, attended by the production of a fluviatile formation, singularly +resembling that which bounds the great plain of the Mississippi, seems to +have occurred in the hydrographical basin of the Rhine, since the time when +that basin had already acquired its present outline of hill and valley. I +allude to the deposit provincially termed _loess_ in part of Germany, or +_lehm_ in Alsace, filled with land and freshwater shells of existing +species. It is a finely comminuted sand or pulverulent loam of a yellowish +grey colour, consisting chiefly of argillaceous matter combined with a +sixth part of carbonate of lime, and a sixth of quartzose and micaceous +sand. It often contains calcareous sandy concretions or nodules, rarely +exceeding the size of a man's head. Its entire thickness amounts, in some +places, to between 200 and 300 feet; yet there are often no signs of +stratification in the mass, except here and there at the bottom, where +there is occasionally a slight intermixture of drifted materials derived +from subjacent rocks. Unsolidified as it is, and of so perishable a nature, +that every streamlet flowing over it cuts out for itself a deep gully, it +usually terminates in a vertical cliff, from the surface of which land +shells are seen here and there to project in relief. In all these features +it presents a precise counterpart to the loess of the Mississippi. It is so +homogeneous as generally to exhibit no signs of stratification, owing, +probably, to its materials having been derived from a common source, and +having been accumulated by a uniform action. Yet it displays in some few +places decided marks of successive deposition, where coarser and finer +materials alternate, especially near the bottom. Calcareous concretions, +also enclosing land-shells, are sometimes arranged in horizontal layers. It +is a remarkable deposit, from its position, wide extent, and thickness, its +homogeneous mineral composition, and freshwater origin. Its distribution +clearly shows that after the great valley of the Rhine, from Schaffhausen +to Bonn, had acquired its present form, having its bottom strewed over with +coarse gravel, a period arrived when it became filled up from side to side +with fine mud, which was also thrown down in the valleys of the principal +tributaries of the Rhine. + +Thus, for example, it may be traced far into Würtemberg, up the valley of +the Neckar, and from Frankfort, up the valley of the Main, to above +Dettelbach. I have also seen it spreading over the country of Mayence, +Eppelsheim, and Worms, on the left bank of the Rhine, and on the opposite +side on the table-land above the Bergstrasse, between Wiesloch and +Bruchsal, where it attains a thickness of 200 feet. Near Strasburg, large +masses of it appear at the foot of the Vosges on the left bank, and at the +base of the mountains of the Black Forest on the right bank. The +Kaiserstuhl, a volcanic mountain which stands in the middle of the plain of +the Rhine near Freiburg, has been covered almost everywhere with this loam, +as have the extinct volcanos between Coblentz and Bonn. Near Andernach, in +the Kirchweg, the loess containing the usual shells alternates with +volcanic matter; and over the whole are strewed layers of pumice, lapilli, +and volcanic sand, from 10 to 15 feet thick, very much resembling the +ejections under which Pompeii lies buried. There is no passage at this +upper junction from the loess into the pumiceous superstratum; and this +last follows the slope of the hill, just as it would have done had it +fallen in showers from the air on a declivity partly formed of loess. + +But, in general, the loess overlies all the volcanic products, even those +between Neuwied and Bonn, which have the most modern aspect; and it has +filled up in part the crater of the Roderberg, an extinct volcano near +Bonn. In 1833 a well was sunk at the bottom of this crater, through 70 feet +of loess, in part of which were the usual calcareous concretions. + +The interstratification above alluded to, of loess with layers of pumice +and volcanic ashes, has led to the opinion that both during and since +its deposition some of the last volcanic eruptions of the Lower Eifel +have taken place. Should such a conclusion be adopted, we should be +called upon to assign a very modern date to these eruptions. This +curious point, therefore, deserves to be reconsidered; since it may +possibly have happened that the waters of the Rhine, swollen by the +melting of snow and ice, and flowing at a great height through a valley +choked up with loess, may have swept away the loose superficial scoriæ +and pumice of the Eifel volcanos, and spread them out occasionally over +the yellow loam. Sometimes, also, the melting of snow on the slope of +small volcanic cones may have given rise to local floods, capable of +sweeping down light pumice into the adjacent low grounds. + +The first idea which has occurred to most geologists, after examining +the loess between Mayence and Basle, is to imagine that a great lake +once extended throughout the valley of the Rhine between those two +places. Such a lake may have sent off large branches up the course of +the Main, Neckar, and other tributary valleys, in all of which large +patches of loess are now seen. The barrier of the lake might be placed +somewhere in the narrow and picturesque gorge of the Rhine between +Bingen and Bonn. But this theory fails altogether to explain the +phenomena; when we discover that that gorge itself has once been filled +with loess, which must have been tranquilly deposited in it, as also in +the lateral valley of the Lahn, communicating with the gorge. The loess +has also overspread the high adjoining platform near the village of +Plaidt above Andernach. Nay, on proceeding farther down to the north, we +discover that the hills which skirt the great valley between Bonn and +Cologne have loess on their flanks, which also covers here and there the +gravel of the plain as far as Cologne, and the nearest rising grounds. + +Besides these objections to the lake theory, the loess is met with near +Basle, capping hills more than 1200 feet above the sea; so that a barrier +of land capable of separating the supposed lake from the ocean would +require to be, at least, as high as the mountains called the Siebengebirge, +near Bonn, the loftiest summit of which, the Oehlberg, is 1209 feet above +the Rhine and 1369 feet above the sea. It would be necessary, moreover, to +place this lofty barrier somewhere below Cologne, or precisely where the +level of the land is now lowest. + +Instead, therefore, of supposing one continuous lake of sufficient extent +and depth to allow of the simultaneous accumulation of the loess, at +various heights, throughout the whole area where it now occurs, I formerly +suggested that, subsequently to the period when the countries now drained +by the Rhine and its tributaries had nearly acquired their actual form and +geographical features, they were again depressed gradually by a movement +like that now in progress on the west coast of Greenland.[119-A] In +proportion as the whole district was lowered, the general fall of the +waters between the Alps and the ocean was lessened; and both the main and +lateral valleys, becoming more subject to river inundations, were partially +filled up with fluviatile silt, containing land and freshwater shells. When +a thickness of many hundred feet of loess had been thrown down slowly by +this operation, the whole region was once more upheaved gradually. During +this upward movement most of the fine loam would be carried off by the +denuding power of rains and rivers; and thus the original valleys might +have been re-excavated, and the country almost restored to its pristine +state, with the exception of some masses and patches of loess such as still +remain, and which, by their frequency and remarkable homogeneousness of +composition and fossils, attest the ancient continuity and common origin of +the whole. By imagining these oscillations of level, we dispense with the +necessity of erecting and afterwards removing a mountain barrier +sufficiently high to exclude the ocean from the valley of the Rhine during +the period of the accumulation of the loess. + +The proportion of land shells of the genera _Helix_, _Pupa_, and _Bulimus_, +is very large in the loess; but in many places aquatic species of the +genera _Lymnea_, _Paludina_, and _Planorbis_ are also found. These may have +been carried away during floods from shallow pools and marshes bordering +the river; and the great extent of marshy ground caused by the wide +overflowings of rivers above supposed would favour the multiplication of +amphibious mollusks, such as the _Succinea_ (fig. 107.), which is almost +everywhere characteristic of this formation, and is sometimes accompanied, +as near Bonn, by another species, _S. amphibia_ (fig. 34. p. 29.). Among +other abundant fossils are _Helix plebeium_ and _Pupa muscorum_. (See +Figures.) Both the terrestrial and aquatic shells preserved in the loess +are of most fragile and delicate structure, and yet they are almost +invariably perfect and uninjured. They must have been broken to pieces had +they been swept along by a violent inundation. Even the colour of some of +the land shells, as that of _Helix nemoralis_, is occasionally preserved. + +[Illustration: Fig. 107. _Succinea elongata._] + +[Illustration: Fig. 108. _Pupa muscorum._] + +[Illustration: Fig. 109. _Helix plebeium._] + +Bones of vertebrated animals are rare in the loess, but those of the +mammoth, horse, and some other quadrupeds have been met with. At the +village of Binningen, and the hills called Bruderholz, near Basle, I found +the vertebræ of fish, together with the usual shells. These vertebræ, +according to M. Agassiz, belong decidedly to the Shark family, perhaps to +the genus _Lamna_. In explanation of their occurrence among land and +freshwater shells, it may be stated that certain fish of this family ascend +the Senegal, Amazon, and other great rivers, to the distance of several +hundred miles from the ocean.[120-A] + +At Cannstadt, near Stuttgart, in a valley also belonging to the +hydrographical basin of the Rhine, I have seen the loess pass downwards +into beds of calcareous tuff and travertin. Several valleys in northern +Germany, as that of the Ilm at Weimar, and that of the Tonna, north of +Gotha, exhibit similar masses of modern limestone filled with recent shells +of the genera _Planorbis_, _Lymnea_, _Paludina_, &c., from 50 to 80 feet +thick, with a bed of loess much resembling that of the Rhine, occasionally +incumbent on them. In these modern limestones used for building, the bones +of _Elephas primigenius_, _Rhinoceros tichorinus_, _Ursus spelæus_, _Hyæna +spelæa_, with the horse, ox, deer, and other quadrupeds, occur; and in 1850 +Mr. H. Credner and I obtained in a quarry at Tonna, at the depth of 15 +feet, inclosed in the calcareous rock and surrounded with dicotyledonous +leaves and petrified leaves, four eggs of a snake of the size of the +largest European Coluber, which, with three others, had been found lying +in a series, or string. + +They are, I believe, the first reptilian remains which have been met with +in strata of this age. + +The agreement of the shells in these cases with recent European species +enables us to refer to a very modern period the filling up and +re-excavation of the valleys; an operation which doubtless consumed +a long period of time, since which the mammiferous fauna has undergone +a considerable change. + + +FOOTNOTES: + +[110-A] See Princ. of Geol. vol. iii. 1st ed. + +[112-A] See Principles, Index, "Serapis." + +[113-A] Geol. Quart. Journ. vol. ii. Memoirs, p. 15. + +[114-A] Quart. Geol. Journ. 4 Mems. p. 48. + +[115-A] Journal, p. 451. + +[116-A] See Principles, 8th ed. pp. 260-268. + +[117-A] Lyell's Second Visit to the United States, vol. ii. chap. xxxiv. + +[119-A] Princ. of Geol. 3d edition, 1834, vol. iii. p. 414. + +[120-A] Proceedings Geol. Soc. No. 43. p. 222. + + + + +CHAPTER 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. + + +Among the different kinds of alluvium described in the seventh chapter, +mention was made of the boulder formation in the north of Europe, the +peculiar characters of which may now be considered, as it belongs in +part to the post-pliocene, and partly to the newer pliocene, period. I +shall first allude briefly to that portion of it which extends from +Finland and the Scandinavian mountains to the north of Russia, and the +low countries bordering the Baltic, and which has been traced southwards +as far as the eastern coast of England. This formation consists of mud, +sand, and clay, sometimes stratified, but often wholly devoid of +stratification, for a depth of more than a hundred feet. To this +unstratified form of the deposit, the name of _till_ has been applied in +Scotland. It generally contains numerous fragments of rocks, some +angular and others rounded, which have been derived from formations of +all ages, both fossiliferous, volcanic, and hypogene, and which have +often been brought from great distances. Some of the travelled blocks +are of enormous size, several feet or yards in diameter; their average +dimensions increasing as we advance northwards. The till is almost +everywhere devoid of organic remains, unless where these have been +washed into it from older formations; so that it is chiefly from +relative position that we must hope to derive a knowledge of its age. + +Although a large proportion of the boulder deposit, or "northern drift," as +it has sometimes been called, is made up of fragments brought from a +distance, and which have sometimes travelled many hundred miles, the bulk +of the mass in each locality consists of the ruins of subjacent or +neighbouring rocks; so that it is red in a region of red sandstone, white +in a chalk country, and grey or black in a district of coal and coal-shale. + +The fundamental rock on which the boulder formation reposes, if it consist +of granite, gneiss, marble, or other hard stone capable of permanently +retaining any superficial markings which may have been imprinted upon it, +is smoothed or polished, and usually exhibits parallel striæ and furrows +having a determinate direction. This direction, both in Europe and North +America, is evidently connected with the course taken by the erratic blocks +in the same district being north or south, or 20 or 30 degrees to the east +or west of north, according as the large angular and rounded stones have +travelled. These stones themselves also are often furrowed and scratched +on more than one side. + +[Illustration: Fig. 110. Limestone polished, furrowed, and scratched by the +glacier of Rosenlaui, in Switzerland. (Agassiz.) + + _a a._ White streaks or scratches, caused by small grains of flint frozen + into the ice. + _b b._ Furrows.] + +In explanation of such phenomena I may refer the student to what was said +of the action of glaciers and icebergs in the Principles of Geology.[122-A] +It is ascertained that hard stones, frozen into a moving mass of ice, and +pushed along under the pressure of that mass, scoop out long rectilinear +furrows or grooves parallel to each other on the subjacent solid rock. (See +fig. 110.) Smaller scratches and striæ are made on the polished surface by +crystals or projecting edges of the hardest minerals, just as a diamond +cuts glass. The recent polishing and striation of limestone by coast-ice +carrying boulders even as far south as the coast of Denmark, has been +observed by Dr. Forchhammer, and helps us to conceive how large icebergs, +running aground on the bed of the sea, may produce similar furrows on a +grander scale. An account was given so long ago as the year 1822, by +Scoresby, of icebergs seen by him drifting along in latitudes 69° and 70° +N., which rose above the surface from 100 to 200 feet, and measured from a +few yards to a mile in circumference. Many of them were loaded with beds of +earth and rock, of such thickness that the weight was conjectured to be +from 50,000 to 100,000 tons.[122-B] A similar transportation of rocks is +known to be in progress in the southern hemisphere, where boulders included +in ice are far more frequent than in the north. One of these icebergs was +encountered in 1839, in mid-ocean, in the antarctic regions, many hundred +miles from any known land, sailing northwards, with a large erratic block +firmly frozen into it. In order to understand in what manner long and +straight grooves may be cut by such agency, we must remember that these +floating islands of ice have a singular steadiness of motion, in +consequence of the larger portion of their bulk being sunk deep under +water, so that they are not perceptibly moved by the winds and waves even +in the strongest gales. Many had supposed that the magnitude commonly +attributed to icebergs by unscientific navigators was exaggerated, but now +it appears that the popular estimate of their dimensions has rather fallen +within than beyond the truth. Many of them, carefully measured by the +officers of the French exploring expedition of the Astrolabe, were between +100 and 225 feet high above water, and from 2 to 5 miles in length. Captain +d'Urville ascertained one of them which he saw floating in the Southern +Ocean to be 13 miles long and 100 feet high, with walls perfectly vertical. +The submerged portions of such islands must, according to the weight of ice +relatively to sea-water, be from six to eight times more considerable than +the part which is visible, so that the mechanical power they might exert +when fairly set in motion must be prodigious.[123-A] + +Glaciers formed in mountainous regions become laden with mud and stones, +and if they melt away at their lower extremity before they reach the sea, +they leave wherever they terminate a confused heap of unstratified rubbish, +called "a moraine," composed of mud and pieces of all the rocks with which +they were loaded. We may expect, therefore, to find a formation of the same +kind, resulting from the liquefaction of icebergs, in tranquil water. But, +should the action of a current intervene at certain points or at certain +seasons, then the materials will be sorted as they fall, and arranged in +layers according to their relative weight and size. Hence there will be +passages from _till_, as it is called in Scotland, to stratified clay, +gravel, and sand, and intercalations of one in the other. + +I have yet to mention another appearance connected with the boulder +formation, which has justly attracted much attention in Norway and other +parts of Europe. Abrupt pinnacles and outstanding ridges of rock are often +observed to be polished and furrowed on the north, or "strike" side as it +is called, or on the side facing the region from which the erratics have +come; while, on the other side, which is usually steeper and often +perpendicular, called the "lee-side," such superficial markings are +wanting. There is usually a collection on this lee-side of boulders and +gravel, or of large angular fragments. In explanation we may suppose that +the north side was exposed, when still submerged, to the action of +icebergs, and afterwards, when the land was upheaved, of coast-ice, which +ran aground upon shoals, or was _packed_ on the beach; so that there would +be great wear and tear on the seaward slope, while, on the other, gravel +and boulders might be heaped up in a sheltered position. + +_Northern origin of erratics._--That the erratics of northern Europe have +been carried southward cannot be doubted; those of granite, for example, +scattered over large districts of Russia and Poland, agree precisely in +character with rocks of the mountains of Lapland and Finland; while the +masses of gneiss, syenite, porphyry, and trap, strewed over the low sandy +countries of Pomerania, Holstein, and Denmark, are identical in mineral +characters with the mountains of Norway and Sweden. + +It is found to be a general rule in Russia, that the smaller blocks are +carried to greater distances from their point of departure than the larger; +the distance being sometimes 800 and even 1000 miles from the nearest rocks +from which they were broken off; the direction having been from N.W. to +S.E., or from the Scandinavian mountains over the seas and low lands to the +south-east. That its accumulation throughout this area took place in part +during the post-pliocene period is proved by its superposition at several +points to strata containing recent shells. Thus, for example, in European +Russia, MM. Murchison and De Verneuil found in 1840, that the flat country +between St. Petersburg and Archangel, for a distance of 600 miles, +consisted of horizontal strata, full of shells similar to those now +inhabiting the arctic sea, on which rested the boulder formation, +containing large erratics. + +In Sweden, in the immediate neighbourhood of Upsala, I observed, in 1834, a +ridge of stratified sand and gravel, in the midst of which is a layer of +marl, evidently formed originally at the bottom of the Baltic, by the slow +growth of the mussel, cockle, and other marine shells, intermixed with some +of freshwater species. The marine shells are all of dwarfish size, like +those now inhabiting the brackish waters of the Baltic; and the marl, in +which myriads of them are imbedded, is now raised more than 100 feet above +the level of the Gulf of Bothnia. Upon the top of this ridge repose several +huge erratics, consisting of gneiss for the most part unrounded, from 9 to +16 feet in diameter, and which must have been brought into their present +position since the time when the neighbouring gulf was already +characterized by its peculiar fauna.[124-A] Here, therefore, we have proof +that the transport of erratics continued to take place, not merely when the +sea was inhabited by the existing testacea, but when the north of Europe +had already assumed that remarkable feature of its physical geography, +which separates the Baltic from the North Sea, and causes the Gulf of +Bothnia to have only one fourth of the saltness belonging to the ocean. In +Denmark, also, recent shells have been found in stratified beds, closely +associated with the boulder clay. + +It was stated that in Russia the erratics diminished generally in size in +proportion as they are traced farther from their source. The same +observation holds true in regard to the average bulk of the Scandinavian +boulders, when we pursue them southwards, from the south of Norway and +Sweden through Denmark and Westphalia. This phenomenon is in perfect +harmony with the theory of ice-islands floating in a sea of variable depth; +for the heavier erratics require icebergs of a larger size to buoy them up; +and, even when there are no stones frozen in, more than seven eighths, and +often nine tenths, of a mass of drift ice is under water. The greater, +therefore, the volume of the iceberg, the sooner would it impinge on some +shallower part of the sea; while the smaller and lighter floes, laden with +finer mud and gravel, may pass freely over the same banks, and be carried +to much greater distances. In those places, also, where in the course of +centuries blocks have been carried southwards by coast-ice, having been +often stranded and again set afloat in the direction of a prevailing +current, the blocks will be worn and diminish in size the farther they +travel from their point of departure. + +The "northern drift" of the most southern latitudes is usually of the +highest antiquity. In Scotland it rests immediately on the older rocks, +and is covered by stratified sand and clay, usually devoid of fossils, +but in which, at certain points near the east and west coast, as, for +example, in the estuaries of the Tay and Clyde, marine shells have been +discovered. The same shells have also been met with in the north, at +Wick in Caithness, and on the shores of the Moray Frith. The principal +deposit on the Clyde occurs at the height of about 70 feet, but a few +shells have been traced in it as high as 554 feet above the sea. +Although a proportion of between 85 or 90 in 100 of the imbedded shells +are of recent species, the remainder are unknown; and even many which +are recent now inhabit more northern seas, where we may, perhaps, +hereafter find living representatives of some of the unknown fossils. +The distance to which erratic blocks have been carried southwards in +Scotland, and the course they have taken, which is often wholly +independent of the present position of hill and valley, favours the idea +that ice-rafts rather than glaciers were in general the transporting +agents. The Grampians in Forfarshire and in Perthshire are from 3000 to +4000 feet high. To the southward lies the broad and deep valley of +Strathmore, and to the south of this again rise the Sidlaw Hills[125-A] +to the height of 1500 feet and upwards. On the highest summits of this +chain, formed of sandstone and shale, and at various elevations, are +found huge angular fragments of mica schist, some 3 and others 15 feet +in diameter, which have been conveyed for a distance of at least 15 +miles from the nearest Grampian rocks from which they could have been +detached. Others have been left strewed over the bottom of the large +intervening vale of Strathmore. + +Still farther south on the Pentland Hills, at the height of 1100 feet +above the sea, Mr. Maclaren has observed a fragment of mica-schist +weighing from 8 to 10 tons, the nearest mountain composed of this +formation being 50 miles distant.[125-B] + +The testaceous fauna of the boulder period, in Scotland, England, and +Ireland, has been shown by Prof. E. Forbes to contain a much smaller +number of species than that now belonging to the British seas, and to have +been also much less rich in species than the Older Pliocene fauna of the +crag which preceded it. Yet the species are nearly all of them now living +either in the British or more northern seas, the shells of more arctic +latitudes being the most abundant and the most wide spread throughout the +entire area of the drift from north to south. + +This extensive range of the fossils can by no means be explained by +imagining the mollusca of the drift to have been inhabitants of a deep +sea, where a more uniform temperature prevailed. On the contrary, many +species were littoral, and others belonged to a shallow sea, not above +100 feet deep, and very few of them lived, according to Prof. E. Forbes, +at greater depths than 300 feet. + +From what was before stated it will appear that the boulder formation +displays almost everywhere, in its mineral ingredients, a strange +heterogeneous mixture of the ruins of adjacent lands, with stones both +angular and rounded, which have come from points often very remote. Thus we +find it in our eastern counties, as in Norfolk, Suffolk, Cambridge, +Huntingdon, Bedford, Hertford, Essex, and Middlesex, containing stones from +the Silurian and Carboniferous strata, and from the lias, oolite, and +chalk, all with their peculiar fossils, together with trap, syenite, +mica-schist, granite, and other crystalline rocks. A fine example of this +singular mixture extends to the very suburbs of London, being seen on the +summit of Muswell Hill, Highgate. But south of London the northern drift is +wanting, as, for example, in the Wealds of Surrey, Kent, and Sussex. + +_Norfolk drift._--The drift can nowhere be studied more advantageously in +England than in the cliffs of the Norfolk coast between Happisburgh and +Cromer. Vertical sections, having an ordinary height of from 50 to 70 feet, +are there exposed to view for a distance of about 20 miles. The name of +diluvium was formerly given to it by those who supposed it to have been +produced by the violent action of a sudden and transient deluge, but the +term drift has been substituted by those who reject this hypothesis. Here, +as elsewhere, it consists for the most part of clay, loam, and sand, in +part stratified, in part devoid of stratification. Pebbles, together with +some large boulders of granite, porphyry, greenstone, lias, chalk, and +other transported rocks, are interspersed, especially through the till. +That some of the granitic and other fragments came from Scandinavia I have +no doubt, after having myself traced the course of the continuous stream of +blocks from Norway and Sweden to Denmark, and across the Elbe, through +Westphalia, to the borders of Holland. We need not be surprised to find +them reappear on our eastern coast, between the Tweed and the Thames, +regions not half so remote from parts of Norway as are many Russian +erratics from the sources whence they came. + +White chalk rubble, unmixed with foreign matter, and even huge fragments +of solid chalk, also occur in many localities in these Norfolk cliffs. +No fossils have been detected in this drift, which can positively be +referred to the era of its accumulation; but at some points it overlies +a freshwater formation containing recent shells, and at others it is +blended with the same in such a manner as to force us to conclude that +both were contemporaneously deposited. + +[Illustration: Fig. 111. The shaded portion consists of Freshwater +beds. Intercalation of freshwater beds and of boulder clay and +sand at Mundesley.] + +This interstratification is expressed in the annexed figure, the dark mass +indicating the position of the freshwater beds, which contain much +vegetable matter, and are divided into thin layers. The imbedded shells +belong to the genera _Planorbis_, _Lymnea_, _Paludina_, _Unio_, _Cyclas_, +and others, all of British species, except a minute _Paludina_ now +inhabiting France. (See fig. 112.) + +[Illustration: Fig. 112. _Paludina marginata_, Michaud. (_P. minuta_, +Strickland.) The middle figure is of the natural size.] + +The _Cyclas_ (fig. 113.) is merely a remarkable variety of the common +English species. The scales and teeth of fish of the genera Pike, +Perch, Roach, and others, accompany these shells; but the species +are not considered by M. Agassiz to be identical with known British +or European kinds. + +[Illustration: Fig. 113. _Cyclas_ (_Pisidium_) _amnica_, var.? The two +middle figures are of the natural size.] + +The series of formations in the cliffs of eastern Norfolk, now under +consideration, beginning with the lowest, is as follows:--First, chalk; +secondly, patches of a marine tertiary formation, called the Norwich +Crag, hereafter to be described; thirdly, the freshwater beds already +mentioned; and lastly, the drift. Immediately above the chalk, or crag, +when that is present, is found here and there a buried forest, or a +stratum in which the stools and roots of trees stand in their natural +position, the trunks having been broken short off and imbedded with +their branches and leaves. It is very remarkable that the strata of the +overlying boulder formation have often undergone great derangement at +points where the subjacent forest bed and chalk remain undisturbed. +There are also cases where the upper portion of the boulder deposit has +been greatly deranged, while the lower beds of the same have continued +horizontal. Thus the annexed section (fig. 114.) represents a cliff +about 50 feet high, at the bottom of which is _till_, or unstratified +clay, containing boulders, having an even horizontal surface, on which +repose conformably beds of laminated clay and sand about 5 feet thick, +which, in their turn, are succeeded by vertical, bent, and contorted +layers of sand and loam 20 feet thick, the whole being covered by flint +gravel. Now the curves of the variously coloured beds of loose sand, +loam, and pebbles are so complicated that not only may we sometimes +find portions of them which maintain their verticality to a height +of 10 or 15 feet, but they have also been folded upon themselves in +such a manner that continuous layers might be thrice pierced in +one perpendicular boring. + +[Illustration: Fig. 114. Cliff 50 feet high between Bacton +Gap and Mundesley.] + +[Illustration: Fig. 115. Folding of the strata between East +and West Runton.] + +[Illustration: Fig. 116. Section of concentric beds west of Cromer. + + 1. Blue clay. + 2. White sand. + 3. Yellow Sand. + 4. Striped loam and clay. + 5. Laminated blue clay.] + +At some points there is an apparent folding of the beds round a central +nucleus, as at _a_, fig. 115., where the strata seem bent round a small +mass of chalk; or, as in fig. 116., where the blue clay, No. 1., is in the +centre; and where the other strata, 2, 3, 4, 5, are coiled round it; the +entire mass being 20 feet in perpendicular height. This appearance of +concentric arrangement around a nucleus is, nevertheless, delusive, being +produced by the intersection of beds bent into a convex shape; and that +which seems the nucleus being, in fact, the innermost bed of the series, +which has become partially visible by the removal of the protuberant +portions of the outer layers. + +To the north of Cromer are other fine illustrations of contorted drift +reposing on a floor of chalk horizontally stratified and having a level +surface. These phenomena, in themselves sufficiently difficult of +explanation, are rendered still more anomalous by the occasional inclosure +in the drift of huge fragments of chalk many yards in diameter. One +striking instance occurs west of Sherringham, where an enormous pinnacle of +chalk, between 70 and 80 feet in height, is flanked on both sides by +vertical layers of loam, clay, and gravel. (Fig. 117.) + +[Illustration: Fig. 117. Included pinnacle of chalk at Old Hythe point, +west of Sherringham. + + _d._ Chalk with regular layers of chalk flints. + _c._ Layer called "the pan," of loose chalk, flints, and marine shells + of recent species, cemented by oxide of iron.] + +This chalky fragment is only one of many detached masses which have been +included in the drift, and forced along with it into their present +position. The level surface of the chalk _in situ_ (_d_) may be traced for +miles along the coast, where it has escaped the violent movements to which +the incumbent drift has been exposed.[129-A] + +We are called upon, then, to explain how any force can have been exerted +against the upper masses, so as to produce movements in which the +subjacent strata have not participated. It may be answered that, if we +conceive the _till_ and its boulders to have been drifted to their +present place by ice, the lateral pressure may have been supplied by the +stranding of ice-islands. We learn, from the observations of Messrs. +Dease and Simpson in the polar regions, that such islands, when they run +aground, push before them large mounds of shingle and sand. It is +therefore probable that they often cause great alterations in the +arrangement of pliant and incoherent strata forming the upper part of +shoals or submerged banks, the inferior portions of the same remaining +unmoved. Or many of the complicated curvatures of these layers of loose +sand and gravel may have been due to another cause, the melting on the +spot of icebergs and coast ice in which successive deposits of pebbles, +sand, ice, snow, and mud, together with huge masses of rock fallen from +cliffs, may have become interstratified. Ice-islands so constituted +often capsize when afloat, and gravel once horizontal may have assumed, +before the associated ice was melted, an inclined or vertical position. +The packing of ice forced up on a coast may lead to similar derangement +in a frozen conglomerate of sand or shingle, and, as Mr. Trimmer has +suggested[130-A], alternate layers of earthy matter may have sunk +down slowly during the liquefaction of the intercalated ice, so as +to assume the most fantastic and anomalous positions, while the +aqueous strata below, and those afterwards thrown down above, may +be perfectly horizontal. + +A buried forest has been adverted to as underlying the drift on the coast +of Norfolk. At the time when the trees grew there must have been dry land +over a large area, which was afterwards submerged, so as to allow a mass of +stratified and unstratified drift, 200 feet and more in thickness, to be +superimposed. The undermining of the cliffs by the sea in modern times has +enabled us to demonstrate, beyond all doubt, the fact of this +superposition, and that the forest was not formed along the present +coast-line. Its situation implies a subsidence of several hundred feet +since the commencement of the drift period, after which there must have +been an upheaval of the same ground; for the forest bed of Norfolk is now +again so high as to be exposed to view at many points at low water; and +this same upward movement may explain why the _till_, which is conceived +to have been of submarine origin, is now met with far inland, and on +the summit of hills. + +The boulder formation of the west of England, observed in Lancashire, +Cheshire, Shropshire, Staffordshire, and Worcestershire, contains in some +places marine shells of recent species, rising to various heights, from 100 +to 350 feet above the sea. The erratics have come partly from the mountains +of Cumberland, and partly from those of Scotland. + +But it is on the mountains of North Wales that the "Northern drift," +with its characteristic marine fossils, reaches its greatest altitude. +On Moel Tryfane, near the Menai Straits, Mr. Trimmer met with shells of +the species commonly found in the drift at the height of 1392 feet above +the level of the sea. + +It is remarkable that in the same neighbourhood where there is evidence of +so great a submergence of the land during part of the glacial period, we +have also the most decisive proofs yet discovered in the British Isles of +subaerial glaciers. Dr. Buckland published in 1842 his reasons for +believing that the Snowdonian mountains in Caernarvonshire were formerly +covered with glaciers, which radiated from the central heights through the +seven principal valleys of that chain, where striæ and flutings are seen +on the polished rocks directed towards as many different points of the +compass. He also described the "moraines" of the ancient glaciers, and the +rounded "bosses" or small flattened domes of polished rock, such as the +action of moving glaciers is known to produce in Switzerland, when gravel, +sand, and boulders, underlying the ice, are forced along over a foundation +of hard stone. Mr. Darwin, and subsequently Prof. Ramsay, have confirmed +Dr. Buckland's views in regard to these Welsh glaciers. Nor indeed was it +to be expected that geologists should discover proofs of icebergs having +abounded in the area now occupied by the British Isles in the Pleistocene +period without sometimes meeting with the signs of contemporaneous glaciers +which covered hills even of moderate elevation between the 50th and 60th +degrees of latitude. + +In Ireland the "drift" exhibits the same general characters and fossil +remains as in Scotland and England; but in the southern part of that +island, Prof. E. Forbes and Capt. James found in it some shells which show +that the glacial sea communicated with one inhabited by a more southern +fauna. Among other species in the south, they mention at Wexford and +elsewhere the occurrence of _Nucula Cobboldiæ_ (see fig. 120. p. 149.) and +_Turritella incrassata_ (a crag fossil); also a southern form of _Fusus_, +and a _Mitra_ allied to a Spanish species.[131-A] + + +FOOTNOTES: + +[122-A] Chap. xvi. and the references there given. + +[122-B] Voyage in 1822, p. 233. + +[123-A] T. L. Hayes, Boston Journ. Nat. Hist. 1844. + +[124-A] See paper by the author, Phil. Trans. 1835, p. 15. + +[125-A] See above, section, p. 48. + +[125-B] Geol. of Fife, &c. p. 220. + +[129-A] For a full account of the drift of East Norfolk, see a paper by the +author, Phil. Mag. No. 104. May, 1840. + +[130-A] Quart. Journ. Geol. Soc. vol. vii. p. 22. + +[131-A] Forbes, Memoirs of Geol. Survey of Great Britain, vol. i. p. 377. + + + + +CHAPTER 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--Whether transported + by glaciers or floating ice--Recent transportation of erratics from + the Andes to Chiloe--Meteorite in Asiatic drift. + + +It will appear from what was said in the last chapter of the marine shells +characterizing the boulder formation, that nine-tenths or more of them +belong to species still living. The superficial position of "the drift" is +in perfect accordance with its imbedded organic remains, leading us to +refer its origin to a modern period. If, then, we encounter so much +difficulty in the interpretation of monuments relating to times so near our +own--if in spite of their recent date they are involved in so much +obscurity--the student may ask, not without reasonable alarm, how we can +hope to decipher the records of remote ages. + +To remove from the mind as far as possible this natural feeling of +discouragement, I shall endeavour in this chapter to prove that what seems +most strikingly anomalous, in the "erratic formation," as some call it, is +really the result of that glacial action which has already been alluded to. +If so, it was to be expected that so long as the true origin of so singular +a deposit remained undiscovered, erroneous theories and terms would be +invented in the effort to solve the problem. These inventions would +inevitably retard the reception of more correct views which a wider field +of observation might afterwards suggest. + +The term "diluvium" was for a time the popular name of the boulder +formation, because it was referred by some geologists to the deluge. +Others retained the name as expressive of their opinion that a series of +diluvial waves raised by hurricanes and storms, or by earthquakes, or by +the sudden upheaval of land from the bed of the sea, had swept over the +continents, carrying with them vast masses of mud and heavy stones, and +forcing these stones over rocky surfaces so as to polish and imprint +upon them long furrows and striæ. + +But no explanation was offered why such agency should have been +developed more energetically in modern times than at former periods of +the earth's history, or why it should be displayed in its fullest +intensity in northern latitudes; for it is important to insist on the +fact, that the boulder formation is a _northern_ phenomenon. Even the +southern extension of the drift, or the large erratics found in the Alps +and the surrounding lands, especially their occurrence round the highest +parts of the chain, offers such an exception to the general rule as +confirms the glacial hypothesis; for it shows that the transportation of +stony fragments to great distances, and the striation, polishing, and +grooving of solid floors of rock, are here again intimately connected +with accumulations of perennial snow and ice. + +That there is some intimate connection between a cold or northern climate +and the various geological appearances now commonly called glacial, cannot +be doubted by any one who has compared the countries bordering the Baltic +with those surrounding the Mediterranean. The smoothing and striation of +rocks, and the erratics, are traced from the sea-shore to the height of +3000 feet above the level of the Baltic, whereas such phenomena are wholly +wanting in countries bordering the Mediterranean; and their absence is +still more marked in the equatorial parts of Asia, Africa, and America; but +when we cross the southern tropic, and reach Chili and Patagonia, we again +encounter the boulder formation, between the latitude 41° S. and Cape Horn, +with precisely the same characters which it assumes in Europe. The evidence +as to climate derived from the organic remains of the drift is, as we have +seen, in perfect harmony with the conclusions above alluded to, the former +habits of the species of mollusca being accurately ascertainable, inasmuch +as they belong to species still living, and known to have at present a wide +range in northern seas. + +But if we are correct in assuming that the northern hemisphere was +considerably colder than now during the period under consideration, owing +probably to the greater area and height of arctic lands, and to the +quantity of icebergs which such a geographical state of things would +generate, it may be well to reflect before we proceed farther on the entire +modification which extreme cold would produce in the operation of those +causes spoken of in the sixth chapter as most active in the formation of +alluvium. A large part of the materials derived from the detritus of rocks, +which in warm climates would go to form deltas, or would be regularly +stratified by marine currents, would, under arctic influences, assume a +superficial and alluvial character. Instead of mud being carried farther +from a coast than sand, and sand farther out than pebbles,--instead of +dense stratified masses being heaped up in limited areas,--nearly the whole +materials, whether coarse or fine, would be conveyed by ice to equal +distances, and huge fragments, which water alone could never move, would be +borne for hundreds of miles without having their edges worn or fractured; +and the earthy and stony masses, when melted out of the frozen rafts, would +be scattered at random over the submarine bottom, whether on mountain tops +or in low plains, with scarcely any relation to the inequalities of the +ground, settling on the crests or ridges of hills in tranquil water as +readily as in valleys and ravines. Occasionally, in those deep and +uninhabited parts of the ocean, never reached by any but the finest +sediment in a normal state of things, the bottom would become densely +overspread by gravel, mud, and boulders. + +In the Western Hemisphere, both in Canada and as far south as the 40th and +even 38th parallel of latitude in the United States, we meet with a +repetition of all the peculiarities which distinguish the European boulder +formation. Fragments of rock have travelled for great distances from north +to south; the surface of the subjacent rock is smoothed, striated, and +fluted; unstratified mud or _till_ containing boulders is associated with +strata of loam, sand, and clay, usually devoid of fossils. Where shells are +present, they are of species still living in northern seas, and half of +them identical with those already enumerated as belonging to European drift +10 degrees of latitude farther north. The fauna also of the glacial epoch +in North America is less rich in species than that now inhabiting the +adjacent sea, whether in the Gulf of St. Lawrence, or off the shores of +Maine, or in the Bay of Massachusetts. At the southern extremity of its +course, moreover, it presents an analogy with the drift of the south of +Ireland, by blending with a more southern fauna, as for example at Brooklyn +near New York, in lat. 41° N., where, according to MM. Redfield and Desor, +_Venus mercenaria_ and other southern species of shells begin to occur as +fossils in the drift. + +The extension on the American continent of the range of erratics during the +Pleistocene period to lower latitudes than they reached in Europe, agrees +well with the present southward deflection of the isothermal lines, or +rather the lines of equal winter temperature. Formerly, as now, a more +extreme climate and a more abundant supply of floating ice prevailed on the +western side of the Atlantic. + +Another resemblance between the distribution of the drift fossils in Europe +and North America has yet to be pointed out. In Norway, Sweden, and +Scotland, as in Canada and the United States, the marine shells are +confined to very moderate elevations above the sea (between 100 and 700 +feet), while the erratic blocks and the grooved and polished surfaces of +rock extend to elevations of several thousand feet. + +[Illustration: Fig. 118. Cross section. + + K. Mr. Ryland's house. + _h_. Clay and sand of higher grounds, with _Saxicava_, &c. + _g_. Gravel with boulders. + _f_. Mass of _Saxicava rugosa_, 12 feet thick. + _e_. Sand and loam with _Mya truncata_, _Scalaria Groenlandica_, &c. + _d_. Drift, with boulders of syenite, &c. + _c_. Yellow sand. + _b_. Laminated clay, 25 feet thick. + A. Horizontal lower Silurian strata. + B. Valley re-excavated.] + +I described in 1839 the fossil shells collected by Captain Bayfield from +strata of drift at Beauport near Quebec, in lat. 47°, and drew from them +the inference that they indicated a more northern climate, the shells +agreeing in great part with those of Uddevalla in Sweden.[134-A] The shelly +beds attain at Beauport and the neighbourhood a height of 200, 300, and +sometimes 400 feet above the sea, and dispersed through some of them are +large boulders of granite, which could not have been propelled by a violent +current, because the accompanying fragile shells are almost all entire. +They seem, therefore, said Captain Bayfield, writing in 1838, to have been +dropped down from melting ice, like similar stones which are now annually +deposited in the St. Lawrence.[134-B] I visited this locality in 1842, and +made the annexed section, fig. 118., which will give an idea of the general +position of the drift in Canada and the United States. I imagine that the +whole of the valley B was once filled up with the beds _b_, _c_, _d_, _e_, +_f_, which were deposited during a period of subsidence, and that +subsequently the higher country (_h_) was submerged and overspread with +drift. The partial re-excavation of B took place when this region was again +uplifted above the sea to its present height. Among the twenty-three +species of fossil shells collected by me from these beds at Beauport, all +were of recent northern species, except one, which is unknown as living, +and may be extinct (see fig. 119.). I also examined the same formation +farther up the valley of the St. Lawrence, in the suburbs of Montreal, +where some of the beds of loam are filled with great numbers of the +_Mytilus edulis_, or our common European mussel, retaining both its valves +and purple colour. This shelly deposit, containing _Saxicava rugosa_ and +other characteristic marine shells, also occurs at an elevated point on +the mountain of Montreal, 450 feet above the level of the sea.[135-A] + +[Illustration: Fig. 119. _Astarte Laurentiana._ + + _a._ Outside. + _b._ Inside of right valve. + _c._ Inside of left valve.] + +In my account of Canada and the United States, published in 1845, I +announced the conclusion to which I had then arrived, that to explain +the position of the erratics and the polished surfaces of rocks, and +their striæ and flutings, we must assume first a gradual submergence of +the land in North America, after it had acquired its present outline of +hill and valley, cliff and ravine, and then its re-emergence from the +ocean. When the land was slowly sinking, the sea which bordered it was +covered with islands of floating ice coming from the north, which, as +they grounded on the coast and on shoals, pushed along such loose +materials of sand and pebbles as lay strewed over the bottom. By this +force all angular and projecting points were broken off, and fragments +of hard stone, frozen into the lower surface of the ice, had power to +scoop out grooves in the subjacent solid rock. The sloping beach, as +well as the floor of the ocean, might be polished and scored by this +machinery; but no flood of water, however violent, or however great the +quantity of detritus or size of the rocky fragments swept along by it, +could produce such long, perfectly straight and parallel furrows, as are +everywhere visible in the Niagara district, and generally in the region +north of the 40th parallel of latitude.[135-B] + +By the hypothesis of such a slow and gradual subsidence of the land we may +account for the fact that almost everywhere in N. America and Northern +Europe the boulder formation rests on a polished and furrowed surface of +rock,--a fact by no means obliging us to imagine, as some think, that the +polishing and grooving action was, as a whole, anterior in date to the +transportation of the erratics. During the successive depression of high +land, varying originally in height from 1000 to 3000 feet above the +sea-level, every portion of the surface would be brought down by turns to +the level of the ocean, so as to be converted first into a coast-line, and +then into a shoal; and at length, after being well scored by the stranding +upon it of thousands of icebergs, might be sunk to a depth of several +hundred fathoms. By the constant depression of land, the coast would recede +farther and farther from the successively formed zones of polished and +striated rock, each outer zone becoming in its turn so deep under water as +to be no longer grated upon by the heaviest icebergs. Such sunken areas +would then simply serve as receptacles of mud, sand, and boulders dropped +from melting ice, perhaps to a depth scarcely, if at all, inhabited by +testacea and zoophytes. Meanwhile, during the formation of the unstratified +and unfossiliferous mass in deeper water, the smoothing and furrowing of +shoals and beaches is still going on elsewhere upon and near the coast in +full activity. If at length the subsidence should cease, and the direction +of the movement of the earth's crust be reversed, the sunken area covered +with drift would be slowly reconverted into land. The boulder deposit, +before emerging, would then for a time be brought within the action of the +waves, tides, and currents, so that its upper portion, being partially +disturbed, would have its materials re-arranged and stratified. Streams +also flowing from the land would in some places throw down layers of +sediment upon the _till_. In that case, the order of superposition will be, +first and uppermost, sand, loam, and gravel occasionally fossiliferous; +secondly, an unstratified and unfossiliferous mass, for the most part of +much older date than the preceding, with angular erratics, or with boulders +interspersed; and, thirdly, beneath the whole, a surface of polished and +furrowed rock. Such a succession of events seems to have prevailed very +widely on both sides of the Atlantic, the travelled blocks having been +carried in general from the North Pole southwards, but mountain chains +having in some cases served as independent centres of dispersion, of which +the Alps present the most conspicuous example. + +It is by no means rare to meet with boulders imbedded in drift which are +worn flat on one or more of their sides, the surface being at the same time +polished, furrowed, and striated. They may have been so shaped in a glacier +before they reached the sea, or when they were fixed in the bottom of an +iceberg as it ran aground. We learn from Mr. Charles Martins that the +glaciers of Spitzbergen project from the coast into a sea between 100 and +400 feet deep; and that numbers of striated pebbles or blocks are there +seen to disengage themselves from the overhanging masses of ice as they +melt, so as to fall at once into deep water.[136-A] + +That they should retain such markings when again upraised above the sea +ought not to surprise us, when we remember that rippled sands, and the +cracks in clay dried between high and low water, and the foot-tracks of +animals and rain-drops impressed on mud, and other superficial markings, +are all found fossil in rocks of various ages. + +On the other hand, it is not difficult to account for the absence in many +districts of striated and scored pebbles and boulders in glacial deposits, +for they may have been exposed to the action of the waves on a coast while +it was sinking beneath or rising above the sea. No shingle on an ordinary +sea-beach exhibits such striæ, and at a very short distance from the +termination of a glacier every stone in the bed of the torrent which gushes +out from the melting ice is found to have lost its glacial markings by +being rolled for a distance even of a few hundred yards. + +The usual dearth of fossil shells in glacial clays well fitted to preserve +organic remains may, perhaps, be owing, as already hinted, to the absence +of testacea in the deep sea, where the undisturbed accumulation of boulders +melted out of very large bergs may take place. In the Ægean and other parts +of the Mediterranean, the zero of animal life, according to Prof. E. +Forbes, is approached at a depth of about 300 fathoms. In tropical seas it +would descend farther down, just as vegetation ascends higher on the +mountains of hot countries. Near the pole, on the other hand, the same zero +would be reached much sooner both on the hills and in the sea. If the ocean +was filled with floating bergs, and a low temperature prevailed in the +northern hemisphere during the glacial period, even the shallow part of the +sea might have been uninhabitable, or very thinly peopled with living +beings. It may also be remarked that the melting of ice in some fiords in +Norway freshens the water so as to destroy marine life, and famines have +been caused in Iceland by the stranding of icebergs drifted from the +Greenland coast, which have required several years to melt, and have not +only injured the hay harvest by cooling the atmosphere, but have driven +away the fish from the shore by chilling and freshening the sea. + +If the cold of the glacial epoch came on slowly, if it was long before it +reached its greatest intensity, and again if it abated gradually, we may +expect to find the earliest and latest formed drift less barren of organic +remains than that deposited during the coldest period. We may also expect +that along the southern limits of the drift during the whole glacial epoch, +there would be an intimate association of transported matter of northern +origin with fossil-bearing sediment, whether marine or freshwater, +belonging to more southern seas, rivers, and continents. + +That in the United States, the _Mastodon giganteus_ was very abundant after +the drift period is evident from the fact that entire skeletons of this +animal are met with in bogs and lacustrine deposits occupying hollows in +the drift. They sometimes occur in the bottom even of small ponds recently +drained by the agriculturist for the sake of the shell marl. I examined one +of these spots at Geneseo in the state of New York, from which the bones, +skull, and tusk of a Mastodon had been procured in the marl below a layer +of black peaty earth, and ascertained that all the associated freshwater +and land shells were of a species now common in the same district. They +consisted of several species of _Lymnea_, of _Planorbis bicarinatus_, +_Physa heterostropha_, &c. + +In 1845 no less than six skeletons of the same species of Mastodon were +found in Warren County, New Jersey, 6 feet below the surface, by a farmer +who was digging out the rich mud from a small pond which he had drained. +Five of these skeletons were lying together, and a large part of the bones +crumbled to pieces as soon as they were exposed to the air. But nearly the +whole of the other skeleton, which lay about 10 feet apart from the rest, +was preserved entire, and proved the correctness of Cuvier's conjecture +respecting this extinct animal, namely, that it had twenty ribs like the +living elephant. From the clay in the interior within the ribs, just where +the contents of the stomach might naturally have been looked for, seven +bushels of vegetable matter were extracted. I submitted some of this matter +to Mr. A. Henfrey of London for microscopic examination, and he informs me +that it consists of pieces of small twigs of a coniferous tree of the +Cypress family, probably the young shoots of the white cedar, _Thuja +occidentalis_, still a native of North America, on which therefore we may +conclude that this extinct Mastodon once fed. + +Another specimen of the same quadruped, the most complete and probably the +largest ever found, was exhumed in 1845 in the town of Newburg, New York, +the length of the skeleton being 25 feet, and its height 12 feet. The +anchylosing of the last two ribs on the right side afforded Dr. John C. +Warren a true gauge for the space occupied by the intervertebrate +substance, so as to enable him to form a correct estimate of the entire +length. The tusks when discovered were 10 feet long, but a part only could +be preserved. The large proportion of animal matter in the tusk, teeth, and +bones of some of these fossil mammalia is truly astonishing. It amounts in +some cases, as Dr. C. T. Jackson has ascertained by analysis, to 27 per +cent., so that when all the earthy ingredients are removed by acids, the +form of the bone remains as perfect, and the mass of animal matter is +almost as firm, as in a recent bone subjected to similar treatment. + +It would be rash, however, to infer from such data that these quadrupeds +were mired in _modern_ times, unless we use that term strictly in a +geological sense. I have shown that there is a fluviatile deposit in the +valley of the Niagara, containing shells of the genera _Melania_, +_Lymnea_, _Planorbis_, _Valvata_, _Cyclas_, _Unio_, and _Helix_, &c., +all of recent species, from which the bones of the great Mastodon have +been taken in a very perfect state. Yet the whole excavation of the +ravine, for many miles below the Falls, has been slowly effected since +that fluviatile deposit was thrown down. + +Whether or not, in assigning a period of more than 30,000 years for the +recession of the Falls from Queenstown to their present site, I have over +or under estimated the time required for that operation, no one can doubt +that a vast number of centuries must have elapsed before so great a series +of geographical changes were brought about as have occurred since the +entombment of this elephantine quadruped. The freshwater gravel which +incloses it is decidedly of much more modern origin than the drift or +boulder clay of the same region.[138-A] + +Other extinct animals accompany the _Mastodon giganteus_ in the +post-glacial deposits of the United States, among which the _Castoroides +ohioensis_, Foster and Wyman, a huge rodent allied to the beaver, and +the _Capybara_ may be mentioned. But whether the "loess," and other +freshwater and marine strata of the Southern States, in which skeletons +of the same Mastodon are mingled with the bones of the Megatherium, +Mylodon, and Megalonyx, were contemporaneous with the drift, or were of +subsequent date, is a chronological question still open to discussion. +It appears clear, however, from what we know of the tertiary fossils of +Europe--and I believe the same will hold true in North America--that +many species of testacea and some mammalia, which existed prior to the +glacial epoch, survived that era. As European examples among the +warm-blooded quadrupeds, the _Elephas primigenius_ and _Rhinoceros +tichorinus_ may be mentioned. As to the shells, whether fresh water, +terrestrial, or marine, they need not be enumerated here, as allusion +will be made to them in the sequel, when the pliocene tertiary fossils +of Suffolk are described. The fact is important, as refuting the +hypothesis that the cold of the glacial period was so intense and +universal as to annihilate all living creatures throughout the globe. + +That the cold was greater for a time than it is now in certain parts of +Siberia, Europe, and North America, will not be disputed; but, before we +can infer the universality of a colder climate, we must ascertain what was +the condition of other parts of the northern, and of the whole southern, +hemisphere at the time when the Scandinavian, British, and Alpine erratics +were transported into their present position. It must not be forgotten that +a great deposit of drift and erratic blocks is now in full progress of +formation in the southern hemisphere, in a zone corresponding in latitude +to the Baltic, and to Northern Italy, Switzerland, France, and England. +Should the uneven bed of the southern ocean be hereafter converted by +upheaval into land, the hills and valleys will be strewed over with +transported fragments, some derived from the antarctic continent, others +from islands covered with glaciers, like South Georgia, which must now be +centres of the dispersion of drift, although situated in a latitude, +agreeing with that of the Cumberland mountains in England. + +Not only are these operations going on between the 45th and 60th parallels +of latitude south of the line, while the corresponding zone of Europe is +free from ice; but, what is still more worthy of remark, we find in the +southern hemisphere itself, only 900 miles distant from South Georgia, +where the perpetual snow reaches to the sea-beach, lands covered with +forests, as in Terra del Fuego. There is here no difference of latitude to +account for the luxuriance of vegetation in one spot, and the absolute want +of it in the other; but among other refrigerating causes in South Georgia +may be enumerated the countless icebergs which float from the antarctic +zone, and which chill, as they melt, the waters of the ocean, and the +surrounding air, which they fill with dense fogs. + +I have endeavoured in the "Principles of Geology," chapters 7. and 8., to +point out the intimate connexion of climate and the physical geography of +the globe, and the dependence of the mean annual temperature, not only on +the height of the dry land, but on its distribution in high or low +latitudes at particular epochs. If, for example, at certain periods of the +past, the antarctic land was less elevated and less extensive than now, +while that at the north pole was higher and more continuous, the conditions +of the northern and southern hemispheres might have been the reverse of +what we now witness in regard to climate, although the mountains of +Scandinavia, Scotland, and Switzerland, may have been less elevated than at +present. But if in both of the polar regions a considerable area of +elevated dry land existed, such a concurrence of refrigerating conditions +in both hemispheres might have created for a time an intensity of cold +never experienced since; and such probably was the state of things during +that period of submergence to which I have alluded in this chapter. + +_Alpine erratics._--Although the arctic regions constitute the great centre +from which erratics have travelled southwards in all directions in Europe +and North America, yet there are some mountains, as I have already stated, +like those of North Wales and the Alps, which have served as separate and +independent centres for the dispersion of blocks. In illustration of this +fact, the Alps deserve particular attention, not only from their magnitude, +but because they lie beyond the ordinary limits of the "northern drift" of +Europe, being situated between the 44th and 47th degrees of north latitude. +On the flanks of these mountains, and on the Subalpine ranges of hills or +plains adjoining them, those appearances which have been so often alluded +to, as distinguishing or accompanying the drift, between the 50th and 70th +parallels of north latitude, suddenly reappear, to assume in a more +southern country their most exaggerated form. Where the Alps are highest, +the largest erratic blocks have been sent forth, as, for example, from the +regions of Mont Blanc and Monte Rosa, into the adjoining parts of France, +Switzerland, Austria, and Italy, while in districts where the great chain +sinks in altitude, as in Carinthia, Carniola, and elsewhere, no such rocky +fragments, or a few only and of smaller bulk, have been detached and +transported to a distance. + +In the year 1821, M. Venetz first announced his opinion that the Alpine +glaciers must formerly have extended far beyond their present limits, and +the proofs appealed to by him in confirmation of this doctrine were +afterwards acknowledged by M. Charpentier, who strengthened them by new +observations and arguments, and declared, in 1836, his conviction that the +glaciers of the Alps must once have reached as far as the Jura, and have +carried thither their moraines across the great valley of Switzerland. M. +Agassiz, after several excursions in the Alps with M. Charpentier, and +after devoting himself some years to the study of glaciers, published, in +1840, an admirable description of them, and of the marks which attest the +former action of great masses of ice over the entire surface of the Alps +and the surrounding country.[140-A] He pointed out that the surface of +every large glacier is strewed over with gravel and stones detached from +the surrounding precipices by frost, rain, lightning, or avalanches. And he +described more carefully than preceding writers the long lines of these +stones, which settle on the sides of the glacier, and are called the +lateral moraines; those found at the lower end of the ice being called +terminal moraines. Such heaps of earth and boulders every glacier pushes +before it when advancing, and leaves behind it when retreating. When the +Alpine glacier reaches a lower and warmer situation, about 3000 or 4000 +feet above the sea, it melts so rapidly that, in spite of the downward +movement of the mass, it can advance no farther. Its precise limits are +variable from year to year, and still more so from century to century; one +example being on record of a recession of half a mile in a single year. We +also learn from M. Venetz, that whereas, between the eleventh and fifteenth +centuries, all the Alpine glaciers were less advanced than now, they began +in the seventeenth and eighteenth centuries to push forward so as to cover +roads formerly open, and to overwhelm forests of ancient growth. + +These oscillations enable the geologist to note the marks which they leave +behind them as they retrograde, and among these the most prominent, as +before stated, are the terminal moraines, or mounds of unstratified earth +and stones, often divided by subsequent floods into hillocks, which cross +the valley like ancient earth-works, or embankments made to dam up the +river. Some of these transverse barriers were formerly pointed out by +Saussure below the glacier of the Rhone, as proving how far it had once +transgressed its present boundaries. On these moraines we see many large +angular fragments, which, having been carried along on the surface of the +ice, have not had their edges worn off by friction; but the greater number +of the boulders, even those of large size, have been well rounded, not by +the power of water, but by the mechanical force of the ice, which has +pushed them against each other, or against the rocks flanking the valley. +Others have fallen down the numerous fissures which intersect the glacier, +where, being subject to the pressure of the whole mass of ice, they have +been forced along, and either well rounded or ground down into sand, or +even the finest mud, of which the moraine is largely constituted. + +As the terminal moraines are the most prominent of all the monuments left +by a receding glacier, so are they the most liable to obliteration; for +violent floods or debacles are often occasioned in the Alps by the sudden +bursting of what are called glacier-lakes. These temporary sheets of water +are caused by the damming up of a river by a glacier which has increased +during a succession of cold seasons, and, descending from a tributary into +the main valley, has crossed it from side to side. On the failure of this +icy barrier, the accumulated waters are let loose, which sweep away and +level all transverse mounds of gravel and loose boulder below, and spread +their materials in confused and irregular beds over the river-plain. + +Another mark of the former action of glaciers, in situations where they +exist no longer, is the polished, striated, and grooved surfaces of rocks +already alluded to. Stones which lie underneath the glacier and are pushed +along by it, sometimes adhere to the ice, and as the mass glides slowly +along at the rate of a few inches, or at the utmost two or three feet, per +day, abrade, groove, and polish the rock, and the larger blocks are +reciprocally grooved and polished by the rock on their lower sides. As the +forces both of pressure and propulsion are enormous, the sand, acting like +emery, polishes the surface; the pebbles, like coarse gravers, scratch and +furrow it; and the large stones scoop out grooves in it. Another effect +also of this action, not yet adverted to, is called "roches moutonnées." +Projecting eminences of rock are smoothed and worn into the shape of +flattened domes, where the glaciers have passed over them. + +Although the surface of almost every kind of rock, when exposed in the open +air, wastes away by decomposition, yet some retain for ages their polished +and furrowed exterior; and, if they are well protected by a covering of +clay or turf, these marks of abrasion seem capable of enduring for ever. +They have been traced in the Alps to great heights above the present +glaciers, and to great horizontal distances beyond them. + +There are also found, on the sides of the Swiss valleys, round and deep +holes, with polished sides, such holes as waterfalls make in the solid +rock, but in places remote from running waters, and where the form of the +surface will not permit us to suppose that any cascade could ever have +existed. Similar cavities are common in hard rocks, such as gneiss, in +Sweden, where they are called _giant caldrons_, and are sometimes 10 feet +and more in depth; but in the Alps and Jura they often pass into +spoon-shaped excavations and prolonged gutters. We learn from M. Agassiz +that hollows of this form are now cut out by streams of water, which flow +along the surface of glaciers, and then fall into fissures which are open +to the bottom. Here, forming a cascade, the stream cuts a round cavity in +the rock with the gravel and sand, which it either finds there or carries +down with it, and causes to rotate; and, as it usually happens that the +glacier is advancing, a locomotive cascade is produced, which converts the +first circular hole into a deep groove. + +Another effect of a glacier is to lodge a ring of stones round the summit +of a conical peak which may happen to project through the ice. If the +glacier is lowered greatly by melting, these circles of large angular +fragments, which are called "perched blocks," are left in a singular +situation near the top of a steep hill or pinnacle, the lower parts of +which may be destitute of boulders. + +_Alpine blocks on the Jura._--Now some or all the marks above +enumerated,--the moraines, erratics, polished surfaces, domes, striæ, +caldrons, and perched rocks, are observed in the Alps at great heights +above the present glaciers, and far below their actual extremities; also +in the great valley of Switzerland, 50 miles broad; and almost +everywhere on the Jura, a chain which lies to the north of this valley. +The average height of the Jura is about one third that of the Alps, and +is now entirely destitute of glaciers, yet it presents almost everywhere +similar moraines, and the same polished and grooved surfaces, and +water-worn cavities. The erratics, moreover, which cover it, present a +phenomenon which has astonished and perplexed the geologist for more +than half a century. No conclusion can be more incontestible than that +these angular blocks of granite, gneiss, and other crystalline +formations, came from the Alps, and that they have been brought for a +distance of 50 miles and upwards across one of the widest and deepest +valleys of the world, so that they are now lodged on the hills and +valleys of a chain composed of limestone and other formations, +altogether distinct from those of the Alps. Their great size and +angularity, after a journey of so many leagues, has justly excited +wonder; for hundreds of them are as large as cottages; and one in +particular, celebrated under the name of Pierre à Bot, rests on the side +of a hill about 900 feet above the lake of Neufchatel, and is no less +than 40 feet in diameter. + +It will be remarked that these blocks on the Jura offer an exception to the +rule before laid down, as applicable in general to erratics, since they +have gone from south to north. Some of the largest masses of granite and +gneiss have been found to contain 50,000 and 60,000 cubic feet of stone, +and one limestone block near Devens, which has travelled 30 miles, contains +161,000 cubic feet, its angles being sharp and unworn.[143-A] + +Von Buch, Escher, and Studer have shown, from an examination of the +mineral composition of the boulders, that those on the western Jura, +near Neufchatel, have come from the region of Mont Blanc and the Valais; +those on the middle parts of the Jura from the Bernese Oberland; and +those on the eastern Jura from the Alps of the small cantons, Glaris, +Schwytz, Uri, and Zug. The blocks, therefore, of these three great +districts have been derived from parts of the Alps nearest to the +localities in the Jura where we now find them, as if they had crossed +the great valley in a direction at right angles to its length: the most +western stream having followed the course of the Rhone; the central, +that of the Aar; and the eastern, that of the two great rivers, Reuss +and Limmat. The non-intermixture of these groups of travelled fragments, +except near their confines, was always regarded as most enigmatical by +those who adopted the opinion of Saussure, that they were all whirled +along by a rapid current of muddy water rushing from the Alps. + +M. Charpentier first suggested, as before mentioned, that the Swiss +glaciers once reached continuously to the Jura, and conveyed to them +these erratics; but at the same time he conceived that the Alps were +formerly higher than now. M. Agassiz, on the other hand, instead of +introducing distinct and separate glaciers, imagines that the whole +valley of Switzerland was filled with ice, and that one great sheet of +it extended from the Alps to the Jura, when the two chains were of the +same height as now relatively to each other. Such an hypothesis labours +under this difficulty, that the difference of altitude, when distributed +over a space of 50 miles, gives an inclination of no more than two +degrees, or far less than that of any known glaciers. It has, however, +since received the able support of Professor James Forbes, in his +excellent work on the Alps, published in 1843. + +In the theory which I formerly advanced, jointly with Mr. Darwin[143-B], +it was suggested that the erratics may have been transferred by floating +ice to the Jura, at the time when the greater part of that chain, and the +whole of the Swiss valley to the south, was under the sea. At that period +the Alps may have attained only half their present altitude, and may yet +have constituted a chain as lofty as the Chilian Andes, which, in a +latitude corresponding to Switzerland, now send down glaciers to the head +of every sound, from which icebergs, covered with blocks of granite, are +floated seaward.[144-A] Opposite that part of Chili where the glaciers +abound is situated the island of Chiloe, 100 miles in length, with a +breadth of 30 miles, running parallel to the continent. The channel which +separates it from the main land is of considerable depth, and 25 miles +broad. Parts of its surface, like the adjacent coast of Chili, are +overspread with recent marine shells, showing an upheaval of the land +during a very modern period; and beneath these shells is a boulder deposit, +in which Mr. Darwin found large travelled blocks. One group of fragments +were of granite, which had evidently come from the Andes, while in another +place angular blocks of syenite were met with. Their arrangement may have +been due to successive crops of icebergs issuing from different sounds, to +the heads of which glaciers descend from the Andes. These icebergs, taking +their departure year after year from distinct points, may have been +stranded repeatedly, in equally distinct groups, in bays or creeks of +Chiloe, and on islets off the coast, so as afterwards to appear, some on +hills and others in valleys, when that country and the bed of the adjacent +sea had been upheaved. A continuance in future of the elevatory movement, +in the region of the Andes and of Chiloe, might cause the former chain to +rival the Alps in altitude, and give to Chiloe a height equal to that of +the Jura. The same rise might dry up the channel between Chiloe and the +main land, so that it would then represent the great valley of Switzerland. +In the course of these changes, all parts of Chiloe and the intervening +strait, having in their turn been a sea-shore, may have been polished and +scratched by coast-ice, and by innumerable icebergs running aground and +grating on the bottom. + +If we apply this hypothesis to Switzerland and the Jura, we are by no means +precluded from the supposition that, in proportion as the land acquired +additional height, and the bed of the sea emerged, the Jura itself may have +had its glaciers; and those existing in the Alps, which had at first +extended to the sea, may, during some part of the period of upheaval, have +been prolonged much farther into the valleys than now. At a later period, +when the climate grew milder, these glaciers may have entirely disappeared +from the Jura, and may have receded in the Alps to their present limits, +leaving behind them in both districts those moraines which now attest the +former extension of the ice.[144-B] + +_Meteorites in drift._--Before concluding my remarks on the northern drift +of the Old World, I shall refer to a fact recently announced, the discovery +of a meteoric stone at a great depth in the alluvium of Northern Asia. + +Erman, in his Archives of Russia for 1841 (p. 314.), cites a very +circumstantial account drawn up by a Russian miner of the finding of a mass +of meteoric iron in the auriferous alluvium of the Altai. Some small +fragments of native iron were first met with in the gold-washings of +Petropawlowsker in the Mrassker Circle; but though they attracted +attention, it was supposed that they must have been broken off from the +tools of the workmen. At length, at the depth of 31 feet 5 inches from the +surface, they dug out a piece of iron weighing 17-1/2 pounds, of a +steel-grey colour, somewhat harder than ordinary iron, and, on analysing +it, found it to consist of native iron, with a small proportion of nickel, +as usual in meteoric stones. It was buried in the bottom of the deposit +where the gravel rested on a flaggy limestone. Much brown iron ore, as well +as gold, occurs in the same gravel, which appears to be part of that +extensive auriferous formation in which the bones of the mammoth, the +_Rhinoceros tichorhinus_, and other extinct quadrupeds abound. No +sufficient data are supplied to enable us to determine whether it be of +Post-Pliocene or Newer Pliocene date. + +We ought not, I think, to feel surprise that we have not hitherto succeeded +in detecting the signs of such aërolites in older rocks, for, besides their +rarity in our own days, those which fell into the sea (and it is with +marine strata that geologists have usually to deal), being chiefly composed +of native iron, would rapidly enter into new chemical combinations, the +water and mud being charged with chloride of sodium and other salts. We +find that anchors, cannon, and other cast-iron implements which have been +buried for a few hundred years off our English coast have decomposed in +part or entirely, turning the sand and gravel which enclosed them into a +conglomerate, cemented together by oxide of iron. In like manner meteoric +iron, although its rusting would be somewhat checked by the alloy of +nickel, could scarcely ever fail to decompose in the course of thousands of +years, becoming oxide, sulphuret or carbonate of iron, and its origin being +then no longer distinguishable. The greater the antiquity of rocks,--the +oftener they have been heated and cooled, permeated by gases or by the +waters of the sea, the atmosphere or mineral springs,--the smaller must be +the chance of meeting with a mass of native iron unaltered; but the +preservation of the ancient meteorite of the Altai, and the presence of +nickel in these curious bodies, renders the recognition of them in deposits +of remote periods less hopeless than we might have anticipated. + + +FOOTNOTES: + +[134-A] Geol. Trans. 2d series, vol. vi. p. 135. Mr. Smith of Jordanhill +had arrived at similar conclusions as to climate from the shells of the +Scotch Pleistocene deposits. + +[134-B] Proceedings of Geol. Soc. No. 63. p. 119. + +[135-A] Travels in N. America, vol. ii. p. 141. + +[135-B] Ibid. p. 99. chap. xix. + +[136-A] Bulletin Soc. Géol. de France, tom. iv. 2de sér. p. 1121. + +[138-A] See Travels in N. America, vol. i. chap. ii. + +[140-A] Agassiz, Etudes sur les Glaciers. + +[143-A] Archiac, Hist. des Progrès, &c. vol. ii. p. 249. + +[143-B] See Elements of Geology, 2d ed. 1841. + +[144-A] Darwin's Journal, p. 283. + +[144-B] More recently Sir R. Murchison, having revisited the Alps, has +declared his opinion that "the great granitic blocks of Mont Blanc were +translated to the Jura when the intermediate country was under +water."--Paper read to Geol. Soc. London, May 30, 1849. + + + + +CHAPTER 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. + + +Having in the last chapter treated of the boulder formation and its +associated freshwater and marine strata as belonging chiefly to the close +of the Newer Pliocene period, we may now proceed to other deposits of the +same or nearly the same age. It should, however, be stated that it is +difficult to draw the line of separation between these modern formations, +especially when we are called upon to compare deposits of marine and +freshwater origin, or these again with the ossiferous contents of caverns. + +If as often as the carcasses of quadrupeds were buried in alluvium during +floods, or mired in swamps, or imbedded in lacustrine strata, a stream of +lava had descended and preserved the alluvial or freshwater deposits, as +frequently happened in Auvergne (see above, p. 80,), keeping them free from +intermixture with strata subsequently formed, then indeed the task of +arranging chronologically the whole series of mammaliferous formations +might have been easy, even though many species were common to several +successive groups. But when there have been oscillations in the levels of +the land, accompanied by the widening and deepening of valleys at more than +one period,--when the same surface has sometimes been submerged beneath the +sea, after supporting forests and land quadrupeds, and then raised again, +and subject during each change of level to sedimentary deposition and +partial denudation,--and when the drifting of ice by marine currents or by +rivers, during an epoch of intense cold, has for a season interfered with +the ordinary mode of transport, or with the geographical range of species, +we cannot hope speedily to extricate ourselves from the confusion in which +the classification of these Pleistocene formations is involved. + +At several points in the valley of the Thames, remnants of ancient +fluviatile deposits occur, which may differ considerably in age, +although the imbedded land and freshwater shells in each are of recent +species. At Brentford, for example, the bones of the Siberian Mammoth, +or _Elephas primigenius_, and the _Rhinoceros tichorhinus_, both of +them quadrupeds of which the flesh and hair have been found preserved in +the frozen soil of Siberia, occur abundantly, with the bones of an +hippopotamus, aurochs, short-horned ox, red deer, rein-deer, and great +cave-tiger or lion.[147-A] A similar group has been found fossil at +Maidstone, in Kent, and other places, agreeing in general specifically +with the fossil bones detected in the caverns of England. When we see +the existing rein-deer and an extinct hippopotamus in the same +fluviatile loam, we are tempted to indulge our imaginations in +speculating on the climatal conditions which could have enabled these +genera to co-exist in the same region. Wherever there is a continuity of +land from polar to temperate and equatorial regions, there will always +be points where the southern limit of an arctic species meets the +northern range of a southern species; and if one or both have migratory +habits, like the Bengal tiger, the American bison, the musk ox, and +others, they may each penetrate mutually far into the respective +provinces of the other. There may also have been several oscillations of +temperature during the periods which immediately preceded and followed +the more intense cold of the glacial epoch. + +The strata bordering the left bank of the Thames at Grays Thurrock, in +Essex, are probably of older date than those of Brentford, although the +associated land and freshwater shells are nearly all, if not all, +identical with species now living. Three of the shells, however, are no +longer inhabitants of Great Britain; namely, _Paludina marginata_ (fig. +112. p. 127.), now living in France; _Unio littoralis_ (fig. 29. p. +28.), now inhabiting the Loire; and _Cyrena consobrina_ (fig. 26. p. +28.). The last-mentioned fossil (a recent Egyptian shell of the Nile) is +very abundant at Grays, and deserves notice, because the genus _Cyrena_ +is now no longer European. + +The rhinoceros occurring in the same beds (_R. leptorhinus_, see fig. 131. +p. 160.) is of a different species from that of Brentford above mentioned, +and the accompanying elephant belongs to the variety called _Elephas +meridionalis_, which, according to MM. Owen and H. von Meyer, two high +authorities, is the same species as the Siberian mammoth, although some +naturalists regard it as distinct. With the above mammalia is also found +the _Hippopotamus major_, and what is most remarkable in so modern and +northern a deposit, a monkey, called by Owen, _Macacus pliocenus_. + +The submerged forest already alluded to (p. 130.) as underlying the drift +at the base of the cliffs of Norfolk is associated with a bed of lignite +and loam, in which a great number of fossil bones occur, apparently of the +same group as that of Grays, just mentioned. It has sometimes been called +"the Elephant bed." One portion of it, which stretches out under the sea at +Happisburgh, was overgrown in 1820 by a bank of recent oysters, and there +the fishermen dredged up, according to Woodward, in the course of thirteen +years, together with the oysters, above 2000 mammoths' grinders.[147-B] +Another portion of the same continuous stratum has yielded at Bacton, +Cromer, and other places on the coast, the bones of a gigantic beaver +(_Trogontherium Cuvierii_, Fischer), as well as the ox, horse, and deer, +and both species of rhinoceros, _R. tichorhinus_ and _R. leptorhinus_. + +In studying these and various other similar assemblages of fossils, we have +a good exemplification of the more rapid rate at which the mammiferous +fauna, as compared to the testaceous, diverges when traced backwards in +time from the recent type. I have before hinted, that the longevity of +species in the class of warm-blooded quadrupeds is less great than in that +of the mollusca, the latter having probably more capacity for enduring +those changes of climate and other external circumstances which take place +in the course of ages on the earth's surface. This phenomenon is by no +means confined to Europe, for Mr. Darwin found at Bahia Blanca, in South +America, lat. 39° S., near the northern confines of Patagonia, fossil +remains of the extinct mammiferous genera Megatherium, Megalonyx, Toxodon, +and others, associated with shells, almost all of species already +ascertained to be still living in the contiguous sea[148-A]; the marine +mollusca, as well as those of rivers, lakes, or the land, having died out +more slowly than the terrestrial mammalia. + +I alluded before (p. 125.) to certain marine strata overlying till near +Glasgow, and at other points on the Clyde, in which the shells are for the +most part British, with an intermixture of some arctic species; while +others, about a tenth of the whole, are supposed to be extinct. This +formation may also be called Newer Pliocene. + +_Fluvio-marine crag of Norwich._--At several places within five miles of +Norwich, on both banks of the Yare, beds of sand, loam, and gravel, +provincially termed "crag," occur, in which there is a mixture of marine, +land, and freshwater shells, with ichthyolites and bones of mammalia. It is +clear that these beds have been accumulated at the bottom of the sea near +the mouth of a river. They form patches of variable thickness, resting on +white chalk, and are covered by a dense mass of stratified flint gravel. +The surface of the chalk is often perforated to the depth of several inches +by the _Pholas crispata_, each fossil shell still remaining at the bottom +of its cylindrical cavity, now filled up with loose sand which has fallen +from the incumbent crag. This species of Pholas still exists and drills the +rocks between high and low water on the British coast. The most common +shells of these strata, such as _Fusus striatus_, _Turritella terebra_, +_Cardium edule_, and _Cyprina islandica_, are now abundant in the British +seas; but with them are some extinct species, such as _Nucula Cobboldiæ_ +(fig. 120.) and _Tellina obliqua_ (fig. 121.). _Natica helicoides_ (fig. +122.) is an example of a species formerly known only as fossil, but which +has now been found living in our seas. + +Among the accompanying bones of mammalia is the _Mastodon_ +_angustidens_[149-A] (see fig. 130.), a portion of the upper jawbone with a +tooth having been found by Mr. Wigham at Postwick, near Norwich. As this +species has also been found in the Red Crag, both at Sutton and at +Felixstow, and had hitherto been regarded as characteristic of formations +older than the Pleistocene, it may possibly have been washed out of the Red +into the Norwich Crag. + +[Illustration: Fig. 120. _Nucula Cobboldiæ._] + +[Illustration: Fig. 121. _Tellina obliqua._] + +[Illustration: Fig. 122. _Natica helicoides_, Johnston.] + +Among the bones, however, respecting the authenticity of which there +seems no doubt, may be mentioned those of the elephant, horse, pig, +deer, and the jaws and teeth of field mice (fig. 141.). I have seen the +tusk of an elephant from Bramerton near Norwich, to which, many serpulæ +were attached, showing that it had lain for some time at the bottom of +the sea of the Norwich Crag. + +At Thorpe, near Aldborough, and at Southwold, in Suffolk, this +fluvio-marine formation is well exposed in the sea-cliffs, consisting of +sand, shingle, loam, and laminated clay. Some of the strata there bear +the marks of tranquil deposition, and in one section a thickness of 40 +feet is sometimes exposed to view. Some of the lamellibranchiate shells +have both valves united, although mixed with land and freshwater +testacea, and with the bones and teeth of elephant, rhinoceros, horse, +and deer. Captain Alexander, with whom I examined these strata in 1835, +showed me a bed rich in marine shells, in which he had found a large +specimen of the _Fusus striatus_, filled with sand, and in the interior +of which was the tooth of a horse. + +Among the freshwater shells I obtained the _Cyrena consobrina_ (fig. 26. +p. 28.), before mentioned, supposed to agree with a species now +living in the Nile. + +I formerly classed the Norwich Crag as older Pliocene, conceiving that more +than a third of the fossil testacea were extinct; but there now seems good +reason for believing that several of the rarer shells obtained from these +strata do not really belong to a contemporary fauna, but have been washed +out of the older beds of the "Red Crag;" while other species, once supposed +to have died out, have lately been met with living in the British seas. +According to Mr. Searles Wood, the total number of marine species does not +exceed seventy-six, of which one tenth only are extinct. Of the fourteen +associated freshwater shells, all the species appear to be living. Strata +containing the same shells as those near Norwich have been found by Mr. +Bean, at Bridlington, in Yorkshire. + +_Newer Pliocene strata of Sicily._--In no part of Europe are the Newer +Pliocene formations seen to enter so largely into the structure of the +earth's crust, or to rise to such heights above the level of the sea, as in +Sicily. They cover nearly half the island, and near its centre, at +Castrogiovanni, they reach an elevation of 3000 feet. They consist +principally of two divisions, the upper calcareous, the lower argillaceous, +both of which may be seen at Syracuse, Girgenti, and Castrogiovanni. + +According to Philippi, to whom we are indebted for the best account of the +tertiary shells of this island, thirty-five species out of one hundred and +twenty-four obtained from the beds in central Sicily are extinct. Of the +remainder, which still live, five species are no longer inhabitants of the +Mediterranean. When I visited Sicily in 1828 I estimated the proportion of +living species as somewhat greater, partly because I confounded with the +tertiary formation of central Sicily the strata at the base of Etna, and +some other localities, where the fossils are now proved to agree entirely +with the present Mediterranean fauna. + +Philippi came to the conclusion, that in Sicily there is a gradual +passage from beds containing 70 per cent. of recent shells, to those in +which the whole of the fossils are identical with recent species; but +his tables appear scarcely to bear out so important a generalization, +several of the places cited by him in confirmation having as yet +furnished no more than twenty or thirty species of testacea. The +Sicilian beds in question probably belong to about the same period as +the Norwich Crag, although a geologist, accustomed to see nearly all the +Pleistocene formations in the north of Europe occupying low grounds and +very incoherent in texture, is naturally surprised to behold formations +of the same age so solid and stony, of such thickness, and attaining so +great an elevation above the level of the sea. + +The upper or calcareous member of this group in Sicily consists in some +places of a yellowish-white stone, like the calcaire grossier of Paris, in +others, of a rock nearly as compact as marble. Its aggregate thickness +amounts sometimes to 700 or 800 feet. It usually occurs in regular +horizontal beds, and is occasionally intersected by deep valleys, such as +those of Sortino and Pentalica, in which are numerous caverns. The fossils +are in every stage of preservation, from shells retaining portions of their +animal matter and colour, to others which are mere casts. + +The limestone passes downwards into a sandstone and conglomerate, below +which is clay and blue marl, like that of the Subapennine hills, from +which perfect shells and corals may be disengaged. The clay sometimes +alternates with yellow sand. + +South of the plain of Catania is a region in which the tertiary beds are +intermixed with volcanic matter, which has been for the most part the +product of submarine eruptions. It appears that, while the clay, sand, +and yellow limestone before mentioned were in course of deposition at +the bottom of the sea, volcanos burst out beneath the waters, like that +of Graham Island, in 1831, and these explosions recurred again and again +at distant intervals of time. Volcanic ashes and sand were showered down +and spread by the waves and currents so as to form strata of tuff, +which are found intercalated between beds of limestone and clay +containing marine shells, the thickness of the whole mass exceeding 2000 +feet. The fissures through which the lava rose may be seen in many +places forming what are called _dikes_. + +In part of the region above alluded to, as, for example, near Lentini, a +conglomerate occurs in which I observed many pebbles of volcanic rocks +covered by full grown _serpulæ_. We may explain the origin of these by +supposing that there were some small volcanic islands which may have been +destroyed from time to time by the waves, as Graham Island has been swept +away since 1831. The rounded blocks and pebbles of solid volcanic matter, +after being rolled for a time on the beach of such temporary islands, were +carried at length into some tranquil part of the sea, where they lay for +years, while the marine _serpulæ_ adhered to them, their shells growing and +covering their surface, as they are seen adhering to the shell figured in +p. 22. Finally, the bed of pebbles was itself covered with strata of shelly +limestone. At Vizzini, a town not many miles distant to the S.W., I +remarked another striking proof of the gradual manner in which these modern +rocks were formed, and the long intervals of time which elapsed between the +pouring out of distinct sheets of lava. A bed of oysters no less than 20 +feet in thickness rests upon a current of basaltic lava. The oysters are +perfectly identifiable with our common eatable species. Upon the oyster +bed, again, is superimposed a second mass of lava, together with tuff or +peperino. In the midst of the same alternating igneous and aqueous +formations is seen near Galieri, not far from Vizzini, a horizontal bed, +about a foot and a half in thickness, composed entirely of a common +Mediterranean coral (_Caryophyllia cæspitosa_, Lam.). These corals stand +erect as they grew; and, after being traced for hundreds of yards, are +again found at a corresponding height on the opposite side of the valley. + +[Illustration: Fig. 123. _Caryophyllia cæspitosa_, Lam. +(_Cladocora cæspitosa_, Ehr.) + + _a._ Stem with young stem growing from its side. + _a*._ Young stem of same twice magnified. + _b._ Portion of branch, twice magnified, with the base of a lateral + branch; the exterior ridges of the main branch appearing through + the lamellæ of the lateral one. + _c._ Transverse section of same, proving, by the integrity of the main + branch, that the lateral one did not originate in a subdivision + of the animal. + _d._ A branch, having at its base another laterally united to it, and + two young corals at its upper part. + _e._ A main branch, with a full grown lateral one. + _f._ A perfect terminal star.] + +The corals are usually branched, but not by the division of the animals as +some have supposed, but by the attachment of young individuals to the sides +of the older ones; and we must understand this mode of increase, in order +to appreciate the time which was required for the building up of the whole +bed of coral during the growth of many successive generations.[152-A] + +Among the other fossil shells met with in these Sicilian strata, which +still continue to abound in the Mediterranean, no shell is more +conspicuous, from its size and frequent occurrence, than the great +scallop, _Pecten jacobæus_ (see fig. 124.), now so common in the +neighbouring seas. We see this shell in the calcareous beds at Palermo +in great numbers, in the limestone at Girgenti, and in that which +alternates with volcanic rocks in the country between Syracuse and +Vizzini, often at great heights above the sea. + +[Illustration: Fig. 124. _Pecten jacobæus_; half natural size.] + +The more we reflect on the preponderating number of these recent shells, +the more we are surprised at the great thickness, solidity, and height +above the sea of the rocky masses in which they are entombed, and the vast +amount of geographical change which has taken place since their origin. It +must be remembered that, before they began to emerge, the uppermost strata +of the whole must have been deposited under water. In order, therefore, to +form a just conception of their antiquity, we must first examine singly the +innumerable minute parts of which the whole is made up, the successive beds +of shells, corals, volcanic ashes, conglomerates, and sheets of lava; and +we must afterwards contemplate the time required for the gradual upheaval +of the rocks, and the excavation of the valleys. The historical period +seems scarcely to form an appreciable unit in this computation, for we +find ancient Greek temples, like those of Girgenti (Agrigentum), built of +the modern limestone of which we are speaking, and resting on a hill +composed of the same; the site having remained to all appearance unaltered +since the Greeks first colonised the island. + +The modern geological date of the rocks in this region leads to another +singular and unexpected conclusion, namely, that the fauna and flora of a +large part of Sicily are of higher antiquity than the country itself, +having not only flourished before the lands were raised from the deep, but +even before their materials were brought together beneath the waters. The +chain of reasoning which conducts us to this opinion may be stated in a few +words. The larger part of the island has been converted from sea into land +since the Mediterranean was peopled with nearly all the living species of +testacea and zoophytes. We may therefore presume that, before this region +emerged, the same land and river shells, and almost all the same animals +and plants, were in existence which now people Sicily; for the terrestrial +fauna and flora of this island are precisely the same as that of other +lands surrounding the Mediterranean. There appear to be no peculiar or +indigenous species, and those which are now established there must be +supposed to have migrated from pre-existing lands, just as the plants and +animals of the Neapolitan territory have colonised Monte Nuovo, since that +volcanic cone was thrown up in the sixteenth century. + +Such conclusions throw a new light on the adaptation of the attributes +and migratory habits of animals and plants to the changes which are +unceasingly in progress in the physical geography of the globe. It is +clear that the duration of species is so great, that they are destined +to outlive many important revolutions in the configuration of the +earth's surface; and hence those innumerable contrivances for enabling +the subjects of the animal and vegetable creation to extend their range; +the inhabitants of the land being often carried across the ocean, and +the aquatic tribes over great continental spaces. It is obviously +expedient that the terrestrial and fluviatile species should not only be +fitted for the rivers, valleys, plains, and mountains which exist at the +era of their creation, but for others that are destined to be formed +before the species shall become extinct; and, in like manner, the marine +species are not only made for the deep and shallow regions of the ocean +existing at the time when they are called into being, but for tracts +that may be submerged or variously altered in depth during the time that +is allotted for their continuance on the globe. + + +OSSEOUS BRECCIAS AND DEPOSITS IN CAVES OF THE PLIOCENE PERIOD. + +_Sicily._--Caverns filled with marine breccias, at the base of ancient +sea-cliffs, have been already mentioned in the sixth chapter; and it was +noticed, respecting the cave of San Ciro, near Palermo (p. 75.), that upon +a bed of sand filled with sea-shells, almost all of recent species, rests +a breccia (_b_, fig. 93.), composed of fragments of calcareous rock, and +the bones of animals. In the sand at the bottom of that cave, Dr. Philippi +found about forty-five marine shells, all clearly identical with recent +species, except two or three. The bones in the incumbent breccia are +chiefly those of the mammoth (_E. primigenius_), with some belonging to an +hippopotamus, distinct from the recent species, and smaller than that +usually found fossil. (See fig. 132.) Several species of deer also, and, +according to some accounts, the remains of a bear, were discovered. These +mammalia are probably referable to the Post-Pliocene period. + +The Newer Pliocene tertiary limestone of the south of Sicily, already +described, is sometimes full of caverns; and the student will at once +perceive that all the quadrupeds of which the remains are found in the +stalactite of these caverns, being of later origin than the rocks, must be +referable to the close of the tertiary epoch, if not of still later date. +The situation of one of these caves, in the valley of Sortino, is +represented in the annexed section. + +[Illustration: Fig. 125. Cross section. + + _a_. Alluvium, } containing the remains of quadrupeds + _b_, _b_. Deposits in caves, } for the most part extinct. + + C. Limestone, containing the remains of shells, of which between 70 and + 80 per cent. are recent.] + +_England._--In a cave at Kirkdale, about twenty-five miles N.N.E. of York, +the remains of about 300 hyænas, belonging to individuals of every age, +have been detected. The species (_Hyæna spelæa_) is extinct, and was larger +than the fierce _Hyæna crocuta_ of South Africa, which it most resembled. +Dr. Buckland, after carefully examining the spot, proved that the Hyænas +must have lived there; a fact attested by the quantity of their dung, +which, as in the case of the living hyæna, is of nearly the same +composition as bone, and almost as durable. In the cave were found the +remains of the ox, young elephant, hippopotamus, rhinoceros, horse, bear, +wolf, hare, water-rat, and several birds. All the bones have the appearance +of having been broken and gnawed by the teeth of the hyænas; and they occur +confusedly mixed in loam or mud, or dispersed through a crust of stalagmite +which covers it. In these and many other cases it is supposed that portions +of herbivorous quadrupeds have been dragged into caverns by beasts of prey, +and have served as their food, an opinion quite consistent with the known +habits of the living hyæna. + +No less than thirty-seven species of mammalia are enumerated by Professor +Owen as having been discovered in the caves of the British islands, of +which eighteen appear to be extinct, while the others still survive in +Europe. They were not washed to the spots where the fossils now occur by a +great flood; but lived and died, one generation after another, in the +places where they lie buried. Among other arguments in favour of this +conclusion may be mentioned the great numbers of the shed antlers of deer +discovered in caves and in freshwater strata throughout England.[155-A] + +Examples also occur of fissures into which animals have fallen from time to +time, or have been washed in from above, together with alluvial matter and +fragments of rock detached by frost, forming a mass which may be united +into a bony breccia by stalagmitic infiltrations. Frequently we discover a +long suite of caverns connected by narrow and irregular galleries, which +hold a tortuous course through the interior of mountains, and seem to have +served as the subterranean channels of springs and engulphed rivers. Many +streams in the Morea are now carrying bones, pebbles, and mud into +underground passages of this kind.[155-B] If, at some future period, the +form of that country should be wholly altered by subterranean movements and +new valleys shaped out by denudation, many portions of the former channels +of these engulphed streams may communicate with the surface, and become the +dens of wild beasts, or the recesses to which quadrupeds retreat to die. +Certain caves of France, Germany, and Belgium, may have passed successively +through these different conditions, and in their last state may have +remained open to the day for several tertiary periods. It is nevertheless +remarkable, that on the continent of Europe, as in England, the fossil +remains of mammalia belong almost exclusively to those of the Newer +Pliocene and Post-Pliocene periods, and not to the Miocene or Eocene +epochs, and when they are accompanied by land or river shells, these agree +in great part, or entirely, with recent species. + +As the preservation of the fossil bones is due to a slow and constant +supply of stalactite, brought into the caverns by water dropping from the +roof, the source and origin of this deposit has been a subject of curious +inquiry. The following explanation of the phenomenon has been recently +suggested by the eminent chemist Liebig. On the surface of Franconia, where +the limestone abounds in caverns, is a fertile soil, in which vegetable +matter is continually decaying. This mould or humus, being acted on by +moisture and air, evolves carbonic acid which is dissolved by rain. The +rain water, thus impregnated, permeates the porous limestone, dissolves a +portion of it, and afterwards, when the excess of carbonic acid evaporates +in the caverns, parts with the calcareous matter, and forms stalactite. + +_Australian cave-breccias._--Ossiferous breccias are not confined to +Europe, but occur in all parts of the globe; and those lately discovered +in fissures and caverns in Australia correspond closely in character +with what has been called the bony breccia of the Mediterranean, in +which the fragments of bone and rock are firmly bound together by a +red ochreous cement. + +Some of these caves have been examined by Sir T. Mitchell in the Wellington +Valley, about 210 miles west of Sidney, on the river Bell, one of the +principal sources of the Macquarie, and on the Macquarie itself. The +caverns often branch off in different directions through the rock, widening +and contracting their dimensions, and the roofs and floors are covered with +stalactite. The bones are often broken, but do not seem to be water-worn. +In some places they lie imbedded in loose earth, but they are usually +included in a breccia. + +The remains found most abundantly are those of the kangaroo, of which there +are four species, besides which the genera _Hypsiprymnus_, _Phalangista_, +_Phascolomys_, and _Dasyurus_, occur. There are also bones, formerly +conjectured by some osteologists to belong to the hippopotamus, and by +others to the dugong, but which are now referred by Mr. Owen to a marsupial +genus, allied to the _Wombat_. + +[Illustration: Fig. 126. _Macropus atlas_, Owen. + +_a._ permanent false molar, in the alveolus.] + +[Illustration: Fig. 127. Lowest jaw of largest living species of kangaroo. +(_Macropus major._)] + +In the fossils above enumerated, several species are larger than the +largest living ones of the same genera now known in Australia. The annexed +figure of the right side of a lower jaw of a kangaroo (_Macropus atlas_, +Owen) will at once be seen to exceed in magnitude the corresponding part of +the largest living kangaroo, which is represented in fig. 127. In both +these specimens part of the substance of the jaw has been broken open, so +as to show the permanent false molar (_a._ fig. 126.) concealed in the +socket. From the fact of this molar not having been cut, we learn that the +individual was young, and had not shed its first teeth. In fig. 128. a +front tooth of the same species of kangaroo is represented. + +[Illustration: Fig. 128. Incisor of _Macropus_.] + +Whether the breccias, above alluded to, of the Wellington Valley, appertain +strictly to the Pliocene period cannot be affirmed with certainty, until we +are more thoroughly acquainted with the recent quadrupeds of the same +district, and until we learn what species of fossil land shells, if any, +are buried in the deposits of the same caves. + +The reader will observe that all these extinct quadrupeds of Australia +belong to the marsupial family, or, in other words, that they are referable +to the same peculiar type of organization which now distinguishes the +Australian mammalia from those of other parts of the globe. This fact is +one of many pointing to a general law deducible from the fossil vertebrate +and invertebrate animals of the eras immediately antecedent to the human, +namely, that the present geographical distribution of organic _forms_ dates +back to a period anterior to the creation of existing _species_; in other +words, the limitation of particular genera or families of quadrupeds, +mollusca, &c., to certain existing provinces of land and sea, began before +the species now contemporary with man had been introduced into the earth. + +Mr. Owen, in his excellent "History of British Fossil Mammals," has called +attention to this law, remarking that the fossil quadrupeds of Europe and +Asia differ from those of Australia or South America. We do not find, for +example, in the Europæo-Asiatic province fossil kangaroos or armadillos, +but the elephant, rhinoceros, horse, bear, hyæna, beaver, hare, mole, and +others, which still characterize the same continent. + +In like manner in the Pampas of South America the skeletons of Megatherium, +Megalonyx, Glyptodon, Mylodon, Toxodon, Macrauchenia, and other extinct +forms, are analogous to the living sloth, armadillo, cavy, capybara, and +llama. The fossil quadrumana, also associated with some of these forms in +the Brazilian caves, belong to the Platyrrhine family of monkeys, now +peculiar to South America. That the extinct fauna of Buenos Ayres and +Brazil was very modern has been shown by its relation to deposits of marine +shells, agreeing with those now inhabiting the Atlantic; and when in +Georgia in 1845, I ascertained that the Megatherium, Mylodon, _Harlanus +americanus_ (Owen), _Equus curvidens_, and other quadrupeds allied to the +Pampean type were posterior in date to beds containing marine shells +belonging to forty-five recent species of the neighbouring sea. + +There are indeed some cosmopolite genera, such as the Mastodon (a genus of +the elephant family), and the horse, which were simultaneously represented +by different fossil species in Europe, North America, and South America; +but these few exceptions can by no means invalidate the rule which has been +thus expressed by Professor Owen, "that in the highest organized class of +animals the same forms were restricted to the same great provinces at the +Pliocene periods as they are at the present day." + +However modern, in a geological point of view, we may consider the +Pleistocene epoch, it is evident that causes more general and powerful +than the intervention of man have occasioned the disappearance of the +ancient fauna from so many extensive regions. Not a few of the species +had a wide range; the same Megatherium, for instance, extended from +Patagonia and the river Plata in South America, between latitudes 31° +and 39° south, to corresponding latitudes in North America, the same +animal being also an inhabitant of the intermediate country of Brazil, +where its fossil remains have been met with in caves. The extinct +elephant, likewise, of Georgia (_Elephas primigenius_) has been traced +in a fossil state northward from the river Alatamaha, in lat. 33° 50' N. +to the polar regions, and then again in the eastern hemisphere from +Siberia to the south of Europe. If it be objected that, notwithstanding +the adaptation of such quadrupeds to a variety of climates and +geographical conditions, their great size exposed them to extermination +by the first hunter tribes, we may observe that the investigations of +Lund and Clausen in the ossiferous limestone caves of Brazil have +demonstrated that these large mammalia were associated with a great many +smaller quadrupeds, some of them as diminutive as field mice, which have +all died out together, while the land shells formerly their +contemporaries still continue to exist in the same countries. As we may +feel assured that these minute quadrupeds could never have been +extirpated by man, so we may conclude that all the species, small and +great, have been annihilated one after the other, in the course of +indefinite ages, by those changes of circumstances in the organic and +inorganic world which are always in progress, and are capable in the +course of time of greatly modifying the physical geography, climate, and +all other conditions on which the continuance upon the earth of any +living being must depend.[158-A] + +The law of geographical relationship above alluded to, between the +living vertebrata of every great zoological province and the fossils of +the period immediately antecedent, even where the fossil species are +extinct, is by no means confined to the mammalia. New Zealand, when +first examined by Europeans, was found to contain no indigenous land +quadrupeds, no kangaroos, or opossums, like Australia; but a wingless +bird abounded there, the smallest living representative of the ostrich +family, called the Xivi, by the natives (_Apteryx_). In the fossils of +the Post-Pliocene and Pleistocene period in this same island, there is +the like absence of kangaroos, opossums, wombats, and the rest; but in +their place a prodigious number of well preserved specimens of gigantic +birds of the struthious order, called by Owen Dinornis and Palapteryx, +which are entombed in superficial deposits. These genera comprehended +many species, some of which were 4, some 7, others 9, and others 11 feet +in height! It seems doubtful whether any contemporary mammalia shared +the land with this population of gigantic feathered bipeds. + +To those who have never studied comparative anatomy it may seem scarcely +credible, that a single bone taken from any part of the skeleton may enable +a skilful osteologist to distinguish, in many cases, the genus, and +sometimes the species, of quadruped to which it belonged. Although few +geologists can aspire to such knowledge, which must be the result of long +practice and study, they will nevertheless derive great advantage from +learning what is comparatively an easy task, to distinguish the principal +divisions of the mammalia by the forms and characters of their teeth. The +annexed figures, all taken from original specimens, may be useful in +assisting the student to recognize the teeth of many genera most frequently +found fossil in Europe:-- + +[Illustration: Fig. 129. _Elephas primigenius_ (or Mammoth); molar of upper +jaw, right side; one third of nat. size. + + _a._ grinding surface. + _b._ side view.] + +[Illustration: Fig. 130. _Mastodon angustidens_ (Norwich Crag, Postwick, +also found in Red Crag, see p. 149.); second true molar, left side, upper +jaw; grinding surface, nat. size. (See p. 149.)] + +[Illustration: Fig. 131. Rhinoceros. + +_Rhinoceros leptorhinus_; fossil from freshwater beds of Grays, Essex +(see p. 147.); penultimate molar, lower jaw, left side; two-thirds +of nat. size.] + +[Illustration: Fig. 132. Hippopotamus. + +Hippopotamus; from cave near Palermo (see p. 154.); molar tooth; two-thirds +of nat. size.] + +[Illustration: Fig. 133. Pig. + +_Sus scrofa_, Lin. (common pig); from shell-marl, Forfarshire; posterior +molar, lower jaw, nat. size.] + +[Illustration: Fig. 134. Horse. + +_Equus caballus_, Lin. (common horse); from the shell marl, Forfarshire; +second molar, lower jaw. + + _a._ grinding surface, two-thirds nat. size. + _b._ side view of same, half nat. size.] + +[Illustration: Fig. 135. Tapir. + +_Tapirus Americanus_; recent; third molar, upper jaw; nat. size.] + +[Illustration: Fig. 136. _a._ _b._ Deer. + +Elk (_Cervus alces_, Lin.); recent; molar of upper jaw. + + _a._ grinding surface. + _b._ side view; two-thirds of nat. size.] + +[Illustration: Fig. 137. _c._ _d._ Ox. + +Ox, common, from shell marl, Forfarshire; true molar upper jaw; +two-thirds nat. size. + + _c._ grinding surface. + _d._ side view.] + +[Illustration: Fig. 138. Bear. + + _a._ canine tooth or tusk of bear (_Ursus spelæus_); from cave + near Liege. + _b._ molar of left side, upper jaw; one third of nat. size.] + +[Illustration: Fig. 139. Tiger. + + _c._ canine tooth of tiger (_Felis tigris_); recent. + _d._ outside view of posterior molar, lower jaw; one-third of nat. size.] + +[Illustration: Fig. 140. _Hyæna spelæa_; second molar, left side, lower +jaw; nat. size. Cave of Kirkdale. (See p. 154.)] + +[Illustration: Fig. 141. Teeth of a new species of _Arvicola_ +(field-mouse); from the Norwich Crag. (See p. 149.) + + _a._ grinding surface. + _b._ side view of same. + _c._ nat. size of a and b.] + + +FOOTNOTES: + +[147-A] Morris, Geol. Soc. Proceed., 1849. + +[147-B] Woodward's Geology of Norfolk. + +[148-A] Zool. of Beagle, part 1. pp. 9. 111. + +[149-A] Owen, Brit. Foss. Mamm. 271. _Mastodon longirostris_, +Kaup, see _ibid._ + +[152-A] I am indebted to Mr. Lonsdale for the details above given +respecting the structure of this coral. + +[155-A] Owen, Brit. Foss. Mam. xxvi., and Buckland, Rel. Dil. 19. 24. + +[155-B] See Principles of Geology. + +[158-A] See Principles of Geology, chaps. xli. to xliv. + + + + +CHAPTER 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--Sub- + apennine 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--Sewâlik Hills in India. + + +The older Pliocene strata, which next claim our attention, are chiefly +confined, in Great Britain, to the eastern part of the county of Suffolk, +where, like the Norwich beds already described, they are called "Crag," a +provincial name given particularly to those masses of shelly sand which +have been used from very ancient times in agriculture, to fertilize soils +deficient in calcareous matter. The relative position of the "red crag" in +Essex to the London clay, may be understood by reference to the +accompanying diagram (fig. 142.). + +[Illustration: Fig. 142. Cross section.] + +These deposits, judging by the shells which they contain, appear, according +to Professor Edward Forbes, to have been formed in a sea of moderate depth, +generally from 15 to 25 fathoms deep, although in some few spots perhaps +deeper. But they may, nevertheless, have been accumulated at the distance +of 40 or 50 miles from land. + +The Suffolk crag is divisible into two masses, the upper of which has been +termed the Red, and the lower the Coralline Crag.[162-A] The upper deposit +consists chiefly of quartzose sand, with an occasional intermixture of +shells, for the most part rolled, and sometimes comminuted. The lower or +Coralline crag is of very limited extent, ranging over an area about 20 +miles in length, and 3 or 4 in breadth, between the rivers Alde and Stour. +It is generally calcareous and marly--a mass of shells and small corals, +passing occasionally into a soft building stone. At Sudbourn, near Orford, +where it assumes this character, are large quarries, in which the bottom of +it has not been reached at the depth of 50 feet. At some places in the +neighbourhood, the softer mass is divided by thin flags of hard limestone, +and corals placed in the upright position in which they grew. + +The Red crag is distinguished by the deep ferruginous or ochreous colour of +its sands and fossils, the Coralline by its white colour. Both formations +are of moderate thickness; the red crag rarely exceeding 40, and the +coralline seldom amounting to 20, feet. But their importance is not to be +estimated by the density of the mass of strata or its geographical extent, +but by the extraordinary richness of its organic remains, belonging to a +very peculiar type, which seems to characterize the state of the living +creation in the north of Europe during the older Pliocene era. + +For a large collection of the fish, echinoderms, shells, and corals of the +deposits in Suffolk, we are indebted to the labours of Mr. Searles Wood. Of +testacea alone he has obtained from 230 species from the Red, and 345 from +the Coralline crag, about 150 being common to each. The proportion of +recent species in the new group is considered by Mr. Wood to be about +70[162-B] per cent., and that in the older or coralline about 60. When I +examined these shells of Suffolk in 1835, with the assistance of Dr. Beck, +Mr. George Sowerby, Mr. Searles Wood, and other eminent conchologists, I +came to the opinion that the extinct species predominated very decidedly in +number over the living. Recent investigations, however, have thrown much +new light on the conchology of the Arctic, Scandinavian, British, and +Mediterranean Seas. Many of the species formerly known only as fossils of +the Crag, and supposed to have died out, have been dredged up in a living +state from depths not previously explored. Other recent species, before +regarded as distinct from the nearest allied Crag fossils, have been +observed, when numerous individuals were procured, to be liable to much +greater variation, both in size and form, than had been suspected, and thus +have been identified. Consequently, the Crag fauna has been found to +approach much more nearly to the recent fauna of the Northern, British, and +Mediterranean Seas than had been imagined. The analogy of the whole group +of testacea to the European type is very marked, whether we refer to the +large development of certain genera in number of species or to their size, +or to the suppression or feeble representation of others. The indication +also afforded by the entire fauna of a climate not much warmer than that +now prevailing in corresponding latitudes, prepares us to believe that they +are not of higher antiquity than the Older Pliocene era.[163-A] + +[Illustration: Fig. 143. Section near Ipswich, in Suffolk. + + _a._ Red crag. + _b._ Coralline crag. + _c._ London clay.] + +The position of the red crag in Essex to the subjacent London clay and +chalk has been already pointed out (fig. 142.). Whenever the two +divisions are met with in the same district, the red crag lies +uppermost; and, in some cases, as in the section represented in fig. +143., it is observed that the older or coralline mass _b_ had suffered +denudation before the newer formation _a_ was thrown down upon it. At D +there is not only a distinct cliff, 8 or 10 feet high, of coralline +crag, running in a direction N.E. and S.W., against which the red crag +abuts with its horizontal layers; but this cliff occasionally overhangs. +The rock composing it is drilled everywhere by _Pholades_, the holes +which they perforated having been afterwards filled with sand and +covered over when the newer beds were thrown down. As the older +formation is shown by its fossils to have accumulated in a deeper sea +(15, and sometimes 25, fathoms deep or more), there must no doubt have +been an upheaval of the sea-bottom before the cliff here alluded to was +shaped out. We may also conclude that so great an amount of denudation +could scarcely take place, in such incoherent materials, without many of +the fossils of the inferior beds becoming mixed up with the overlying +crag, so that considerable difficulty must be occasionally experienced +by the palæontologist in deciding which species belong severally to each +group. The red crag being formed in a shallower sea, often resembles in +structure a shifting sand bank, its layers being inclined diagonally, +and the planes of stratification being sometimes directed in the same +quarry to the four cardinal points of the compass, as at Butley. That +in this and many other localities, such a structure is not deceptive or +due to any subsequent concretionary re-arrangement of particles, or to +mere lines of colour, is proved by each bed being made up of flat pieces +of shell which lie parallel to the planes of the smaller strata. + +Some fossils, which are very abundant in the red crag, have never been +found in the white or coralline division; as, for example, the _Fusus +contrarius_ (fig. 144.), and several species of _Buccinum_ (or _Nassa_) +and _Murex_ (see figs. 145, 146.), which two genera seem wanting in +the lower crag. + +[4 Illustrations: Fossils characteristic of the Red Crag. + +Fig. 144. _Fusus contrarius._ + +Fig. 145. _Murex alveolatus._ + +Fig. 146. _Nassa granulata._ + +Fig. 147. _Cypræa coccinelloides._ + +Fig. 144. half nat. size; the others nat. size.] + +Among the bones and teeth of fishes are those of large sharks +(_Carcharias_), and a gigantic skate of the extinct genus _Myliobates_, and +many other forms, some common to our seas, and many foreign to them. + +The distinctness of the fossils of the coralline crag arises in part from +higher antiquity, and, in some degree, from a difference in the +geographical conditions of the submarine bottom. The prolific growth of +corals, echini, and a prodigious variety of testacea, implies a region of +deeper and more tranquil water; whereas, the red crag may have formed +afterwards on the same spot, when the water was shallower. In the mean time +the climate may have become somewhat cooler, and some of the zoophytes +which flourished in the first period may have disappeared, so that the +fauna of the red crag acquired a character somewhat more nearly resembling +that of our northern seas, as is implied by the large development of +certain sections of the genera _Fusus_, _Buccinum_, _Purpura_, and +_Trochus_, proper to higher latitudes, and which are wanting or feebly +represented in the inferior crag. + +Some of the corals of the lower crag of Suffolk belong to genera unknown in +the living creation, and of a very peculiar structure; as, for example, +that represented in the annexed fig. (148.), which is one of several +species having a globular form. The great number and variety of these +zoophytes probably indicate an equable climate, free from intense cold in +winter. On the other hand, that the heat was never excessive is confirmed +by the prevalence of northern forms among the testacea, such as the +_Glycimeris_, _Cyprina_, and _Astarte_. Of the genus last mentioned (see +fig. 149.) there are about fourteen species, many of them being rich in +individuals; and there is an absence of genera peculiar to hot climates, +such as _Conus_, _Oliva_, _Mitra_, _Fasciolaria_, _Crassatella_, and +others. The cowries (_Cypræa_, fig. 147.), also, are small, and belong to a +section (_Trivia_) now inhabiting the colder regions. A large volute, +called _Voluta Lamberti_ (fig. 150.), may seem an exception; but it differs +in form from the volutes of the torrid zone, and may, like the living +_Voluta Magellanica_, have been fitted for an extra-tropical climate. + +[Illustration: Fig. 148. _Fascicularia aurantium_, Milne Edwards. Family, +_Tubuliporidæ_, of same author. + +Coral of extinct genus, from the inferior or coralline crag, Suffolk. + + _a._ exterior. + _b._ vertical section of interior. + _c._ portion of exterior magnified. + _d._ portion of interior magnified, showing that it is made up of long, + thin, straight tubes, united in conical bundles.] + +[Illustration: Fig. 149. _Astarte_ (_Crassina_, Lam.); species common to +upper and lower crag. + +_Astarte Omalii_, Lajonkaire; Syn. _A. bipartita_, Sow. Min. Con. T. +521. f. 3.; a very variable species most characteristic of the +coralline crag, Suffolk.] + +[Illustration: Fig. 150. _Voluta Lamberti_, young individ.] + +The occurrence of a species of _Lingula_ at Sutton is worthy of remark, +as these _Brachiopoda_ seem now confined to more equatorial latitudes, +and the same may be said still more decidedly of a species of _Pyrula_, +allied to _P. reticulata_. Whether, therefore, we may incline to the +belief that the mean annual temperature was higher or lower than now, +we may at least infer that the climate and geographical conditions were +by no means the same at the period of the Suffolk crag as those now +prevailing in the same region. + +Of the echinoderms of the coralline crag about eleven species are known, +but some of these are in too fragmentary a condition to admit of exact +comparison. Of six which are the most perfect, Prof. E. Forbes has been +able to identify three with recent species: one of which, of the genus +_Echinus_, is British; a second, _Echinocyamus_, British and Mediterranean; +and a third, _Echinus monilis_, a Mediterranean species, also found fossil +in the faluns of Touraine. + +One of the most interesting conclusions deduced from a careful comparison +of the shells of these British Older Pliocene strata and those now +inhabiting our seas, has been pointed out by Prof. E. Forbes. It appears +that, during the glacial period, a period intermediate, as we have seen, +between that of the crag and our own times, many shells, previously +established in the temperate zone, retreated southwards to avoid an +uncongenial climate. The Professor has given a list of fifty shells which +inhabited the British seas while the coralline and red crag were forming, +and which are wanting in the Pleistocene or glacial deposits. They must, +therefore, after their migration to the south, have made their way +northwards again. In corroboration of these views, it is stated that all +these fifty species occur fossil in the Newer Pliocene strata of Sicily, +Southern Italy, and the Grecian Archipelago, where they may have enjoyed, +during the era of floating icebergs, a climate resembling that now +prevailing in higher European latitudes.[166-A] + +In the red crag at Felixstow, in Suffolk, Professor Henslow has found the +ear-bones of no less than four species of cetacea, which, according to Mr. +Owen, are the remains of true whales of the family _Balænidæ_. Mr. Wood is +of opinion that these cetacea may be of the age of the red crag, or if not +that they may be derived from the destruction of beds of coralline crag. I +agree with him that the supposition of their having been washed out of the +London clay, in which no _Balænidæ_ have yet been met with, is improbable. + +Strata containing fossil shells, like those of the Suffolk crag, above +described, have been found at Antwerp, and on the banks of the Scheldt +below that city. In 1840 I observed a small patch of them near Valognes, in +Normandy; and there is also a deposit containing similar fossils at St. +George Bohon, and several places a few leagues to the S. of Carentan, in +Normandy; but they have never been traced farther southwards. + +_Subapennine strata._--The Apennines, it is well known, are composed +chiefly of secondary rocks, forming a chain which branches off from the +Ligurian Alps and passes down the middle of the Italian peninsula. At the +foot of these mountains, on the side both of the Adriatic and the +Mediterranean, are found a series of tertiary strata, which form, for the +most part, a line of low hills occupying the space between the older chain +and the sea. Brocchi, as we have seen (p. 105.), was the first Italian +geologist who described this newer group in detail, giving it the name of +the Subapennines; and he classed all the tertiary strata of Italy, from +Piedmont to Calabria, as parts of the same system. Certain mineral +characters, he observed, were common to the whole; for the strata consist +generally of light brown or blue marl, covered by yellow calcareous sand +and gravel. There are also, he added, some species of fossil shells which +are found in these deposits throughout the whole of Italy. + +We have now, however, satisfactory evidence that the Subapennine beds of +Brocchi belong, at least, to three periods. To the Miocene we can refer a +portion of the strata of Piedmont, those of the hill of the Superga, for +example; to the Older Pliocene, part of the strata of northern Italy, of +Tuscany, and of Rome; while the tufaceous formations of Naples, of Ischia, +and the calcareous strata of Otranto, are referable to the Newer Pliocene, +and in great part to the Post-Pliocene period. + +That there is a considerable correspondence in the mineral composition of +these different Italian groups is undeniable; but not that exact +resemblance which should lead us to assume a precise identity of age, +unless the fossil remains agreed very closely. It is now indispensable that +a new scrutiny should be made in each particular district, of the fossils +derived from the upper and lower beds--especially such localities as Asti +and Parma, where the formation attains a great thickness; and at Sienna, +where the shells of the incumbent yellow sand are generally believed to +approach much more nearly, as a whole, to the recent fauna of the +Mediterranean than those in the subjacent blue marl. + +The greyish brown or blue marl of the Subapennine formation is very +aluminous, and usually contains much calcareous matter and scales of mica. +Near Parma it attains a thickness of 2000 feet, and is charged throughout +with marine shells, some of which lived in deep, others in shallow water, +while a few belong to freshwater genera, and must have been washed in by +rivers. Among these last I have seen the common _Limnea palustris_ in the +blue marl, filled with small marine shells. The wood and leaves, which +occasionally form beds of lignite in the same deposit, may have been +carried into the sea by similar causes. The shells, in general, are soft +when first taken from the marl, but they become hard when dried. The +superficial enamel is often well preserved, and many shells retain their +pearly lustre, part of their external colour, and even the ligament which +unites the valves. No shells are more usually perfect than the microscopic +foraminifera, which abound near Sienna, where more than a thousand +full-grown individuals may be sometimes poured out of the interior of a +single univalve of moderate dimensions. + +The other member of the Subapennine group, the yellow sand and +conglomerate, constitutes, in most places, a border formation near the +junction of the tertiary and secondary rocks. In some cases, as near the +town of Sienna, we see sand and calcareous gravel resting immediately on +the Apennine limestone, without the intervention of any blue marl. +Alternations are there seen of beds containing fluviatile shells, with +others filled exclusively with marine species; and I observed oysters +attached to many limestone pebbles. This appears to have been a point +where a river, flowing from the Apennines, entered the sea when the +tertiary strata were formed. + +The sand passes in some districts into a calcareous sandstone, as at San +Vignone. Its general superposition to the marl, even in parts of Italy and +Sicily where the date of its origin is very distinct, may be explained if +we consider that it may represent the deltas of rivers and torrents, which +gained upon the bed of the sea where blue marl had previously been +deposited. The latter, being composed of the finer and more transportable +mud, would be conveyed to a distance, and first occupy the bottom, over +which sand and pebbles would afterwards be spread, in proportion as rivers +pushed their deltas farther outwards. In some large tracts of yellow sand +it is impossible to detect a single fossil, while in other places they +occur in profusion. Occasionally the shells are silicified, as at San +Vitale, near Parma, from whence I saw two individuals of recent species, +one freshwater and the other marine (_Limnea palustris_, and _Cytherea +concentrica_, Lam.), both perfectly converted into flint. + +_Rome._--The seven hills of Rome are composed partly of marine tertiary +strata, those of Monte Mario, for example, of the Older Pliocene period, +and partly of superimposed volcanic tuff, on the top of which are usually +cappings of a fluviatile and lacustrine deposit. Thus, on Mount Aventine, +the Vatican, and the Capitol, we find beds of calcareous tufa with +incrusted reeds, and recent terrestrial shells, at the height of about 200 +feet above the alluvial plain of the Tiber. The tusk of the mammoth has +been procured from this formation, but the shells appear to be all of +living species, and must have been embedded when the summit of the Capitol +was a marsh, and constituted one of the lowest hollows of the country as it +then existed. It is not without interest that we thus discover the +extremely recent date of a geological event which preceded an historical +era so remote as the building of Rome. + + +MIOCENE FORMATIONS. + +_Faluns of Touraine._--The Miocene strata, corresponding with those named +by many geologists "Middle Tertiary," will next claim our attention. Near +the towns of Dinan and Rennes, in Brittany, and again in the provinces +bordering the Loire, a tertiary formation, containing another assemblage of +fossils, is met with, to which the name of _Faluns_ has been long given by +the French agriculturists, who spread the shelly sand and marl over the +land, in the same manner as the crag was formerly much used in Suffolk. +Isolated masses of these faluns occur from near the mouth of the Loire, +near Nantes, as far as a district south of Tours. They are also found at +Pontlevoy, on the Cher, about 70 miles above the junction of that river +with the Loire, and 30 miles S.E. of Tours. I have visited all the +localities above mentioned, and found the beds to consist principally of +sand and marl, in which are shells and corals, some entire, some rolled, +and others in minute fragments. In certain districts, as at Doué, in the +department of Maine and Loire, 10 miles S.W. of Saumur, they form a soft +building-stone, chiefly composed of an aggregate of broken shells, corals, +and echinoderms, united by a calcareous cement; the whole mass being very +like the coralline crag near Aldborough and Sudbourn in Suffolk. The +scattered patches of faluns are of slight thickness, rarely exceeding 50 +feet; and between the district called Sologne and the sea they repose on a +great variety of older rocks; being seen to rest successively upon gneiss, +clay-slate, and various secondary formations, including the chalk; and, +lastly, upon the upper freshwater limestone of the Parisian tertiary +series, which, as before mentioned (p. 106.), stretches continuously from +the basin of the Seine to that of the Loire. + +At some points, as at Louans, south of Tours, the shells are stained of a +ferruginous colour, not unlike that of the red crag of Suffolk. The species +are, for the most part, marine, but a few of them belong to land and +fluviatile genera. Among the former, _Helix turonensis_ (fig. 45. p. 30.) +is the most abundant. Remains of terrestrial quadrupeds are here and there +intermixed, belonging to the genera Deinotherium, Mastodon, Rhinoceros, +Hippopotamus, Chæropotamus, Dichobune, Deer, and others, and these are +accompanied by cetacea, such as the Lamantine, Morse, Sea-calf, and +Dolphin, all of extinct species. + +Professor E. Forbes, after studying the fossil testacea which I obtained +from these beds; informs me that he has no doubt they were formed partly on +the shore itself at the level of low water, and partly at very moderate +depths, not exceeding 10 fathoms below that level. The molluscous fauna of +the "faluns" is on the whole much more littoral than that of the red and +coralline crag of Suffolk, and implies a shallower sea. It is, moreover, +contrasted with the Suffolk crag by the indications it affords of an +extra-European climate. Thus it contains seven species of _Cypræa_, some +larger than any existing cowry of the Mediterranean, several species of +_Oliva_, _Ancillaria_, _Mitra_, _Terebra_, _Pyrula_, _Fasciolaria_, and +_Conus_. Of the cones there are no less than eight species, some very +large, whereas the only European cone is of diminutive size. The genus +_Nerita_, and many others, are also represented by individuals of a type +now characteristic of equatorial seas, and wholly unlike any Mediterranean +forms. These proofs of a more elevated temperature seem to imply the higher +antiquity of the faluns as compared with the Suffolk crag, and are in +perfect accordance with the fact of the smaller proportion of testacea of +recent species found in the faluns. + +Out of 290 species of shells, collected by myself, in 1840, at +Pontlevoy, Louans, Bossée, and other villages 20 miles south of Tours; +and at Savigné, about 15 miles north-west of that place; 72 only could +be identified with recent species, which is in the proportion of 25 per +cent. A large number of the 290 species are common to all the +localities, those peculiar to each not being more numerous than we might +expect to find in different bays of the same sea. + +The total number of mollusca from the faluns, in my possession, is 302, +of which 45 only were found by Mr. Wood to be common to the Suffolk +crag. The number of corals obtained by me at Doué, and other localities +before adverted to, amounts to 43, as determined by Mr. Lonsdale, of +which 7 agree specifically with those of the Suffolk crag. Only one has, +as yet, been identified with a living species. But it is difficult, if +not impossible, to institute at present a satisfactory comparison +between fossil and recent _Polyparia_, from the deficiency of our +knowledge of the living species. Some of the genera occurring fossil in +Touraine, as the _Astrea_, _Lunulites_, and _Dendrophyllia_, have not +been found in European seas north of the Mediterranean; nevertheless the +_Polyparia_ of the faluns do not seem to indicate on the whole so warm a +climate as would be inferred from the shells. + +It was stated that, on comparing about 300 species of Touraine shells +with about 450 from the Suffolk crag, 45 only were found to be common to +both, which is in the proportion of only 15 per cent. The same small +amount of agreement is found in the corals also. I formerly endeavoured +to reconcile this marked difference in species with the supposed +co-existence of the two faunas, by imagining them to have severally +belonged to distinct zoological provinces or two seas, the one opening +to the north, and the other to the south, with a barrier of land between +them, like the Isthmus of Suez, separating the Red Sea and the +Mediterranean. But I now abandon that idea for several reasons; +among others, because I succeeded in 1841 in tracing the Crag fauna +southwards in Normandy to within 70 miles of the Falunian type, near +Dinan, yet found that both assemblages of fossils retained their +distinctive characters, showing no signs of any blending of species +or transition of climate. + +On a comparison of 280 Mediterranean shells with 600 British species, made +for me by an experienced conchologist in 1841, 160 were found to be common +to both collections, which is in the proportion of 57 per cent., a fourfold +greater specific resemblance than between the seas of the crag and the +faluns, notwithstanding the greater geographical distance between England +and the Mediterranean than between Suffolk and the Loire. The principal +grounds, however, for referring the English crag to the older Pliocene and +the French faluns to the Miocene epochs, consist in the predominance of +fossil shells in the British strata identifiable with species, not only +still living, but which are now inhabitants of neighbouring seas, while the +accompanying extinct species are of genera such as characterize Europe. In +the faluns, on the contrary, the recent species are in a decided minority, +and many of them, like the associated extinct testacea, are much less +European in character, and point to the prevalence of a warmer climate,--in +other words, to a state of things receding farther from the present +condition of Europe, geographically and climatologically, and doubtless, +therefore, receding farther in time. + +_Bordeaux._--A great extent of country between the Pyrenees and the Gironde +is overspread by tertiary deposits, which have been more particularly +studied in the environs of Bordeaux and Dax, from whence about 700 species +of shells have been obtained. A large proportion of these shells belong to +the same zoological type as those of Touraine; but many are peculiar, and +the whole may possibly constitute a somewhat older division of the Miocene +period than the faluns of the Loire. We must wait, however, for farther +investigations, in order to decide this question with accuracy. + +_Piedmont._--Many of the shells peculiar to the hill of the Superga, near +Turin, agree with those found at Bordeaux and Dax; but the proportion of +recent species is much less. The strata of the Superga consist of a bright +green sand and marl, and a conglomerate with pebbles, chiefly of green +serpentine, and are inclined at an angle of more than 70°. This formation, +which attains a great thickness in the valley of the Bormida, is probably +one of the oldest Miocene groups hitherto discovered. + +_Molasse of Switzerland._--If we cross the Alps, and pass from Piedmont +to Savoy, we find there, at the northern base of the great chain, and +throughout the lower country of Switzerland, a soft green sandstone much +resembling some of the beds of the basin of the Bormida, above +described, and associated in a similar manner with marls and +conglomerate. This formation is called in Switzerland "molasse," said to +be derived from "mol," "_soft_" because the stone is easily cut in the +quarry. It is of vast thickness, and probably divisible into several +formations. How large a portion of these belong to the Miocene period +cannot yet be determined, as fossil shells are often entirely wanting. +In some places a decided agreement of the fossil fishes of the molasse +and faluns has been observed. Among those common to both, M. Agassiz +pointed out to me _Lamna contortidens_, _Myliobates Studeri_, _Spherodus +cinctus_, _Notidanus primigenius_, and others. + +_Lisbon._--Marine tertiary strata near Lisbon contain shells which agree +very closely with those of Bordeaux, and are therefore referred to the +Miocene era. Thus, out of 112 species collected by Mr. Smith of Jordanhill, +between 60 and 70 were found to be common to the strata of Bordeaux and +Dax, the recent species being in the proportion of 21 per cent. + +_Older Pliocene and Miocene formations in the United States._--Between the +Alleghany mountains, formed of older rocks, and the Atlantic, there +intervenes, in the United States, a low region occupied principally by beds +of marl, clay, and sand, consisting of the cretaceous and tertiary +formations, and chiefly of the latter. The general elevation of this plain +bordering the Atlantic does not exceed 100 feet, although it is sometimes +several hundred feet high. Its width in the middle and southern states is +very commonly from 100 to 150 miles. It consists, in the South, as in +Georgia, Alabama, and South Carolina, almost exclusively of Eocene +deposits; but in North Carolina, Maryland, Virginia, and Delaware, more +modern strata predominate, which I have assimilated in age to the English +crag and Faluns of Touraine.[172-A] If, chronologically speaking, they can +be truly said to be the representatives of these two European formations, +they may range in age from the Older Pliocene to the Miocene epoch, +according to the classification of European strata adopted in this chapter. + +The proportion of fossil shells agreeing with recent, out of 147 species +collected by me, amounted to about 17 per cent., or one-sixth of the +whole; but as the fossils so assimilated were almost always the same as +species now living in the neighbouring Atlantic, the number may +hereafter be augmented, when the recent fauna of that ocean is better +known. In different localities, also, the proportion of recent +species varied considerably. + +[Illustration: Fig. 151. _Fulgur canaliculatus._ Maryland.] + +[Illustration: Fig. 152. _Fusus quadricostatus_, Say. Maryland.] + +On the banks of the James River, in Virginia, about 20 miles below +Richmond, in a cliff about 30 feet high, I observed yellow and white sands +overlying an Eocene marl, just as the yellow sands of the crag lie on the +blue London clay in Suffolk and Essex in England. In the Virginian sands, +we find a profusion of an Astarte (_A. undulata_, Conrad), which resembles +closely, and may possibly be a variety of, one of the commonest fossils of +the Suffolk crag (_A. bipartita_); the other shells also, of the genera +_Natica_, _Fissurella_, _Artemis_, _Lucina_, _Chama_, _Pectunculus_, and +_Pecten_, are analogous to shells both of the English crag and French +faluns, although the species are almost all distinct. Out of 147 of these +American fossils I could only find 13 species common to Europe, and these +occur partly in the Suffolk crag, and partly in the faluns of Touraine; but +it is an important characteristic of the American group, that it not only +contains many peculiar extinct forms, such as _Fusus quadricostatus_, Say +(see fig. 152.), and _Venus tridacnoides_, abundant in these same +formations, but also some shells which, like _Fulgur carica_ of Say, and +_F. canaliculatus_ (see fig. 151.), _Calyptræa costata_, _Venus +mercenaria_, Lam., _Modiola glandula_, Totten, and _Pecten magellanicus_, +Lam., are recent species, yet of forms now confined to the western side of +the Atlantic, a fact implying that the beginning of the present +geographical distribution of mollusca dates back to a period as remote as +that of the Miocene strata. + +Of ten species of zoophytes which I procured on the banks of the James +River, two were identical with species of the Faluns of Touraine. With +respect to climate, Mr. Lonsdale regards these corals as indicating a +temperature exceeding that of the Mediterranean, and the shells would +lead to similar conclusions. Those occurring on the James River are in +the 37th degree of N. latitude, while the French faluns are in the 47th; +yet the forms of the American fossils would scarcely imply so warm a +climate as must have prevailed in France, when the Miocene strata +of Touraine originated. + +Among the remains of fish in these Post-Eocene strata of the United +States are several large teeth of the shark family, not distinguishable +specifically from fossils of the faluns of Touraine, and the +Maltese tertiaries. + +_India._--The freshwater deposits of the Sub-Himalayan or Sewâlik Hills, +described by Dr. Falconer and Captain Cautley, may perhaps be regarded as +Miocene. Like the faluns of Touraine, they contain the Deinotherium and +Mastodon. Whether any of the associated freshwater and land shells are of +recent species is not yet determined. The occurrence in them of a fossil +giraffe and hippopotamus, genera now only living in Africa, as well as of a +camel, implies a geographical state of things very different from that now +established in the same parts of India. The huge Sivatherium of the same +era appears to have been a ruminating quadruped bigger than the rhinoceros, +and provided with a large upper lip, or probably a short proboscis, and +having two pair of horns, resembling those of antelopes. Several species of +monkey belonged to the same fauna; and among the reptiles, several +crocodiles, larger than any now living, and an enormous tortoise, _Testudo +Atlas_, the curved shell of which measured 20 feet across. + + +FOOTNOTES: + +[162-A] See paper by E. Charlesworth, Esq.; London and Ed. Phil. Mag. No. +xxxviii. p. 81., Aug. 1835. + +[162-B] See Monograph on the Crag Mollusca. Searles Wood, Paleont. +Soc. 1848. + +[163-A] In regarding the Suffolk crag, both red and coralline, as +older Pliocene instead of Miocene, I am only returning to the +classification adopted by me in the Principles and Elements of +Geology up to the year 1838. + +[166-A] E. Forbes, Mem. Geol. Survey, Gt. Brit., vol. i. 386. + +[172-A] Proceedings of the Geol. Soc. vol. iv. part 3. 1845, p. 547. + + + + +CHAPTER 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. + + +[Illustration: Fig. 153. Map of the principal tertiary basins of +the Eocene period. + +N. B. The space left blank is occupied by secondary formations from the +Devonian or old red sandstone to the chalk inclusive.] + +The tertiary strata described in the preceding chapters are all of them +characterized by fossil shells, of which a considerable proportion are +specifically identical with the living mollusca; and the greater the +number, the more nearly does the entire fauna approach in species and +genera to that now inhabiting the adjoining seas. But in the Eocene +formations next to be considered, the proportion of recent species is very +small, and sometimes scarcely appreciable, and those agreeing with the +fossil testacea often belong to remote parts of the globe, and to various +zoological provinces. This difference in conchological character implies a +considerable interval of time between the Eocene and Miocene periods, +during which the whole fauna and flora underwent other changes as great, +and often greater, than those exhibited by the mollusca. In the +accompanying map, the position of several Eocene areas is pointed out, such +as the basin of the Thames, part of Hampshire, part of the Netherlands, +and the country round Paris. The deposits, however, occupying these spaces +comprise a great succession of marine and freshwater formations, which, +although they may all be termed Eocene, as being newer than the chalk, and +older than the faluns, are nevertheless divisible into separate groups, of +high geological importance. + +The newest of these, like the Faluns of the Loire, have no true +representatives, or exact chronological equivalents, in the British Isles. +Their place in the series will best be understood by referring to the order +of superposition of the successive deposits found in the neighbourhood of +Paris. The area which has been called the Paris basin is about 180 miles in +its greatest length from north-east to south-west, and about 90 miles from +east to west. This space may be described as a depression in the chalk, +which has been filled up by alternating groups of marine and freshwater +strata. MM. Cuvier and Brongniart attempted, in 1810, to distinguish five +different formations, comprising three freshwater and two marine, which +alternated with each other. It was imagined that the waters of the ocean +had been by turns admitted and excluded from the same region; but the +subsequent investigations of several geologists, especially of M. Constant +Prevost,[175-A] have led to great modifications in these theoretical views; +and now that the true order of succession is better understood, it appears +that several of the deposits, which were supposed to have originated one +after the other, were, in fact, in progress at the same time by the joint +action of the sea and rivers. + +The whole series of strata may be divided into three groups, as expressed +in the following table:-- + + { _a._ Upper freshwater limestone, marls, and siliceous + 1. Upper Eocene { millstone. + { _b._ Upper marine sands, or Fontainebleau sandstone + { and sand. + + { _a._ Lower freshwater limestone and marl, or + { gypseous series. + { _b._ Sandstone and sands with marine shells (_Sables_ + 2. Middle Eocene { _moyens_, or _grès de Beauchamp_). + { _c._ Calcaire grossier, limestone with marine shells. + { _d._ Calcaire siliceux, hard siliceous freshwater + { limestone, for the most part contemporaneous + { with _c_. + + { _a._ Lower sands with marine shelly beds (_Sables_ + 3. Lower Eocene { _inférieurs et lits coquilliers_). + { _b._ Lower sands, with lignite and plastic clay + { (_Sables inférieurs et argiles plastiques_). + +Postponing to the next chapter the consideration of the Middle and Lower +Eocene groups, I shall now speak of the Upper Eocene of Paris, and +its foreign equivalents. + +The upper freshwater marls and limestone (1. _a_) seem to have been formed +in a great number of marshes and shallow lakes, such as frequently +overspread the newest parts of great deltas. It appears that many layers of +marl, tufaceous limestone, and travertin, with beds of flint, continuous +or in nodules, accumulated in these lakes. _Charæ_, aquatic plants, already +alluded to (see p. 32.) left their stems and seed-vessels imbedded both in +the marl and flint, together with freshwater and land shells. Some of the +siliceous rocks of this formation are used extensively for millstones. The +flat summits or platforms of the hills round Paris, large areas in the +forest of Fontainebleau, and the Plateau de la Beauce, between the Seine +and the Loire, are chiefly composed of these upper freshwater strata. + +The upper marine sands (1. _b_), consist chiefly of micaceous and quartzose +sands, 80 feet thick. As they succeed throughout an extensive area deposit +of a purely freshwater origin (2 _a_.), they appear to mark a subsidence of +the subjacent soil, whether it had formed the bottom of an estuary or a +lake. The sea, which afterwards took possession of the same space, was +inhabited by testacea, almost all of them differing from those found in the +lower formations (2. _b_ and 2. _c_) and equally or still more distinct +from the Miocene Faluns of subsequent date. One of these upper Eocene +strata in the neighbourhood of Paris has been called the oyster bed, +"couche à _Ostrea cyathula_, Lamk.," which is spread over a remarkably wide +area. From the manner in which the oysters lie, it is inferred that they +did not grow on the spot, but that some current swept them away from a bed +of oysters formed in some other part of the bay. The strata of sand which +immediately repose on the oyster-bed are quite destitute of organic +remains; and nothing is more common in the Paris basin, and in other +formations, than alternations of shelly beds with others entirely devoid of +them. The temporary extinction and renewal of animal life at successive +periods have been rashly inferred from such phenomena, which may +nevertheless be explained, as M. Prevost justly remarks, without appealing +to any such extraordinary revolutions in the state of the animate creation. +A current one day scoops out a channel in a bed of shelly sand and mud, and +the next day, by a slight alteration of its course, ceases to prey upon the +same bank. It may then become charged with sand unmixed with shells, +derived from some dune, or brought down by a river. In the course of ages +an indefinite number of transitions from shelly strata to those without +shells may thus be caused. + +Besides these oysters, M. Deshayes has described 29 species of shells, +in his work (Coquilles fossiles de Paris), as belonging to this +formation, all save one regarded by him as differing from fossils of the +calcaire grossier. Since that time the railway cuttings near Etampes +have enabled M. Hébert to raise the number to 90. I have myself +collected fossils in that district, where the shells are very entire, +and detachable from the yellow sandy matrix. M. Hébert first pointed out +that most of them agree specifically with those of Kleyn Spauwen, Boom, +and other localities of Limburg in Flanders, where they have been +studied by MM. Nyst and De Koninck.[176-A] + +The position in Belgium of this formation above the older Eocene group is +well seen in the small hill of Pellenberg, rising abruptly from the great +plain, half a mile south-east of the city of Louvain, where I examined it +in company with M. Nyst in 1850. At the top of the hill, a thin bed of dark +greyish green tile-clay is seen 1-1/2 foot thick, with casts of _Nucula +Deshaysiana_. This clay rests on 12 feet of yellow sand, separated, by a +band of flint and quartz pebbles, from a mass of subjacent white sand 15 +feet thick, in which casts of the Kleyn Spauwen fossils have been met with. +Under this is a bed of yellow sand 12 feet thick, and, at a lower level, +the railway cuttings have passed through calcareous sands like those of +Brussels, in which the _Nautilus Burtini_, and various shells common to the +older Eocene strata of the neighbourhood of London, have been obtained. +Every new fact which throws light on the true paleontological relations of +the strata now under consideration, (the Upper Marine or Fontainebleau beds +of the Paris basin, 1. _b_, p. 175.), deserves more particular attention, +because geologists of high authority differ in opinion as to whether they +should be classed as Eocene or Miocene. + +Professor Beyrich has lately described a formation of the same age, +occurring within 7 miles of the gates of Berlin, near the village of +Hermsdorf, where, in the midst of the sands of which that country +chiefly consists, a mass of tile-clay, more than 40 feet thick, and of a +dark blueish grey colour, is found, full of shells, among which the +genera _Fusus_ and _Pleurotoma_ predominate, and among the bivalves, +_Nucula Deshaysiana_, Nyst, an extremely common shell in the Belgian +beds above-mentioned. M. Beyrich has identified eighteen out of +forty-five species of the Hermsdorf fossils with the Belgian species; +and I believe that a much larger proportion agree with the Upper Eocene +of Belgium, France, and the Rhine. On the other hand, eight of the +forty-five species are supposed by him to agree with English Eocene +shells. Messrs. Morris, Edwards, and S. Wood have compared a small +collection, which I obtained of these Berlin shells, with the Eocene +fossils of their museums, and confirmed the result of M. Beyrich, the +species common to the English fossils belonging not simply to the +uppermost of our marine beds, or those of Barton, but some of them to +lower parts of the series, such as Bracklesham and Highgate. On the +other hand, while these testacea, like those of Kleyn Spauwen and +Etampes, present many analogies to the Middle and Lower Eocene group, +they differ widely from the Falun shells,--a fact the more important in +reference to Etampes, as that locality approaches within 70 miles of +Pontlevoy, near Blois, and within 100 miles of Savigné, near Tours, +where Falun shells are found. It is evident that the discordance of +species cannot be attributed to distance or geographical causes, but +must be referred to time, or the different epoch at which the upper +marine beds of the Paris basin and the Faluns of the Loire originated. + +_Mayence._--The true chronological relation of many tertiary strata on +the banks of the Rhine has always presented a problem of considerable +difficulty. They occupy a tract from 5 to 12 miles in breadth, extending +along the left bank of the Rhine from Mayence to the neighbourhood of +Manheim, and are again found to the east, north, and south-west of +Frankfort. In some places they have the appearance of a freshwater +formation; but in others, as at Alzey, the shells are for the most part +marine. _Cerithia_ are in great profusion, which indicates that the sea +where the deposit was formed was fed by rivers; and the great quantity +of fossil land shells, chiefly of the genus _Helix_, confirm the same +opinion. The variety in the species of shells is small, while the +individuals are exceedingly numerous; a fact which accords perfectly +with the idea that the formation may have originated in a gulf or sea +which, like the Baltic, was brackish in some parts, and almost fresh in +others. A species of _Paludina_ (fig. 154.), very nearly resembling the +recent _Littorina ulva_, is found throughout this basin. These shells +are like grains of rice in size, and are often in such quantity as to +form entire beds of marl and limestone. They are as thick as grains of +sand, in stratified masses from 15 to 30 feet in thickness. + +[Illustration: Fig. 154. _Paludina._ Mayence.] + +That these Rhenish tertiary formations agree more nearly with the Upper +Eocene deposits above enumerated, than with any others, I have no doubt, +since I had the advantage of comparing (August, 1850), with the +assistance of M. De Koninck of Liége, the fossils from Kleyn Spauwen, +Boom, and other Limburg localities, with those from Mayence, Alzey, +Weinheim, and other Rhenish strata. Among the common Belgian and Rhenish +shells which are identical, I may mention _Cassidaria depressa_, +_Tritonium flandricum_ De Koninck, _Cerithium tricinctum_ Nyst, +_Tornatella simulata_, _Rostellaria Sowerbyi_, _Nucula Deshaysiana_, +_Corbula pisum_, and _Pectunculus terebratularis_. + +From these Upper Eocene deposits of the Rhine M. H. von Meyer has +obtained a great number of characteristic fossil mammalia, such as +_Palæomæryx medius_, _Hyotherium Meissneri_, _Tapirus Helveticus_, +_Anthracotherium Alsaticum_, and others. The three first of these are +species common to some of the lignite, or brown coal beds in +Switzerland, commonly classed with the molasse, but of which the true +age has not yet been distinctly made out. + +The fossils of the sandy beds of Eppelsheim, comprising bones of the +Deinotherium, Mastodon, and other quadrupeds, are regarded by H. von Meyer +as belonging to a mammiferous fauna quite distinct from that of the Mayence +basin, and they are probably referable to the Miocene period. + +The upper freshwater strata (1. _a_, p. 175.), of the neighbourhood of +Paris, stretch southwards from the valley of the Seine to that of the +Loire, and in the last-mentioned region are seen to be older than the +marine faluns, so that the perforating shells of the Miocene sea have +sometimes bored the hard compact freshwater limestones; and fragments of +the Upper Eocene rocks are found at Pontlevoy and elsewhere, which have +been rolled in the bed of the Miocene sea. + +[Illustration: Fig. 155. Simplified geological map south of Paris.] + +_Central France._--Lacustrine strata belonging, for the most part, to +the same Upper Eocene series, are again met with in Auvergne, Cantal, +and Velay, the sites of which may be seen in the annexed map. They +appear to be the monuments of ancient lakes, which, like some of those +now existing in Switzerland, once occupied the depressions in a +mountainous region, and have been each fed by one or more rivers and +torrents. The country where they occur is almost entirely composed of +granite and different varieties of granitic schist, with here and there +a few patches of secondary strata, much dislocated, and which have +probably suffered great denudation. There are also some vast piles +of volcanic matter (see the map), the greater part of which is newer +than the freshwater strata, and is sometimes seen to rest upon them, +while a small part has evidently been of contemporaneous origin. +Of these igneous rocks I shall treat more particularly in another +part of this work. + +Before entering upon any details, I may observe, that the study of these +regions possesses a peculiar interest, very distinct in kind from that +derivable from the investigation either of the Parisian or English tertiary +strata. For we are presented in Auvergne with the evidence of a series of +events of astonishing magnitude and grandeur, by which the original form +and features of the country have been greatly changed, yet never so far +obliterated but that they may still, in part at least, be restored in +imagination. Great lakes have disappeared,--lofty mountains have been +formed, by the reiterated emission of lava, preceded and followed by +showers of sand and scoriæ,--deep valleys have been subsequently furrowed +out through masses of lacustrine and volcanic origin,--at a still later +date, new cones have been thrown up in these valleys,--new lakes have been +formed by the damming up of rivers,--and more than one creation of +quadrupeds, birds, and plants, Eocene, Miocene, and Pliocene, have followed +in succession; yet the region has preserved from first to last its +geographical identity; and we can still recall to our thoughts its external +condition and physical structure before these wonderful vicissitudes began, +or while a part only of the whole had been completed. There was first a +period when the spacious lakes, of which we still may trace the boundaries, +lay at the foot of mountains of moderate elevation, unbroken by the bold +peaks and precipices of Mont Dor, and unadorned by the picturesque outline +of the Puy de Dome, or of the volcanic cones and craters now covering the +granitic platform. During this earlier scene of repose deltas were slowly +formed; beds of marl and sand, several hundred feet thick, deposited; +siliceous and calcareous rocks precipitated from the waters of mineral +springs; shells and insects imbedded, together with the remains of the +crocodile and tortoise, the eggs and bones of water birds, and the +skeletons of quadrupeds, some of them belonging to the same genera as those +entombed in the Eocene gypsum of Paris. To this tranquil condition of the +surface succeeded the era of volcanic eruptions, when the lakes were +drained, and when the fertility of the mountainous district was probably +enhanced by the igneous matter ejected from below, and poured down upon the +more sterile granite. During these eruptions, which appear to have taken +place after the disappearance of the Eocene fauna, and in the Miocene +epoch, the mastodon, rhinoceros, elephant, tapir, hippopotamus, together +with the ox, various kinds of deer, the bear, hyæna, and many beasts of +prey, ranged the forest, or pastured on the plain, and were occasionally +overtaken by a fall of burning cinders, or buried in flows of mud, such as +accompany volcanic eruptions. Lastly, these quadrupeds became extinct, and +gave place to Pliocene mammalia, and these, in their turn, to species now +existing. There are no signs, during the whole time required for this +series of events, of the sea having intervened, nor of any denudation which +may not have been accomplished by currents in the different lakes, or by +rivers and floods accompanying repeated earthquakes, during which the +levels of the district have in some places been materially modified, and +perhaps the whole upraised relatively to the surrounding parts of France. + +_Auvergne._--The most northern of the freshwater groups is situated in the +valley-plain of the Allier, which lies within the department of the Puy de +Dome, being the tract which went formerly by the name of the Limagne +d'Auvergne. It is inclosed by two parallel mountain ranges,--that of the +Forèz, which divides the waters of the Loire and Allier, on the east; and +that of the Monts Domes, which separates the Allier from the Sioule, on the +west.[181-A] The average breadth of this tract is about 20 miles; and it is +for the most part composed of nearly horizontal strata of sand, sandstone, +calcareous marl, clay, and limestone, none of which observe a fixed and +invariable order of superposition. The ancient borders of the lake, wherein +the freshwater strata were accumulated, may generally be traced with +precision, the granite and other ancient rocks rising up boldly from the +level country. The actual junction, however, of the lacustrine and granitic +beds is rarely seen, as a small valley usually intervenes between them. The +freshwater strata may sometimes be seen to retain their horizontality +within a very slight distance of the border-rocks, while in some places +they are inclined, and in few instances vertical. The principal divisions +into which the lacustrine series may be separated are the following:--1st, +Sandstone, grit, and conglomerate, including red marl and red sandstone. +2dly, Green and white foliated marls. 3dly, Limestone or travertin, often +oolitic. 4thly, Gypseous marls. + +1. _a_. _Sandstone and conglomerate._--Strata of sand and gravel, sometimes +bound together into a solid rock, are found in great abundance around the +confines of the lacustrine basin, containing, in different places, pebbles +of all the ancient rocks of the adjoining elevated country; namely, +granite, gneiss, mica-schist, clay-slate, porphyry, and others. But these +strata do not form one continuous band around the margin of the basin, +being rather disposed like the independent deltas which grow at the mouths +of torrents along the borders of existing lakes. + +At Chamalieres, near Clermont, we have an example of one of these deltas, +or littoral deposits, of local extent, where the pebbly beds slope away +from the granite, as if they had formed a talus beneath the waters of the +lake near the steep shore. A section of about 50 feet in vertical height +has been laid open by a torrent, and the pebbles are seen to consist +throughout of rounded and angular fragments of granite, quartz, primary +slate, and red sandstone; but without any intermixture of those volcanic +rocks which now abound in the neighbourhood, and which could not have been +there when the conglomerate was formed. Partial layers of lignite and +pieces of wood are found in these beds. + +At some localities on the margin of the basin quartzose grits are found; +and, where these rest on granite, they are sometimes formed of separate +crystals of quartz, mica, and felspar, derived from the disintegrated +granite, the crystals having been subsequently bound together by a +siliceous cement. In these cases the granite seems regenerated in a new and +more solid form; and so gradual a passage takes place between the rock of +crystalline and that of mechanical origin, that we can scarcely distinguish +where one ends and the other begins. + +In the hills called the Puy de Jussat and La Roche, we have the advantage +of seeing a section continuously exposed for about 700 feet in thickness. +At the bottom are foliated marls, white and green, about 400 feet thick; +and above, resting on the marls, are the quartzose grits, cemented by +calcareous matter, which is sometimes so abundant as to form imbedded +nodules. These sometimes constitute spheroidal concretions 6 feet in +diameter, and pass into beds of solid limestone, resembling the Italian +travertins, or the deposits of mineral springs. This section is close to +the confines of the basin; so that the lake must here have been filled up +near the shore with fine mud, before the coarse superincumbent sand was +introduced. There are other cases where sand is seen below the marl. + +1. _b._ _Red marl and sandstone_.--But the most remarkable of the +arenaceous groups is one of red sandstone and red marl, which are identical +in all their mineral characters with the secondary _New Red sandstone_ and +marl of England. In these secondary rocks the red ground is sometimes +variegated with light greenish spots, and the same may be seen in the +tertiary formation of freshwater origin at Coudes, on the Allier. The marls +are sometimes of a purplish-red colour, as at Champheix, and are +accompanied by a reddish limestone, like the well-known "cornstone," which +is associated with the Old Red sandstone of English geologists. The red +sandstone and marl of Auvergne have evidently been derived from the +degradation of gneiss and mica-schist, which are seen _in situ_ on the +adjoining hills, decomposing into a soil very similar to the tertiary red +sand and marl. We also find pebbles of gneiss, mica-schist, and quartz in +the coarser sandstones of this group, clearly pointing to the parent rocks +from which the sand and marl are derived. The red beds, although destitute +themselves of organic remains, pass upwards into strata containing Eocene +fossils, and are certainly an integral part of the lacustrine formation. +From this example the student will learn how small is the value of mineral +character alone, as a test of the relative age of rocks. + +2. _Green and white foliated marls._--The same primary rocks of Auvergne, +which, by the partial degradation of their harder parts, gave rise to the +quartzose grits and conglomerates before mentioned, would, by the reduction +of the same materials into powder, and by the decomposition of their +felspar, mica, and hornblende, produce aluminous clay, and, if a sufficient +quantity of carbonate of lime was present, calcareous marl. This fine +sediment would naturally be carried out to a greater distance from the +shore, as are the various finer marls now deposited in Lake Superior. And, +as in the American lake, shingle and sand are annually amassed near the +northern shores, so in Auvergne the grits and conglomerates before +mentioned were evidently formed near the borders. + +[Illustration: Fig. 156. _Cypris unifasciata_, a living species, +greatly magnified. + + _a._ Upper part. + _b._ Side view of the same.] + +[Illustration: Fig. 157. _Cypris vidua_, a living species, +greatly magnified.[183-A]] + +The entire thickness of these marls is unknown; but it certainly exceeds, +in some places, 700 feet. They are, for the most part, either light-green +or white, and usually calcareous. They are thinly foliated,--a character +which frequently arises from the innumerable thin shells, or +carapace-valves, of that small animal called _Cypris_; a genus which +comprises several species, of which some are recent, and may be seen +swimming swiftly through the waters of our stagnant pools and ditches. The +antennæ, at the end of which are fine pencils of hair, are the principal +organs of motion, and are seen to vibrate with great rapidity. This animal +resides within two small valves, not unlike those of a bivalve shell, and +moults its integuments periodically, which the conchiferous mollusks do +not. This circumstance may partly explain the countless myriads of the +shells of _Cypris_ which were shed in the ancient lakes of Auvergne, so as +to give rise to divisions in the marl as thin as paper, and that, too, in +stratified masses several hundred feet thick. A more convincing proof of +the tranquillity and clearness of the waters, and of the slow and gradual +process by which the lake was filled up with fine mud, cannot be desired. +But we may easily suppose that, while this fine sediment was thrown down in +the deep and central parts of the basin, gravel, sand, and rocky fragments +were hurried into the lake, and deposited near the shore, forming the group +described in the preceding section. + +Not far from Clermont, the green marls, containing the _Cypris_ in +abundance, approach to within a few yards of the granite which forms the +borders of the basin. The occurrence of these marls so near the ancient +margin may be explained by considering that, at the bottom of the +ancient lake, no coarse ingredients were deposited in spaces +intermediate between the points where rivers and torrents entered, but +finer mud only was drifted there by currents. The _verticality_ of some +of the beds in the above section bears testimony to considerable local +disturbance subsequent to the deposition of the marls; but such inclined +and vertical strata are very rare. + +[Illustration: Fig. 158. Vertical strata of marl, at Champradelle, +near Clermont. + + A. Granite. + B. Space of 60 feet, in which no section is seen. + C. Green marl, vertical and inclined. + D. White marl.] + +3. _Limestone, travertin, oolite._--Both the preceding members of the +lacustrine deposit, the marls and grits, pass occasionally into +limestone. Sometimes only concretionary nodules abound in them; but +these, where there is an increase in the quantity of calcareous matter, +unite into regular beds. + +On each side of the basin of the Limagne, both on the west at Gannat, and +on the east at Vichy, a white oolitic limestone is quarried. At Vichy, the +oolite resembles our Bath stone in appearance and beauty; and, like it, is +soft when first taken from the quarry, but soon hardens on exposure to the +air. At Gannat, the stone contains land-shells and bones of quadrupeds, +resembling those of the Paris gypsum. At Chadrat, in the hill of La Serre, +the limestone is pisolitic, the small spheroids combining both the radiated +and concentric structure. + +_Indusial limestone._--There is another remarkable form of freshwater +limestone in Auvergne, called "indusial," from the cases, or _indusiæ_, +of caddis-worms (the larvæ of _Phryganea_); great heaps of which have +been incrusted, as they lay, by carbonate of lime, and formed into a +hard travertin. The rock is sometimes purely calcareous, but there is +occasionally an intermixture of siliceous matter. Several beds of it are +frequently seen, either in continuous masses, or in concretionary +nodules, one upon another, with layers of marl interposed. The annexed +drawing (fig. 159.) will show the manner in which one of these indusial +beds (_a_) is laid open at the surface, between the marls (_b b_), near +the base of the hill of Gergovia; and affords, at the same time, an +example of the extent to which the lacustrine strata, which must once +have filled a hollow, have been denuded, and shaped out into hills and +valleys, on the site of the ancient lakes. + +[Illustration: Fig. 159. Bed of indusial limestone, interstratified with +freshwater marl, near Clermont (Kleinschrod.)] + +[Illustration: Fig. 160. Larva of recent Phryganea.[185-A]] + +[Illustration: Fig. 161. + + _a_. Indusial limestone of Auvergne. + _b_. Fossil _Paludina_ magnified.] + +We may often observe in our ponds the _Phryganea_ (or Caddis-fly), in its +caterpillar state, covered with small freshwater shells, which they have +the power of fixing to the outside of their tubular cases, in order, +probably, to give them weight and strength. The individual figured in the +annexed cut, which belongs to a species very abundant in England, has +covered its case with shells of a small _Planorbis_. In the same manner a +large species of caddis-worm, which swarmed in the Eocene lakes of +Auvergne, was accustomed to attach to its dwelling the shells of a small +spiral univalve of the genus _Paludina_. A hundred of these minute shells +are sometimes seen arranged around one tube, part of the central cavity of +which is often empty, the rest being filled up with thin concentric layers +of travertin. The cases have been thrown together confusedly, and often +lie, as in fig. 161., at right angles one to the other. When we consider +that ten or twelve tubes are packed within the compass of a cubic inch, and +that some single strata of this limestone are 6 feet thick, and may be +traced over a considerable area, we may form some idea of the countless +number of insects and mollusca which contributed their integuments and +shells to compose this singularly constructed rock. It is unnecessary to +suppose that the _Phryganeæ_ lived on the spots where their cases are now +found; they may have multiplied in the shallows near the margin of the +lake, or in the streams by which it was fed, and their cases may have been +drifted by a current far into the deep water. + +In the summer of 1837, when examining, in company with Dr. Beck, a small +lake near Copenhagen, I had an opportunity of witnessing a beautiful +exemplification of the manner in which the tubular cases of Auvergne were +probably accumulated. This lake, called the Fuure-Soe, occurring in the +interior of Seeland, is about twenty English miles in circumference, and in +some parts 200 feet in depth. Round the shallow borders an abundant crop of +reeds and rushes may be observed, covered with the indusiæ of the +_Phryganea grandis_ and other species, to which shells are attached. The +plants which support them are the bullrush, _Scirpus lacustris_, and common +reed, _Arundo phragmitis_, but chiefly the former. In summer, especially in +the month of June, a violent gust of wind sometimes causes a current by +which these plants are torn up by the roots, washed away, and floated off +in long bands, more than a mile in length, into deep water. The _Cypris_ +swarms in the same lake; and calcareous springs alone are wanting to form +extensive beds of indusial limestone, like those of Auvergne. + +4. _Gypseous marls._--More than 50 feet of thinly laminated gypseous marls, +exactly resembling those in the hill of Montmartre, at Paris, are worked +for gypsum at St. Romain, on the right bank of the Allier. They rest on a +series of green cypriferous marls which alternate with grit, the united +thickness of this inferior group being seen, in a vertical section on the +banks of the river, to exceed 250 feet. + +_General arrangement, origin, and age of the freshwater formations of +Auvergne._--The relations of the different groups above described cannot be +learnt by the study of any one section; and the geologist who sets out with +the expectation of finding a fixed order of succession may perhaps complain +that the different parts of the basin give contradictory results. The +arenaceous division, the marls, and the limestone, may all be seen in some +places to alternate with each other; yet it can, by no means, be affirmed +that there is no order of arrangement. The sands, sandstone, and +conglomerate, constitute in general a littoral group; the foliated white +and green marls, a contemporaneous central deposit; and the limestone is +for the most part subordinate to the newer portions of both. The uppermost +marls and sands are more calcareous than the lower; and we never meet with +calcareous rocks covered by a considerable thickness of quartzose sand or +green marl. From the resemblance of the limestones to the Italian +travertins, we may conclude that they were derived from the waters of +mineral springs,--such springs as even now exist in Auvergne, and which may +be seen rising up through the granite, and precipitating travertin. They +are sometimes thermal, but this character is by no means constant. + +It seems that, when the ancient lake of the Limagne first began to be +filled with sediment, no volcanic action had yet produced lava and +scoriæ on any part of the surface of Auvergne. No pebbles, therefore, of +lava were transported into the lake,--no fragments of volcanic rocks +embedded in the conglomerate. But at a later period, when a considerable +thickness of sandstone and marl had accumulated, eruptions broke out, +and lava and tuff were deposited, at some spots, alternately with the +lacustrine strata. It is not improbable that cold and thermal springs, +holding different mineral ingredients in solution, became more numerous +during the successive convulsions attending this development of volcanic +agency, and thus deposits of carbonate and sulphate of lime, silex, and +other minerals were produced. Hence these minerals predominate in the +uppermost strata. The subterranean movements may then have continued +until they altered the relative levels of the country, and caused the +waters of the lakes to be drained off, and the farther accumulation of +regular freshwater strata to cease. + +We may easily conceive a similar series of events to give rise to +analogous results in any modern basin, such as that of Lake Superior, +for example, where numerous rivers and torrents are carrying down the +detritus of a chain of mountains into the lake. The transported +materials must be arranged according to their size and weight, the +coarser near the shore, the finer at a greater distance from land; but +in the gravelly and sandy beds of Lake Superior no pebbles of modern +volcanic rocks can be included, since there are none of these at present +in the district. If igneous action should break out in that country, and +produce lava, scoriæ, and thermal springs, the deposition of gravel, +sand, and marl might still continue as before; but, in addition, there +would then be an intermixture of volcanic gravel and tuff, and of rocks +precipitated from the waters of mineral springs. + +Although the freshwater strata of the Limagne approach generally to a +horizontal position, the proofs of local disturbance are sufficiently +numerous and violent to allow us to suppose great changes of level since +the lacustrine period. We are unable to assign a northern barrier to the +ancient lake, although we can still trace its limits to the east, west, +and south, where they were formed of bold granite eminences. Nor need we +be surprised at our inability to restore entirely the physical geography +of the country after so great a series of volcanic eruptions; for it is +by no means improbable that one part of it, the southern, for example, +may have been moved upwards bodily, while others remained at rest, or +even suffered a movement of depression. + +Whether all the freshwater formations of the Limagne d'Auvergne belong to +one period, I cannot pretend to decide, as large masses both of the +arenaceous and marly groups are often devoid of fossils. Much light has +been thrown on the mammiferous fauna by the labours of MM. Bravard and +Croizet, and by those of M. Pomel. The last-mentioned naturalist has +pointed out the specific distinction of all, or nearly all, the species of +mammalia, from those of the gypseous series near Paris. Nevertheless, many +of the forms are analogous to those of Eocene quadrupeds. The +_Cainotherium_, for example, is not far removed from the _Anoplotherium_, +and is, according to Waterhouse, the same as the genus _Microtherium_ of +the Germans. There are two species of marsupial animals allied to +_Didelphys_, a genus also found in the Paris gypsum. The _Amphitragulus +elegans_ of Pomel, has been identified with a Rhenish species from +Weissenau near Mayence, called by Kaup _Dorcatherium nanum_; and other +Auvergne fossils, e.g., _Microtherium Reuggeri_, and a small rodent, +_Titanomys_, are specifically the same with mammalia of the Mayence basin. + +_Cantal._--A freshwater formation, very analogous to that of Auvergne, +is situated in the department of Haute Loire, near the town of Le Puy, +in Velay, and another occurs near Aurillac, in Cantal. The leading +feature of the formation last mentioned, as distinguished from those of +Auvergne and Velay, is the immense abundance of silex associated with +calcareous marls and limestone. + +The whole series may be separated into two divisions; the lower, composed +of gravel, sand, and clay, such as might have been derived from the wearing +down and decomposition of the granitic schists of the surrounding country; +the upper system, consisting of siliceous and calcareous marls, contains +subordinately gypsum, silex, and limestone. + +The resemblance of the freshwater limestone of the Cantal, and its +accompanying flint, to the upper chalk of England, is very instructive, and +well calculated to put the student upon his guard against relying too +implicitly on mineral character alone as a safe criterion of relative age. + +When we approach Aurillac from the west, we pass over great heathy plains, +where the sterile mica-schist is barely covered with vegetation. Near +Ytrac, and between La Capelle and Viscamp, the surface is strewed over with +loose broken flints, some of them black in the interior, but with a white +external coating; others stained with tints of yellow and red, and in +appearance precisely like the flint gravel of our chalk districts. When +heaps of this gravel have thus announced our approach to a new formation, +we arrive at length at the escarpment of the lacustrine beds. At the bottom +of the hill which rises before us, we see strata of clay and sand, resting +on mica-schist; and above, in the quarries of Belbet, Leybros, and Bruel, a +white limestone, in horizontal strata, the surface of which has been +hollowed out into irregular furrows, since filled up with broken flint, +marl, and dark vegetable mound. In these cavities we recognize an exact +counterpart to those which are so numerous on the furrowed surface of our +own white chalk. Advancing from these quarries along a road made of the +white limestone, which reflects as glaring a light in the sun, as do our +roads composed of chalk, we reach, at length, in the neighbourhood of +Aurillac, hills of limestone and calcareous marl, in horizontal strata, +separated in some places by regular layers of flint in nodules, the coating +of each nodule being of an opaque white colour, like the exterior of the +flinty nodules of our chalk. + +It will be remembered that the siliceous stone of Bilin, called _tripoli_, +is a freshwater deposit, and has been shown, by Ehrenberg, to be of +infusorial origin (see p. 24.). What is true of the Bohemian flint and +opal, where the beds attain a thickness of 14 feet, may also, perhaps, be +found to hold good respecting the silex of Aurillac, which may also have +been immediately derived from the minute cases of microscopic animalcules. +But even if this conclusion be established, the abundant supply both of +siliceous, calcareous, and gypseous matter, which the ancient lakes of +France received, may have been connected with the subterranean volcanic +agency of which those regions were so long the theatre, and which may have +impregnated the springs with mineral matter, even before the great outbreak +of lava. It is well known that the hot springs of Iceland, and many other +countries, contain silex in solution; and it has been lately affirmed, that +steam at a high temperature is capable of dissolving quartzose rocks +without the aid of any alkaline or other flux.[189-A] + +Travellers not unfrequently mention, in their accounts of India, Australia, +and other distant lands, that they have seen chalk with flints, which they +have assumed to be of the same age as the Cretaceous system of Europe. A +hasty observation of the white limestone and flint of Aurillac might convey +the same idea; but when we turn from the mineral aspect and composition to +the organic remains, we find in the flints of the Cantal the seed-vessels +of the freshwater _Chara_, instead of the marine zoophytes so abundantly +imbedded in chalk flints; and in the limestone we meet with shells of +_Limnea_, _Planorbis_, and other lacustrine genera, instead of the oyster, +terebratula, and echinus of the Cretaceous period. + +_Proofs of gradual deposition_.--Some sections of the foliated marls in the +valley of the Cer, near Aurillac, attest, in the most unequivocal manner, +the extreme slowness with which the materials of the lacustrine series were +amassed. In the hill of Barrat, for example, we find an assemblage of +calcareous and siliceous marls; in which, for a depth of at least 60 feet, +the layers are so thin, that thirty are sometimes contained in the +thickness of an inch; and when they are separated, we see preserved in +every one of them the flattened stems of _Charæ_, or other plants, or +sometimes myriads of small _Paludinæ_ and other freshwater shells. These +minute foliations of the marl resemble precisely some of the recent +laminated beds of the Scotch marl lakes, and may be compared to the pages +of a book, each containing a history of a certain period of the past. The +different layers may be grouped together in beds from a foot to a foot and +a half in thickness, which are distinguished by differences of composition +and colour, the tints being white, green, and brown. Occasionally there is +a parting layer of pure flint, or of black carbonaceous vegetable matter, +about an inch thick, or of white pulverulent marl. We find several hills in +the neighbourhood of Aurillac composed of such materials, for the height of +more than 200 feet from their base, the whole sometimes covered by rocky +currents of trachytic or basaltic lava.[190-A] + +Thus wonderfully minute are the separate parts of which some of the most +massive geological monuments are made up! When we desire to classify, it +is necessary to contemplate entire groups of strata in the aggregate; +but if we wish to understand the mode of their formation, and to explain +their origin, we must think only of the minute subdivisions of which +each mass is composed. We must bear in mind how many thin leaf-like +seams of matter, each containing the remains of myriads of testacea and +plants, frequently enter into the composition of a single stratum, and +how vast a succession of these strata unite to form a single group! We +must remember, also, that piles of volcanic matter, like the Plomb du +Cantal, which rises in the immediate neighbourhood of Aurillac, are +themselves equally the result of successive accumulation, consisting of +reiterated sheets of lava, showers of scoriæ, and ejected fragments of +rock.--Lastly, we must not forget that continents and mountain-chains, +colossal as are their dimensions, are nothing more than an assemblage of +many such igneous and aqueous groups, formed in succession during an +indefinite lapse of ages, and superimposed upon each other. + + +FOOTNOTES: + +[175-A] Bulletin des Sci. de la Soc. Philom., May, 1825, p. 74. + +[176-A] Hébert. Bulletin. 1849, vol. vi. 2d series, p. 459. + +[181-A] Scrope, Geology of Central France, p. 15. + +[183-A] See Desmarest's Crustacea, plate 55. + +[185-A] I believe that the British specimen here figured is +P. _rhombica_, Linn. + +[189-A] See Proceedings of Roy. Soc., No. 44. p. 233. + +[190-A] Lyell and Murchison, sur les Dépôts Lacust. Tertiaries du Cantal, +&c. Ann. des Sci. Nat. Oct. 1829. + + + + +CHAPTER 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 sands 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. + + +From what was said in the two preceding chapters, it has already +appeared that we have in England no true chronological representative of +the Miocene faluns of the Loire, and none of the Upper Eocene group +described in the last chapter. But, when we descend to the middle and +inferior divisions of the Eocene system of France, we find that they +have their equivalents in Great Britain. + + +MIDDLE EOCENE.--FRANCE. + +_Gypseous series_ (2. _a_, Table, p. 175.).--Next below the upper marine +sands of the neighbourhood of Paris, we find a series of white and green +marls, with subordinate beds of gypsum. These are most largely developed in +the central parts of the Paris basin, and, among other places, in the Hill +of Montmartre, where its fossils were first studied by M. Cuvier. + +The gypsum quarried there for the manufacture of plaster of Paris occurs as +a granular crystalline rock, and, together with the associated marls, +contains land and fluviatile shells, together with the bones and skeletons +of birds and quadrupeds. Several land plants are also met with, among which +are fine specimens of the fan palm or palmetto tribe (_Flabellaria_). The +remains also of freshwater fish and of crocodiles and other reptiles, occur +in the gypsum. The skeletons of mammalia are usually isolated, often +entire, the most delicate extremities being preserved; as if the carcasses, +clothed with their flesh and skin, had been floated down soon after death, +and while they were still swoln by the gases generated by their first +decomposition. The few accompanying shells are of those light kinds which +frequently float on the surface of rivers, together with wood. + +M. Prevost has therefore suggested that a river may have swept away the +bodies of animals, and the plants which lived on its borders, or in the +lakes which it traversed, and may have carried them down into the centre +of the gulf into which flowed the waters impregnated with sulphate of +lime. We know that the Fiume Salso in Sicily enters the sea so charged +with various salts that the thirsty cattle refuse to drink of it. A +stream of sulphureous water, as white as milk, descends into the sea +from the volcanic mountain of Idienne on the east of Java; and a great +body of hot water, charged with sulphuric acid, rushed down from the +same volcano on one occasion, and inundated a large tract of country, +destroying, by its noxious properties, all the vegetation.[191-A] In +like manner the Pusanibio, or "Vinegar River," of Colombia, which rises +at the foot of Puracé, an extinct volcano, 7,500 feet above the level of +the sea, is strongly impregnated with sulphuric and muriatic acids and +with oxide of iron. We may easily suppose the waters of such streams to +have properties noxious to marine animals, and in this manner the entire +absence of marine remains in the ossiferous gypsum may be +explained.[191-B] There are no pebbles or coarse sand in the gypsum; a +circumstance which agrees well with the hypothesis that these beds were +precipitated from water holding sulphate of lime in solution, and +floating the remains of different animals. + +In this formation the relics of about fifty species of quadrupeds, +including the genera _Paleotherium_, _Anoplotherium_, and others, have been +found, all extinct, and nearly four-fifths of them belonging to a division +of the order _Pachydermata_, which is now represented by only four living +species; namely three tapirs and the daman of the Cape. With them a few +carnivorous animals are associated, among which are a species of fox and +gennet. Of the _Rodentia_, a dormouse and a squirrel; of the _Insectivora_, +a bat; and of the _Marsupialia_ (an order now confined to America, +Australia, and some contiguous islands), an opossum, have been discovered. + +Of birds, about ten species have been ascertained, the skeletons of some of +which are entire. None of them are referable to existing species.[192-A] +The same remark applies to the fish, according to MM. Cuvier, and Agassiz, +as also to the reptiles. Among the last are crocodiles and tortoises of the +genera _Emys_ and _Trionyx_. + +The tribe of land quadrupeds most abundant in this formation is such as +now inhabits alluvial plains and marshes, and the banks of rivers and +lakes, a class most exposed to suffer by river inundations. Whether the +disproportion of carnivorous animals can be ascribed to this cause, or +whether they were comparatively small in number and dimensions, as in +the indigenous fauna of Australia, when first known to Europeans, is a +point on which it would be rash, perhaps, to offer an opinion in the +present state of our knowledge. + +[Illustration: Fig. 162. _Paleotherium magnum._] + +The Paleothere, above alluded to, resembled the living tapir in the form of +the head, and in having a short proboscis, but its molar teeth were more +like those of the rhinoceros (see fig. 163.). _Paleotherium magnum_ was of +the size of a horse, 3 or 4 feet high. The annexed woodcut, fig. 162., is +one of the restorations which Cuvier attempted of the outline of the living +animal, derived from the study of the entire skeleton. When the French +osteologist declared in the early part of the present century, that all the +fossil quadrupeds of the gypsum of Paris were extinct, the announcement of +so startling a fact, on such high authority, created a powerful sensation, +and from that time a new impulse was given throughout Europe to the +progress of geological investigation. Eminent naturalists, it is true, had +long before maintained that the shells and zoophytes, met with in many +ancient European rocks, had ceased to be inhabitants of the earth, but the +majority even of the educated classes continued to believe that the species +of animals and plants now contemporary with man, were the same as those +which had been called into being when the planet itself was created. It was +easy to throw discredit upon the new doctrine by asking whether corals, +shells, and other creatures previously unknown, were not annually +discovered? and whether living forms corresponding with the fossils might +not yet be dredged up from seas hitherto unexamined? But from the era of +the publication of Cuvier's Ossements Fossiles, and still more his popular +Treatise called "A Theory of the Earth," sounder views began to prevail. It +was clearly demonstrated that most of the mammalia found in the gypsum of +Montmartre differed even generically from any now existing, and the extreme +improbability that any of them, especially the larger ones, would ever be +found surviving in continents yet unexplored, was made manifest. Moreover, +the non-admixture of a single living species in the midst of so rich a +fossil fauna was a striking proof that there had existed a state of the +earth's surface zoologically unconnected with the present order of things. + +[Illustration: Fig. 163. Upper molar tooth of _Paleotherium magnum_ from +Isle of Wight. (Owen's Brit. Foss. p. 317.) + +Reduced one-third.] + +_Grès de Beauchamp_ (2. _b_, Table, p. 175.).--In some parts of the Paris +basin, sands and marls, called the Grès de Beauchamp, or Sables Moyens, +divide the gypseous beds from the underlying Calcaire grossier. These sands +contain more than 300 species of marine shells, many of them peculiar, but +others common to the underlying marine deposit (No. 2. _c_.). + +_Calcaire grossier_ (2. _c_, Table, p. 175.).--The formation called +Calcaire grossier consists of a coarse limestone, often passing into sand. +It contains the greater number of the fossil shells which characterize the +Paris basin. No less than 400 distinct species have been procured from a +single spot near Grignon, where they are embedded in a calcareous sand, +chiefly formed of comminuted shells, in which, nevertheless, individuals in +a perfect state of preservation, both of marine, terrestrial, and +freshwater species, are mingled together. Some of the marine shells may +have lived on the spot; but the _Cyclostoma_ and _Limnea_ must have been +brought thither by rivers and currents, and the quantity of triturated +shells implies considerable movement in the waters. + +Nothing is more striking in this assemblage of fossil testacea than the +great proportion of species referable to the genus _Cerithium_ (see fig. +164.). There occur no less than 137 species of this genus in the Paris +basin, and almost all of them in the calcaire grossier. Now the living +_Cerithia_ inhabit the sea near the mouths of rivers, where the waters +are brackish; so that their abundance in the marine strata now under +consideration is in harmony with the hypothesis, that the Paris basin +formed a gulf into which several rivers flowed, the sediment of some of +which gave rise to the beds of clay and lignite before mentioned; while +a distinct freshwater limestone, called calcaire siliceux, which will +presently be described, was precipitated from the waters of others +situated farther to the south. + +[Illustration: Fig. 164. Cerithium cinctum.[194-A]] + +[4 Illustrations: EOCENE FORAMINIFERA. + +Fig. 165. _Calcarina rarispina_, Desh. + _b_. natural size. + _a_, _c_. same magnified. + +Fig. 166. _Spirolina stenostoma_, Desh. + B. natural size. + A, C, D. same magnified. + +Fig. 167. _Triloculina inflata_, Desh. + _b_. natural size. + _a_, _c_, _d_, same magnified. + +Fig. 168. _Clavulina corrugata_, Desh. + _a_. natural size. + _b_, _c_. same magnified.] + +In some parts of the calcaire grossier round Paris, certain beds occur of a +stone used in building, and called by the French geologists "Miliolite +limestone." It is almost entirely made up of millions of microscopic +shells, of the size of minute grains of sand, which all belong to the class +Foraminifera. Figures of some of these are given in the annexed woodcut. As +this miliolitic stone never occurs in the Faluns, or Miocene strata of +Brittany and Touraine, it often furnishes the geologist with a useful +criterion for distinguishing the detached Eocene and Miocene formations, +scattered over those and other adjoining provinces. The discovery of the +remains of Paleotherium and other mammalia in some of the upper beds of the +calcaire grossier shows that these land animals began to exist before the +deposition of the overlying gypseous series had commenced. + +_Calcaire siliceux_.--This compact siliceous limestone extends over +a wide area. It resembles a precipitate from the waters of mineral +springs, and is often traversed by small empty sinuous cavities. +It is, for the most part, devoid of organic remains, but in some +places contains freshwater and land species, and never any marine +fossils. The siliceous limestone and the calcaire grossier occupy +distinct parts of the Paris basin, the one attaining its fullest +development in those places where the other is of slight thickness. +They also alternate with each other towards the centre of the basin, +as at Sergy and Osny; and there are even points where the two rocks are +so blended together that portions of each may be seen in hand specimens. +Thus, in the same bed, at Triel, we have the compact freshwater +limestone, characterized by its _Limneæ_, mingled with the coarse marine +limestone, with its small multilocular shells, or "miliolites," +dispersed through it in countless numbers. These microscopic testacea +are also accompanied by _Cerithia_ and other shells of the calcaire +grossier. It is very extraordinary that in this instance both kinds of +sediment must have been thrown down together on the same spot, yet each +retains its own peculiar organic remains. + +From these facts we may conclude, that while to the north, where the bay +was probably open to the sea, a marine limestone was formed, another +deposit of freshwater origin was introduced to the southward, or at the +head of the bay; for it appears that during the Eocene period, as now, the +ocean was to the north, and the continent, where the great lakes existed, +to the south. From that southern region we may suppose a body of fresh +water to have descended, charged with carbonate of lime and silica, the +water being perhaps in sufficient volume to freshen the upper end of the +bay. The gypseous series (2. _a_, Table, p. 175.), before described, was +once supposed to be entirely subsequent in origin to the two groups, called +calcaire grossier and calcaire siliceux. But M. Prevost has pointed out +that in some localities they alternate repeatedly with both. + +The gypsum, with its associated marl and limestone, is in greatest force +towards the centre of the basin, where the calcaire grossier and calcaire +siliceux are less fully developed. Hence M. Prevost infers, that while +those two principal deposits were gradually in progress, the one towards +the north, and the other towards the south, a river descending from the +east may have brought down the gypseous and marly sediment. + +It must be admitted, as highly probable, that a bay or narrow sea, 180 +miles in length, would receive, at more points than one, the waters of the +adjoining continent. At the same time, we must be prepared to find that +the simultaneous deposition of two or more sets of strata in one basin, +some freshwater and others marine, must have produced very complex results. +But, in proportion as it is more difficult in these cases to discover any +fixed order of superposition in the associated mineral masses, so also is +it more easy to explain the manner of their origin, and to reconcile their +relations to the agency of known causes. Instead of the successive +irruptions and retreats of the sea, and changes in the chemical nature of +the fluid, and other speculations of the earlier geologists, we are now +simply called upon to imagine a gulf, into one extremity of which the sea +entered, and at the other a large river, while other streams may have +flowed in at different points, whereby an indefinite number of alternations +of marine and freshwater beds would be occasioned. + + +LOWER EOCENE, FRANCE. + +_Lits coquilliers_ (3. _a_, Table, p. 175.).--Below the calcaire grossier +are extensive deposits of sand, in the upper parts of which some marine +beds, called "lits coquilliers," occur, in which M. d'Archiac has +discovered 200 species of shells. Many of these are peculiar, but the +larger portion appear to agree with species of the calcaire grossier, so +that the line of demarcation usually adopted between the French Lower and +Middle Eocene formations, seems not to be very strongly drawn. _Sands and +plastic clay_ (3. _b_, Table, p. 175.)--At the base of the tertiary system +in France are extensive deposits of sands, with occasional beds of clay +used for pottery, and called "argile plastique." Fossil oysters (_Ostrea +bellovacina_) abound in some places, and in others there is a mixture of +fluviatile shells, such as _Cyrena cuneiformis_ (fig. 187. p. 204.), +_Melania inquinata_ (fig. 188.), and others, frequently met with in beds +occupying the same position in the valley of the Thames. Layers of lignite +also accompany the inferior clays and sands. + +Immediately upon the chalk at the bottom of all the tertiary strata +there is often a conglomerate or breccia of rolled and angular chalk +flints, cemented by siliceous sand. These beds appear to be of littoral +origin, and imply the previous emergence of some portions of the chalk, +and its waste by denudation. + +[Illustration: Fig. 169. _Cardium porulosum_. Paris and London basins.] + +The lower sandy beds of the Paris basin are often called the sands of the +Soissonais, from a district so named 50 miles N.E. of Paris. One of the +shells of the formation is adduced by M. Deshayes as an example of the +changes which certain species underwent in the successive stages of their +existence. It seems that different varieties of the _Cardium porulosum_ are +characteristic of different formations. In the Lower Eocene of the +Soissonais this shell acquires but a small volume, and has many +peculiarities, which disappear in the lowest beds of the calcaire grossier. +In these the shell attains its full size, and many distinctive characters, +which are again modified in the uppermost beds of the calcaire grossier; +and these last modifications of form are preserved throughout the whole of +the "upper marine" (or Upper Eocene) series.[197-A] + + +ENGLISH EOCENE FORMATIONS. + +The Eocene areas of Hampshire and London are delineated in the map +(fig. 153. p. 174.). + +The following table will show the succession of the principal deposits +found in our island. The true place of the Bagshot sands, in this +series, was never accurately ascertained till Mr. Prestwich published, +in 1847, his classification of the English Eocene strata, dividing them +into three principal formations, in which the Bagshot sands occupied +the central place.[197-B] + + Localities. + 1. Upper Eocene. Wanting in Great Britain. + + { _a._ Freshwater and Headon Hill, Isle of + { fluvio-marine beds. Wight; and Hordwell + { Cliff, Hants. + 2. Middle Eocene { _b._ Barton beds. Barton Cliff, Hants. + { _c._ Bagshot and Bracklesham Bagshot Heath, Surrey; + { sands and clays. Bracklesham Bay, + { Sussex. + + { _a._ London Clay Proper, Highgate Hill, + { and Bognor beds. Middlesex; I. of + { Sheppey; Bognor, + 3. Lower Eocene { Sussex. + { _b._ Mottled and Plastic Newhaven, Sussex; + { clays and sands. Reading, Berks; + { Woolwich, Kent. + +[Illustration: Fig. 170. _Lymnea longiscata._ + +Freshwater Eocene strata, Isle of Wight.] + +_Freshwater beds_ (2. _a_, Table, p. 175.).--In the northern part of the +Isle of Wight, beds of marl, clay, and sand, and a friable limestone, +containing freshwater shells, are seen, containing shells of the genera +_Lymnea_ (see fig. 170.), _Planorbis_, _Melanopsis_, _Cyrena_, &c., several +of them of the same species as those occurring in the Eocene beds of the +Paris basin. Gyrogonites, also, or seed-vessels of _Chara_, exhibiting a +similar specific identity, occur. At Headon Hill, on the western side of +the island, where these beds are seen in the sea-cliffs, some of the strata +contain a few marine and estuary shells, such as _Cytheræa_, _Corbula_, +&c., showing a temporary occupation of the area by brackish or salt water, +after which the river or a lake seems again to have prevailed. A species +of fan-palm, _Flabellaria Lamanonis_, Brong., like one which characterizes +the Parisian Eocene beds, has been recently detected by Dr. Mantell in this +formation, in Whitecliff Bay, at the eastern end of the island. + +Several of the species of extinct quadrupeds already alluded to as +characterizing the gypsum of Montmartre have been discovered by Messrs. +Pratt and Fox, in the Isle of Wight, chiefly at Binstead, near Ryde, as +_Palæotherium magnum_, _P. medium_, _P. minus_, _P. minimum_, _P. curtum_, +_P. crassum_, also _Anoplotherium commune_, _A. secundarium_, _Dichobune +cervinum_, and _Chæropotamus Cuvieri_. In Hordwell cliff, also on the +Hampshire coast, several of these species, with other quadrupeds of new +genera, such as _Paloplotherium_, Owen, have been met with; and remains of +a remarkable carnivorous genus, _Hyænodon_. These fossils are accompanied +by the bones of _Trionyx_, and other tortoises, and by two land snakes of +the genus _Paleryx_, Owen, from 3 to 4 feet long, also a species of +crocodile, and an alligator. Among other fossils collected by Lady +Hastings, Sir Philip Egerton has recognized the well-known gar or bony pike +of the American rivers, a ganoid fish of the genus _Lepidotus_, with its +hard shining scales. The shells of Hordwell are similar to those of the +freshwater beds of the Isle of Wight, and among them are a few specifically +undistinguishable from recent testacea, as _Paludina lenta_ and _Helix +labyrinthica_, the latter discovered by Mr. S. Wood, and identified with an +existing N. American helix. + +The white and green marls of this freshwater series in Hampshire, and some +of the accompanying limestones, often resemble those of France in mineral +character and colour in so striking a manner, as to suggest the idea that +the sediment was derived from the same region, or produced +contemporaneously under very similar geographical circumstances. + +_Barton beds._--Both in the cliffs of Headon Hill and Hordwell, already +mentioned, the freshwater series rests on a mass of pure white sand without +fossils, and this is seen in Barton Cliff to overlie a marine deposit, in +which 209 species of testacea have been found. More than half of these are +peculiar; and, according to Mr. Prestwich, only 11 of them common to the +London Clay proper, being in the proportion of only 5 per cent. On the +other hand, 70 of them agree with the _calcaire grossier_ shells. As this +is the newest purely marine bed of the Eocene series known in England, we +might have expected that some of its peculiar fossils would be found to +agree with the upper Eocene strata described in the last chapter, and +accordingly some identifications have been cited with testacea, both of the +Berlin and Belgian strata. It is nearly a century since Brander published, +in 1766, an account of the organic remains collected from these cliffs, and +his excellent figures of the shells then deposited in the British Museum +are justly admired by conchologists for their accuracy. + +_Bagshot Sands_ (2. _c_, Table, p. 197.).--These beds, consisting +chiefly of siliceous sand, occupy extensive tracts round Bagshot, in +Surrey, and in the New Forest, Hampshire. They succeed next in +chronological order, and may be separated into three divisions, the +upper and lower consisting of light yellow sands, and the central of +dark green sands and brown clays, the whole reposing on the London clay +proper.[199-A] Although the Bagshot beds are usually devoid of fossils, +they contain marine shells in some places, among which _Venericardia +planicosta_ (see fig. 171.) is abundant, with _Turritella sulcifera_ and +_Nummulites lævigatus_. (See fig. 174. p. 200.) + +[Illustration: Fig. 171. _Venericardia planicosta_, Lamck. + +_Cardita planicosta_, Deshayes.] + +At Bracklesham Bay, near Chichester, in Sussex, the characteristic shells +of this member of the Eocene series are best seen; among others, the huge +_Cerithium giganteum_, so conspicuous in the calcaire grossier of Paris, +where it is sometimes 2 feet in length. The volutes and cowries of this +formation, as well as the lunulites and other corals, seem to favour the +idea of a warm climate having prevailed, which is borne out by the +discovery of a serpent _Palæophis typhæus_, exceeding, according to Mr. +Owen, 20 feet in length, and allied to the Boa, Python, Coluber, and +Hydrus. The compressed form and diminutive size of certain caudal vertebræ +indicate so much analogy with Hydrus as to induce the Hunterian professor +to pronounce the extinct ophidian to have been marine.[199-B] He had +previously combated with so much success the evidence advanced, to prove +the existence in the Northern Ocean of sea-serpents in our own times, that +he will not be suspected of any undue bias in contending for their former +existence in the British Eocene seas. The climate, however, of the Middle +Eocene period was evidently far more genial; and amongst the companions of +the sea-serpent of Bracklesham was an extinct Gavial (_Gavialis Dixoni_, +Owen), and numerous fish, such as now frequent the seas of warm latitudes, +as the sword-fish (see fig. 172. p. 200.) and gigantic rays of the genus +Miliobates. (See fig. 173.) + +Out of 193 species of testacea procured from the Bagshot and Bracklesham +beds in England, 126 occur in the French calcaire grossier. It was +clearly, therefore, coeval with that part of the Parisian series more +nearly than with any other. The _Nummulites lævigatus_ (see fig. 174.), +a fossil characteristic of the lower beds of the calcaire grossier, +is abundant at Bracklesham. + +[Illustration: Fig. 172. Prolonged premaxillary bone or "sword" of +a fossil sword-fish (_Cælorhynchus_). Bracklesham. Dixon's Fossils +of Sussex, pl. 8.] + +[Illustration: Fig. 173. Dental plates of _Myliobates Edwardsi_. +Bracklesham Bay. Ibid. pl. 8.] + +[Illustration: Fig. 174. _Nummulites_ (_Nummularia_) _lævigatus._ +Bracklesham. Ibid. pl. 8. + + _a._ section of the nummulite. + _b._ group, with an individual showing the exterior of the shell.] + +_London clay proper_ (3. _a_, Table, p. 197.).--This formation underlies +the preceding, and consists of tenacious brown and blueish grey clay, with +layers of concretions called septaria, which abound chiefly in the brown +clay, and are obtained in sufficient numbers from the cliffs near Harwich, +and from shoals of the Essex coast, to be used for making Roman cement. The +principal localities of fossils in the London clay are Highgate Hill, near +London, the island of Sheppey, and Bognor in Hampshire. Out of 133 fossil +shells, Mr. Prestwich found only 20 to be common to the calcaire grossier +(from which 600 species have been obtained), while 33 are common to the +lits coquilliers (p. 196.), in which only 200 species are known in France. +We may presume, therefore, that the London clay proper is older than the +calcaire grossier. This may perhaps remove a difficulty which M. Adolphe +Brongniart has experienced when comparing the Eocene Flora of the +neighbourhoods of London and Paris. The fossil species of the island of +Sheppey, he observes, indicate a much more tropical climate than the Eocene +Flora of France, which has been derived principally from the "gypseous +series." The latter resembles the vegetation of the borders of the +Mediterranean rather than that of an equatorial region. + +Mr. Bowerbank, in a valuable publication on the fossil fruits and seeds of +the island of Sheppey, near London, has described no less than thirteen +fruits of palms of the recent type _Nipa_, now only found in the Molucca +and Philippine islands. (See fig. 175.) These plants are allied to the +cocoa-nut tribe on the one side, and on the other to the _Pandanus_, or +screw-pine. Species of cocoa-nuts are also met with, and other kinds of +palms; also three species of _Anona_, or custard-apple; cucurbitaceous +fruits, also (the gourd and melon family), are in considerable abundance. +Fruits of various species of _Acacia_ are in profusion; and, although less +decidedly tropical, imply a warm climate. + +[Illustration: Fig. 175. _Nipadites ellipticus._ Bow. Fossil +palm of Sheppey.] + +The contiguity of land may be inferred not only from these vegetable +productions, but also from the teeth and bones of crocodiles and turtles, +since these creatures, as Mr. Conybeare has remarked, must have resorted to +some shore to lay their eggs. Of turtles there were numerous species +referred to extinct genera, and, for the most part, not equal in size to +the largest living tropical turtles. A snake, which must have been 13 feet +long, of the genus _Palæophis_ before mentioned, has also been described by +Mr. Owen from Sheppey, of a different species from that of Bracklesham. A +true crocodile, also, _Crocodilus toliapicus_, and another Saurian more +nearly allied to the gravial, accompany the above fossils. A bird allied to +the vultures, and a quadruped of the new genus _Hyracotherium_, allied to +the Hyrax, Hog, and Chæropotamus, are also among the additions made of late +years to the palæontology of this division. + +[3 Illustrations: FOSSIL SHELLS OF THE LONDON CLAY. + +Fig. 176. _Mitra scabra_. + +Fig. 177. _Rostellaria macroptera_, Sow. One-third of nat. size. + +Fig. 178. _Crassatella sulcata._] + +The marine shells of the London clay confirm the inference derivable from +the plants and reptiles of a high temperature. Thus, many species of +_Conus_, _Mitra_, and _Voluta_ occur, a large _Cypræa_, a very large +_Rostellaria_, and shells of the genera _Terebellum_, _Cancellaria_, +_Crassatella_, and others, with four or more species of _Nautilus_ (see +fig. 182.) and other cephalopoda of extinct genera, one of the most +remarkable of which is the _Belosepia_.[202-A] (See fig. 183.) + +[Illustration: Fig. 179. _Nautilus centralis._] + +[Illustration: Fig. 180. _Voluta athleta._] + +[Illustration: Fig. 181. _Terebellum fusiforme._] + +[Illustration: Fig. 182. _Aturia zigzag._ Bronn. Syn. _Nautilus zigzag._ +Sow. London clay. Sheppey.] + +[Illustration: Fig. 183. _Belosepia sepiodea_, De Blainv. +London clay. Sheppey.] + +The above shells are accompanied by a sword-fish (_Tetrapterus priscus_, +Agassiz), about 8 feet long, and a saw-fish (_Pristis bisulcatus_, Ag.), +about 10 feet in length; genera now foreign to the British seas. On the +whole, no less than 50 species of fish have been described by +M. Agassiz from these beds in Sheppey, and they indicate, in his +opinion, a warm climate. + +[Illustration: Fig. 184. Molar of monkey (_Macacus_).] + +_Strata of Kyson in Suffolk._--At Kyson, a few miles east of Woodbridge, a +bed of Eocene clay, 12 feet thick, underlies the red crag. Beneath it is a +deposit of yellow and white sand, of considerable interest, in consequence +of many peculiar fossils contained in it. Its geological position is +probably the lowest part of the London clay proper. In this sand has been +found the first example of a fossil quadrumanous animal discovered in Great +Britain, namely, the teeth and part of a jaw, shown by Mr. Owen to belong +to a monkey of the genus _Macacus_ (see fig. 184.). The mammiferous +fossils, first met with in the same bed, were those of an opossum +(_Didelphys_) (see fig. 185.), and an insectivorous bat (fig. 186.), +together with many teeth of fishes of the shark family. Mr. Colchester in +1840 obtained other mammalian relics from Kyson, among which Mr. Owen has +recognized several teeth of the genus _Hyracotherium_, and the vertebræ of +a large serpent, probably a _Palæophis_. As the remains both of the +_Hyracotherium_ and _Palæophis_ were afterwards met with in the London +clay, as before remarked, these fossils confirmed the opinion previously +entertained, that the Kyson sand belongs to the Eocene period. The +_Macacus_, therefore, constitutes the first example of any quadrumanous +animal found in strata as old as the Eocene, or so far from the equator as +lat. 52° N. It was not until after the year 1836 that the existence of any +fossil quadrumana was brought to light. Since that period they have been +found in France, India, and Brazil. + +[Illustration: Fig. 185. Molar tooth and part of jaw of opossum. +From Kyson.[203-A]] + +[Illustration: Fig. 186. Molars of insectivorous bats, twice nat. size. +From Kyson, Suffolk.] + +_Mottled or Plastic Clays_, _&c._ (3. _b_, Table, p. 197.).--No +formations can be more dissimilar on the whole in mineral character than +the Eocene deposits of England and Paris; those of our own island being +almost exclusively of mechanical origin,--accumulations of mud, sand, +and pebbles; while in the neighbourhood of Paris we find a great +succession of strata composed of a coarse white limestone, and compact +siliceous limestone with beds of crystalline gypsum and siliceous +sandstone, and sometimes pure flint used for millstones. Hence it is by +no means an easy task to institute an exact comparison between the +various members of the English and French series, and to settle their +respective ages. It is clear that a continual change was going on in the +fauna and flora by the coming in of new species and the dying out of +others; and contemporaneous changes of geographical conditions were also +in progress in consequence of the rising and sinking of the land and +bottom of the sea. A particular subdivision, therefore, of time was +occasionally represented in one area by land, in another by an estuary, +in a third by the sea, and even where the conditions were in both +areas of a marine character, there was often shallow water in one, +and deep sea in another, producing a want of agreement in the state +of animal life. + +At the commencement, however, of the Eocene formations in France and +England, we find an exception to this rule, for a marked similarity of +mineral character reigns in the lowest division, whether in the basins +of Paris, Hampshire, or London. This uniformity of aspect must be seen +in order to be fully appreciated, since the beds consist simply of sand, +mottled clays, and well-rolled flint pebbles, derived from the chalk, +and varying in size from that of a pea to an egg. These strata may be +seen at Reading, at Blackheath, near London, and at Woolwich. In some +of the lowest of them, banks of oysters are observed, consisting of +_Ostrea bellovicina_, so common in France in the same relative position, +and _Ostrea edulina_, scarcely distinguishable from the living eatable +species. In this formation at Bromley, Dr. Buckland found one large +pebble to which five full-grown oysters were affixed, in such a manner +as to show that they had commenced their first growth upon it, and +remained attached to it through life. + +In several places, as at Woolwich on the Thames, at Newhaven in Sussex, and +elsewhere, a mixture of marine and freshwater testacea distinguishes this +member of the series. Among the latter, _Melania inquinata_ (see fig. 188.) +and _Cyrena cuneiformis_ are very common. They probably indicate points +where rivers entered the Eocene sea. + +[Illustration: Fig. 187. _Cyrena cuneiformis_, Min. Con. Natural size.] + +[Illustration: Fig. 188. _Melania inquinata_, Des. Nat. size. + +Syn. _Cerithium melanoides_, Min. Con.] + +With us as in France, clay of this formation is used in some places, as +near Poole in Dorsetshire, for pottery; and hence the name of plastic clay +was adopted for the group by Mr. T. Webster. Lignite also is associated +with it in some spots, as in the Paris basin. + +Before the minds of geologists had become familiar with the theory of the +gradual sinking of the land, and its conversion into sea at different +periods, and the consequent change from shallow to deep water, the +freshwater and littoral character of this inferior group appeared strange +and anomalous. After passing through many hundred feet of London clay, +proved by its fossils to have been deposited in salt water of considerable +depth, we arrive at beds of fluviatile origin. Thick masses, also, of +shingle indicate the proximity of land, where the flints of the chalk were +rolled into sand and pebbles, and spread continuously over wide spaces, as +in the Isle of Wight, in the south of Hampshire, and near London, always +appearing at the bottom of the Eocene series. It may be asked why they did +not constitute simply a narrow littoral zone, such as we might look for in +strata formed at a moderate distance from the shore. In answer to this +inquiry, the student must be reminded, that wherever a gently-sloping land +is gradually sinking and becoming submerged, shingle may be heaped up +successively over a wide area, although marine currents have no power of +dispersing it simultaneously over a large space. In such cases it is not +the shingle which recedes from the coast, but the coast which recedes from +the shingle, which is formed one mass after another as often as successive +portions of the land are converted into sea and others into a sea beach. + +The London area appears to have been upraised before that of Hampshire, so +that it never became the receptacle of the Barton clays, nor of the +overlying fluvio-marine and freshwater beds of Hordwell and the north part +of the Isle of Wight. On the other hand, the Hampshire Eocene area seems to +have emerged before that of Paris, so that no marine beds of the Upper +Eocene era were ever thrown down in Hampshire. + +_Nummulitic formation of the Alps and Pyrenees._--It has long been +matter of controversy, whether the nummulitic rocks of the Alps and +Pyrenees should be regarded as Eocene or Cretaceous; but the number of +geologists of high authority who regard this important group as +belonging to the lowest tertiary system of Europe has for many years +been steadily increasing. The late M. Alex. Brongniart first declared +the specific identity of many of the shells of this formation with those +of the marine strata near Paris, although he obtained them from the +summit of the Diablerets, one of the loftiest of the Swiss Alps, which +rises more than 10,000 feet above the level of the sea. + +Deposits of the same age, found on the flanks of the Pyrenees, contain also +a great number of shells common to the Paris and London areas, and three or +four species only which are common to the cretaceous formation. + +The calcareous division consists often of a compact crystalline marble, +full of nummulites (see fig. 189.), shells of the class _Foraminifera_. + +[Illustration: Fig. 189. _Nummulites_. Peyrehorade, Pyrenees. + + _a._ external surface of one of the nummulites, of which longitudinal + sections are seen in the limestone. + _b._ transverse section of same.] + +The nummulitic limestone of the Alps is often of great thickness, and is +immediately covered by another series of strata of dark-coloured slates, +marls, and fucoidal sandstones, to the whole of which the provincial name +of "flysch" has been given in parts of Switzerland. The researches of Sir +Roderick Murchison in the Alps in 1847 enable us to refer the whole of +these beds to the Eocene period, and it seems probable that they most +nearly coincide in age with the Lower Eocene. They enter into the disturbed +and loftiest portions of the Alpine chain, to the elevation of which they +enable us therefore to assign a comparatively modern date. + +The nummulitic formation, with its characteristic fossils, plays a far more +conspicuous part than any other tertiary group in the solid framework of +the earth's crust, whether in Europe, Asia, or Africa. It often attains a +thickness of many thousand feet, and extends from the Alps to the +Apennines. It is found in the Carpathians, and in full force in the north +of Africa, as, for example, in Algeria and Morocco. It has also been traced +from Egypt into Asia Minor, and across Persia by Bagdad to the mouths of +the Indus. It occurs not only in Cutch, but in the mountain ranges which +separate Scinde from Persia, and which form the passes leading to Caboul; +and it has been followed still farther eastward into India. + +Some members of this lower tertiary formation in the central Alps, +including even the superior strata called _flysch_, have been converted +into crystalline rocks, and changed into saccharoid marble, quartz, +rock, and mica-schist.[206-A] + + +EOCENE STRATA IN THE UNITED STATES. + +In North America the Eocene formations occupy a large area bordering the +Atlantic, which increases in breadth and importance as it is traced +southwards from Delaware and Maryland to Georgia and Alabama. They also +occur in Louisiana and other states both east and west of the valley of the +Mississippi. At Claiborne in Alabama no less than four hundred species of +marine shells, with many echinoderms and teeth of fish, characterize one +member of this system. Among the shells the _Cardita planicosta_, before +mentioned (fig. 171. p. 199.), is in abundance; and this fossil, and some +others identical with European species, or very nearly allied to them, make +it highly probable that the Claiborne beds agree in age with the central or +Bracklesham group of England, and the calcaire grossier of Paris.[206-B] + +Higher in the series is a remarkable calcareous rock, formerly called "the +nummulite limestone," from the great number of discoid bodies resembling +nummulites which it contains, fossils now referred by A. d'Orbigny to the +genus _Orbitoides_, which has been demonstrated by Dr. Carpenter to belong +to the Foraminifera.[206-C] The following section will enable the reader to +understand the position of the three subdivisions of the series, Nos. 1, +2, and 3., the relations of which I ascertained in Clarke County, between +the rivers Alabama and Tombeckbee. + +[Illustration: Fig. 190. Cross section. + + 1. Sand, marl, &c., with numerous fossils. } + 2. White or rotten limestone, with _Zeuglodon_. } Eocene. + 3. Orbitoidal, or so called nummulitic limestone. } + 4. Overlying formation of sand and clay without fossils. Age unknown.] + +The lowest set of strata, No. 1., having a thickness of more than 100 +feet, comprise marly beds, in which the _Ostrea sellæformis_ occurs, a +shell ranging from Alabama to Virginia, and being a representative form +of the _Ostrea flabellula_ of the Eocene group of Europe. In others beds +of No. 1., two European shells, _Cardita planicosta_, before mentioned, +and _Solarium canaliculatum_ are found, with a great many other species +peculiar to America. Numerous corals, also, and the remains of placoid +fish and of rays occur, and the "swords," as they are called, of sword +fishes, all bearing a great generic likeness to those of the Eocene +strata of England and France. + +No. 2. (fig. 190.) is a white limestone, sometimes soft and argillaceous, +but in parts very compact and calcareous. It contains several peculiar +corals, and a large Nautilus allied to _N. zigzag_, also in its upper bed a +gigantic cetacean, called _Zeuglodon_ by Owen.[207-A] + +[2 Illustrations: _Zeuglodon cetoides_, Owen. _Basilosaurus_, Harlan.] + +Fig. 191. Molar tooth, natural size.] + +Fig. 192. Vertebra, reduced.] + +The colossal bones of this cetacean are so plentiful in the interior of +Clarke County as to be characteristic of the formation. The vertebral +column of one skeleton found by Dr. Buckley at a spot visited by me, +extended to the length of nearly 70 feet, and not far off part of another +backbone nearly 50 feet long was dug up. I obtained evidence, during a +short excursion, of so many localities of this fossil animal within a +distance of 10 miles, as to lead me to conclude that they must have +belonged to at least forty distinct individuals. + +Mr. Owen first pointed out that the huge animal was not reptilian, since +each tooth was furnished with double roots (see fig. 191.), implanted in +corresponding double sockets; and his opinion of the cetacean nature of +the fossil was afterwards confirmed by Dr. Wyman and Professor R. W. +Gibbes. That it was an extinct species of the whale tribe has since been +placed beyond all doubt by the discovery of the entire skull of another +fossil of the same family, found to have the double occipital condyles +only met with in mammals, and the convoluted tympanic bones which are +characteristic of cetaceans. + +Near the junction of No. 2. and the incumbent limestone, No. 3., next to be +mentioned, are strata characterized by the following shells: Spondylus +dumosus (_Plagiostoma dumosum_, Morton), _Pecten Poulsoni_, _Pecten +perplanus_, and _Ostrea cretacea_. + +No. 3. (fig. 190.) is a white limestone, for the most part made up of the +_Orbitoides_ of d'Orbigny before mentioned (p. 206.), formerly supposed to +be a nummulite, and called _N. Mantelli_, mixed with a few lunulites and +small corals and shells.[208-A] The origin of this cream-coloured soft +stone, like that of our white chalk, which it much resembles, is, I +believe, due to the decomposition of the orbitoides. The surface of the +country where it prevails is sometimes marked by the absence of wood, like +our chalk downs, or is covered exclusively by the _Juniperus Virginiana_, +as certain chalk districts in England by yew trees and juniper. + +Some of the shells of this limestone are common to the Claiborne beds, but +many of them are peculiar. + +It will be seen in the section (fig. 190. p. 155.) that the strata, Nos. 1, +2, 3., are, for the most part, overlaid by a dense formation of sand or +clay without fossils. In some points of the bluff or cliff of the Alabama +river, at Claiborne, the beds Nos. 1, 2., are exposed nearly from top to +bottom, whereas at other points the newer formation, No. 4., occupies the +face of nearly the whole cliff. The age of this overlying mass has not yet +been determined, as it has hitherto proved destitute of organic remains. + +The burr-stone strata of the Southern States contain so many fossils +agreeing with those of Claiborne, that it doubtless belongs to the same +part of the Eocene group, though I was not fortunate enough to see the +relations of the two deposits in a continuous section. Mr. Tuomey considers +it as the lower portion of the series. It may, perhaps, be a form of the +Claiborne beds in places where lime was wanting, and where silex, derived +from the decomposition of felspar, predominated. It consists chiefly of +slaty clays, quartzose sands, and loam, of a brick red colour, with layers +of chert or burr-stone, used in some places for millstones. + + +FOOTNOTES: + +[191-A] Leyde Magaz. voor Wetensch Konst en Lett., partie v. cahier i. p. +71. Cited by Rozet, Journ. de Géologie, tom. i. p. 43. + +[191-B] M. C. Prevost, Submersions Itératives, &c. Note 23. + +[192-A] Cuvier, Oss. Foss., tom. iii. p. 255. + +[194-A] This species is found both in the Paris and London basins. + +[197-A] Coquilles caractérist. des Terrains, 1831. + +[197-B] Quarterly Geol. Journal, vol. iii. p. 353. + +[199-A] Prestwich, Quart. Geol. Journ. vol. iii. p. 386. + +[199-B] Palæont. Soc. Monograph. Rept. pt. ii. p. 61. + +[202-A] For description of Eocene Cephalopoda, see Monograph by F. E. +Edwards, Palæontograph. Soc. 1849. + +[203-A] Annals of Nat. Hist. vol. iv. No. 23. Nov. 1839. + +[206-A] Murchison, Quart. Journ. of Geol. Soc. vol. v., and Lyell, vol. vi. +1850. Anniversary Address. + +[206-B] See paper by the author, Quart. Journ. Geol. Soc. vol. iv, p. 12.; +and Second Visit to the U. S. vol. ii. p. 59. + +[206-C] Quart. Journ. Geol Soc. vol. vi. p. 32. + +[207-A] See Memoir by R. W. Gibbes, Journ. of Acad. Nat. Sci. Philad. +vol. i. 1847. + +[208-A] Lyell, Quart. Journ. Geol. Soc. 1847, vol. iv. p. 15. + + + + +CHAPTER XVII. + +CRETACEOUS GROUP. + + Divisions of the cretaceous series in North-Western Europe--Upper + cretaceous strata--Maestricht beds--Chalk of Faxoe--White + chalk--Characteristic fossils--Extinct cephalopoda--Sponges and corals + of the chalk--Signs of open and deep sea--Wide area of white + chalk--Its origin from corals and shells--Single pebbles in + chalk--Siliceous sandstone in Germany contemporaneous with white + chalk--Upper greensand and gault--Lower cretaceous strata--Atherfield + section, Isle of Wight--Chalk of South of Europe--Hippurite + limestone--Cretaceous Flora--Chalk of United States. + + +Having treated in the preceding chapters of the tertiary strata, we have +next to speak of the uppermost of the secondary groups, called the Chalk +or Cretaceous (No. 6. Table, p. 103.), because in those parts of Europe +where it was first studied its upper members are formed of that +remarkable white earthy limestone, termed chalk (_creta_). The inferior +division consists, for the most part, of clays and sands, called +Greensand, because some of the sands derive a bright green colour from +intermixed grains of chloritic matter. The cretaceous strata in the +north-west of Europe may be thus divided[209-A]: + + _Upper Cretaceous._ + + 1. Maestricht beds and Faxoe limestone. + 2. Upper white chalk, with flints. + 3. Lower white chalk, without flints, passing downwards into chalk marl, + which is slightly argillaceous. + 4. Upper greensand. + 5. Gault. + + _Lower Cretaceous._ + + 6. Lower greensand--Ironsand, clay, and occasional beds of limestone + (Kentish rag). + +_Maestricht Beds._--On the banks of the Meuse, at Maestricht, reposing on +ordinary white chalk with flints, we find an upper calcareous formation +about 100 feet thick, the fossils of which are, on the whole, very +peculiar, and all distinct from tertiary species. Some few are of species +common to the inferior white chalk, among which may be mentioned +_Belemnites mucronatus_ (see fig. 197.) and _Pecten quadricostatus_. +Besides the Belemnite there are other _genera_, such as Ammonite, Baculite, +and Hamite, never found in strata newer than the cretaceous, but frequently +met with in these Maestricht beds. On the other hand, Volutes and other +genera of univalve shells, usually met with only in tertiary strata, occur. + +The upper part of the rock, about 20 feet thick, as seen in St. Peter's +Mount, in the suburbs of Maestricht, abounds in corals, often detachable +from the matrix; and these beds are succeeded by a soft yellowish +limestone 50 feet thick, extensively quarried from time immemorial for +building. The stone below is whiter, and contains occasional nodules of +grey chert or chalcedony. + +M. Bosquet, with whom I lately examined this formation (August, 1850), +pointed out to me a layer of chalk from 2 to 4 inches thick, containing +green earth and numerous encrinital stems, which forms the line of +demarcation between the strata containing the fossils peculiar to +Maestricht and the white chalk below. The latter is distinguished by +regular layers of black flint in nodules, and by several shells, such as +_Terebratula carnea_ (see fig. 201.), wholly wanting in beds higher than +the green band. Some of the organic remains, however, for which St. Peter's +Mount is celebrated, occur both above and below that parting layer, and, +among others, the great marine reptile, called _Mosasaurus_, a saurian +supposed to have been 24 feet in length, of which the entire skull and a +great part of the skeleton have been found. Such remains are chiefly met +with in the soft freestone, the principal member of the Maestricht beds. + +_Chalk of Faxoe._--In the island of Seeland, in Denmark, the newest member +of the chalk series, seen in the sea-cliffs at Stevens Klint resting on +white chalk with flints, is a yellow limestone, a portion of which, at +Faxoe, where it is used as a building-stone, is composed of corals, even +more conspicuously than is usually observed in recent coral reefs. It has +been quarried to the depth of more than 40 feet, but its thickness is +unknown. The imbedded shells are chiefly casts, many of them of univalve +mollusca, which, as they strictly belong to the Cretaceous era, are worthy +of notice, since such forms, whether spiral or patelliform, are wanting in +the white chalk of Europe generally. Thus, there are two species of +_Cypræa_, one of _Oliva_, two of _Mitra_, four of the genus _Cerithium_, +six of _Fusus_, two of _Trochus_, one _Patella_, one _Emarginula_, &c., on +the whole, more than thirty univalves, spiral or patelliform, not one of +which is common to the white chalk. At the same time, a large proportion of +the accompanying bivalve shells, echinoderms, and zoophytes, are +specifically identical with fossils of older parts of the Cretaceous +series. Among the cephalopoda of Faxoe, may be mentioned _Baculites +Faujasii_ and _Belemnites mucronatus_, shells of the white chalk. + +The claws and entire shell of a small crab, _Brachyurus rugosus_ +(Schlotheim), are scattered through the Faxoe stone, reminding us of +similar crustaceans enclosed in the rocks of many modern coral +reefs.[211-A] Some small portions of this coralline formation consist of +white earthy chalk; it is, therefore, clear that this substance must +have been produced simultaneously, a fact of some importance, as bearing +on the theory of the origin of white chalk; for the decomposition of +such corals as we see at Faxoe is capable, we know, of forming white +mud, undistinguishable from chalk, and which we may suppose to have +been dispersed far and wide through the ocean, in which such reefs as +that of Faxoe grew. + +[Illustration: Fig. 193. Section from Hertfordshire, in England, to +Sena, in France.] + +_White Chalk_ (2. and 3. Tab. p. 209.).--The highest beds of chalk in +England and France consist of a pure, white, calcareous mass, usually +too soft for a building stone, but sometimes passing into a more solid +state. It consists, almost purely, of carbonate of lime; the +stratification is often obscure, except where rendered distinct by +interstratified layers of flint, a few inches thick, occasionally in +continuous beds, but oftener in nodules, and recurring at intervals from +2 to 4 feet distant from each other. + +This upper chalk is usually succeeded, in the descending order, by a great +mass of white chalk without flints, below which comes the chalk marl, in +which there is a slight admixture of argillaceous matter. The united +thickness of the three divisions in the south of England equals, in some +places, 1000 feet.[211-B] + +The annexed section, fig. 193., will show the manner in which the white +chalk extends from England into France, covered by the tertiary strata +described in former chapters, and reposing on lower cretaceous beds. + +Among the conspicuous forms of mollusca wholly foreign to the tertiary and +recent periods, and which we meet with in the white chalk, are the +Belemnite, Ammonite, Baculite, and Turrilite, all genera of _Cephalopoda_, +a family to which the living cuttle-fish and nautilus belong. + +[Illustration: Fig. 194. Portion of _Baculites Faujasii_. Maestricht and +Faxoe beds and white chalk.] + +[Illustration: Fig. 195. Portion of _Baculites anceps_. Maestricht and +Faxoe beds and white chalk.] + +[Illustration: Fig. 196. Turrilites. + + _a._ _Turrilites costatus._ Chalk marl. + _b._ Same, showing the indented border of the partition of the chambers.] + +[Illustration: Fig. 197. Belemnites. + + _a._ _Belemnites mucronatus._ + _b._ Same, showing internal structure. + +Maestricht, Faxoe, and white chalk.] + +Among the brachiopoda in the white chalk, the _Terebratulæ_ are very +abundant. These shells are known to live at the bottom of the sea, where +the water is tranquil and of some depth (see figs. 198, 199, 200, 201.). +With these are associated some forms of oyster (see figs. 202. and 204.), +and other bivalves (figs. 203, 205, 206, 207, 208.). + +[Illustration: Fig. 198. _Terebratula plicatilis_, dorsal view. +Upper white chalk.] + +[Illustration: Fig. 199. _Terebratula plicatilis_, side view.] + +[Illustration: Fig. 200. _Terebratula pumilus._ (_Magas pumilus_, Sow.) +Upper white chalk.] + +[Illustration: Fig. 201. _Terebratula carnea._ Upper white chalk.] + +[Illustration: Fig. 202. _Ostrea vesicularis._ _Gryphæa globosa_, Min. Con. +Upper chalk and upper greensand.] + +[Illustration: Fig. 203. _Pecten 5-costatus._ White chalk, upper +and lower greensands.] + +[Illustration: Fig. 204. _Ostrea carinata._ Chalk marl, upper +and lower greensands.] + +[Illustration: Fig. 205. _Crania Parisiensis_, inferior or attached valve. +Upper white chalk.] + +[Illustration: Fig. 206. _Plagiostoma Hoperi_, Sow. Syn. _Lima Hoperi_. +White chalk and upper greensand.] + +[Illustration: Fig. 207. _Plagiostoma spinosum_, Sow. Syn. _Spondylus +spinosus_. Upper white chalk.] + +Among the rest, no form marks the cretaceous era in Europe, America, and +India, in a more striking manner than the extinct genus _Inoceramus_ +(_Catillus_ of Lamk.), the shells of which are distinguished by a +fibrous texture, and are often met with in fragments, having, probably, +been extremely friable. + +[Illustration: Fig. 208. _Inoceramus Lamarckii._ + +Syn. _Catillus Lamarckii_. + +White Chalk (Dixon's Geol. Sussex, Tab. 28. fig. 29.)] + +[Illustration: Fig. 209. _Eschara disticha._ + +_a._ Natural size. +_b._ Portion magnified. + +White chalk.] + +[2 Illustrations: Fig. 210. Fig. 211. A branching sponge in a flint, from +the white chalk. From the collection of Mr. Bowerbank.] + +With these mollusca are many corals (figs. 209, 210, 211.) and sea urchins +(fig. 212.), which are alike marine, and, for the most part, indicative of +a deep sea. They are dispersed indifferently through the soft chalk, and +hard flint, and some of the flinty nodules owe their irregular forms to +inclosed zoophytes, as in the specimen represented in fig. 211., where the +hollows in the exterior are caused by the branches of a sponge seen on +breaking open the flint, fig. 210. + +[Illustration: Fig. 212. _Ananchytes ovata_. White chalk, upper and lower. + + _a_. Side view. + _b_. Bottom of the shell on which both the oral and anal apertures are + placed; the anal being more round, and at the smaller end.] + +Of the singular family called _Rudistes_, by Lamarck, hereafter to be +mentioned, as extremely characteristic of the chalk of Southern Europe, +a single representative only (fig. 213.) has been discovered in the +white chalk of England. + +[4 Illustrations: _Hippurites Mortoni_, Mantell. Houghton, Sussex. White +chalk. Diameter one seventh of nat. size. + +Fig. 213. Two individuals deprived of their opercula, adhering together. + +Fig. 214. Same seen from above. + +Fig. 215. Transverse section of part of the wall of the shell, magnified + to show the structure. + +Fig. 216. Vertical section of the same. + +On the side where the shell is thinnest, there is one external furrow and +corresponding internal ridge, a, b. figs. 213, 214.; but they are usually +less prominent than in these figures. This species has been referred to +_Hippurites_, but does not, I believe, fully agree in character with that +genus. I have never seen the opercular piece, or _valve_, as it is called +by those conchologists who regard the _Rudistes_ as bivalve mollusca. The +specimen above figured was discovered by the late Mr. Dixon.] + +The remains of fishes of the Upper Cretaceous formations consist chiefly +of teeth of the shark family of genera, in part common to the tertiary, +and partly distinct. But we meet with no bones of land animals, nor any +terrestrial or fluviatile shells, nor any plants, except sea weeds, and +here and there a piece of drift wood. All the appearances concur in +leading us to conclude that the white chalk was the product of an open +sea of considerable depth. + +The existence of turtles and oviparous saurians, and of a Pterodactyl or +winged-lizard, found in the white chalk of Maidstone, implies, no doubt, +some neighbouring land; but a few small islets in mid-ocean, like +Ascension, so much frequented by migratory droves of turtles, might perhaps +have afforded the required retreat where these creatures might lay their +eggs in the sand, or from which the flying species may have been blown out +to sea. Of the vegetation of such islands we have scarcely any indication, +but it consisted partly of cycadeous plants; for a fragment of one of these +was found by Capt. Ibbetson in the chalk marl of the Isle of Wight, and is +referred by A. Brongniart to _Clathraria Lyellii_, Mantell, a species +common to the antecedent Wealden period. + +_Geographical extent and origin of the While Chalk._--The area over +which the white chalk preserves a nearly homogeneous aspect is so vast, +that the earlier geologists despaired of discovering any analogous +deposits of recent date. Pure chalk, of nearly uniform aspect and +composition, is met with in a north-west and south-east direction, from +the north of Ireland to the Crimea, a distance of about 1140 +geographical miles; and in an opposite direction it extends from the +south of Sweden to the south of Bordeaux, a distance of about 840 +geographical miles. In Southern Russia, according to Sir R. Murchison, +it is sometimes 600 feet thick, and retains the same mineral character +as in France and England, with the same fossils, including _Inoceramus +Cuvieri_, _Belemnites mucronatus_, and _Ostrea vesicularis_. + +But it would be an error to imagine, that the chalk was ever spread out +continuously over the whole of the space comprised within these limits, +although it prevailed in greater or less thickness over large portions of +that area. On turning to those regions of the Pacific where coral reefs +abound, we find some archipelagoes of lagoon islands, such as that of the +Dangerous Archipelago, for instance, and that of Radack, with several +adjoining groups, which are from 1100 to 1200 miles in length, and 300 or +400 miles broad; and the space to which Flinders proposed to give the name +of the Corralline Sea is still larger; for it is bounded on the east by the +Australian barrier--all formed of coral rock,--on the west by New +Caledonia, and on the north by the reefs of Louisiade. Although the islands +in these areas may be thinly sown, the mud of the decomposing zoophytes may +be scattered far and wide by oceanic currents. That this mud would resemble +chalk I have already hinted when speaking of the Faxoe limestone, p. 211.; +and it was also remarked in an early part of this volume, that some even of +that chalk which appears to an ordinary observer quite destitute of organic +remains, is nevertheless, when seen under the microscope, full of fragments +of corals and sponges; together with the valves of entomostraca, the shells +of foraminifera, and still more minute infusoria.[215-A] (See p. 26.) + +Now it had been often suspected, before these discoveries, that white +chalk might be of animal origin, even where every trace of organic +structure has vanished. This bold idea was partly founded on the fact, +that the chalk consisted of pure carbonate of lime, such as would result +from the decomposition of testacea, echini, and corals; and partly on +the passage observable between these fossils when half decomposed and +chalk. But this conjecture seemed to many naturalists quite vague and +visionary, until its probability was strengthened by new evidence +brought to light by modern geologists. + +We learn from Lieutenant Nelson, that, in the Bermuda Islands, there are +several basins or lagoons almost surrounded and enclosed by reefs of +coral. At the bottom of these lagoons a soft white calcareous mud is +formed by the decomposition of _Eschara_, _Flustra_, _Cellepora_, and +other corallines. This mud, when dried, is undistinguishable from common +white earthy chalk; and some portions of it, presented to the Museum of +the Geological Society of London, might, after full examination, be +mistaken for ancient chalk, but for the labels attached to them. About +the same time Mr. C. Darwin observed similar facts in the coral islands +of the Pacific; and came also to the opinion, that much of the soft +white mud found at the bottom of the sea near coral reefs has passed +through the bodies of worms, by which the stony masses of coral are +everywhere bored; and other portions through the intestines of fishes; +for certain gregarious fishes of the genus _Sparus_ are visible through +the clear water, browsing quietly, in great numbers, on living corals, +like grazing herds of graminivorous quadrupeds. On opening their bodies, +Mr. Darwin found their intestines filled with impure chalk. This +circumstance is the more in point, when we recollect how the fossilist +was formerly puzzled by meeting, in chalk, with certain bodies, called +cones of the larch, which were afterwards recognized by Dr. Buckland to +be the excrement of fish.[216-A] These spiral coprolites (see figures), +like the scales and bones of fossil fish in the chalk, are composed +chiefly of phosphate of lime. + +[2 Illustrations: Fig. 217. Fig. 218. Coprolites of fish called +_Iulo-eido-copri_, from the chalk.] + +Mr. Dana, when describing the elevated coral reef of Oahu, in the Sandwich +Islands, says, that some varieties of the rock consist of aggregated +shells, imbedded in a compact calcareous base as firm in texture as any +secondary limestone; while others are like chalk, having its colour, its +earthy fracture, its soft homogeneous texture, and being an equally good +writing material. The same author describes, in many growing coral reefs, a +similar formation of modern chalk, undistinguishable from the +ancient.[216-B] The extension over a wide submarine area of the calcareous +matrix of the chalk, as well as of the imbedded fossils, would take place +the more readily, in consequence of the low specific gravity of the shells +of mollusca and zoophytes, when compared with ordinary sand and mineral +matter. The mud also derived from their decomposition would be much lighter +than argillaceous and other inorganic mud, and very easily transported by +currents, especially in salt water. + +_Single pebbles in chalk._--The general absence of sand and pebbles in +the white chalk has been already mentioned; but the occurrence here and +there, in the south-east of England, of a few isolated pebbles of quartz +and green schist, some of them 2 or 3 inches in diameter, has justly +excited much wonder. If these had been carried to the spots where we now +find them by waves or currents from the lands once bordering the +cretaceous sea, how happened it that no sand or mud were transported +thither at the same time? We cannot conceive such rounded stones to have +been drifted like erratic blocks by ice[217-A], for that would imply a +cold climate in the Cretaceous period; a supposition inconsistent with +the luxuriant growth of large chambered univalves, numerous corals, and +many fish, and other fossils of tropical forms. + +Now in Keeling Island, one of those detached masses of coral which rise up +in the wide Pacific, Captain Ross found a single fragment of greenstone, +where every other particle of matter was calcareous; and Mr. Darwin +concludes that it must have come there entangled in the roots of a large +tree. He reminds us that Chamisso, the distinguished naturalist who +accompanied Kotzebue, affirms, that the inhabitants of the Radack +archipelago, a group of lagoon islands, in the midst of the Pacific, +obtained stones for sharpening their instruments by searching the roots of +trees which are cast up on the beach.[217-B] + +It may perhaps be objected, that a similar mode of transport cannot have +happened in the cretaceous sea, because fossil wood is very rare in the +chalk. Nevertheless wood is sometimes met with, and in the same parts of +the chalk where the pebbles are found, both in soft stone and in a +silicified state in flints. In these cases it has often every appearance of +having been floated from a distance, being usually perforated by +boring-shells, such as the _Teredo_ and _Fistulana_.[217-C] + +The only other mode of transport which suggests itself is sea-weed. Dr. +Beck informs me, that in the Lym-Fiord, in Jutland, the _Fucus +vesiculosus_, often called kelp, sometimes grows to the height of 10 +feet, and the branches rising from a single root form a cluster several +feet in diameter. When the bladders are distended, the plant becomes so +buoyant as to float up loose stones several inches in diameter, and +these are often thrown by the waves high up on the beach. The _Fucus +giganteus_ of Solander, so common in Terra del Fuego, is said by Captain +Cook to attain the length of 360 feet, although the stem is not much +thicker than a man's thumb. It is often met with floating at sea, with +shells attached, several hundred miles from the spots where it grew. +Some of these plants, says Mr. Darwin, were found adhering to large +loose stones in the inland channels of Terra del Fuego, during the +voyage of the Beagle in 1834; and that so firmly, that the stones were +drawn up from the bottom into the boat, although so heavy that they +could scarcely be lifted in by one person. Some fossil sea-weeds have +been found in the Cretaceous formation, but none, as yet, of large size. + +But we must not imagine that because pebbles are so rare in the white +chalk of England and France there are no proofs of sand, shingle, and +clay having been accumulated contemporaneously even in the European +seas. The siliceous sandstone, called "upper quader" by the Germans, +overlies white argillaceous chalk, or "pläner-kalk," a deposit +resembling in composition and organic remains the chalk marl of the +English series. This sandstone contains as many fossil shells common to +our white chalk as could be expected in a sea-bottom formed of such +different materials. It sometimes attains a thickness of 600 feet, and +by its jointed structure and vertical precipices, plays a conspicuous +part in the picturesque scenery of Saxon Switzerland, near Dresden. + +_Upper greensand_ (4. Tab. p. 209.).--The lower chalk without flints passes +gradually downwards, in the south of England, into an argillaceous +limestone, "the chalk marl," already alluded to, in which ammonites and +other cephalopoda, so rare in the higher parts of the series, appear. This +marly deposit passes in its turn into beds containing green particles of a +chloritic mineral, called the upper greensand. In parts of Surrey +calcareous matter is largely intermixed, forming a stone called +_firestone_. In the cliffs of the southern coast of the Isle of Wight, this +upper greensand is 100 feet thick, and contains bands of siliceous +limestone and calcareous sandstone with nodules of chert. + +[2 Illustrations: Fossils of the Upper Greensand. + +Fig. 219. + + _a._ _Terebratula lyra._ } Upper greensand. + _b._ Same, seen in profile. } France. + +Fig. 220. _Ammonites Rhotomagensis._ Upper greensand.] + +[Illustration: Fig. 221. _Hamites spiniger_ (Fitton); +near Folkstone. Gault.] + +_Gault._--The lowest member of the upper Cretaceous group, usually about +100 feet thick in the S.E. of England, is provincially termed Gault. It +consists of a dark blue marl, sometimes intermixed with greensand. Many +peculiar forms of cephalopoda, such as the _Hamite_ (fig. 221.) and +_Scaphite_, with other fossils, characterize this formation, which, small +as is its thickness, can be traced by its organic remains to distant parts +of Europe, as, for example, to the Alps. + +The phosphate of lime, found lately near Farnham, in Surrey, in such +abundance as to be used largely by the agriculturist for fertilizing soils, +occurs exclusively, according to Mr. R. A. C. Austen, in the upper +greensand and gault. It is doubtless of animal origin, and partly +coprolitic, probably derived from the excrement of fish. + + +LOWER CRETACEOUS DIVISION. (No. 6. Tab. p. 209.) + +That part of the Cretaceous series which is older than the Gault has been +commonly called the Lower Greensand. The greater number of its fossils are +specifically distinct from those of the upper cretaceous system. Dr. +Fitton, to whom we are indebted for an excellent monograph on this +formation as developed in England, gives the following as the succession of +rocks seen in parts of Kent. + + No. 1. Sand, white, yellowish, or ferruginous, with + concretions of limestone and chert 70 feet. + 2. Sand with green matter 70 to 100 feet. + 3. Calcareous stone, called Kentish rag 60 to 80 feet. + +In his detailed description of the fine section displayed at Atherfield, +in the south of the Isle of Wight, we find the limestone wholly wanting; +in fact, the variations in the mineral composition of this group, even +in contiguous districts, is very great; and on comparing the Atherfield +beds with corresponding strata at Hythe in Kent, distant 95 miles, +the whole series has lost half its thickness, and presents a very +dissimilar aspect.[219-A] + +On the other hand, Professor E. Forbes has shown that when the sixty-three +strata at Atherfield are severally examined, the total thickness of which +he gives as 843 feet, there are some fossils which range through the whole +series, others which are peculiar to particular divisions. As a proof that +all belong chronologically to one system, he states that whenever similar +conditions are repeated in overlying strata the same species reappear. +Changes of depth, or of the mineral nature of the bottom, the presence or +absence of lime or of peroxide of iron, the occurrence of a muddy, or a +sandy, or a gravelly bottom, are marked by the banishment of certain +species and the predominance of others. But these differences of conditions +being mineral, chemical, and local in their nature, have nothing to do with +the extinction, throughout a large area, of certain animals or plants. The +rule laid down by this eminent naturalist for enabling us to test the +arrival of a new state of things in the animate world, is the +representation by new and different species of corresponding genera of +mollusca or other beings. When the forms proper to loose sand or soft clay, +or a stony or calcareous bottom, or a moderate or a great depth of water, +recur with all the same species, the interval of time has been, +geologically speaking, small, however dense the mass of matter accumulated. +But if, the genera remaining the same, the species are changed, we have +entered upon a new period; and no similarity of climate, or of geographical +and local conditions, can then recall the old species which a long series +of destructive causes in the animate and inanimate world has gradually +annihilated. On passing from the lower greensand to the gault, we suddenly +reach one of these new epochs, scarcely any of the fossil species being +common to the lower and upper cretaceous systems, a break in the chain +implying no doubt many missing links in the series of geological monuments +which we may some day be able to supply. + +One of the largest and most abundant shells in the lowest strata of the +lower greensand, as displayed in the Atherfield section, is the large +_Perna mulleti_ of which a reduced figure is here given (fig. 222.). + +[Illustration: Fig. 222. _Perna mulleti._ Desh. in Leym. + + _a._ Exterior. + _b._ Hinge of upper valve.] + +In the south of England, during the accumulation of the lower greensand +above described, the bed of the sea appears to have been continually +sinking, from the commencement of the period, when the freshwater +Wealden beds were submerged, to the deposition of those strata on which +the gault immediately reposes. + +Pebbles of quartzose sandstone, jasper, and flinty slate, together with +grains of chlorite and mica, speak plainly of the nature of the +pre-existing rocks, from the wearing down of which the greensand beds +were derived. The land, consisting of such rocks, was doubtless +submerged before the origin of the white chalk, as corals can only +multiply in the clear waters of the sea in spaces to which no mud or +sand are conveyed by currents. + + +HIPPURITE LIMESTONE. + +_Difference between the chalk of the north and south of Europe._--By the +aid of the three tests of relative age, namely, superposition, mineral +character, and fossils, the geologist has been enabled to refer to the same +Cretaceous period certain rocks in the north and south of Europe, which +differ greatly, both in their fossil contents and in their mineral +composition and structure. + +If we attempt to trace the cretaceous deposits from England and France to +the countries bordering the Mediterranean, we perceive, in the first place, +that the chalk and Greensand in the neighbourhood of London and Paris form +one great continuous mass, the Straits of Dover being a trifling +interruption, a mere valley with chalk cliffs on both sides. We then +observe that the main body of the chalk which surrounds Paris stretches +from Tours to near Poitiers (see the annexed map, fig. 223., in which the +shaded part represents chalk). + +[Illustration: Fig. 223. Map of south-western France.] + +Between Poitiers and La Rochelle, the space marked A on the map separates +two regions of chalk. This space is occupied by the Oolite and certain +other formations older than the Chalk, and has been supposed by M. E. de +Beaumont to have formed an island in the cretaceous sea. South of this +space we again meet with a formation which we at once recognize by its +mineral character to be chalk, although there are some places where the +rock becomes oolitic. The fossils are, upon the whole, very similar; +especially certain species of the genera _Spatangus_, _Ananchytes_, +_Cidarites_, _Nucula_, _Ostrea_, _Gryphæa_ (_Exogyra_), _Pecten_, +_Plagiostoma_ (_Lima_), _Trigonia_, _Catillus_, (_Inoceramus_), and +_Terebratula_.[221-A] But _Ammonites_, as M. d'Archiac observes, of which +so many species are met with in the chalk of the north of France, are +scarcely ever found in the southern region; while the genera _Hamite_, +_Turrilite_, and _Scaphite_, and perhaps _Belemnite_, are entirely wanting. + +On the other hand, certain forms are common in the south which are rare or +wholly unknown in the north of France. Among these may be mentioned many +_Hippurites_, _Sphærulites_, and other members of that great family of +mollusca called _Rudistes_ by Lamarck, to which nothing analogous has been +discovered in the living creation, but which is quite characteristic of +rocks of the Cretaceous era in the south of France, Spain, Sicily, Greece, +and other countries bordering the Mediterranean. + +[Illustration: Fig. 224. + + _a._ _Radiolites radiosus_, D'Orb. (_Hippurites_, Lamk.) + _b._ Opercular valve of same. + +White chalk of France.] + +[Illustration: Fig. 225. _Radiolites foliaceus_, D'Orb. Syn. _Sphærulites +agariciformis_, Blainv. White chalk of France.] + +[Illustration: Fig. 226. _Hippurites organisans_, Desmoulins. Upper +chalk:--chalk marl of Pyrenees?[222-A] + + _a._ Young individual; when full grown they occur in groups adhering + laterally to each other. + _b._ Upper side of the opercular valve, showing a reticulated structure + in those parts, _b_, where the external coating is worn off. + _c._ Upper side of the lower and cylindrical valve. + _d._ Cast of the interior of the lower conical valve.] + +The species called _Hippurites organisans_ (fig. 226.) is more abundant +than any other in the south of Europe; and the geologist should make +himself well acquainted with the cast _d_, which is far more common in many +compact marbles of the upper cretaceous period than the shell itself, which +has often wholly disappeared. The flutings, or smooth, rounded, +longitudinal ribs, representing the form of the interior, are wholly +unlike the hippurite itself, and in some individuals, which attain a great +size and length, are very conspicuous. + +Between the region of chalk last mentioned in which Perigueux is situated, +and the Pyrenees, the space B intervenes. (See Map, p. 221.) Here the +tertiary strata cover, and for the most part conceal, the cretaceous rocks, +except in some spots where they have been laid open by the denudation of +newer formations. In these places they are seen still preserving the form +of a white chalky rock, which is charged in part with grains of green sand. +Even as far south as Tercis, on the Adour, near Dax, where I examined them +in 1828, the cretaceous rocks retain this character. In that region M. +Grateloup has found in them _Ananchytes ovata_ (fig. 212.), and other +fossils of the English chalk, together with _Hippurites_. + + +FLORA OF THE CRETACEOUS PERIOD. + +Although the fossil plants of the Cretaceous era at present known are +few in number, the rocks being principally marine, they suffice, +according to M. Ad. Brongniart, to show a transition character between +the vegetation of the secondary and that of the tertiary formations. The +tertiary strata, when compared to the older rocks, are marked by the +predominance of _Exogens_, which now constitute three-fourths of the +living plants of the globe.[223-A] + +These exogens are wanting in the secondary strata generally, but in the +Cretaceous period they equal in number the _Gymnogens_ (_Coniferæ_ and +_Cycadeæ_) which abounded so much in the preceding Oolitic period, and +disappeared before the Eocene rocks were formed.[223-B] The discovery of a +tree-fern in the ferruginous sands of the Lower Cretaceous group of the +department of Ardennes in France is one of many signs of the contrast of +the flora, and doubtless of the climate, of this era with that of the +Pliocene and Modern periods. + + +CRETACEOUS ROCKS IN THE UNITED STATES. + +If we pass to the American continent, we find in the state of New Jersey a +series of sandy and argillaceous beds wholly unlike our Upper Cretaceous +system; which we can, nevertheless, recognize as referable, +paleontologically, to the same division. + +That they were about the same age generally as the European chalk and +greensand, was the conclusion to which Dr. Morton and Mr. Conrad came after +their investigation of the fossils in 1834. The strata consist chiefly of +greensand and green marl, with an overlying coralline limestone of a pale +yellow colour, and the fossils, on the whole, agree most nearly with those +of the upper European series, from the Maestricht beds to the gault +inclusive. I collected sixty shells from the New Jersey deposits in 1841; +five of which were identical with European species--_Ostrea larva_, _O. +vesicularis_, _Gryphæa costata_, _Pecten quinque-costatus_, _Belemnites +mucronatus_. As some of these have the greatest vertical range in Europe, +they might be expected more than any others to recur in distant parts of +the globe. Even where the species are different, the generic forms, such as +the Baculite and certain sections of Ammonites, as also the Inoceramus (see +above, fig. 208.) and other bivalves, have a decidedly cretaceous aspect. +Fifteen out of the sixty shells above alluded to, were regarded by +Professor Forbes as good geographical representatives of well-known +cretaceous fossils of Europe. The correspondence, therefore, is not small, +when we reflect that the part of the United States where these strata occur +is between 3000 and 4000 miles distant from the chalk of Central and +Northern Europe, and that there is a difference of ten degrees in the +latitude of the places compared on opposite sides of the Atlantic.[224-A] + +Fish of the genera _Lamna_, _Galeus_, and _Carcharias_ are common to New +Jersey and the European cretaceous rocks. So also is the genus _Mosasaurus_ +among reptiles, and _Pliosaurus_ (Owen), another saurian likewise obtained +from the English chalk. From New Jersey the cretaceous formation extends +southwards to North Carolina, Georgia, and Alabama, cropping out at +intervals from beneath the tertiary strata, between the Appalachian +Mountains and the Atlantic. They then sweep round the southern extremity of +that chain, and stretch northwards again to Tennessee and Kentucky. They +have also been traced far up the valley of the Missouri 275 English miles +above its mouth, to the neighbourhood of Fort Leavenworth; and southwards +to Texas, according to the observations of Ferdinand Römer; so that already +the area which they are ascertained to occupy in North America may perhaps +equal their extent in Europe. So little do they resemble mineralogically +the European white chalk, that limestone in North America is, upon the +whole, an exception to the rule; and, even in Alabama, where I saw a +calcareous member of this group, the marlstones are much more like the +English and French Lias than any other secondary deposit of the Old World. + +At the base of the system in Alabama I found dense masses of shingle, +perfectly loose and unconsolidated, derived from the waste of paleozoic +(or carboniferous) rocks, a mass in no way distinguishable, except +by its position, from ordinary alluvium, but covered with marls +abounding in Inocerami. + +In Texas, according to F. Römer, the chalk assumes a new lithological type, +a large portion of it consisting of hard siliceous limestone, but the +organic remains leaving no doubt in regard to its age. + +In South America the cretaceous strata have been discovered in Columbia, as +at Bogota and elsewhere, containing Ammonites, Hamites, Inocerami, and +other characteristic shells.[225-A] + +In the South of India, also, at Pondicherry, Verdachellum, and +Trinconopoly, Messrs. Kaye and Egerton have collected fossils belonging to +the cretaceous system. Taken in connection with those from the United +States they prove, says Prof. E. Forbes, that those powerful causes which +stamped a peculiar character on the forms of marine animal life at this +period, exerted their full intensity through the Indian, European, and +American seas.[225-B] Here, as in North and South America, the cretaceous +character can be recognized even where there is no specific identity in the +fossils; and the same may be said of the organic type of those rocks in +Europe and India which succeed next in the ascending and descending order, +the Eocene and the Oolitic. + + +FOOTNOTES: + +[209-A] M. Alcide d'Orbigny, in his valuable work entitled Paléontologie +Française, has adopted new terms for the French subdivisions of the +Cretaceous Series, which, so far as they can be made to tally with English +equivalents, seem explicable thus: + + Danien. Maestricht beds. + Senonien. Upper and lower white chalk, and chalk marl. + Turonien. Part of the chalk marl and the upper greensand, the latter + being in his last work (Cours Elémentaire) termed + Cénomanien. + Albien. Gault. + Aptien. Upper part of lower greensand. + Neocomien. Lower part of same. + +[211-A] See paper by the author, Trans. of Geol. Soc., vol. v. +p. 246., 1840. + +[211-B] Fitton, Geol. Trans., 2d series, vol. iv. p. 319. + +[215-A] Proceedings of Geol. Soc., vol. iii. pp. 7, 8., 1842. + +[216-A] Geol. Trans. Second Series, vol. iii. p. 232. plate 31. +figs. 3. and 11. + +[216-B] Geol. of U. S. Exploring Exped. p. 252. 1849. + +[217-A] See Chapters X. and XI. + +[217-B] Darwin, p. 549. Kotzebue's First Voyage, vol. iii. p. 155. + +[217-C] Mantell, Geol. of S. E. of England, p. 96. + +[219-A] Dr. Fitton, Quart. Geol. Journ., vol. i. p. 179., ii. p. 55., +and iii. p. 289., where comparative sections and a valuable table +showing the vertical range of the various fossils of the lower greensand +at Atherfield is given. + +[221-A] Archiac, sur la Form. Crétacée du S. O. de la France, Mém. de la +Soc. Géol. de France, tom. ii. + +[222-A] D'Orbigny's Paléontologie Française, pl. 533. + +[223-A] In this and subsequent remarks on fossil plants I shall often +use Dr. Lindley's terms, as most familiar in this country; but as those +of M. A. Brongniart are much cited, it may be useful to geologists to +give a table explaining the corresponding names of groups so much spoken +of in palæontology. + + | Brongniart. + | | |Lindley. + | | | Examples. + Cryptogamic. + | |1. Cryptogamous amphigens, or cellular cryptogamic. + | | |Thallogens. + | | | |Lichens, sea-weeds, fungi. + | |2. Cryptogamous acrogens. + | | |Acrogens. + | | | |Mosses, equisetums, ferns, + | | | |lycopodiums--Lepidodendron. + | | | | + Phanerogamic. + | |3. Dicotyledonous gymnosperms. + | | |Gymnogens. + | | | |Conifers and Cycads. + | |4. Dicot. Angiosperms. + | | |Exogens. + | | | |Compositæ, leguminosæ, umbelliferæ, + | | | |cruciferæ, heaths, &c. All native + | | | |European trees except conifers. + | |5. Monocotyledons. + | | |Endogens. + | | | |Palms, lilies, aloes, rushes, + | | | |grasses, &c. + +[223-B] A. Brongniart, Veget. Foss. Dict. Univ., p. 111., 1849. + +[224-A] See a paper by the author, Quart. Journ. Geol. Soc., vol. i. p. 55. + +[225-A] Proceed. Geol. Soc. iv. p. 391. + +[225-B] See Forbes, Quart. Geol. Journ. vol. i. p. 79. + + + + +CHAPTER XVIII. + +WEALDEN GROUP. + + The Wealden divisible into Weald Clay, Hastings Sand, and Purbeck + Beds--Intercalated between two marine formations--Weald clay and + Cypris-bearing strata--Iguanodon--Hastings sands--Fossil fish--Strata + formed in shallow water--Brackish water-beds--Upper, middle, and lower + Purbeck--Alternations of brackish water, freshwater, and + land--Dirt-bed, or ancient soil--Distinct species of fossils in each + subdivision of the Wealden--Lapse of time implied--Plants and insects + of Wealden--Geographical extent of Wealden--Its relation to the + cretaceous and oolitic periods--Movements in the earth's crust to + which it owed its origin and submergence. + + +Beneath the cretaceous rocks in the S.E. of England, a freshwater formation +is found, called the Wealden (see Nos. 5. and 6. Map, p. 242.), which, +although it occupies a small horizontal area in Europe, as compared to the +chalk, is nevertheless of great geological interest, not only from its +position, as being interpolated between two great marine formations (Nos. +7. and 9. Table, p. 103.), but also because the imbedded fossils indicate a +grand succession of changes in organic life, effected during its +accumulation. It is composed of three minor divisions, the Weald Clay, the +Hastings, and the Purbeck Beds, of which the aggregate thickness in some +districts may be 700 or 800 feet; but which would be much more considerable +(perhaps 2000 feet), were we to add together the extreme thickness acquired +by each of them in their fullest development. + +The common name of Wealden was given to the whole, because it was first +studied in parts of Kent, Surrey, and Sussex, called the Weald, (see Map, +p. 242.), and we are indebted to Dr. Mantell for having shown in 1822, in +his Geology of Sussex, that the whole group was of fluviatile origin. In +proof of this he called attention to the entire absence of Ammonites, +Belemnites, Terebratulæ, Echinites, Corals, and other marine fossils, so +characteristic of the cretaceous rocks above, and of the Oolitic strata +below, and to the presence of Paludinæ, Melaniæ, and various fluviatile +shells, as well as the bones of terrestrial reptiles and the trunks and +leaves of land plants. + +[Illustration: Fig. 227. Position of the Wealden between two +marine formations.] + +The evidence of so unexpected a fact as the infra-position of a dense mass +of purely freshwater origin to a deep-sea deposit (a phenomenon with which +we have since become familiar, in other chapters of the earth's +autobiography), was received, at first, with no small doubt and +incredulity. But the relative position of the beds is unequivocal; the +Weald Clay being distinctly seen to pass beneath the Greensand in various +parts of Surrey, Kent, and Sussex; and if we proceed from Sussex westward +to the Vale of Wardour, we there again observe the same formation, or, at +least, the lower division of it, the Purbeck, occupying the same relative +position, and resting on the Oolite (see fig. 228.). Or if we pass from the +base of the South Downs in Sussex, and cross to the Isle of Wight, we there +again meet with the Wealden series reappearing beneath the Greensand, and +cannot doubt that the beds are prolonged subterraneously, as indicated by +the dotted lines in fig. 229. + +[Illustration: Fig. 228. Cross section. + + O, Oolite. + G S, Greensand, or Lower Cretaceous.] + +[Illustration: Fig. 229. Cross section.] + +The minor groups into which the Wealden has been commonly divided in +England are, as before stated, three, and they succeed each other in the +following descending order[227-A]:-- + + Thickness. + 1st. Weald Clay, sometimes including thin beds of + sand and shelly limestone 140 to 280 ft. + 2d. Hastings Sand, in which occur some clays and + calcareous grits 400 to 500 ft. + 3d. Purbeck Beds, consisting of various kinds of + limestones and marls 150 to 200 ft. + + +_Weald Clay._ + +The first division, or Weald Clay, is of purely freshwater origin. The +uppermost beds are not only conformable, as Dr. Fitton observes, to the +inferior strata of the Lower Greensand, but of similar mineral composition. +To explain this, we may suppose, that as the delta of a great river was +tranquilly subsiding, so as to allow the sea to encroach upon the space +previously occupied by freshwater, the river still continued to carry down +the same sediment into the sea. In confirmation of this view it may be +stated, that the remains of the _Iguanodon Mantelli_, a gigantic +terrestrial reptile, very characteristic of the Wealden, has been +discovered near Maidstone, in the overlying Kentish rag, or marine +limestone of the Lower Greensand. Hence we may infer that some of the +saurians which inhabited the country of the great river continued to live +when part of the country had become submerged beneath the sea. Thus, in our +own times, we may suppose the bones of large alligators to be frequently +entombed in recent freshwater strata in the delta of the Ganges. But if +part of that delta should sink down so as to be covered by the sea, marine +formations might begin to accumulate in the same space where freshwater +beds had previously been formed; and yet the Ganges might still pour down +its turbid waters in the same direction, and carry seaward the carcasses of +the same species of alligator, in which case their bones might be included +in marine as well as in subjacent freshwater strata. + +The Iguanodon, first discovered by Dr. Mantell, has left more of its +remains in the Wealden strata of the south-eastern counties, and Isle of +Wight, than any other genus of associated saurians. It was an +herbivorous reptile, and regarded by Cuvier as more extraordinary than +any with which he was acquainted; for the teeth, though bearing a great +analogy to the modern Iguanas which now frequent the tropical woods of +America and the West Indies, exhibit many striking and important +differences (see fig. 230.). It appears that they have been worn by +mastication; whereas the existing herbivorous reptiles clip and gnaw off +the vegetable productions on which they feed, but do not chew them. +Their teeth, when worn, present an appearance of having been chipped +off, and never, like the fossil teeth of the Iguanodon, have a flat +ground surface (see fig. 231.), resembling the grinders of herbivorous +mammalia. Dr. Mantell computes that the teeth and bones of this animal +which have passed under his examination during the last twenty years, +must have belonged to no less than seventy-one distinct individuals; +varying in age and magnitude from the reptile just burst from the egg, +to one of which the femur measured 24 inches in circumference. Yet +notwithstanding that the teeth were more numerous than any other bones, +it is remarkable that it was not till the relics of all these +individuals had been found, that a solitary example of part of a +jaw-bone was obtained. More recently remains both of the upper and lower +jaw have been met with in the Hastings Beds in Tilgate Forest. Their +size was somewhat greater than had been anticipated, and even allowing +that the tail was short, which Professor Owen infers from the short +bodies of the caudal vertebræ, Dr. Mantell estimates the probable length +of some of these saurians at between 30 and 40 feet. The largest femur +yet found measures 4 feet 8 inches in length, the circumference of the +shaft being 25 inches, and round the condyles 42 inches. + +[2 Illustrations: Teeth of Iguanodon. + +Fig. 230. Partially worn tooth of a young animal. (Mantell.) + +Fig. 231. Crown of tooth in adult, worn down. (Mantell.)] + +Occasionally bands of limestone, called Sussex Marble, occur in the Weald +Clay, almost entirely composed of a species of _Paludina_, closely +resembling the common _P. vivipara_ of English rivers. + +[Illustration: Fig. 232. _Cypris spinigera_, Fitton.] + +[Illustration: Fig. 233. _Cypris Valdensis_, Fitton. (_C. faba_, +Min. Con. 485.)] + +[Illustration: Fig. 234. _Cypris tuberculata_, Fitton.] + +[Illustration: Fig. 235. Sample with lamination.] + +Shells of the _Cypris_, an animal belonging to the Crustacea, and before +mentioned (p. 31.) as abounding in lakes and ponds, are also plentifully +scattered through the clays of the Wealden, sometimes producing, like +the plates of mica, a thin lamination (see fig. 235.). Similar +cypriferous marls are found in the lacustrine tertiary beds of +Auvergne (see above, p. 183.). + + +_Hastings Sands._ + +This middle division of the Wealden consists of sand, calciferous grit, +clay, and shale; the argillaceous strata, notwithstanding the name, +being nearly in the same proportion as the arenaceous. The calcareous +sandstone and grit of Tilgate Forest, near Cuckfield, in which the +remains of the Iguanodon and Hyleosaurus were first found, constitute an +upper member of this formation. The white "sand-rock" of the Hastings +cliffs, about 100 feet thick, is one of the lower members of the same. +The reptiles, which are very abundant in it, consist partly of saurians, +already referred by Owen and Mantell to eight genera, among which, +besides those already enumerated, we find the Megalosaurus and +Plesiosaurus. The Pterodactyl, also a flying reptile, is met with in the +same strata, and many remains of Testudinata of the genera _Trionyx_ and +_Emys_, now confined to tropical regions. + +[Illustration: Fig. 236. _Lepidotus Mantelli_, Agass. Wealden. + + _a._ palate and teeth. + _b._ side view of teeth. + _c._ scale.] + +The fishes of the Wealden belong partly to the genera _Pycnodus_ and +_Hybodus_ (see figure of genus in Chap. XXI.), forms common to the Wealden +and Oolite; but the teeth and scales of a species of _Lepidotus_ are most +widely diffused (see fig. 236.). The general form of these fish was that of +the carp tribe, although perfectly distinct in anatomical character, and +more allied to the pike. The whole body was covered with large rhomboidal +scales, very thick, and having the exposed part covered with enamel. Most +of the species of this genus are supposed to have been either river fish, +or inhabitants of the coasts, having not sufficient powers of swimming to +advance into the deep sea. + +[Illustration: Fig. 237. _Corbula alata_, Fitton. Magnified.] + +The shells of the Hastings beds belong to the genera _Melanopsis_, +_Melania_, _Paludina_, _Cyrena_, _Cyclas_, _Unio_, and others, which +inhabit rivers or lakes; but one band has been found in Dorsetshire +indicating a brackish state of the water, and, in some places, even a +saltness, like that of the sea, where the genera _Corbula_ (see fig. 237.), +_Mytilus_, and _Ostrea_ occur. At different heights in the Hastings Sand, +in the middle of the Wealden, we find again and again slabs of sandstone +with a strong ripple-mark, and between these slabs beds of clay many yards +thick. In some places, as at Stammerham, near Horsham, there are +indications of this clay having been exposed so as to dry and crack before +the next layer was thrown down upon it. The open cracks in the clay have +served as moulds, of which casts have been taken in relief, and which are, +therefore, seen on the lower surface of the sandstone (see fig. 238.). + +[Illustration: Fig. 238. Underside of slab of sandstone about one yard in +diameter. Stammerham, Sussex.] + +Near the same place a reddish sandstone occurs in which are innumerable +traces of a fossil vegetable, apparently _Sphenopteris_, the stems and +branches of which are disposed as if the plants were standing erect on the +spot where they originally grew, the sand having been gently deposited upon +and around them; and similar appearances have been remarked in other places +in this formation.[230-A] In the same division also of the Wealden, at +Cuckfield, is a bed of gravel or conglomerate, consisting of water-worn +pebbles of quartz and jasper, with rolled bones of reptiles. These must +have been drifted by a current, probably in water of no great depth. + +[Illustration: Fig. 239. _Sphenopteris gracilis_ (Fitton), from +near Tunbridge Wells. + +_a._ portion of the same magnified.] + +From such facts we may infer that, notwithstanding the great thickness of +this division of the Wealden (and the same observation applies to the Weald +Clay and Purbeck Beds), the whole of it was a deposit in water of a +moderate depth, and often extremely shallow. This idea may seem startling +at first, yet such would be the natural consequence of a gradual and +continuous sinking of the ground in an estuary or bay, into which a great +river discharged its turbid waters. By each foot of subsidence, the +fundamental rock, such as the Portland Oolite, would be depressed one foot +farther from the surface; but the bay would not be deepened, if newly +deposited mud and sand should raise the bottom one foot. On the contrary, +such new strata of sand and mud might be frequently laid dry at low water, +or overgrown for a season by a vegetation proper to marshes. + + +_Purbeck Beds._ + +Immediately below the Hastings Sands we find a series of calcareous +slates, marls, and limestones, called the Purbeck Beds, because well +exposed to view in the sea-cliffs of the Peninsula of Purbeck, +especially in Durlestone Bay, near Swanage. They may also be +advantageously studied at Lulworth Cove and the neighbouring bays +between Weymouth and Dorchester. At Meup's Bay in particular, Prof. E. +Forbes has recently examined minutely the organic remains of the three +members of the Purbeck group, displayed there in a vertical section 155 +feet thick. To the information previously supplied in the works of +Messrs. Webster, Fitton, De la Beche, Buckland, and Mantell, he has made +most ample and important additions, so that it will be desirable to give +them at some length, it appearing that the Upper, Middle, and Lower +Purbecks are each marked by peculiar species of organic remains, these +again being different, so far as a comparison has yet been instituted, +from the fossils of the overlying Hastings Sands and Weald Clay. This +result cannot fail to excite much wonder, and it leads us to suspect +that the Wealden period, which many geologists have scarcely deigned +to notice in their classification, may comprehend the history of a +lapse of time as great as that of the Oolitic or Cretaceous +eras respectively.[231-A] + +_Upper Purbeck._--The highest of the three divisions is purely freshwater, +the strata, about 50 feet in thickness, containing shells of the genera +_Paludina_, _Physa_, _Lymnea_, _Planorbis_, _Valvata_, _Cyclas_, and +_Unio_, with cyprides, and fish. + +_Middle Purbeck._--To these succeed the Middle Purbeck, about 30 feet +thick, the uppermost part of which consists of freshwater limestone, with +cyprides, turtles, and fish of different species from those in the +preceding strata. Below the limestone are brackish-water beds full of +_Cyrena_, and traversed by bands abounding in _Corvulæ_ and _Melaniæ_. +These are based on a purely marine deposit, with _Pecten_, _Modiola_, +_Avicula_, and _Thracia_, all undescribed shells. Below this, again, come +limestones and shales, partly of brackish and partly of freshwater origin, +in which many fish, especially species of _Lepidotus_ and _Microdon +radiatus_, are found, and a reptile named _Macrorhyncus_. Among the +mollusks, a remarkable ribbed _Melania_, of the section _Chilira_, occurs. + +Immediately below is the great and conspicuous stratum, 12 feet thick, long +familiar to geologists under the local name of "Cinder-bed," formed of a +vast accumulation of shells of _Ostrea distorta_ (fig. 240.). In the +uppermost part of this bed Mr. Forbes discovered the first echinoderm as +yet known in the Purbeck series, a species of _Hemicidaris_, a genus +characteristic of the Oolitic period. It was accompanied by a species of +_Perna_. Below the Cinder-bed freshwater strata are again seen, filled in +many places with species of _Cypris_, _Valvata_, _Paludina_, _Planorbis_, +_Lymnea_, _Physa_, and _Cyclas_, all different from any we had previously +seen above. Thick siliceous beds of chert, filled with these fossils, occur +in a beautiful state of preservation, often converted into chalcedony. +Among these Mr. Forbes met with gyrogonites (the spore vesicles of +_Charæ_), plants never before discovered in rocks older than the Eocene. +Again, beneath these freshwater strata, a very thin band of greenish +shales, with marine shells and impressions of leaves, like those of a large +_Zostera_, succeeds, forming the base of the Middle Purbeck. + +[Illustration: Fig. 240. Ostrea distorta. Cinder-bed.] + +_Lower Purbeck._--Beneath the thin marine band last mentioned, purely +freshwater marls occur, containing species of _Cypris_, _Valvata_, and +_Lymnea_, different from those of the Middle Purbeck. This is the beginning +of the Inferior division, which is about 80 feet thick. Below the marls are +seen more than 30 feet of brackish-water beds, at Meup's Bay, abounding in +a species of _Serpula_, allied to, if not identical with, _Serpula +coacervites_, found in the Wealden of Hanover. There are also shells of the +genus _Rissoa_ (of the subgenus _Hydrobia_), and a little _Cardium_ of the +subgenus _Protocardium_, in the same beds, together with _Cypris_. Some of +the cypris-bearing shales are strangely contorted and broken up, at the +west end of the Isle of Purbeck. The great dirt-bed or vegetable soil +containing the roots and stools of _Cycadeæ_, which I shall presently +describe, underlies these marls, resting upon the lowest freshwater +limestone, a rock about 8 feet thick, containing _Cyclades_, _Valvata_, and +_Lymnea_, of the same species as those of the uppermost part of the Lower +Purbeck. This rock rests upon the top beds of the Portland stone, which is +purely marine, and between which and the Purbecks there is no passage. + +The most remarkable of all the varied successions of beds enumerated in the +above list, is that called by the quarrymen "the dirt," or "black dirt," +which was evidently an ancient vegetable soil. It is from 12 to 18 inches +thick, is of a dark brown or black colour, and contains a large proportion +of earthy lignite. Through it are dispersed rounded fragments of stone, +from 3 to 9 inches in diameter, in such numbers that it almost deserves the +name of gravel. Many silicified trunks of coniferous trees, and the remains +of plants allied to _Zamia_ and _Cycas_, are buried in this dirt-bed (see +figure of living _Zamia_, fig. 241.). + +These plants must have become fossil on the spots where they grew. The +stumps of the trees stand erect for a height of from 1 to 3 feet, and even +in one instance to 6 feet, with their roots attached to the soil at about +the same distances from one another as the trees in a modern +forest.[233-A] The carbonaceous matter is most abundant immediately around +the stumps, and round the remains of fossil _Cycadeæ_.[233-B] + +[Illustration: Fig. 241. Zamia spiralis; Southern Australia.[233-C]] + +Besides the upright stumps above mentioned, the dirt-bed contains the stems +of silicified trees laid prostrate. These are partly sunk into the black +earth, and partly enveloped by a calcareous slate which covers the +dirt-bed. The fragments of the prostrate trees are rarely more than 3 or 4 +feet in length; but by joining many of them together, trunks have been +restored, having a length from the root to the branches of from 20 to 23 +feet, the stems being undivided for 17 or 20 feet, and then forked. The +diameter of these near the roots is about 1 foot.[233-D] Root-shaped +cavities were observed by Professor Henslow to descend from the bottom of +the dirt-bed into the subjacent freshwater stone, which, though now solid, +must have been in a soft and penetrable state when the trees grew.[233-E] + +[Illustration: Fig. 242. Section in Isle of Portland, Dorset. +(Buckland and De la Beche.)] + +The thin layers of calcareous slate (fig. 242.) were evidently deposited +tranquilly, and would have been horizontal but for the protrusion of the +stumps of the trees, around the top of each of which they form +hemispherical concretions. + +The dirt-bed is by no means confined to the island of Portland, where it +has been most carefully studied, but is seen in the same relative position +in the cliffs east of Lulworth Cove, in Dorsetshire, where, as the strata +have been disturbed, and are now inclined at an angle of 45°, the stumps of +the trees are also inclined at the same angle in an opposite direction--a +beautiful illustration of a change in the position of beds originally +horizontal (see fig. 243.). Traces of the dirt-bed have also been observed +by Dr. Buckland, about two miles north of Thame, in Oxfordshire; and by Dr. +Fitton, in the cliffs of the Boulonnois, on the French coast; but, as might +be expected, this freshwater deposit is of limited extent when compared to +most marine formations. + +[Illustration: Fig. 243. Section in cliff east of Lulworth Cove. +(Buckland and De la Beche.)] + +From the facts above described, we may infer, first, that the superior +beds of the Oolite, called "the Portland," which are full of marine +shells, were overspread with fluviatile mud, which became dry land, and +covered by a forest, throughout a portion of the space now occupied by +the south of England, the climate being such as to admit the growth of +the _Zamia_ and _Cycas_. 2dly. This land at length sank down and was +submerged with its forests beneath a body of fresh water, from which +sediment was thrown down enveloping fluviatile shells. 3dly. The regular +and uniform preservation of this thin bed of black earth over a distance +of many miles, shows that the change from dry land to the state of a +freshwater lake or estuary, was not accompanied by any violent +denudation, or rush of water, since the loose black earth, together with +the trees which lay prostrate on its surface, must inevitably have been +swept away had any such violent catastrophe then taken place. + +The dirt-bed has been described above in its most simple form, but in some +sections the appearances are more complicated. The forest of the dirt-bed +was not everywhere the first vegetation which grew in this region. Two +other beds of carbonaceous clay, one of them containing _Cycadeæ_, in an +upright position, have been found below it, and one above it[234-A], which +implies other oscillations in the level of the same ground, and its +alternate occupation by land and water more than once. + +_Table showing the changes of medium in which the strata were formed, +from the Lower Greensand to the Portland Stone inclusive, in the +south-east of England._ + + 1. Marine Lower greensand. + + 2. Freshwater Weald clay. + + 3. Freshwater } + Brackish } Hastings sand. + Freshwater } + + 4. Freshwater Upper Purbeck. + + 5. Freshwater } + Brackish } + Marine } + Brackish } Middle Purbeck. + Marine } + Freshwater } + Marine } + + 6. Freshwater } + Brackish } + Land } + Freshwater } + Land (dirt-bed) } Lower Purbeck. + Freshwater } + Land } + Freshwater } + Land } + Freshwater } + + 7. Marine Portland stone. + +The annexed tabular view will enable the reader to take in at a glance the +successive changes from sea to river, and from river to sea, or from these +again to a state of land, which have occurred in this part of England +between the Cretaceous and Oolitic periods. That there have been at least +four changes in the species of testacea during the deposition of the +Wealden, seems to follow from the observations recently made by Professor +E. Forbes, so that, should we hereafter find the signs of many more +alternate occupations of the same area by different elements, it is no more +than we might expect. Even during a small part of a zoological period, not +sufficient to allow time for many species to die out, we find that the same +area has been laid dry, and then submerged, and then again laid dry, as in +the deltas of the Po and Ganges, the history of which has been brought to +light by Artesian borings.[235-A] We also know that similar revolutions +have occurred within the present century (1819) in the delta of the Indus +in Cutch[235-B], where land has been laid permanently under the waters both +of the river and sea, without its soil or shrubs having been swept away. +Even, independently of any vertical movements of the ground, we see in the +principal deltas, such as that of the Mississippi, that the sea extends its +salt waters annually for many months over considerable spaces, which, at +other seasons, are occupied by the river during its inundations. + +It will be observed that the division of the Purbecks into upper, middle, +and lower, has been made by Professor E. Forbes, strictly on the principle +of the entire distinctness of the species of organic remains which they +include. The lines of demarcation are not lines of disturbance, nor +indicated by any striking physical characters or mineral changes. The +features which attract the eye in the Purbecks, such as the dirt-beds, the +dislocated strata at Lulworth, and the Cinder-bed, do not indicate any +breaks in the distribution of organized beings. "The causes which led to a +complete change of life three times during the deposition of the freshwater +and brackish strata must," says this naturalist, "be sought for, not simply +in either a rapid or a sudden change of their area into land or sea, but +in the great lapse of time which intervened between the epochs of +deposition at certain periods during their formation." + +Each dirt-bed may, no doubt, be the memorial of many thousand years or +centuries, because we find that 2 or 3 feet of vegetable soil is the only +monument which many a tropical forest has left of its existence ever since +the ground on which it now stands was first covered with its shade. Yet, +even if we imagined the fossil soils of the Lower Purbeck to represent as +many ages, we need not expect on that account to find them constituting the +lines of separation between successive strata characterized by different +zoological types. The preservation of a layer of vegetable soil, when in +the act of being submerged, must be regarded as a rare exception to a +general rule. It is of so perishable a nature, that it must usually be +carried away by the denuding waves or currents of the sea or by a river; +and many dirt-beds were probably formed in succession, and annihilated in +the Wealden, besides those few which now remain. + +[Illustration: Fig. 244. Cone from the Isle of Purbeck, resembling the +_Dammara_ of the Moluccas. (Fitton.)] + +The plants of the Wealden, so far as our knowledge extends at present, +consist chiefly of Ferns, Coniferæ (see fig. 244.), and Cycadeæ, without +any exogens; the whole more allied to the Oolitic than to the Cretaceous +vegetation, although some of the species seem to be common to the chalk. +But the vertebrate and invertebrate animals indicate, in like manner, a +relationship to both these periods, though a nearer affinity to the +Oolitic. Mr. Brodie has found the remains of beetles and several insects of +the homopterous and trichopterous orders, some of which now live on plants, +like those of the Wealden, while others hover over the surface of our +present rivers. But no bones of mammalia have been met with among those of +land-reptiles. Yet, as the reader will learn, in Chapter XX., that the +relics of marsupial quadrupeds have been detected in still older beds, and, +as it was so long before a single portion of the jaw of an iguanodon was +met with in the Tilgate quarries (see p. 228.), we need by no means despair +of discovering hereafter some evidence of the existence of warm-blooded +quadrupeds at this era. It is, at least, too soon to infer, on mere +negative evidence, that the mammalia were foreign to this fauna. + +In regard to the geographical extent of the Wealden, it cannot be +accurately laid down; because so much of it is concealed beneath the newer +marine formations. It has been traced about 200 English miles from west to +east, from Lulworth Cove to near Boulogne, in France; and about 220 miles +from north-west to south-east, from Whitchurch, in Buckinghamshire, to +Beauvais, in France. If the formation be continuous throughout this space, +which is very doubtful, it does not follow that the whole was +contemporaneous; because, in all likelihood, the physical geography of the +region underwent frequent change throughout the whole period, and the +estuary may have altered its form, and even shifted its place. Dr. Dunker, +of Cassel, and H. Von Meyer, in an excellent monograph on the Wealdens of +Hanover and Westphalia, have shown that they correspond so closely, not +only in their fossils, but also in their mineral characters, with the +English series, that we can scarcely hesitate to refer the whole to one +great delta. Even then, the magnitude of the deposit may not exceed that of +many modern rivers. Thus, the delta of the Quorra or Niger, in Africa, +stretches into the interior for more than 170 miles, and occupies, it is +supposed, a space of more than 300 miles along the coast, thus forming a +surface of more than 25,000 square miles, or equal to about one half of +England.[237-A] Besides, we know not, in such cases, how far the fluviatile +sediment and organic remains of the river and the land may be carried out +from the coast, and spread over the bed of the sea. I have shown, when +treating of the Mississippi, that a more ancient delta, including species +of shells, such as now inhabit Louisiana, has been upraised, and made to +occupy a wide geographical area, while a newer delta is forming[237-B]; and +the possibility of such movements, and their effects, must not be lost +sight of when we speculate on the origin of the Wealden. + +If it be asked where the continent was placed from the ruins of which +the Wealden strata were derived, and by the drainage of which a great +river was fed, we are half tempted to speculate on the former existence +of the Atlantis of Plato. The story of the submergence of an ancient +continent, however fabulous in history, must have been true again and +again as a geological event. + +The real difficulty consists in the persistence of a large hydrographical +basin, from whence a great body of fresh water was poured into the sea, +precisely at a period when the neighbouring area of the Wealden was +gradually going downwards 1000 feet or more perpendicularly. If the +adjoining land participated in the movement, how could it escape being +submerged, or how could it retain its size and altitude so as to continue +to be the source of such an inexhaustible supply of fresh water and +sediment? In answer to this question, we are fairly entitled to suggest +that the neighbouring land may have been stationary, or may even have +undergone a contemporaneous slow upheaval. There may have been an ascending +movement in one region, and a descending one in a contiguous parallel zone +of country; just as the northern part of Scandinavia is now rising, while +the middle portion (that south of Stockholm) is unmoved, and the southern +extremity in Scania is sinking, or at least has sunk within the historical +period.[237-C] We must, nevertheless, conclude, if we adopt the above +hypothesis, that the depression of the land became general throughout a +large part of Europe at the close of the Wealden period, a subsidence which +brought in the cretaceous ocean. + + +FOOTNOTES: + +[227-A] Dr. Fitton, Geol. Trans. vol. iv. p. 320. Second Series. + +[230-A] Mantell, Geol. of S. E. of England, p. 244. + +[231-A] "On the Dorsetshire Purbecks," by Prof. E. Forbes, Edinb. Brit. +Assoc., Aug. 1850. + +[233-A] Mr. Webster first noticed the erect position of the trees and +described the Dirt-bed. + +[233-B] Fitton, Geol. Trans., Second Series, vol. iv. pp. 220, 221. + +[233-C] See Flinders' Voyage. + +[233-D] Fitton, ibid. + +[233-E] Buckland and De la Beche, Geol. Trans., Second Series, vol. iv. +p. 16. Mr. Forbes has ascertained that the subjacent rock is a +freshwater limestone, and not a portion of the Portland oolite, as +was previously imagined. + +[234-A] E. Forbes, ibid. + +[235-A] See Principles of Geol., 8th ed. pp. 260-268. + +[235-B] Ibid. p. 443. + +[237-A] Fitton, Geol. of Hastings, p. 58.; who cites Lander's Travels. + +[237-B] See above, p. 85.; and Second Visit to the U. S. vol. ii. +chap. xxxiv. + +[237-C] See the Author's Anniv. Address, Geol. Soc. 1850, Quart. Geol. +Journ. vol. vi. p. 52. + + + + +CHAPTER XIX. + +DENUDATION OF THE CHALK AND WEALDEN. + + Physical geography of certain districts composed of Cretaceous and + Wealden strata--Lines of inland chalk-cliffs on the Seine in + Normandy--Outstanding pillars and needles of chalk--Denudation of the + chalk and Wealden in Surrey, Kent, and Sussex--Chalk once continuous + from the North to the South Downs--Anticlinal axis and parallel + ridges--Longitudinal and transverse valleys--Chalk escarpments--Rise + and denudation of the strata gradual--Ridges formed by harder, valleys + by softer beds--Why no alluvium, or wreck of the chalk, in the central + district of the Weald--At what periods the Weald valley was + denuded--Land has most prevailed where denudation has been + greatest--Elephant bed, Brighton. + + +All the fossiliferous formations may be studied by the geologist in two +distinct points of view: first, in reference to their position in the +series, their mineral character and fossils; and, secondly, in regard to +their physical geography, or the manner in which they now enter, as mineral +masses, into the external structure of the earth; forming the bed of lakes +and seas, or the surface and foundation of hills and valleys, plains and +table-lands. Some account has already been given on the first head of the +Tertiary, the Cretaceous, and Wealden strata; and we may now proceed to +consider certain features in the physical geography of these groups as they +occur in parts of England and France. + +The hills composed of white chalk in the S.E. of England have a smooth +rounded outline, and being usually in the state of sheep pastures, are free +from trees or hedgerows; so that we have an opportunity of observing how +the valleys by which they are drained ramify in all directions, and become +wider and deeper as they descend. Although these valleys are now for the +most part dry, except during heavy rains and the melting of snow, they may +have been due to aqueous denudation, as explained in the sixth chapter; +having been excavated when the chalk emerged gradually from the sea. This +opinion is confirmed by the occasional occurrence of long lines of inland +cliffs, in which the strata are cut off abruptly in steep and often +vertical precipices. The true nature of such escarpments is nowhere more +obvious than in parts of Normandy, where the river Seine and its +tributaries flow through deep winding valleys, hollowed out of chalk +horizontally stratified. Thus, for example, if we follow the Seine for a +distance of about 30 miles from Andelys to Elboeuf, we find the valley +flanked on both sides by a deep slope of chalk, with numerous beds of +flint, the formation being laid open for a thickness of about 250 and 300 +feet. Above the chalk is an overlying mass of sand, gravel, and clay, from +30 to 100 feet thick. The two opposite slopes of the hills _a_ and _b_, +where the chalk appears at the surface, are from 2 to 4 miles apart, and +they are often perfectly smooth and even, like the steepest of our downs in +England; but at many points they are broken by one, two, or more ranges of +vertical and even overhanging cliffs of bare white chalk with flints. At +some points detached needles and pinnacles stand in the line of the cliffs, +or in front of them, as at _c_, fig. 245. On the right bank of the Seine, +at Andelys, one range, about 2 miles long, is seen varying from 50 to 100 +feet in perpendicular height, and having its continuity broken by a number +of dry valleys or coombs, in one of which occurs a detached rock or needle, +called the Tête d'Homme (see figs. 246, 247.). The top of this rock +presents a precipitous face towards every point of the compass; its +vertical height being more than 20 feet on the side of the downs, and 40 +towards the Seine, the average diameter of the pillar being 36 feet. Its +composition is the same as that of the larger cliffs in its neighbourhood, +namely, white chalk, having occasionally a crystalline texture like marble, +with layers of flint in nodules and tabular masses. The flinty beds often +project in relief 4 or 5 feet beyond the white chalk, which is generally in +a state of slow decomposition, either exfoliating or being covered with +white powder, like the chalk cliffs on the English coast; and, as in them, +this superficial powder contains in some places common salt. + +[Illustration: Fig. 245. Section across Valley of Seine.] + +[Illustration: Fig. 246. View of the Tête d'Homme, Andelys, +seen from above.] + +Other cliffs are situated on the right bank of the Seine, opposite +Tournedos, between Andelys and Pont de l'Arche, where the precipices are +from 50 to 80 feet high: several of their summits terminate in pinnacles; +and one of them, in particular, is so completely detached as to present a +perpendicular face 50 feet high towards the sloping down. On these cliffs +several ledges are seen, which mark so many levels at which the waves of +the sea may be supposed to have encroached for a long period. At a still +greater height, immediately above the top of this range, are three much +smaller cliffs, each about 4 feet high, with as many intervening terraces, +which are continued so as to sweep in a semicircular form round an +adjoining coomb, like those in Sicily before described (p. 76.). + +[Illustration: Fig. 247. Side view of the Tête d'Homme. +White chalk with flints.] + +[Illustration: Fig. 248. Chalk pinnacle at Senneville.] + +[Illustration: Fig. 249. Roches d'Orival, Elboeuf.] + +If we then descend the river from Vatteville to a place called Senneville, +we meet with a singular needle about 50 feet high, perfectly isolated on +the escarpment of chalk on the right bank of the Seine (see fig. 248.). +Another conspicuous range of inland cliffs is situated about 12 miles below +on the left bank of the Seine, beginning at Elboeuf, and comprehending the +Roches d'Orival (see fig. 249.). Like those before described, it has an +irregular surface, often overhanging, and with beds of flint projecting +several feet. Like them, also, it exhibits a white powdery surface, and +consists entirely of horizontal chalk with flints. It is 40 miles inland, +its height, in some parts, exceeding 200 feet, and its base only a few feet +above the level of the Seine. It is broken, in one place, by a pyramidal +mass or needle, 200 feet high, called the Roche de Pignon, which stands out +about 25 feet in front of the upper portion of the main cliffs, with which +it is united by a narrow ridge about 40 feet lower than its summit (see +fig. 250.). Like the detached rocks before mentioned at Senneville, +Vatteville, and Andelys, it may be compared to those needles of chalk which +occur on the coast of Normandy, as well as in the Isle of Wight and in +Purbeck[241-A] (see fig. 251.). + +[Illustration: Fig. 250. View of the Roche de Pignon, seen from the south.] + +[Illustration: Fig. 251. Needle and Arch of Etretat, in the chalk cliffs of +Normandy. Height of Arch 100 feet. (Passy.)[241-B]] + +The foregoing description and drawings will show, that the evidence of +certain escarpments of the chalk having been originally sea-cliffs, is far +more full and satisfactory in France than in England. If it be asked why, +in the interior of our own country, we meet with no ranges of precipices +equally vertical and overhanging, and no isolated pillars or needles, we +may reply that the greater hardness of the chalk in Normandy may, no doubt, +be the chief cause of this difference. But the frequent absence of all +signs of littoral denudation in the valley of the Seine itself is a +negative fact of a far more striking and perplexing character. The cliffs, +after being almost continuous for miles, are then wholly wanting for much +greater distances, being replaced by a green sloping down, although the +beds remain of the same composition, and are equally horizontal; and +although we may feel assured that the manner of the upheaval of the land, +whether intermittent or not, must have been the same at those intermediate +points where no cliffs exist, as at others where they are so fully +developed. But, in order to explain such apparent anomalies, the reader +must refer again to the theory of denudation, as expounded in the 6th +chapter; where it was shown, first, that the undermining force of the waves +and marine currents varies greatly at different parts of every coast; +secondly, that precipitous rocks have often decomposed and crumbled down; +and thirdly, that many terraces and small cliffs may now lie concealed +beneath a talus of detrital matter. + +_Denudation of the Weald Valley._--No district is better fitted to +illustrate the manner in which a great series of strata may have been +upheaved and gradually denuded than the country intervening between the +North and South Downs. This region, of which a ground plan is given in the +accompanying map (fig. 252.), comprises within it the whole of Sussex, and +parts of the counties of Kent, Surrey, and Hampshire. The space in which +the formations older than the White Chalk, or those from the Gault to the +Hastings sand inclusive, crop out, is bounded everywhere by a great +escarpment of chalk, which is continued on the opposite side of the channel +in the Bas Boulonnais in France, where it forms the semicircular boundary +of a tract in which older strata also appear at the surface. The whole of +this district may therefore be considered geologically as one and the same. + +[Illustration: Fig. 252. Geological Map of the south-east of England and +part of France, exhibiting the denudation of the Weald. + + 1. Tertiary. + 2. Chalk and upper greensand. + 3. Gault. + 4. Lower Greensand. + 5. Weald clay. + 6. Hastings sand. + 7. Purbeck beds. + 8. Oolite.] + +[Illustration: Fig. 253. Section from the London to the Hampshire basin +across the valley of the Weald. + + 1. Tertiary strata. + 2. Chalk and firestone. + 3. Gault. + 4. Lower greensand. + 5. Weald clay. + 6. Hastings sands.] + +[Illustration: Fig. 254. Highest point of South Downs, 858 feet. + +Anticlinal axis of the Weald. Crowborough Hill, 804 feet. + +Highest point of North Downs, 880 feet.[243-A] + +Section of the country from the confines of the basin of London to +that of Hants, with the principal heights above the level of the sea +on a true scale.[243-B]] + +The space here inclosed within the escarpment of the chalk affords an +example of what has been sometimes called a "valley of elevation" (more +properly "of denudation"); where the strata, partially removed by +aqueous excavation, dip away on all sides from a central axis. Thus, it +is supposed that the area now occupied by the Hastings sand (No. 6.) was +once covered by the Weald clay (No. 5.), and this again by the Greensand +(No. 4.), and this by the Gault (No. 3.); and, lastly, that the Chalk +(No. 2.) extended originally over the whole space between the North and +the South Downs. This theory will be better understood by consulting the +annexed diagram (fig. 253.), where the dark lines represent what now +remains, and the fainter ones those portions of rock which are believed +to have been carried away. + +At each end of the diagram the tertiary strata (No. 1.) are exhibited +reposing on the chalk. In the middle are seen the Hastings sands (No. 6.) +forming an anticlinal axis, on each side of which the other formations are +arranged with an opposite dip. It has been necessary, however, in order to +give a clear view of the different formations, to exaggerate the +proportional height of each in comparison to its horizontal extent; and a +true scale is therefore subjoined in another diagram (fig. 254.), in order +to correct the erroneous impression which might otherwise be made on the +reader's mind. In this section the distance between the North and South +Downs is represented to exceed forty miles; for the Valley of the Weald is +here intersected in its longest diameter, in the direction of a line +between Lewes and Maidstone. + +Through the central portion, then, of the district supposed to be denuded +runs a great anticlinal line, having a direction nearly east and west, on +both sides of which the beds 5, 4, 3, and 2, crop out in succession. But, +although, for the sake of rendering the physical structure of this region +more intelligible, the central line of elevation has alone been introduced, +as in the diagrams of Smith, Mantell, Conybeare, and others, geologists +have always been well aware that numerous minor lines of dislocation and +flexure run parallel to the great central axis. + +In the central area of the Hastings sand the strata have undergone the +greatest displacement; one fault being known, where the vertical shift +of a bed of calcareous grit is no less than 60 fathoms.[244-A] Much of +the picturesque scenery of this district arises from the depth of the +narrow valleys and ridges to which the sharp bends and fractures of +the strata have given rise; but it is also in part to be attributed +to the excavating power exerted by water, especially on the +interstratified argillaceous beds. + +Besides the series of longitudinal valleys and ridges in the Weald, there +are valleys which run in a transverse direction, passing through the chalk +to the basin of the Thames on the one side, and to the English Channel on +the other. In this manner the chain of the North Downs is broken by the +rivers Wey, Mole, Darent, Medway, and Stour; the South Downs by the Arun, +Adur, Ouse, and Cuckmere.[244-B] If these transverse hollows could be +filled up, all the rivers, observes Mr. Conybeare, would be forced to take +an easterly course, and to empty themselves into the sea by Romney Marsh +and Pevensey Levels.[245-A] + +Mr. Martin has suggested that the great cross fractures of the chalk, which +have become river channels, have a remarkable correspondence on each side +of the valley of the Weald; in several instances the gorges in the North +and South Downs appearing to be directly opposed to each other. Thus, for +example, the defiles of the Wey in the North Downs, and of the Arun in the +South, seemed to coincide in direction; and, in like manner, the Ouse +corresponds to the Darent, and the Cuckmere to the Medway.[245-B] + +[Illustration: Fig. 255. View of the chalk escarpment of the South Downs. +Taken from the Devil's Dike, looking towards the west and south-west. + + _a._ The town of Steyning is hidden by this point. + _b._ Edburton church. + _c._ Road. + _d._ River Adur.] + +Although these coincidences may, perhaps, be accidental, it is by no means +improbable, as hinted by the author above mentioned, that great amount of +elevation towards the centre of the Weald district gave rise to transverse +fissures. And as the longitudinal valleys were connected with that linear +movement which caused the anticlinal lines running east and west, so the +cross fissures might have been occasioned by the intensity of the upheaving +force towards the centre of the line. + +But before treating of the manner in which the upheaving movement may +have acted, I shall endeavour to make the reader more intimately +acquainted with the leading geographical features of the district, so +far as they are of geological interest. + +In whatever direction we travel from the tertiary strata of the basins of +London and Hampshire towards the valley of the Weald, we first ascend a +slope of white chalk, with flints, and then find ourselves on the summit of +a declivity consisting, for the most part, of different members of the +chalk formation; below which the upper greensand, and sometimes, also, the +gault, crop out. This steep declivity is the great escarpment of the chalk +before mentioned, which overhangs a valley excavated chiefly out of the +argillaceous or marly bed, termed Gault (No. 3.). The escarpment is +continuous along the southern termination of the North Downs, and may be +traced from the sea, at Folkestone, westward to Guildford and the +neighbourhood of Petersfield, and from thence to the termination of the +South Downs at Beachy Head. In this precipice or steep slope the strata are +cut off abruptly, and it is evident that they must originally have extended +farther. In the woodcut (fig. 255. p. 245.), part of the escarpment of the +South Downs is faithfully represented, where the denudation at the base of +the declivity has been somewhat more extensive than usual, in consequence +of the upper and lower greensand being formed of very incoherent materials, +the upper, indeed, being extremely thin and almost wanting. + +[Illustration: Fig. 256. Chalk escarpment, as seen from the hill above +Steyning, Sussex. The castle and village of Bramber in the foreground.] + +The geologist cannot fail to recognize in this view the exact likeness of a +sea cliff; and if he turns and looks in an opposite direction, or eastward, +towards Beachy Head (see fig. 256.), he will see the same line of heights +prolonged. Even those who are not accustomed to speculate on the former +changes which the surface has undergone may fancy the broad and level plain +to resemble the flat sands which were laid dry by the receding tide, and +the different projecting masses of chalk to be the headlands of a coast +which separated the different bays from each other. + +In regard to the transverse valleys before mentioned, as intersecting the +chalk hills, some idea of them may be derived from the subjoined sketch +(fig. 257.), of the gorge of the river Adur, taken from the summit of the +chalk downs, at a point in the bridle-way leading from the towns of Bramber +and Steyning to Shoreham. If the reader will refer again to the view given +in a former woodcut (fig. 255. p. 245.), he will there see the exact point +where the gorge of which I am now speaking interrupts the chalk escarpment. +A projecting hill, at the point _a_, hides the town of Steyning, near which +the valley commences where the Adur passes directly to the sea at Old +Shoreham. The river flows through a nearly level plain, as do most of the +others which intersect the hills of Surrey, Kent, and Sussex; and it is +evident that these openings, so far at least as they are due to aqueous +erosion, have not been produced by the rivers, many of which, like the Ouse +near Lewes, have filled up arms of the sea, instead of deepening the +hollows which they traverse. + +[Illustration: Fig. 257. Transverse Valley of the Adur in the South Downs. + + _a._ Town of Steyning. + _b._ River Adur. + _c._ Old Shoreham.] + +Now, in order to account for the manner in which the five groups of +strata, 2, 3, 4, 5, 6, represented in the map, fig. 252. and in the +section fig. 253., may have been brought into their present position, +the following hypothesis has been very generally adopted:--Suppose the +five formations to lie in horizontal stratification at the bottom of the +sea; then let a movement from below press them upwards into the form of +a flattened dome, and let the crown of this dome be afterwards cut off, +so that the incision should penetrate to the lowest of the five groups. +The different beds would then be exposed on the surface, in the manner +exhibited in the map, fig. 252.[247-A] + +The quantity of denudation or removal by water of stratified masses assumed +to have once reached continuously from the North to the South Downs is so +enormous, that the reader may at first be startled by the boldness of the +hypothesis. But the difficulty vanishes when once sufficient time is +allowed for the gradual and successive rise of the strata, during which the +waves and currents of the ocean might slowly accomplish an operation, which +no sudden diluvial rush of waters could possibly have effected. + +Among other proofs of the action of water, it may be stated that the great +longitudinal valleys follow the outcrop of the softer and more incoherent +beds, while ridges or lines of cliff usually occur at those points where +the strata are composed of harder stone. Thus, for example, the chalk with +flints, together with the subjacent upper greensand, which is often used +for building, under the provincial name of "firestone," has been cut into a +steep cliff on that side on which the sea encroached. This escarpment +bounds a deep valley, excavated chiefly out of the soft argillaceous or +marly bed, termed gault (No. 3.). In some places the upper greensand is in +a loose and incoherent state, and there it has been as much denuded as the +gault; as, for example, near Beachy Head; but farther to the westward it is +of great thickness, and contains hard beds of blue chert and calcareous +sandstone or firestone. Here, accordingly, we find that it produces a +corresponding influence on the scenery of the country; for it runs out like +a step beyond the foot of the chalk-hills, and constitutes a lower terrace, +varying in breadth from a quarter of a mile to three miles, and following +the sinuosities of the chalk escarpment.[248-A] + +[Illustration: Fig. 258. Cross section. + + _a._ Chalk with flints. + _b._ Chalk without flints. + _c._ Upper greensand, or firestone. + _d._ Gault.] + +It is impossible to desire a more satisfactory proof that the escarpment +is due to the excavating power of water during the rise of the strata; +for I have shown, in my account of the coast of Sicily, in what manner +the encroachments of the sea tend to efface that succession of terraces +which must otherwise result from the intermittent upheaval of a coast +preyed upon by the waves.[248-B] During the interval between two +elevatory movements, the lower terrace will usually be destroyed, +wherever it is composed of incoherent materials; whereas the sea will +not have time entirely to sweep away another part of the same terrace, +or lower platform, which happens to be composed of rocks of a harder +texture, and capable of offering a firmer resistance to the erosive +action of water. As the yielding clay termed gault would be readily +washed away, we find its outcrop marked everywhere by a valley which +skirts the base of the chalk hills, and which is usually bounded on the +opposite side by the lower greensand; but as the upper beds of this last +formation are most commonly loose and incoherent, they also have usually +disappeared and increased the breadth of the valley. But in those +districts where chert, limestone, and other solid materials enter +largely into the composition of this formation (No. 4.), they give rise +to a range of hills parallel to the chalk, which sometimes rival the +escarpment of the chalk itself in height, or even surpass it, as in +Leith Hill, near Dorking. This ridge often presents a steep escarpment +towards the soft argillaceous deposit called the Weald clay (No. 5.; +see the strong lines in fig. 253. p. 243.), which usually forms a broad +valley, separating the lower greensand from the Hastings sands or Forest +ridge; but where subordinate beds of sandstone of a firmer texture +occur, the uniformity of the plain of No. 5. is broken by waving +irregularities and hillocks. + +It will be easy to show how closely the superficial inequalities agree with +those which we might naturally expect to originate during the gradual rise +of the Wealden district. Suppose the line of the most energetic movement to +have coincided with what is now the central ridge of the Weald valley; in +that case the first land which emerged must have been situated where the +Forest ridge is now placed. Here many shoals and reefs may first have +existed, and islands of chalk devoured in the course of ages by the ocean +(see fig. 253.); so that the top of the shattered dome which first appeared +above water may have been utterly destroyed, and the masses represented by +the fainter lines (fig. 253.) removed. + +[2 Illustrations: Fig. 259., Fig. 260. + +The dotted lines represent the sea-level.] + +The upper greensand is represented (fig. 259.) as forming on the left +hand a single precipice with the chalk; while on the right there are two +cliffs, with an intervening terrace, as before described in fig. 258. +Two strips of land would then remain on each side of a channel, +presenting ranges of white cliffs facing each other. A powerful current +might then scoop out a channel in the gault (No. 2.). This softer bed +would yield with ease in proportion as parts of it were brought up from +time to time and exposed to the fury of the waves, so that large spaces +occupied by the harder formation or greensand (No. 3.) would be laid +bare. This last rock, opposing a more effectual resistance, would next +emerge; while the chalk cliffs, at the base of which the gault is +rapidly undermined, would recede farther from each other, after which +four parallel strips of land, or rows of islands, would be caused, which +are represented by the masses which in fig. 260. rise above the dotted +line indicating the sea-level. In this diagram, however, the inclination +of the upper surface of the formations (Nos. 1. and 3.), is exaggerated. +Originally this surface must have been level, like the submarine +terraces produced by denudation, and described before (p. 74. and 77.); +but they were afterwards more and more tilted by that general movement +to which the region of the Weald owes its structure. At length, by the +farther elevation of the dome-shaped mass, the clay (No. 4.) would be +brought within reach of the waves, which would probably gain the more +easy access to the subjacent deposit by the rents which would be caused +in No. 3., and in the central part of the ridge where the uplifting +force had been exerted with the greatest energy. The opposite cliffs, in +which the greensand (No. 3.) terminates, would now begin to recede from +each other, having at their base a yielding stratum of clay (No. 4.). +Lastly, the sea would penetrate to the sand (No. 5.), and then the +state of things indicated in the dark lines of the upper section +(fig. 253.), would be consummated. + +[Illustration: Fig. 261. The Coomb, near Lewes.] + +It was stated that there are many lines of flexure and dislocation, running +east and west, or parallel to the central axis of the Wealden. They are +numerous in the district of the Hastings sand, and sometimes occur in the +chalk itself. One of the latter kind has given rise to the ravine called +the Coomb, near Lewes, and was first traced out by Dr. Mantell, in whose +company I examined it. This coomb is seen on the eastern side of the valley +of the Ouse, in the suburbs of the town of Lewes. The steep declivities on +each side are covered with green turf, as is the bottom, which is perfectly +dry. No outward signs of disturbance are visible; and the connection of the +hollow with subterranean movements would not have been suspected by the +geologist, had not the evidence of great convulsions been clearly exposed +in the escarpment of the valley of the Ouse, and the numerous chalk pits +worked at the termination of the Coomb. By the aid of these we discover +that the ravine coincides precisely with a line of fault, on one side of +which the chalk with flints (_a_, fig. 262.), appears at the summit of the +hill, while it is thrown down to the bottom on the other. + +Mr. Martin, in his work on the geology of Western Sussex, published in +1828, threw much light on the structure of the Wealden by tracing out +continuously for miles the direction of many anticlinal lines and cross +fractures; and the same course of investigation has since been followed +out in greater detail by Mr. Hopkins. The mathematician last-mentioned +has shown that the observed direction of the lines of flexure and +dislocation in the Weald district coincide with those which might have +been anticipated theoretically on mechanical principles, if we assume +certain simple conditions under which the strata were lifted up by an +expansive subterranean force. He finds by calculation that if this force +was applied so as to act uniformly upwards within an elliptic area, the +longitudinal fissures thereby produced would nearly coincide with the +outlines of the ellipse, forming cracks, which are portions of smaller +concentric ellipses, parallel to the margin of the larger one. These +longitudinal fissures would also be intercepted by others running at +right angles to them, and both lines of fracture may have been produced +at the same time.[251-A] In this illustration it is supposed that the +expansive force acted simultaneously and with equal intensity at every +point within the upheaved area, and not with greater energy along the +central axis or region of principal elevation. + +[Illustration: Fig. 262. Fault in the cliff hills near Lewes. Mantell. + + _a._ Chalk with flints. + _b._ Lower chalk.[251-B]] + +The geologist cannot fail to derive great advantage in his speculations +from the mathematical investigation of a problem of this kind, where +results free from all uncertainty are obtained on the assumption of certain +simple conditions. Such results, when once ascertained by mathematical +methods, may serve as standard cases, to which others occurring in nature +of a more complicated kind may be referred. In order that a uniform force +should cause the strata to attain in the centre of the ellipse a height so +far exceeding that which they have reached round the margin, it is +necessary to assume that the mass of upheaved strata offered originally a +very unequal degree of resistance to the subterranean force. This may have +happened either from their being more fractured in one place than in +another, or from being pressed down by a less weight of incumbent strata; +as if we suppose, what is far from improbable, that great denudation had +taken place in the middle of the Wealden before the final and principal +upheaval occurred. It is suggested that the beds may have been acted upon +somewhat in the manner of a carpet spread out loosely on a floor, and +nailed down round the edges, which would swell into the shape of a dome if +pressed up equally at every point by air admitted from beneath. But when we +are reasoning on the particular phenomena of the Weald, we have no +geological data for determining whether it be more probable that originally +the resistance to be overcome was so extremely unequal in different +places, or whether the subterranean force, instead of being everywhere +uniform, was not applied with very different degrees of intensity beneath +distinct portions of the upraised area. + +The opinion that both the longitudinal and transverse lines of fracture +may have been produced simultaneously, accords well with that expressed +by M. Thurmann, in his work on the anticlinal ridges and valleys of +elevation of the Bernese Jura.[252-A] For the accuracy of his map and +sections I can vouch, from personal examination, in 1835, of part of the +region surveyed by him. Among other results, at which this author +arrived, it appears that the breadth of all the numerous anticlinal +ridges and dome-shaped masses in the Jura is invariably great in +proportion to the number of the formations exposed to view; or, in other +words, to the depth to which the superimposed groups of secondary strata +have been laid open. (See fig. 71. p. 55. for structure of Jura.) He +also remarks, that the anticlinal lines are occasionally oblique and +cross each other, in which case the greatest dislocation of the beds +takes place. Some of the cross fractures are imagined by him to have +been contemporaneous, others subsequent to the longitudinal ones. + +I have assumed, in the former part of this chapter, that the rise of the +Weald was gradual, whereas many geologists have attributed its elevation to +a single effort of subterranean violence. There appears to them such a +unity of effect in this and other lines of deranged strata in the +south-east of England, such as that of the Isle of Wight, as is +inconsistent with the supposition of a great number of separate movements +recurring after long intervals of time. But we know that earthquakes are +repeated throughout a long series of ages in the same spots, like volcanic +eruptions. The oldest lavas of Etna were poured out many thousands, perhaps +myriads of years before the newest, and yet they, and the movements +accompanying their emission, have produced a symmetrical mountain; and if +rivers of melted matter thus continue to flow in the same direction, and +towards the same point, for an indefinite lapse of ages, what difficulty is +there in conceiving that the subterranean volcanic force, occasioning the +rise or fall of certain parts of the earth's crust, may, by reiterated +movements, produce the most perfect unity of result? + +_Alluvium of the Weald._--Our next inquiry may be directed to the alluvium +strewed over the surface of the supposed area of denudation. Has any wreck +been left behind of the strata removed? To this we may answer, that the +chalk downs even on their summits are covered every where with gravel +composed of unrounded and partially rounded chalk flints, such as might +remain after masses of white chalk had been softened and removed by water. +This superficial accumulation of the hard or siliceous materials of the +disintegrated strata may be due in some degree to pluvial action; for +during extraordinary rains a rush of water charged with calcareous matter, +of a milk-white colour, may be seen to descend even gently sloping hills of +chalk. If a layer no thicker than the tenth of an inch be removed once in a +century, a considerable mass may in the course of indefinite ages melt +away, leaving nothing save a layer of flinty nodules to attest its former +existence. These unrolled flints may remain mixed with others more or less +rounded, which the waves left originally on the surface of the chalk, when +it first emerged from the sea. A stratum of fine clay sometimes covers the +surface of slight depressions and the bottom of valleys in the white chalk, +which may represent the aluminous residue of the rock, after the pure +carbonate of lime has been dissolved by rain water, charged with excess of +carbonic acid derived from decayed vegetable matter.[253-A] + +Although flint gravel is so abundant on the chalk itself, it is usually +wanting in the deep longitudinal valleys at the foot of the chalk +escarpment, although, in some few instances, the detritus of the chalk has +been traced in patches over the gault, and even the lower greensand, for a +distance of several miles from the escarpment of the North and South Downs. +But no vestige of the chalk and its flints has been seen on the central +ridge of the Weald or the Hastings sands, but merely gravel derived from +the rocks immediately subjacent. This distribution of alluvium, and +especially the absence of chalk detritus in the central district, agrees +well with the theory of denudation before set forth; for to return to fig. +259., if the chalk (No. 1.) were once continuous and covered every where +with flint gravel, this superficial covering would be the first to be +carried away from the highest part of the dome long before any of the gault +(No. 2.) was laid bare. Now if some ruins of the chalk remain at first on +the gault, these would be, in a great degree, cleared away before any part +of the lower greensand (No. 3.) is denuded. Thus in proportion to the +number and thickness of the groups removed in succession, is the +probability lessened of our finding any remnants of the highest group +strewed over the bared surface of the lowest. + +As an exception to the general rule of the small distance to which any +wreck of the chalk can be traced from the escarpments of the North and +South Downs, I may mention a thick bed of chalk flints which occurs near +Barcombe, about three miles to the north of Lewes (see fig. 263.), a place +which I visited with Dr. Mantell, to whom I am indebted for the +accompanying section. Even here it will be seen that the gravel reaches no +farther than the Weald Clay. The same section shows one of the minor east +and west anticlinal lines before alluded to (p. 244.). + +[Illustration: Fig. 263. Section from the north escarpment of the South +Downs to Barcombe. + + 1. Gravel composed of partially rounded chalk flints. + 2. Chalk with and without flints. + 3. Lowest chalk or chalk marl (upper greensand wanting). + 4. Gault. + 5. Lower greensand. + 6. Weald clay.] + +_At what period the Weald Valley was denuded._--If we inquire at what +geological period the denudation of the Weald was effected, we shall +immediately perceive that the question is limited to this point, whether it +took place during or subsequent to the deposition of the Eocene strata of +the south of England. For in the basins of London and Hampshire the Eocene +strata are conformable to the chalk, being horizontal where the beds of +chalk are horizontal, and vertical where they are vertical, so that both +series of rocks appear to have participated in nearly the same movements. +At the eastern extremity of the Isle of Wight, some beds even of the +freshwater series have been thrown on their edges, like those of the London +clay. Nevertheless we can by no means infer that all the tertiary deposits +of the London and Hampshire basins once extended like the chalk over the +entire valley of the Weald, because the denudation of the chalk and +greensand may have been going on in the centre of that area, while +contiguous parts of the sea were sufficiently deep to receive and retain +the matter derived from that waste. Thus while the waves and currents were +excavating the longitudinal valleys D and C (fig. 264.), the deposits _a_ +may have been thrown down to the bottom of the contiguous deep water E, the +sediment being drifted through transverse fissures, as before explained. In +this case, the rise of the formations Nos. 1, 2, 3, 4, 5, may have been +going on contemporaneously with the excavation of the valleys C and D, and +with the accumulation of the tertiary strata _a_. + +[Illustration: Fig. 264. Cross section.] + +This idea receives some countenance from the fact of the tertiary strata, +near their junction with the chalk of the London and Hampshire basins, +often consisting of dense beds of sand and shingle, as at Blackheath and in +the Addington Hills near Croydon. They also contain occasionally freshwater +shells and the remains of land animals and plants, which indicate the +former presence of land at no great distance, some part of which may have +occupied the centre of the Weald. + +Such masses of well-rolled pebbles occurring in the lowest Eocene +strata, or those called "the plastic clay and sands" before described +(No. 3. _b_, Tab. p. 197.), imply the neighbourhood of an ancient shore. +They also indicate the destruction of pre-existing chalk with flints. At +the same time fossil shells of the genera _Melania_, _Cyclas_, and +_Unio_, appearing here and there in beds of the same age, together with +plants and the bones of land animals, bear testimony to contiguous land, +which probably constituted islands scattered over the space now occupied +by the tertiary basins of the Seine and Thames. The stage of denudation +represented in fig. 259., p. 249., may explain the state of things +prevailing at points where such islands existed. By the alternate rising +and sinking of the white chalk and older beds, a large area may have +become overspread with gravelly sandy, and clayey beds of fluvio-marine +and shallow-water origin, before any of the London clay proper (or +Calcaire grossier in France) were superimposed. This may account for the +fact that patches of "plastic clay and sand" (No. 3. _b_, Tab. p. 197.), +are scattered over the surface of the chalk, reaching in some places to +great heights, and approaching even the edges of the escarpments. We +must suppose that subsequently a gradual subsidence took place in +certain areas, which allowed the London clay proper to accumulate over +the Lower Eocene sands and clays, in a deep sea. During this sinking +down (the vertical amount of which equalled 800, and in parts of the +Isle of Wight, according to Mr. Prestwich, 1800 feet), the work of +denudation would be unceasing, being always however confined to those +areas where land or islands existed. At length, when the Bagshot sand +had been in its turn thrown down on the London clay, the space covered +by these two formations was again upraised from the sea to about the +height which it has since retained. During this upheaval, the waves +would again exert their power, not only on the white chalk and lower +cretaceous and Wealden strata, but also on the Eocene formations of +the London basin, excavating valleys and undermining cliffs as the +strata emerged from the deep. + +There are grounds, as before stated (p. 205.), for presuming that the +tertiary area of London was converted into land before that of +Hampshire, and for this reason it contains no marine Eocene deposits so +modern as those of Barton Cliff, or the still newer freshwater and +fluvio-marine beds of Hordwell and the Isle of Wight. These last seem +unequivocally to demonstrate the local inequality of the upheaving and +depressing movements of the period alluded to; for we find, in spite of +the evidence afforded in Alum and White Cliff Bays, of continued +depression to the extent of 1800 or 2000 feet, that at the close of the +Eocene period a dense formation of freshwater strata was produced. The +fossils of these strata bear testimony to rivers draining adjacent +lands, and the existence of numerous quadrupeds on those lands. +Instead of such phenomena, the signs of an open sea might naturally +have been expected as the consequence of so much subsidence, had +not the depression been accompanied or followed by upheaval in a +region immediately adjoining. + +When we attempt to speculate on the geographical changes which took place +in the earlier part of the Eocene epoch, and to restore in imagination the +former state of the physical geography of the south-east of England, we +shall do well to bear in mind that wherever there are proofs of great +denudation, there also the greatest area of land has probably existed. In +the same space, moreover, the oscillations of level, and the alternate +submergence and emergence of coasts, may be presumed to have been most +frequent; for these fluctuations facilitate the wasting and removing power +of waves, currents, and rivers. + +We should also remember that there is always a tendency in the last +denuding operations, to efface all signs of preceding denudation, or at +least all those marks of waste from which alone a geologist can ascertain +the date of the removal of the missing strata within the denuded area. It +may often be difficult to settle the chronology even of the last of a +series of such acts of removal, but it must be, in the nature of things, +almost always impossible to assign a date to each of the antecedent +denudations. If we wish to determine the times of the destruction of rocks, +we must look any where rather than to the spaces once occupied by the +missing rocks. We must inquire to what regions the ruins of the white +chalk, greensand, Wealden, and other strata which have disappeared were +transported. We are then led at once to the examination of all the deposits +newer than the chalk, and first to the oldest of these, the Lower Eocene, +and its sand, shingle, and clay. In them, so largely developed in the +immediate neighbourhood of the denuded area, we discover the wreck we are +in search of, regularly stratified, and inclosing, in some of its layers, +organic remains of a littoral, and sometimes fluviatile character. What +more can we desire? The shores must have consisted of chalk, greensand, and +Wealden, since these were the only superficial rocks in the south-east of +England, at the commencement of the Eocene epoch. The waves of the sea, +therefore, and the rivers were grinding down chalk-flints and chert from +the greensand into shingle and sand, or were washing away calcareous and +argillaceous matter from the cretaceous and Wealden beds, during the whole +of the Eocene period. Thus we obtain the date of a great part at least of +that enormous amount of denudation of which we have such striking monuments +in the space intervening between the North and South Downs. + +[Illustration: Fig. 265. Cross section. + + A. Chalk with layers of flint dipping slightly to the south. + _b._ Ancient beach, consisting of fine sand, from one to four feet thick, + covered by shingle from five to eight feet thick of pebbles of + chalk-flint, granite, and other rocks, with broken shells of recent + marine species, and bones of cetacea. + _c._ Elephant bed, about fifty feet thick, consisting of layers of white + chalk rubble, with broken chalk-flints, in which deposit are found + bones of ox, deer, horse, and mammoth. + _d._ Sand and shingle of modern beach.] + +There have been some movements of land on a smaller scale since the Eocene +period in the south-east of England. One of the latest of these happened in +the Pleistocene, or even perhaps as late as the Post-Pliocene period. The +formation called by Dr. Mantell the Elephant Bed, at the foot of the chalk +cliffs at Brighton, is not merely a talus of calcareous rubble collected at +the base of an inland cliff, but exhibits every appearance of having been +spread out in successive horizontal layers by water in motion. + +The deposit alluded to skirts the shores between Brighton and Rottingdean, +and another mass apparently of the same age occurs at Dover. The phenomena +appear to me to suggest the following conclusions:--First, the +south-eastern part of England had acquired its actual configuration when +the ancient chalk cliff A _a_ was formed, the beach of sand and shingle _b_ +having then been thrown up at the base of the cliff. Afterwards the whole +coast, or at least that part of it where the elephant bed now extends, +subsided to the depth of 50 or 60 feet; and during the period of +submergence successive layers of white calcareous rubble _c_ were +accumulated, so as to cover the ancient beach _b_. Subsequently, the coast +was again raised, so that the ancient shore was elevated to a level +somewhat higher than its original position.[257-A] + + +FOOTNOTES: + +[241-A] An account of these cliffs was read by the author to the British +Assoc. at Glasgow, Sept. 1840. + +[241-B] Seine-Inferieure, p. 142. and pl. 6. fig. 1. + +[243-A] Botley Hill, near Godstone, in Surrey, was found by +trigonometrical measurement to be 880 feet above the level of the sea; +and Wrotham Hill, near Maidstone, which appears to be next in height of +the North Downs, 795 feet. + +[243-B] My friend Dr. Mantell has kindly drawn up this scale at my request. + +[244-A] Fitton, Geol. of Hastings, p. 55. + +[244-B] Conybeare, Outlines of Geol., p. 81. + +[245-A] Ibid., p. 145. + +[245-B] Geol. of Western Sussex, p. 61. + +[247-A] See illustrations of this theory by Dr. Fitton, Geol. +Sketch of Hastings. + +[248-A] Sir E. Murchison, Geol. Sketch of Sussex, &c., Geol. Trans., Second +Series, vol. ii. p. 98. + +[248-B] See fig. 94. p. 76. + +[251-A] Geol. Soc. Proceed. No. 74. p. 363. 1841, and G. S. Trans. +2 Ser. v. 7. + +[251-B] For farther information, see Mantell's Geol. of S. E. +of England, p. 352. + +[252-A] Soulèvemens Jurassiques. Paris, 1832. + +[253-A] See above, p. 82. + +[257-A] See Mantell's Geol. of S. E. of England, p. 32. After +re-examining the elephant bed in 1834, I was no longer in doubt of its +having been a regular subaqueous deposit. In 1828, Dr. Mantell +discovered in the shingle below the chalk-rubble the jawbone of a whale +12 feet long, which must have belonged to an individual from 60 to 70 +feet in length, Medals of Creation, p. 825. + + + + +CHAPTER 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--Nerinæan limestone--Diceras limestone--Oxford clay, + Ammonites and Belemnites--Lower Oolite, Crinoideans--Great Oolite and + Bradford clay--Stonesfield slate--Fossil mammalia, placental and + marsupial--Resemblance to an Australian fauna--Doctrine of progressive + development--Collyweston slates--Yorkshire Oolitic coal-field--Brora + coal--Inferior Oolite and fossils. + + +_OOLITIC OR JURASSIC GROUP._--Below the freshwater group called the +Wealden, or, where this is wanting, immediately beneath the Cretaceous +formation, a great series of marine strata, commonly called "the Oolite," +occurs in England and many other parts of Europe. This group has been so +named, because, in the countries where it was first examined, the +limestones belonging to it had an oolitic structure (see p. 12.). These +rocks occupy in England a zone which is nearly 30 miles in average breadth, +and extends across the island, from Yorkshire in the north-east, to +Dorsetshire in the south-west. Their mineral characters are not uniform +throughout this region; but the following are the names of the principal +subdivisions observed in the central and south-eastern parts of England:-- + + OOLITE. + + Upper { _a._ Portland stone and sand. + { _b._ Kimmeridge clay. + + Middle { _c._ Coral rag. + { _d._ Oxford clay. + + Lower { _e._ Cornbrash and Forest marble. + { _f._ Great Oolite and Stonesfield slate. + { _g._ Fuller's earth. + { _h._ Inferior Oolite. + + The Lias then succeeds to the Inferior Oolite. + +The Upper oolitic system of the above table has usually the Kimmeridge +clay for its base; the Middle oolitic system, the Oxford clay. The Lower +system reposes on the Lias, an argillo-calcareous formation, which some +include in the Lower Oolite, but which will be treated of separately in +the next chapter. Many of these subdivisions are distinguished by +peculiar organic remains; and though varying in thickness, may be traced +in certain directions for great distances, especially if we compare the +part of England to which the above-mentioned type refers with the +north-east of France, and the Jura mountains adjoining. In that country, +distant above 400 geographical miles, the analogy to the English type, +notwithstanding the thinness, or occasional absence of the clays, is +more perfect than in Yorkshire or Normandy. + +_Physical geography._--The alternation, on a grand scale, of distinct +formations of clay and limestone, has caused the oolitic and liassic series +to give rise to some marked features in the physical outline of parts of +England and France. Wide valleys can usually be traced throughout the long +bounds of country where the argillaceous strata crop out; and between these +valleys the limestones are observed, composing ranges of hills, or more +elevated grounds. These ranges terminate abruptly on the side on which the +several clays rise up from beneath the calcareous strata. + +[Illustration: Fig. 266. Cross section.] + +The annexed diagram will give the reader an idea of the configuration of +the surface now alluded to, such as may be seen in passing from London to +Cheltenham, or in other parallel lines, from east to west, in the southern +part of England. It has been necessary, however, in this drawing, greatly +to exaggerate the inclination of the beds, and the height of the several +formations, as compared to their horizontal extent. It will be remarked, +that the lines of cliff, or escarpment, face towards the west in the great +calcareous eminences formed by the Chalk and the Upper, Middle, and Lower +Oolites; and at the base of which we have respectively the Gault, +Kimmeridge clay, Oxford clay, and Lias. This last forms, generally, a broad +vale at the foot of the escarpment of inferior oolite, but where it +acquires considerable thickness, and contains solid beds of marlstone, it +occupies the lower part of the escarpment. + +The external outline of the country which the geologist observes in +travelling eastward from Paris to Metz is precisely analogous, and is +caused by a similar succession of rocks intervening between the tertiary +strata and the Lias; with this difference, however, that the escarpments +of Chalk, Upper, Middle, and Lower Oolites, face towards the east +instead of the west. + +The Chalk crops out from beneath the tertiary sands and clays of the Paris +basin, near Epernay, and the Gault from beneath the Chalk and Upper +Greensand at Clermont-en-Argonne; and passing from this place by Verdun and +Etain to Metz, we find two limestone ranges, with intervening vales of +clay, precisely resembling those of southern and central England, until we +reach the great plain of Lias at the base of the Inferior Oolite at Metz. + +It is evident, therefore, that the denuding causes have acted similarly +over an area several hundred miles in diameter, sweeping away the softer +clays more extensively than the limestones, and undermining these last so +as to cause them to form steep cliffs wherever the harder calcareous rock +was based upon a more yielding and destructible clay. This denudation +probably occurred while the land was slowly rising out of the sea.[259-A] + + +_Upper Oolite._ + +The Portland stone has already been mentioned as forming in Dorsetshire +the foundation on which the freshwater limestone of the Lower Purbeck +reposes (see p. 232.). It supplies the well-known building stone of +which St. Paul's and so many of the principal edifices of London are +constructed. This upper member, characterized by peculiar marine +fossils, rests on a dense bed of sand, called the Portland sand, below +which is the Kimmeridge clay. In England these Upper Oolite formations +are almost wholly confined to the southern counties. Corals are rare in +them, although one species is found plentifully at Tisbury, in +Wiltshire, in the Portland sand converted into flint and chert, the +original calcareous matter being replaced by silex (fig. 267.). + +[Illustration: Fig. 267. _Columnaria oblonga_, Blainv. + +As seen on a polished slab of chert from the sand of the +Upper Oolite, Tisbury.] + +Among the characteristic fossils of the Upper Oolite, may be mentioned +the _Ostrea deltoidea_ (fig. 269.), found in the Kimmeridge clay +throughout England and the north of France, and also in Scotland, near +Brora. The _Gryphæa virgula_ (fig. 268.), also met with in the same clay +near Oxford, is so abundant in the Upper Oolite of parts of France as to +have caused the deposit to be termed "marnes à gryphées virgules." Near +Clermont, in Argonne, a few leagues from St. Menehould, where these +indurated marls crop out from beneath the gault, I have seen them, on +decomposing, leave the surface of every ploughed field literally strewed +over with this fossil oyster. + +[2 Illustrations: Upper Oolite: Kimmeridge clay. 1/4 nat. size. + +Fig. 268. _Gryphæa virgula._ + +Fig. 269. _Ostrea deltoidea._] + +[Illustration: Fig. 270. _Trigonia gibbosa._ 1/2 nat. size. _a._ the hinge. + +Portland Oolite, Tisbury.] + +The Kimmeridge clay consists, in great part, of a bituminous shale, +sometimes forming an impure coal several hundred feet in thickness. In +some places in Wiltshire it much resembles peat; and the bituminous +matter may have been, in part at least, derived from the decomposition +of vegetables. But as impressions of plants are rare in these shales, +which contain ammonites, oysters, and other marine shells, the bitumen +may perhaps be of animal origin. + +The celebrated lithographic stone of Solenhofen, in Bavaria, belongs to one +of the upper divisions of the oolite, and affords a remarkable example of +the variety of fossils which may be preserved under favourable +circumstances, and what delicate impressions of the tender parts of certain +animals and plants may be retained where the sediment is of extreme +fineness. Although the number of testacea in this slate is small, and the +plants few, and those all marine, Count Munster had determined no less than +237 species of fossils when I saw his collection in 1833; and among them no +less than seven _species_ of flying lizards, or pterodactyls, six saurians, +three tortoises, sixty species of fish, forty-six of crustacea, and +twenty-six of insects. These insects, among which is a libellula, or +dragon-fly, must have been blown out to sea, probably from the same land to +which the flying lizards, and other contemporaneous reptiles, resorted. + + +_Middle Oolite._ + +_Coral Rag._--One of the limestones of the Middle Oolite has been called +the "Coral Rag," because it consists, in part, of continuous beds of +petrified corals, for the most part retaining the position in which they +grew at the bottom of the sea. They belong chiefly to the genera +_Caryophyllia_ (fig. 271.), _Agaricia_, and _Astrea_, and sometimes form +masses of coral 15 feet thick. In the annexed figure of an _Astrea_, from +this formation, it will be seen that the cup-shaped cavities are deepest on +the right-hand side, and that they grow more and more shallow, till those +on the left side are nearly filled up. The last-named stars are supposed to +be Polyparia of advanced age. These coralline strata extend through the +calcareous hills of the N.W. of Berkshire, and north of Wilts, and again +recur in Yorkshire, near Scarborough. + +[Illustration: Fig. 271. _Caryophyllia annularis_, Parkin. Coral +rag, Steeple Ashton.] + +[Illustration: Fig 272. _Astrea._ Coral rag.] + +One of the limestones of the Jura, referred to the age of the English +coral rag, has been called "Nerinæan limestone" (Calcaire à Nérinées) by +M. Thirria; _Nerinæa_ being an extinct genus of univalve shells, much +resembling the _Cerithium_ in external form. The annexed section (fig. +273.) shows the curious form of the hollow part of each whorl, and also +the perforation which passes up the middle of the columella. _N. +Goodhallii_ (fig. 274.) is another English species of the same genus, +from a formation which seems to form a passage from the Kimmeridge clay +to the coral rag.[261-A] + +[Illustration: Fig. 273. _Nerinæa hieroglyphica._ Coral rag.] + +[Illustration: Fig. 274. _Nerinæa Goodhallii_, Fitton. Coral rag, +Weymouth. 1/4 nat. size.] + +A division of the oolite in the Alps, regarded by most geologists as coeval +with the English coral rag, has been often named "Calcaire à Dicerates," or +"Diceras limestone," from its containing abundantly a bivalve shell (see +fig. 275.) of a genus allied to the _Chama_. + +[Illustration: Fig. 275. Cast of _Diceras arietina_. Coral rag, France.] + +[Illustration: Fig. 276. _Cidaris coronata._ Coral rag.] + +_Oxford Clay._--The coralline limestone, or "coral rag," above described, +and the accompanying sandy beds, called "calcareous grits" of the Middle +Oolite, rests on a thick bed of clay, called the Oxford clay, sometimes not +less than 500 feet thick. In this there are no corals, but great abundance +of cephalopoda of the genera Ammonite and Belemnite. (See fig. 277.) In +some of the clay of very fine texture ammonites are very perfect, although +somewhat compressed, and are seen to be furnished on each side of the +aperture with a single horn-like projection (see fig. 278.). These were +discovered in the cuttings of the Great Western Railway, near Chippenham, +in 1841, and have been described by Mr. Pratt.[262-A] + +[Illustration: Fig. 277. _Belemnites hastatus._ Oxford Clay.] + +[Illustration: Fig. 278. _Ammonites Jason_, Reinecke. Syn. _A. Elizabethæ_, +Pratt. Oxford clay, Christian Malford, Wiltshire.] + +[Illustration: Fig. 279. _Belemnites Puzosianus_, D'Orb. Oxford +Clay, Christian Malford. + + _a, a._ projecting processes of the shell or phragmocone. + _b, c._ broken exterior of a conical shell called the phragmocone, which + is chambered within, or composed of a series of shallow concave + cells pierced by a siphuncle. + _c, d._ The guard or osselet, which is commonly called the belemnite.] + +Similar elongated processes have been also observed to extend from the +shells of some belemnites discovered by Dr. Mantell in the same clay +(see fig. 279.), who, by the aid of this and other specimens, has been +able to throw much light on the structure of this singular extinct +form of cuttle-fish.[263-A] + + +_Lower Oolite._ + +The upper division of this series, which is more extensive than the +preceding or Middle Oolite, is called in England the Cornbrash. It +consists of clays and calcareous sandstones, which pass downwards into +the Forest marble, an argillaceous limestone, abounding in marine +fossils. In some places, as at Bradford, this limestone is replaced by a +mass of clay. The sandstones of the Forest Marble of Wiltshire are often +ripple-marked and filled with fragments of broken shells and pieces of +drift-wood, having evidently been formed on a coast. Rippled slabs of +fissile oolite are used for roofing, and have been traced over a broad +band of country from Bradford, in Wilts, to Tetbury, in Gloucestershire. +These calcareous tile-stones are separated from each other by thin seams +of clay, which have been deposited upon them, and have taken their form, +preserving the undulating ridges and furrows of the sand in such +complete integrity, that the impressions of small footsteps, apparently +of crabs, which walked over the soft wet sands, are still visible. In +the same stone the claws of crabs, fragments of echini, and other signs +of a neighbouring beach are observed.[263-B] + +_Great Oolite._--Although the name of coral-rag has been appropriated, +as we have seen, to a member of the Upper Oolite before described, +some portions of the Lower Oolite are equally intitled in many places +to be called coralline limestones. Thus the Great Oolite near Bath +contains various corals, among which the _Eunomia radiata_ (fig. 280.) +is very conspicuous, single individuals forming masses several feet +in diameter; and having probably required, like the large existing +brain-coral (_Meandrina_) of the tropics, many centuries before +their growth was completed. + +[Illustration: Fig. 280. _Eunomia radiata_, Lamouroux. + + _a._ section transverse to the tubes. + _b._ vertical section, showing the radiation of the tubes. + _c._ portion of interior of tubes magnified, showing striated surface.] + +[Illustration: Fig. 281. _Apiocrinites rotundus_, or Pear Encrinite; +Miller. Fossil at Bradford, Wilts. + + _a._ Stem of _Apiocrinites_, and one of the articulations, natural size. + _b._ Section at Bradford of great oolite and overlying clay, containing + the fossil encrinites. See text. + _c._ Three perfect individuals of Apiocrinites, represented as they grew + on the surface of the Great Oolite. + _d._ Body of the _Apiocrinites rotundus_.] + +Different species of _Crinoideans_, or stone-lilies, are also common in the +same rocks with corals; and, like them, must have enjoyed a firm bottom, +where their root, or base of attachment, remained undisturbed for years +(_c_, fig. 281.). Such fossils, therefore, are almost confined to the +limestones; but an exception occurs at Bradford, near Bath, where they are +enveloped in clay. In this case, however, it appears that the solid upper +surface of the "Great Oolite" had supported, for a time, a thick submarine +forest of these beautiful zoophytes, until the clear and still water was +invaded by a current charged with mud, which threw down the stone-lilies, +and broke most of their stems short off near the point of attachment. The +stumps still remain in their original position; but the numerous +articulations once composing the stem, arms, and body of the zoophyte, were +scattered at random through the argillaceous deposit in which some now lie +prostrate. These appearances are represented in the section _b_, fig. 281., +where the darker strata represent the Bradford clay, which some geologists +class with the Forest marble, others with the Great Oolite. The upper +surface of the calcareous stone below is completely incrusted over with a +continuous pavement, formed by the stony roots or attachments of the +Crinoidea; and besides this evidence of the length of time they had lived +on the spot, we find great numbers of single joints, or circular plates of +the stem and body of the encrinite, covered over with _serpulæ_. Now these +_serpulæ_ could only have begun to grow after the death of some of the +stone-lilies, parts of whose skeletons had been strewed over the floor of +the ocean before the irruption of argillaceous mud. In some instances we +find that, after the parasitic _serpulæ_ were full grown, they had become +incrusted over with a coral, called _Berenicea diluviana_; and many +generations of these polyps had succeeded each other in the pure water +before they became fossil. + +[Illustration: Fig. 282. + + _a._ Single plate or articulation of an Encrinite overgrown with + _serpulæ_ and corals. Natural size Bradford clay. + _b._ Portion of the same magnified, showing the coral _Berenicea_ + _diluviana_ covering one of the _serpulæ_.] + +We may, therefore, perceive distinctly that, as the pines and cycadeous +plants of the ancient "dirt bed," or fossil forest, of the Lower Purbeck +were killed by submergence under fresh water, and soon buried beneath +muddy sediment, so an invasion of argillaceous matter put a sudden stop +to the growth of the Bradford Encrinites, and led to their preservation +in marine strata.[265-A] + +Such differences in the fossils as distinguish the calcareous and +argillaceous deposits from each other, would be described by naturalists +as arising out of a difference in the _stations_ of species; but besides +these, there are variations in the fossils of the higher, middle, and +lower part of the oolitic series, which must be ascribed to that great +law of change in organic life by which distinct assemblages of species +have been adapted, at successive geological periods, to the varying +conditions of the habitable surface. In a single district it is +difficult to decide how far the limitation of species to certain minor +formations has been due to the local influence of _stations_, or how +far it has been caused by time or the creative and destroying law above +alluded to. But we recognize the reality of the last-mentioned +influence, when we contrast the whole oolitic series of England with +that of parts of the Jura, Alps, and other distant regions, where there +is scarcely any lithological resemblance; and yet some of the same +fossils remain peculiar in each country to the Upper, Middle, and Lower +Oolite formations respectively. Mr. Thurmann has shown how remarkably +this fact holds true in the Bernese Jura, although the argillaceous +divisions, so conspicuous in England, are feebly represented there, +and some entirely wanting. + +[Illustration: Fig. 283. _Terebratula digona._ Bradford clay. Nat. size.] + +The Bradford clay above alluded to is sometimes 60 feet thick, but, in many +places, it is wanting; and, in others, where there are no limestones, it +cannot easily be separated from the clays of the overlying "forest marble" +and underlying "fuller's earth." + +The calcareous portion of the Great Oolite consists of several shelly +limestones, one of which, called the Bath Oolite, is much celebrated as +a building stone. In parts of Gloucestershire, especially near +Minchinhampton, the Great Oolite, says Mr. Lycett, "must have been +deposited in a shallow sea, where strong currents prevailed, for there +are frequent changes in the mineral character of the deposit, and some +beds exhibit false stratification. In others, heaps of broken shells are +mingled with pebbles of rocks foreign to the neighbourhood, and with +fragments of abraded madrepores, dicotyledonous wood, and crabs' claws. +The shelly strata, also, have occasionally suffered denudation, and the +removed portions have been replaced by clay."[266-A] In such +shallow-water beds cephalopoda are rare, and, instead of ammonites and +belemnites, numerous genera of carnivorous trachelipods appear. Out of +one hundred and forty-two species of univalves obtained from the +Minchinhampton beds, Mr. Lycett found no less than forty-one to be +carnivorous. They belong principally to the genera _Buccinum_, +_Pleurotoma_, _Rostellaria_, _Murex_, and _Fusus_, and exhibit a +proportion of zoophagous species not very different from that which +obtains in warm seas of the recent period. These conchological results +are curious and unexpected, since it was imagined that we might look +in vain for the carnivorous trachelipods in rocks of such high +antiquity as the Great Oolite, and it was a received doctrine that +they did not begin to appear in considerable numbers till the Eocene +period when those two great families of cephalopoda, the ammonites +and belemnites, had become extinct. + +_Stonesfield slate._--The slate of Stonesfield has been shown by Mr. +Lonsdale to lie at the base of the Great Oolite.[266-B] It is a slightly +oolitic shelly limestone, forming large spheroidal masses imbedded in +sand, only 6 feet thick, but very rich in organic remains. It contains some +pebbles of a rock very similar to itself, and which may be portions of the +deposit, broken up on a shore at low water or during storms, and +redeposited. The remains of belemnites, trigoniæ, and other marine shells, +with fragments of wood, are common, and impressions of ferns, cycadeæ, and +other plants. Several insects, also, and, among the rest, the wing-covers +of beetles, are perfectly preserved (see fig. 284.), some of them +approaching nearly to the genus _Buprestis_.[267-A] The remains, also, of +many genera of reptiles, such as _Plesiosaur_, _Crocodile_, and +_Pterodactyl_, have been discovered in the same limestone. + +[Illustration: Fig. 284. Elytron of _Buprestis_? Stonesfield.] + +[Illustration: Fig. 285. Bone of a reptile, formerly supposed to be the +ulna of a Cetacean; from the Great Oolite of Enstone, near Woodstock.] + +But the remarkable fossils for which the Stonesfield slate is most +celebrated, are those referred to the mammiferous class. The student should +be reminded that in all the rocks described in the preceding chapters as +older than the Eocene, no bones of any land quadruped, or of any cetacean, +have been discovered. Yet we have seen that terrestrial plants were not +rare in the lower cretaceous formation, and that in the Wealden there was +evidence of freshwater sediment on a large scale, containing various +plants, and even ancient vegetable soils with the roots and erect stumps of +trees. We had also in the same Wealden many land-reptiles and +winged-insects, which renders the absence of terrestrial quadrupeds the +more striking. The want, however, of any bones of whales, seals, dolphins, +and other aquatic mammalia, whether in the chalk or in the upper or middle +oolite, is certainly still more remarkable. Formerly, indeed, a bone from +the great oolite of Enstone, near Woodstock, in Oxfordshire, was cited, on +the authority of Cuvier, as referable to this class. Dr. Buckland, who +stated this in his Bridgewater Treatise[267-B], had the kindness to send me +the supposed ulna of a whale, that Mr. Owen might examine into its claims +to be considered as cetaceous. It is the opinion of that eminent +comparative anatomist that it cannot have belonged to the cetacea, because +the fore-arm in these marine mammalia is invariably much flatter, and +devoid of all muscular depressions and ridges, one of which is so prominent +in the middle of this bone, represented in the above cut (fig. 285.). In +saurians, on the contrary, such ridges exist for the attachment of muscles; +and to some animal of that class the bone is probably referable. + +[Illustration: Fig. 286. _Amphitherium Prevostii_. Stonesfield +Slate. Natural size. + + _a_. coronoid process. + _b_. condyle. + _c_. angle of jaw. + _d_. double-fanged molars.] + +These observations are made to prepare the reader to appreciate more justly +the interest felt by every geologist in the discovery in the Stonesfield +slate of no less than seven specimens of lower jaws of mammiferous +quadrupeds, belonging to three different species and to two distinct +genera, for which the names of _Amphitherium_ and _Phascolotherium_ have +been adopted. When Cuvier was first shown one of these fossils in 1818, he +pronounced it to belong to a small ferine mammal, with a jaw much +resembling that of an opossum, but differing from all known ferine genera, +in the great number of the molar teeth, of which it had at least ten in a +row. Since that period, a much more perfect specimen of the same fossil, +obtained by Dr. Buckland (see fig. 286.), has been examined by Mr. Owen, +who finds that the jaw contained on the whole twelve molar teeth, with the +socket of a small canine, and three small incisors, which are _in situ_, +altogether amounting to sixteen teeth on each side of the lower jaw. + +[Illustration: Fig. 287. _Amphitherium Broderipii_. Natural size. +Stonesfield Slate.] + +The only question which could be raised respecting the nature of these +fossils was, whether they belonged to a mammifer, a reptile, or a fish. Now +on this head the osteologist observes that each of the seven half jaws is +composed of but one single piece, and not of two or more separate bones, as +in fishes and most reptiles, or of two bones, united by a suture, as in +some few species belonging to those classes. The condyle, moreover (_b_, +fig. 286.), or articular surface, by which the lower jaw unites with the +upper, is convex in the Stonesfield specimens, and not concave as in fishes +and reptiles. The coronoid process (_a_, fig. 286.) is well developed, +whereas it is wanting or very small, in the inferior classes of vertebrata. +Lastly, the molar teeth in the _Amphitherium_ and _Phascolotherium_ have +complicated crowns, and two roots (see _d_, fig. 286.), instead of being +simple and with single fangs.[269-A] + +[Illustration: Fig. 288. _Tupaia Tana._ Right ramus of lower jaw, natural +size. A recent insectivorous mammal from Sumatra.] + +[2 Illustrations: Part of lower jaw of _Tupaia Tana_; twice natural size. + +Fig. 289. End view seen from behind, showing the very slight inflection of +the angle at _c_. + +Fig. 290. Side view of same.] + +[2 Illustrations: Part of lower jaw of _Didelphis Azaræ_; recent, +Brazil. Natural size. + +Fig. 291. End view seen from behind, showing the inflection of the angle of +the jaw, _c. d._ + +Fig. 292. Side view of same.] + +The only question, therefore, which could fairly admit of controversy +was limited to this point, whether the fossil mammalia found in the +lower oolite of Oxfordshire ought to be referred to the marsupial +quadrupeds, or to the ordinary placental series. Cuvier had long ago +pointed out a peculiarity in the form of the angular process (_c_, figs. +291. and 292.) of the lower jaw, as a character of the genus +_Didelphys_; and Mr. Owen has since established its generality in the +entire marsupial series. In all these pouched quadrupeds, this process +is turned inwards, as at _c d_, fig. 291. in the Brazilian opossum, +whereas in the placental series, as at _c_, figs. 290. and 289. there is +an almost entire absence of such inflection. The _Tupaia Tana_ of +Sumatra has been selected by my friend Mr. Waterhouse, for this +illustration, because that small insectivorous quadruped bears a great +resemblance to those of the Stonesfield _Amphitherium_. By clearing away +the matrix from the specimen of _Amphitherium Prevostii_ above +represented (fig. 286.), Mr. Owen ascertained that the angular process +(_c_) bent inwards in a slighter degree than in any of the known +marsupialia; in short, the inflection does not exceed that of the mole +or hedgehog. This fact turns the scale in favour of its affinities to +the placental insectivora. Nevertheless, the _Amphitherium_ offers some +points of approximation in its osteology to the marsupials, especially +to the _Myrmecobius_, a small insectivorous quadruped of Australia, +which has nine molars on each side of the lower jaw, besides a canine +and three incisors.[269-B] + +Another species of _Amphitherium_ has been found at Stonesfield (fig. +287. p. 268.), which differs from the former (fig. 286.) principally +in being larger. + +[Illustration: Fig. 293. _Phascolotherium Bucklandi_, Owen. + + _a._ natural size. + _b._ molar of same magnified.] + +The second mammiferous genus discovered in the same slates was named +originally by Mr. Broderip _Didelphys Bucklandi_ (see fig. 293.), and +has since been called _Phascolotherium_ by Owen. It manifests a much +stronger likeness to the marsupials in the general form of the jaw, and +in the extent and position of its inflected angle, while the agreement +with the living genus _Didelphys_ in the number of the premolar and +molar teeth, is complete.[270-A] + +On reviewing, therefore, the whole of the osteological evidence, it will be +seen that we have every reason to presume that the _Amphitherium_ and +_Phascolotherium_ of Stonesfield represent both the placental and marsupial +classes of mammalia; and if so, they warn us in a most emphatic manner, not +to found rash generalizations respecting the non-existence of certain +classes of animals at particular periods of the past, on mere negative +evidence. The singular accident of our having as yet found nothing but the +lower jaws of seven individuals, and no other bones of their skeletons, is +alone sufficient to demonstrate the fragmentary manner in which the +memorials of an ancient terrestrial fauna are handed down to us. We can +scarcely avoid suspecting that the two genera above described, may have +borne a like insignificant proportion to the entire assemblage of +warm-blooded quadrupeds which flourished in the islands of the oolitic sea. + +Mr. Owen has remarked that as the marsupial genera, to which the +_Phascolotherium_ is most nearly allied, are now confined to New South +Wales and Van Diemen's Land, so also is it in the Australian seas, that we +find the _Cestracion_, a cartilaginous fish which has a bony palate, allied +to those called _Acrodus_ and _Psammodus_ (see figs. 307, 308. p. 275.), so +common in the oolite and lias. In the same Australian seas, also, near the +shore, we find the living _Trigonia_, a genus of mollusca so frequently met +with in the Stonesfield slate. So, also, the Araucarian pines are now +abundant, together with ferns, in Australia and its islands, as they were +in Europe in the oolitic period. Many botanists incline to the opinion, +that the _Thuja_, _Pine_, _Cycas_, _Zamia_, in short, all the gymnogens, +belong to a less highly developed type of flowering plants than do the +exogens; but even if this be admitted, no naturalist can ascribe a low +standard of organization to the oolitic flora, since we meet with endogens +of the most perfect structure in oolitic rocks, both above and below the +Stonesfield slate, as, for example, the _Podocarya_ of Buckland, a fruit +allied to the _Pandanus_, found in the Inferior Oolite (see fig. 294.), and +the _Carpolithes conica_ of the Coral rag. The doctrine, therefore, of a +regular series of progressive development at successive eras in the animal +and vegetable kingdoms, from beings of a more simple to those of a more +complex organization, receives a check, if not a refutation, from the facts +revealed to us by the study of the Lower Oolites. + +[Illustration: Fig. 294. Portion of a fossil fruit of _Podocarya_ +magnified. (Buckland's Bridgew. Treat. Pl. 63.) Inferior Oolite, +Charmouth, Dorset.] + +The Stonesfield slate, in its range from Oxfordshire to the north-east, +is represented by flaggy and fissile sandstones, as at Collyweston in +Northamptonshire, where, according to the researches of Messrs. Ibbetson +and Morris, it contains many shells, such as _Trigonia angulata_, also +found at Stonesfield. But the Northamptonshire strata of this age assume +a more marine character, or appear at least to have been formed farther +from land. They inclose, however, some fossil ferns, such as _Pecopteris +polypodioides_, of species common to the oolites of the Yorkshire +coast[271-A], where rocks of this age put on all the aspect of a true +coal-field; thin seams of coal having actually been worked in them +for more than a century. + +[Illustration: Fig. 295. _Pterophyllum comptum._ (Syn. _Cycadites +comptus_.) Upper sandstone and shale, Gristhorpe, near Scarborough.] + +In the north-west of Yorkshire, the formation alluded to consists of an +upper and a lower carbonaceous shale, abounding in impressions of +plants, divided by a limestone considered by many geologists as the +representative of the Great Oolite; but the scarcity of marine fossils +makes all comparisons with the subdivisions adopted in the south +extremely difficult. A rich harvest of fossil ferns has been obtained +from the upper carbonaceous shales and sandstones at Gristhorpe, near +Scarborough (see figs. 295, 296.). The lower shales are well exposed in +the sea-cliffs at Whitby, and are chiefly characterized by ferns and +cycadeæ. They contain, also, a species of calamite, and a fossil called +_Equisetum columnare_, which maintains an upright position in sandstone +strata over a wide area. Shells of the genus _Cypris_ and _Unio_, +collected by Mr. Bean from these Yorkshire coal-bearing beds, point to +the estuary or fluviatile origin of the deposit. + +[Illustration: Fig. 296. _Hemitelites Brownii_, Goepp. Syn. +_Phlebopteris contigua_, Lind. & Hutt. Upper carbonaceous strata, Lower +Oolite, Gristhorpe, Yorkshire.] + +At Brora, in Sutherlandshire, a coal formation, probably coeval with the +above, or belonging to some of the lower divisions of the Oolitic period, +has been mined extensively for a century or more. It affords the thickest +stratum of pure vegetable matter hitherto detected in any secondary rock in +England. One seam of coal of good quality has been worked 3-1/2 feet thick, +and there are several feet more of pyritous coal resting upon it. + +_Inferior Oolite._--Between the Great and Inferior Oolite, near Bath, an +argillaceous deposit called "the fuller's earth," occurs, but is wanting in +the north of England. The Inferior Oolite is a calcareous freestone, +usually of small thickness, which sometimes rests upon, or is replaced by, +yellow sands, called the sands of the Inferior Oolite. These last, in their +turn, repose upon the lias in the south and west of England. + +Among the characteristic shells of the Inferior Oolite, I may instance +_Terebratula spinosa_ (fig. 297.), and _Pholadomya fidicula_ (fig. 298.). +The extinct genus _Pleurotomaria_ is also a form very common in this +division as well as in the Oolitic system generally. It resembles the +_Trochus_ in form, but is marked by a singular cleft (_a_, fig. 299.) on +the right side of the mouth. + +[Illustration: Fig. 297. _Terebratula spinosa._ Inferior Oolite.] + +[Illustration: Fig. 298. + + _a._ _Pholadomya fidicula_, 1/3 nat. size. Inf. Ool. + _b._ Heart-shaped anterior termination of the same.] + +[Illustration: Fig. 299. _Pleurotomaria ornata._ Ferruginous Oolite, +Normandy. Inferior Oolite, England.] + +As illustrations of shells having a great vertical range, I may allude to +_Trigonia clavellata_, found in the Upper and Inferior Oolite, and _T. +costata_, common to the Upper, Middle, and Lower Oolite; also _Ostrea +Marshii_ (fig. 300.), common to the Cornbrash of Wilts and the Inferior +Oolite of Yorkshire; and _Ammonites striatulus_ (fig. 301.) common to the +Inferior Oolite and Lias. + +[Illustration: Fig. 300. _Ostrea Marshii._ 1/2 nat. size. Middle +and Lower Oolite.] + +[Illustration: Fig. 301. _Ammonites striatulus_, Sow. 1/3 nat. size. +Inferior Oolite and Lias.] + +Such facts by no means invalidate the general rule, that certain fossils +are good chronological tests of geological periods; but they serve to +caution us against attaching too much importance to single species, some +of which may have a wider, others a more confined vertical range. We +have before seen that, in the successive tertiary formations, there are +species common to older and newer groups, yet these groups are +distinguishable from one another by a comparison of the whole assemblage +of fossil shells proper to each. + + +FOOTNOTES: + +[259-A] See Chapters VI. and XIX. + +[261-A] Fitton, Geol. Trans., Second Series, vol. iv. pl. 23. fig. 12. + +[262-A] S. P. Pratt, Annals of Nat. Hist., November, 1841. + +[263-A] See Phil. Trans. 1850, p. 393. + +[263-B] P. Scrope, Geol. Proceed., March, 1831. + +[265-A] For a fuller account of these Encrinites, see Buckland's +Bridgewater Treatise, vol. i. p. 429. + +[266-A] Lycett, Quart. Geol. Journ. vol. iv. p. 183. + +[266-B] Proceedings Geol. Soc. vol. i. p. 414. + +[267-A] See Buckland's Bridgewater Treatise; and Brodie's Fossil Insects, +where it is suggested that these elytra may belong to _Priomus_. + +[267-B] Vol. i. p. 115. + +[269-A] I have given a figure in the Principles of Geology, chap. ix., of +another Stonesfield specimen of _Amphitherium Prevostii_, in which the +sockets and roots of the teeth are finely exposed. + +[269-B] A figure of this recent _Myrmecobius_ will be found in the +Principles, chap. ix. + +[270-A] Owen's British Fossil Mammals, p. 62. + +[271-A] Ibbetson and Morris, Report of Brit. Ass., 1847, p. 131. + + + + +CHAPTER XXI. + +OOLITE AND LIAS--_continued_. + + Mineral character of Lias--Name of Gryphite limestone--Fossil shells + and fish--Ichthyodorulites--Reptiles of the Lias--Ichthyosaur and + Plesiosaur--Marine Reptile of the Galapagos Islands--Sudden + destruction and burial of fossil animals in Lias--Fluvio-marine beds + in Gloucestershire and insect limestone--Origin of the Oolite and + Lias, and of alternating calcareous and argillaceous + formations--Oolitic coal-field of Virginia, in the United States. + + +_LIAS._--The English provincial name of Lias has been very generally +adopted for a formation of argillaceous limestone, marl, and clay, which +forms the base of the Oolite, and is classed by many geologists as part of +that group. They pass, indeed, into each other in some places, as near +Bath, a sandy marl called the marlstone of the Lias being interposed, and +partaking of the mineral characters of the upper lias and inferior oolite. +These last-mentioned divisions have also some fossils in common, such as +the _Avicula inæquivalvis_ (fig. 302.). Nevertheless the Lias may be traced +throughout a great part of Europe as a separate and independent group, of +considerable thickness, varying from 500 to 1000 feet, containing many +peculiar fossils, and having a very uniform lithological aspect. Although +usually conformable to the oolite, it is sometimes, as in the Jura, +unconformable. In the environs of Lons-le-Saulnier, for instance, in the +department of Jura, the strata of lias are inclined at an angle of about +45°, while the incumbent oolitic marls are horizontal. + +[Illustration: Fig. 302. _Avicula inæquivalvis_, Sow.] + +The peculiar aspect which is most characteristic of the Lias in England, +France, and Germany, is an alternation of thin beds of blue or grey +limestone with a surface becoming light-brown when weathered, these beds +being separated by dark-coloured narrow argillaceous partings, so that +the quarries of this rock, at a distance, assume a striped and +riband-like appearance.[274-A] + +Although the prevailing colour of the limestone of this formation is blue, +yet some beds of the lower lias are of a yellowish white colour, and have +been called white lias. In some parts of France, near the Vosges mountains, +and in Luxembourg, M. E. de Beaumont has shown that the lias containing +_Gryphæa arcuata_, _Plagiostoma giganteum_ (see fig. 303.), and other +characteristic fossils, becomes arenaceous; and around the Hartz, in +Westphalia and Bavaria, the inferior parts of the lias are sandy, and +sometimes afford a building stone. + +[Illustration: Fig. 303. _Plagiostoma giganteum._ Lias.] + +[Illustration: Fig. 304. _Gryphæa incurva_, Sow. (_G. arcuata_, Lam.)] + +[Illustration: Fig. 305. _Nautilus truncatus._ Lias.] + +The name of Gryphite limestone has sometimes been applied to the lias, in +consequence of the great number of shells which it contains of a species of +oyster, or _Gryphæa_ (fig. 304., see also fig. 30. p. 29.). Many +cephalopoda, also, such as _Ammonite_, _Belemnite_, and _Nautilus_ (fig. +305.), prove the marine origin of the formation. + +[Illustration: Fig. 306. Scales of _Lepidotus gigas_, Agas. + +_a._ two of the scales detached.] + +The fossil fish resemble generically those of the oolite, belonging all, +according to M. Agassiz, to extinct genera, and differing remarkably from +the ichthyolites of the Cretaceous period. Among them is a species of +_Lepidotus_ (_L. gigas_, Agas.) (fig. 306.), which is found in the lias of +England, France, and Germany.[275-A] This genus was before mentioned (p. +229.) as occurring in the Wealden, and is supposed to have frequented both +rivers and coasts. The teeth of a species of _Acrodus_, also, are very +abundant in the lias (fig. 307.). + +[Illustration: Fig. 307. _Acrodus nobilis_, Agas. (tooth); commonly called +fossil leach. Lias, Lyme Regis, and Germany.] + +[Illustration: Fig. 308. _Hybodus reticulatus_, Agas. Lias, Lyme Regis. + + _a._ Part of fin, commonly called Ichthyodorulite. + _b._ Tooth.] + +But the remains of fish which have excited more attention than any others, +are those large bony spines called _ichthyodorulites_ (_a_, fig. 308.), +which were once supposed by some naturalists to be jaws, and by others +weapons, resembling those of the living _Balistes_ and _Silurus_; but which +M. Agassiz has shown to be neither the one nor the other. The spines, in +the genera last mentioned, articulate with the backbone, whereas there are +no signs of any such articulation in the ichthyodorulites. These last +appear to have been bony spines which formed the anterior part of the +dorsal fin, like that of the living genera _Cestracion_ and _Chimæra_ (see +_a_, fig. 309.). In both of these genera, the posterior concave face is +armed with small spines like that of the fossil _Hybodus_ (fig. 308.), one +of the shark family found fossil at Lyme Regis. Such spines are simply +imbedded in the flesh, and attached to strong muscles. "They serve," says +Dr. Buckland, "as in the _Chimæra_ (fig. 309.), to raise and depress the +fin, their action resembling that of a moveable mast, raising and lowering +backwards the sail of a barge."[276-A] + +[Illustration: Fig. 309. _Chimæra monstrosa._[276-B] + +_a._ Spine forming anterior part of the dorsal fin.] + +_Reptiles of the Lias._--It is not, however, the fossil fish which form the +most striking feature in the organic remains of the Lias; but the reptiles, +which are extraordinary for their number, size, and structure. Among the +most singular of these are several species of _Ichthyosaurus_ and +_Plesiosaurus_. The genus _Ichthyosaurus_, or fish-lizard, is not confined +to this formation, but has been found in strata as high as the chalk-marl +and gault of England, and as low as the muschelkalk of Germany, a formation +which immediately succeeds the lias in the descending order.[276-C] It is +evident from their fish-like vertebræ, their paddles, resembling those of a +porpoise or whale, the length of their tail, and other parts of their +structure, that the habits of the Ichthyosaurs were aquatic. Their jaws and +teeth show that they were carnivorous; and the half-digested remains of +fishes and reptiles, found within their skeletons, indicate the precise +nature of their food.[276-D] + +A specimen of the hinder fin or paddle of _Ichthyosaurus communis_ was +discovered in 1840 at Barrow-on-Soar, by Sir P. Egerton, which +distinctly exhibits on its posterior margin the remains of cartilaginous +rays that bifurcate as they approach the edge, like those in the fin of +a fish (see _a_, fig. 312.). It had previously been supposed, says Mr. +Owen, that the locomotive organs of the Ichthyosaurus were enveloped, +while living, in a smooth integument, like that of the turtle and +porpoise, which has no other support than is afforded by the bones and +ligaments within; but it now appears that the fin was much larger, +expanding far beyond its osseous framework, and deviating widely in its +fish-like rays from the ordinary reptilian type. In fig. 312. the +posterior bones, or digital ossicles of the paddle, are seen near _b_; +and beyond these is the dark carbonized integument of the terminal half +of the fin, the outline of which is beautifully defined.[277-A] Mr. Owen +believes that, besides the fore-paddles, these short-and stiff-necked +saurians were furnished with a tail-fin without bones and purely +tegumentary, expanding in a vertical direction; an organ of motion which +enabled them to turn their heads rapidly.[277-B] + +[Illustration: Fig. 310. _Ichthyosaurus communis_, restored by +Conybeare and Cuvier. + +_a._ costal vertebræ.] + +[Illustration: Fig. 311. _Plesiosaurus dolichodeirus_, restored by +Rev. W. D. Conybeare. + +_a._ cervical vertebra.] + +[Illustration: Fig. 312. Posterior part of hind fin or paddle of +_Ichthyosaurus communis_.] + +Mr. Conybeare was enabled, in 1824, after examining many skeletons nearly +perfect, to give an ideal restoration of the osteology of this genus, and +of that of the _Plesiosaurus_.[278-A] (See figs. 310, 311.) The latter +animal had an extremely long neck and small head, with teeth like those of +the crocodile, and paddles analogous to those of the _Ichthyosaurus_, but +larger. It is supposed to have lived in shallow seas and estuaries, and to +have breathed air like the Ichthyosaur, and our modern cetacea.[278-B] Some +of the reptiles above mentioned were of formidable dimensions. One specimen +of _Ichthyosaurus platyodon_, from the lias at Lyme, now in the British +Museum, must have belonged to an animal more than 24 feet in length; and +another of the _Plesiosaurus_, in the same collection, is 11 feet long. The +form of the _Ichthyosaurus_ may have fitted it to cut through the waves +like the porpoise; but it is supposed that the _Plesiosaurus_, at least the +long-necked species (fig. 311.), was better suited to fish in shallow +creeks and bays defended from heavy breakers. + +In many specimens both of Ichthyosaur and Plesiosaur the bones of the head, +neck, and tail, are in their natural position, while those of the rest of +the skeleton are detached and in confusion. Mr. Stutchburg has suggested +that their bodies after death became inflated with gases, and, while the +abdominal viscera were decomposing, the bones, though disunited, were +retained within the tough dermal covering as in a bag, until the whole, +becoming water-logged, sank to the bottom.[278-C] As they belonged to +individuals of all ages they are supposed, by Dr. Buckland, to have +experienced a violent death; and the same conclusion might also be drawn +from their having escaped the attacks of their own predaceous race, or of +fishes, found fossil in the same beds. + +[Illustration: Fig 313. _Amblyrhynchus cristatus_, Bell. Length varying +from 3 to 4 feet. The only existing marine lizard now known. + +_a._ Tooth, natural size and magnified.] + +For the last twenty years, anatomists have agreed that these extinct +saurians must have inhabited the sea; and it was argued that, as there +are now chelonians, like the tortoise, living in fresh water, and +others, as the turtle, frequenting the ocean, so there may have been +formerly some saurians proper to salt, others to fresh water. The common +crocodile of the Ganges is well known to frequent equally that river and +the brackish and salt water near its mouth; and crocodiles are said in +like manner to be abundant both in the rivers of the Isla de Pinos (or +Isle of Pines), south of Cuba, and in the open sea round the coast. More +recently a saurian has been discovered of aquatic habits and exclusively +marine. This creature was found in the Galapagos Islands, during the +visit of H. M. S. Beagle to that archipelago, in 1835, and its habits +were then observed by Mr. Darwin. The islands alluded to are situated +under the equator, nearly 600 miles to the westward of the coast of +South America. They are volcanic, some of them being 3000 or 4000 feet +high; and one of them, Albemarle Island, 75 miles long. The climate is +mild; very little rain falls; and, in the whole archipelago, there is +only one rill of fresh water that reaches the coast. The soil is for the +most part dry and harsh, and the vegetation scanty. The birds, reptiles, +plants, and insects are, with very few exceptions, of species found no +where else in the world, although all partake, in their general form, of +a South American type. Of the mammalia, says Mr. Darwin, one species +alone appears to be indigenous, namely, a large and peculiar kind of +mouse; but the number of lizards, tortoises, and snakes is so great, +that it may be called a land of reptiles. The variety, indeed, of +species is small; but the individuals of each are in wonderful +abundance. There is a turtle, a large tortoise (_Testudo Indicus_), four +lizards, and about the same number of snakes, but no frogs or toads. Two +of the lizards belong to the family _Iguanidæ_ of Bell, and to a +peculiar genus (_Amblyrhynchus_) established by that naturalist, and so +named from their obtusely truncated head and short snout.[279-A] Of +these lizards one is terrestrial in its habits, and burrows in the +ground, swarming everywhere on the land, having a round tail, and a +mouth somewhat resembling in form that of the tortoise. The other is +aquatic, and has its tail flattened laterally for swimming (see fig. +313.). "This marine saurian," says Mr. Darwin, "is extremely common on +all the islands throughout the archipelago. It lives exclusively on the +rocky sea-beaches, and I never saw one even ten yards inshore. The usual +length is about a yard, but there are some even 4 feet long. It is of a +dirty black colour, sluggish in its movements on the land; but, when in +the water, it swims with perfect ease and quickness by a serpentine +movement of its body and flattened tail, the legs during this time being +motionless, and closely collapsed on its sides. Their limbs and strong +claws are admirably adapted for crawling over the rugged and fissured +masses of lava which everywhere form the coast. In such situations, a +group of six or seven of these hideous reptiles may oftentimes be seen +on the black rocks, a few feet above the surf, basking in the sun with +outstretched legs. Their stomachs, on being opened, were found to be +largely distended with minced sea-weed, of a kind which grows at the +bottom of the sea at some little distance from the coast. To obtain +this, the lizards go out to sea in shoals. One of these animals was sunk +in salt water, from the ship, with a heavy weight attached to it, and +on being drawn up again after an hour it was quite active and unharmed. +It is not yet known by the inhabitants where this animal lays its eggs; +a singular fact, considering its abundance, and that the natives are +well acquainted with the eggs of the terrestrial _Amblyrhynchus_, +which is also herbivorous."[280-A] + +In those deposits now forming by the sediment washed away from the +wasting shores of the Galapagos Islands the remains of saurians, both of +the land and sea, as well as of chelonians and fish, may be mingled with +marine shells, without any bones of land quadrupeds or batrachian +reptiles; yet even here we should expect the remains of marine mammalia +to be imbedded in the new strata, for there are seals, besides several +kinds of cetacea, on the Galapagian shores; and, in this respect, the +parallel between the modern fauna, above described, and the ancient one +of the lias, would not hold good. + +_Sudden destruction of saurians._--It has been remarked, and truly, that +many of the fish and saurians, found fossil in the lias, must have met with +sudden death and immediate burial; and that the destructive operation, +whatever may have been its nature, was often repeated. + +"Sometimes," says Dr. Buckland, "scarcely a single bone or scale has been +removed from the place it occupied during life; which could not have +happened had the uncovered bodies of these saurians been left, even for a +few hours, exposed to putrefaction, and to the attacks of fishes, and other +smaller animals at the bottom of the sea."[280-B] Not only are the +skeletons of the Ichthyosaurs entire, but sometimes the contents of their +stomachs still remain between their ribs, as before remarked, so that we +can discover the particular species of fish on which they lived, and the +form of their excrements. Not unfrequently there are layers of these +coprolites, at different depths in the lias, at a distance from any entire +skeletons of the marine lizards from which they were derived; "as if," says +Sir H. De la Beche, "the muddy bottom of the sea received small sudden +accessions of matter from time to time, covering up the coprolites and +other exuviæ which had accumulated during the intervals."[281-A] It is +farther stated that, at Lyme Regis, those surfaces only of the coprolites +which lay uppermost at the bottom of the sea have suffered partial decay, +from the action of water before they were covered and protected by the +muddy sediment that has afterwards permanently enveloped them.[281-B] + +Numerous specimens of the pen-and-ink fish (_Sepia loligo_, Lin.; _Loligo +vulgaris_, Lam.) have also been met with in the lias at Lyme, with the +ink-bags still distended, containing the ink in a dried state, chiefly +composed of carbon, and but slightly impregnated with carbonate of lime. +These cephalopoda, therefore, must, like the saurians, have been soon +buried in sediment; for, if long exposed after death, the membrane +containing the ink would have decayed.[281-C] + +As we know that river fish are sometimes stifled, even in their own +element, by muddy water during floods, it cannot be doubted that the +periodical discharge of large bodies of turbid fresh water into the sea may +be still more fatal to marine tribes. In the Principles of Geology I have +shown that large quantities of mud and drowned animals have been swept down +into the sea by rivers during earthquakes, as in Java, in 1699; and that +undescribable multitudes of dead fishes have been seen floating on the sea +after a discharge of noxious vapours during similar convulsions.[281-D] +But, in the intervals between such catastrophes, strata may have +accumulated slowly in the sea of the lias, some being formed chiefly of one +description of shell, such as ammonites, others of gryphites. + +From the above remarks the reader will infer that the lias is for the most +part a marine deposit. Some members, however, of the series, especially in +the lowest part of it, have an estuary character, and must have been formed +within the influence of rivers. In Gloucestershire, where there is a good +type of the lias of the West of England, it may be divided into an upper +mass of shale with a base of marlstone, and a lower series of shales with +underlying limestones and shales. We learn from the researches of the Rev. +P. B. Brodie[281-E], that in the superior of these two divisions numerous +remains of insects and plants have been detected in several places, mingled +with marine shells; but in the inferior division similar fossils are still +more plentiful. One band, rarely exceeding a foot in thickness, has been +named the "insect limestone." It passes upwards into a shale containing +_Cypris_ and _Estheria_, and is charged with the wing-cases of several +genera of coleoptera, and with some nearly entire beetles, of which the +eyes are preserved. The nervures of the wings of neuropterous insects +(fig. 314.) are beautifully perfect in this bed. Ferns, with leaves of +monocotyledonous plants, and freshwater shells, such as _Cyclas_ and +_Unio_, accompany the insects in some places, while in others marine shells +predominate, the fossils varying apparently as we examine the bed nearer or +farther from the ancient land, or the source whence the fresh water was +derived. There are two, or even three, bands of "insect limestone" in +several sections, and they have been ascertained by Mr. Brodie to retain +the same lithological and zoological characters when traced from the centre +of Warwickshire to the borders of the southern part of Wales. After +studying 300 specimens of these insects from the lias, Mr. Westwood +declares that they comprise both wood-eating and herb-devouring beetles of +the Linnean genera _Elater_, _Carabus_, &c., besides grasshoppers +(_Gryllus_), and detached wings of dragon-flies and may-flies, or insects +referable to the Linnean genera _Libellula_, _Ephemera_, _Hemerobius_, and +_Panorpa_, in all belonging to no less than twenty-four families. The size +of the species is usually small, and such as taken alone would imply a +temperate climate; but many of the associated organic remains of other +classes must lead to a different conclusion. + +[Illustration: Fig. 314. Wing of a neuropterous insect, from the Lower +Lias, Gloucestershire. (Rev. B. Brodie.)] + +_Fossil plants._--Among the vegetable remains of the Lias, several species +of _Zamia_ have been found at Lyme Regis, and the remains of coniferous +plants at Whitby. Fragments of wood are common, and often converted into +limestone. That some of this wood, though now petrified, was soft when it +first lay at the bottom of the sea, is shown by a specimen now in the +museum of the Geological Society (see fig. 315.), which has the form of an +_ammonite_ indented on its surface. + +[Illustration: Fig. 315. Petrified wood.] + +M. Ad. Brongniart enumerates forty-seven liassic acrogens, most of them +ferns; and fifty gymnogens, of which thirty-nine are cycads, and eleven +conifers. Among the cycads the predominance of _Zamites_ and _Nilsonia_, +and among the ferns the numerous genera with leaves having reticulated +veins (as in fig. 296. p. 272.), are mentioned as botanical +characteristics of this era.[282-A] + +_Origin of the Oolite and Lias._--If we now endeavour to restore, in +imagination, the ancient condition of the European area at the period of +the Oolite and Lias, we must conceive a sea in which the growth of coral +reefs and shelly limestones, after proceeding without interruption for +ages, was liable to be stopped suddenly by the deposition of clayey +sediment. Then, again, the argillaceous matter, devoid of corals, was +deposited for ages, and attained a thickness of hundreds of feet, until +another period arrived when the same space was again occupied by +calcareous sand, or solid rocks of shell and coral, to be again succeeded +by the recurrence of another period of argillaceous deposition. Mr. +Conybeare has remarked of the entire group of Oolite and Lias, that it +consists of repeated alternations of clay, sandstone, and limestone, +following each other in the same order. Thus the clays of the lias are +followed by the sands of the inferior oolite, and these again by shelly and +coralline limestone (Bath oolite, &c.); so, in the middle oolite, the +Oxford clay is followed by calcareous grit and "coral rag;" lastly, in the +upper oolite, the Kimmeridge clay is followed by the Portland sand and +limestone.[283-A] The clay beds, however, as Sir H. De la Beche remarks, +can be followed over larger areas than the sands or sandstones.[283-B] It +should also be remembered that while the oolitic system becomes arenaceous, +and resembles a coal-field in Yorkshire, it assumes, in the Alps, an almost +purely calcareous form, the sands and clays being omitted; and even in the +intervening tracts, it is more complicated and variable than appears in +ordinary descriptions. Nevertheless, some of the clays and intervening +limestones do, in reality, retain a pretty uniform character, for distances +of from 400 to 600 miles from east to west and north to south. + +According to M. Thirria, the entire oolitic group in the department of the +Haute-Saône, in France, may be equal in thickness to that of England; but +the importance of the argillaceous divisions is in the inverse ratio to +that which they exhibit in England, where they are about equal to twice the +thickness of the limestones, whereas, in the part of France alluded to, +they reach only about a third of that thickness.[283-C] In the Jura the +clays are still thinner; and in the Alps they thin out and almost vanish. + +In order to account for such a succession of events, we may imagine, +first, the bed of the ocean to be the receptacle for ages of fine +argillaceous sediment, brought by oceanic currents, which may have +communicated with rivers, or with part of the sea near a wasting coast. +This mud ceases, at length, to be conveyed to the same region, either +because the land which had previously suffered denudation is depressed +and submerged, or because the current is deflected in another direction +by the altered shape of the bed of the ocean and neighbouring dry land. +By such changes the water becomes once more clear and fit for the growth +of stony zoophytes. Calcareous sand is then formed from comminuted shell +and coral, or, in some cases, arenaceous matter replaces the clay; +because it commonly happens that the finer sediment, being first drifted +farthest from coasts, is subsequently overspread by coarse sand, after +the sea has grown shallower, or when the land, increasing in extent, +whether by upheaval or by sediment filling up parts of the sea, has +approached nearer to the spots first occupied by fine mud. + +In order to account for another great formation, like the Oxford clay, +again covering one of coral limestone, we must suppose a sinking down +like that which is now taking place in some existing regions of coral +between Australia and South America. The occurrence of subsidences, on +so vast a scale, may have caused the bed of the ocean and the adjoining +land, throughout great parts of the European area, to assume a shape +favourable to the deposition of another set of clayey strata; and this +change may have been succeeded by a series of events analogous to that +already explained, and these again by a third series in similar order. +Both the ascending and descending movements may have been extremely +slow, like those now going on in the Pacific; and the growth of every +stratum of coral, a few feet of thickness, may have required centuries +for its completion, during which certain species of organic beings +disappeared from the earth, and others were introduced in their place; +so that, in each set of strata, from the Upper Oolite to the Lias, some +peculiar and characteristic fossils were embedded. + + +_Oolite and Lias of the United States._ + +[Illustration: Fig. 316. Section showing the geological position of the +James River, or East Virginian Coal-field. + + A. Granite, gneiss, &c. + B. Coal-measures. + C. Tertiary strata. + D. Drift or _ancient alluvium_.] + +There are large tracts on the globe, as in Russia and the United States, +where all the members of the oolitic series are unrepresented. In the state +of Virginia, however, at the distance of about 13 miles eastward of +Richmond, the capital of that State, there is a regular coal-field +occurring in a depression of the granite rocks (see section, fig. 316.), +which Professor W. B. Rogers first correctly referred to the age of the +lower part of the Jurassic group. This opinion I was enabled to confirm +after collecting a large number of fossil plants, fish, and shells, and +examining the coal-field throughout its whole area. It extends 26 miles +from north to south, and from 4 to 12, from east to west. The plants +consist chiefly of zamites, calamites, and equisetums, and these last are +very commonly met with in a vertical position more or less compressed +perpendicularly. It is clear that they grew in the places where they now +lie buried in strata of hardened sand and mud. I found them maintaining +their erect attitude, at points many miles distant from others, in beds +both above and between the seams of coal. In order to explain this fact we +must suppose such shales and sandstones to have been gradually accumulated +during the slow and repeated subsidence of the whole region. + +It is worthy of remark that the _Equisetum columnare_ of these Virginian +rocks appears to be undistinguishable from the species found in the +oolitic sandstones near Whitby in Yorkshire, where it also is met with +in an upright position. One of the American ferns, _Pecopteris +Whitbyensis_, is also a species common to the Yorkshire oolites.[285-A] +These Virginian coal-measures are composed of grits, sandstones, and +shales, exactly resembling those of older or primary date in America and +Europe, and they rival or even surpass the latter in the richness and +thickness of the seams. One of these, the main seam, is in some places +from 30 to 40 feet thick, composed of pure bituminous coal. On +descending a shaft 800 feet deep, in the Blackheath mines in +Chesterfield county, I found myself in a chamber more than 40 feet high, +caused by the removal of this coal. Timber props of great strength +supported the roof, but they were seen to bend under the incumbent +weight. The coal is like the finest kinds shipped at Newcastle, and when +analysed yields the same proportions of carbon and hydrogen, a fact +worthy of notice when we consider that this fuel has been derived from +an assemblage of plants very distinct specifically, and in part +generically, from those which have contributed to the formation of +the ancient or paleozoic coal. + +The fossil fish of these Richmond strata belong to the liassic genus +_Tetragonolepis_, and to a new genus which I have called _Dictyopyge_. +Shells are very rare, as usually in all coal-bearing deposits, but a +species of _Posidonomya_ is in such profusion in some shaley beds as to +divide them like the plates of mica in micaceous shales (see fig. 317.). + +[Illustration: Fig. 317. Oolitic coal-shale, Richmond, Virginia. + + _a._ _Posidonomya._ + _b._ young of same.] + +In India, especially in Cutch, a formation occurs clearly referable to the +oolitic and liassic type, as shown by the shells, corals, and plants; and +there also coal has been procured from one member of the group. + + +FOOTNOTES: + +[274-A] Conyb. and Phil. p. 261. + +[275-A] Agassiz, Pois. Fos. vol. ii. tab. 28, 29. + +[276-A] Bridgewater Treatise, p. 290. + +[276-B] Agassiz, Poissons Fossiles, vol. iii. tab. C. fig. 1. + +[276-C] Ibid. p. 168. + +[276-D] Ibid. p. 187. + +[277-A] Geol. Soc. Proceedings, vol. iii. p. 157. 1839. + +[277-B] Geol. Trans. Second Series, vol. v. p. 511. + +[278-A] Geol. Trans., Second Series, vol. i. pl. 49. + +[278-B] Conybeare and De la Beche. Geol. Trans.; and Buckland, +Bridgew. Treat., p. 203. + +[278-C] Quart. Geol. Journ. vol. ii. p. 411. + +[279-A] +Amblys+, _amblys_, blunt; and +rhygchos+, _rhynchus_, snout. + +[280-A] Darwin's Journal, chap. xix. + +[280-B] Bridgew. Treat., p. 125. + +[281-A] Geological Researches, p. 334. + +[281-B] Buckland, Bridgew. Treat., p. 307. + +[281-C] Ibid. + +[281-D] See Principles, _Index_, Lancerote, Graham Island, Calabria. + +[281-E] A History of Fossil Insects, &c. 1845. London. + +[282-A] Tableau des Veg. Fos. 1849, p. 105. + +[283-A] Con. and Phil., p. 166. + +[283-B] Geol. Researches, p. 337. + +[283-C] Burat's D'Aubuisson, tom. ii. p. 456. + +[285-A] See description of the coal-field by the author, and the plants by +C. J. F. Bunbury, Esq., Quart. Geol. Journ., vol. iii. p. 281. + + + + +CHAPTER XXII. + +TRIAS OR NEW RED SANDSTONE GROUP. + + Distinction between New and Old Red Sandstone--Between Upper and Lower + New Red--The Trias and its three divisions--Most largely developed in + Germany--Keuper and its fossils--Muschelkalk--Fossil plants of + Bunter--Triassic group in England--Bone-bed of Axmouth and Aust--Red + Sandstone of Warwickshire and Cheshire--Footsteps of _Chirotherium_ in + England and Germany--Osteology of the _Labyrinthodon_--Identification + of this Batrachian with the Chirotherium--Origin of Red Sandstone and + Rock-salt--Hypothesis of saline volcanic exhalations--Theory of the + precipitation of salt from inland lakes or lagoons--Saltness of the + Red Sea--New Red Sandstone in the United States--Fossil footprints of + birds and reptiles in the Valley of the Connecticut--Antiquity of the + Red Sandstone containing them. + + +Between the Lias and the Coal, or Carboniferous group, there is interposed, +in the midland and western counties of England, a great series of red +loams, shales, and sandstones, to which the name of the New Red Sandstone +formation was first given, to distinguish it from other shales and +sandstones called the "Old Red" (_c_, fig. 318.), often identical in +mineral character, which lie immediately beneath the coal (_b_). + +[Illustration: Fig. 318. Cross section. + + _a._ New red sandstone. + _b._ Coal. + _c._ Old red.] + +The name of "Red Marl" has been incorrectly applied to the red clays of +this formation, as before explained (p. 13.), for they are remarkably free +from calcareous matter. The absence, indeed, of carbonate of lime, as well +as the scarcity of organic remains, together with the bright red colour of +most of the rocks of this group, causes a strong contrast between it and +the Jurassic formations before described. + +Before the distinctness of the fossil remains characterizing the upper and +lower part of the English New Red had been clearly recognized, it was found +convenient to have a common name for all the strata intermediate in +position between the Lias and Coal; and the term "Poikilitic" was proposed +by Messrs. Conybeare and Buckland[286-A], from +poikilos+, poikilos, +_variegated_, some of the most characteristic strata of this group having +been called _variegated_ by Werner, from their exhibiting spots and streaks +of light-blue, green, and buff colour, in a red base. + +A single term, thus comprehending both Upper and Lower New Red, or the +Triassic and Permian groups of modern classifications, may still be useful +in describing districts where we have to speak of masses of red sandstone +and shale, referable, in part, to both these eras, but which, in the +absence of fossils, it is impossible to divide. + + +_Trias or Upper New Red Sandstone Group._ + +The accompanying table will explain the subdivisions generally adopted for +the uppermost of the two systems above alluded to, and the names given to +them in England and on the Continent. + + Synonyms. + German. French. + + { _a._ Saliferous and } } + { gypseous shales and } Keuper } Marnes irisées. + Trias or Upper { sandstone } } + New Red { + Sandstone { _b._ (wanting in England) } Muschelkalk { Muschelkalk, ou + { { calcaire + { { coquillier. + { + { _c._ Sandstone and } Bunter- } Grès bigarré. + { quartzose conglomerate } sandstein } + +I shall first describe this group as it occurs in South Western and North +Western Germany, for it is far more fully developed there than in England +or France. It has been called the Trias by German writers, or the Triple +Group, because it is separable into three distinct formations, called the +"Keuper," the "Muschelkalk," and the "Bunter-sandstein." + +[Illustration: Fig. 319. _Equisetites columnaris._ (Syn. _Equisetum +columnare_.) Fragment of stem, and small portion of same +magnified. Keuper.] + +_The Keuper_, the first or newest of these, is 1000 feet thick in +Würtemberg, and is divided by Alberti into sandstone, gypsum, and +carbonaceous slate-clay.[287-A] Remains of Reptiles, called _Nothosaurus_ +and _Phytosaurus_, have been found in it with _Labyrinthodon_; the detached +teeth, also, of placoid fish and of rays, and of the genera _Saurichthys_ +and _Gyrolepis_ (figs. 325, 326, p. 289.). The plants of the Keuper are +generically very analogous to those of the lias and oolite, consisting of +ferns, equisetaceous plants, cycads, and conifers, with a few doubtful +monocotyledons. A few species, such as _Equisetites columnaris_, are common +to this group, and the oolite. + +_The Muschelkalk_ consists chiefly of a compact, greyish limestone, but +includes beds of dolomite in many places, together with gypsum and +rock-salt. This limestone, a rock wholly unrepresented in England, abounds +in fossil shells, as the name implies. Among the cephalopoda there are no +belemnites, and no ammonites with foliated sutures, as in the incumbent +lias and oolite, but a genus allied to the Ammonite, called _Ceratite_ by +De Haan, in which the descending lobes (see _a_, _b_, _c_, fig. 320.) +terminate in a few small denticulations pointing inwards. Among the +bivalve shells, the _Posidonia minuta_, Goldf. (_Posidonomya minuta_, +Bronn) (see fig. 321.), is abundant, ranging through the Keuper, +Muschelkalk, and Bunter-sandstein; and _Avicula socialis_, fig. 322., +having a similar range, is very characteristic of the Muschelkalk in +Germany, France, and Poland. + +[Illustration: Fig. 320. _Ceratites nodosus._ Muschelkalk. + + _a._ Side view. + _b._ Front view. + _c._ Partially denticulated outline of the septa dividing the chambers.] + +[Illustration: Fig. 321. _Posidonia minuta_, Goldf. (_Posidonomya +minuta_, Bronn.)] + +[Illustration: Fig. 322. Avicula. Characteristic of the Muschelkalk. + + _a._ _Avicula socialis._ + _b._ Side view of same.] + +The abundance of the heads and stems of lily encrinites, _Encrinus +liliiformis_ (or _Encrinites moniliformis_), show the slow manner in which +some beds of this limestone have been formed in clear sea-water. + +[Illustration: Fig. 323. Voltzia. Bunter-sandstein. + + _a._ _Voltzia heterophylla._ (Syn. _Voltzia brevifolia_.) + _b._ portion of same magnified to show fructification. Sulzbad.] + +_The Bunter-sandstein_ consists of various coloured sandstones, +dolomites, and red-clays, with some beds, especially in the Hartz, of +calcareous pisolite or roe-stone, the whole sometimes attaining a +thickness of more than 1000 feet. The sandstone of the Vosges, according +to Von Meyer, is proved, by the presence of _Labyrinthodon_, to belong +to this lowest member of the Triassic group. At Sulzbad (or +Soultz-les-bains), near Strasburg, on the flanks of the Vosges, many +plants have been obtained from the "bunter," especially conifers of the +extinct genus _Voltzia_, peculiar to this period, in which even the +fructification has been preserved. (See fig. 323.) + +Out of thirty species of ferns, cycads, conifers, and other plants, +enumerated by M. Ad. Brongniart, in 1849, as coming from the "grès +bigarré," or Bunter, not one is common to the Keuper.[288-A] + +The footprints of a reptile (_Labyrinthodon_) have been observed on the +clays of this member of the Trias, near Hildburghausen, in Saxony, +impressed on the upper surface of the beds, and standing out as casts in +relief from the under sides of incumbent slabs of sandstone. To these I +shall again allude in the sequel; they attest, as well as the accompanying +ripple-marks, and the cracks which traverse the clays, the gradual +formation in shallow water, and sometimes between high and low water, of +the beds of this formation. + + +_Triassic group in England._ + +In England the Lias is succeeded by conformable strata of red and green +marl, or clay. There intervenes, however, both in the neighbourhood of +Axmouth, in Devonshire, and in the cliffs of Westbury and Aust, in +Gloucestershire, on the banks of the Severn, a dark-coloured stratum, well +known by the name of the "bone-bed." It abounds in the remains of saurians +and fish, and was formerly classed as the lowest bed of the Lias; but Sir +P. Egerton has shown that it should be referred to the Upper New Red +Sandstone, for it contains an assemblage of fossil fish which are either +peculiar to this stratum, or belong to species well known in the +Muschelkalk of Germany. These fish belong to the genera _Acrodus_, +_Hybodus_, _Gyrolepis_, and _Saurichthys_. + +Among those common to the English bone-bed and the Muschelkalk of Germany +are _Hybodus plicatilis_ (fig. 324.), _Saurichthys apicalis_ (fig. 325.), +_Gyrolepis tenuistriatus_ (fig. 326.), and _G. Albertii_. Remains of +saurians have also been found in the bone-bed, and plates of an _Encrinus_. + +[Illustration: Fig. 324. _Hybodus plicatilis._ Teeth. Bone-bed, +Aust and Axmouth.] + +[Illustration: Fig. 325. _Saurichthys apicalis._ Tooth; nat. size, +and magnified. Axmouth.] + +[Illustration: Fig. 326. _Gyrolepis tenuistriatus._ Scale; nat. size, +and magnified. Axmouth.] + +The strata of red and green marl, which follow the bone-bed in the +descending order at Axmouth and Aust, are destitute of organic remains; +as is the case, for the most part, in the corresponding beds in almost +every part of England. But fossils have lately been found at a few +localities in sandstones of this formation, in Worcestershire and +Warwickshire, and among them the bivalve shell called _Posidonia +minuta_, Goldf., before mentioned (fig. 321. p. 288.). + +The upper member of the English "New Red" containing this shell, in +those parts of England, is, according to Messrs. Murchison and +Strickland, 600 feet thick, and consists chiefly of red marl or +slate, with a band of sandstone. Spines of _Hybodus_, called +_ichthyodorulites_, teeth of fishes, and footprints of reptiles, with +remains of a saurian called _Rhyncosaurus_, were observed by the same +geologists in these strata.[290-A] + +In Cheshire and Lancashire the gypseous and saliferous red shales and loams +of the Trias are between 1000 and 1500 feet thick. In some places +lenticular masses of rock-salt are interpolated between the argillaceous +beds, the origin of which will be spoken of in the sequel. + +[Illustration: Fig. 327. Single footstep of _Chirotherium_. Bunter +Sandstein, Saxony; one eighth of nat. size.] + +[Illustration: Fig. 328. Line of footsteps on slab of sandstone. +Hildburghausen, in Saxony.] + +The lower division or English representative of the "Bunter" attains a +thickness of 600 feet in the counties last mentioned. Besides red and green +shales and red sandstones, it comprises much soft white quartzose +sandstone, in which the trunks of silicified trees have been met with at +Allesley Hill, near Coventry. Several of them were a foot and a half in +diameter, and some yards in length, decidedly of coniferous wood, and +showing rings of annual growth.[290-B] Impressions, also, of the footsteps +of animals have been detected in Lancashire and Cheshire in this formation. +Some of the most remarkable occur a few miles from Liverpool, in the +whitish quartzose sandstone of Storton Hill, on the west side of the +Mersey. They bear a close resemblance to tracks first observed in a member +of the Upper New Red Sandstone, at the village of Hesseberg, near +Hildburghausen, in Saxony, to which I have already alluded. For many years +these footprints have been referred to a large unknown quadruped, +provisionally named _Chirotherium_ by Professor Kaup, because the marks +both of the fore and hind feet resembled impressions made by a human hand. +(See fig. 327.) The footmarks at Hesseberg are partly concave and partly in +relief; the former, or the depressions, are seen upon the upper surface of +the sandstone slabs, but those in relief are only upon the lower surfaces, +being in fact natural casts, formed in the subjacent footprints as in +moulds. The larger impressions, which seem to be those of the hind foot, +are generally 8 inches in length, and 5 in width, and one was 12 inches +long. Near each large footstep, and at a regular distance (about an inch +and a half), before it, a smaller print of a fore foot, 4 inches long and +3 inches wide, occurs. The footsteps follow each other in pairs, each pair +in the same line, at intervals of 14 inches from pair to pair. The large as +well as the small steps show the great toes alternately on the right and +left side; each step makes the print of five toes, the first or great toe +being bent inwards like a thumb. Though the fore and hind foot differ so +much in size, they are nearly similar in form. + +The similar footmarks afterwards observed in a rock of corresponding age at +Storton Hill, were imprinted on five thin beds of clay, superimposed one +upon the other in the same quarry, and separated by beds of sandstone. On +the lower surface of the sandstone strata, the solid casts of each +impression are salient, in high relief, and afford models of the feet, +toes, and claws of the animals which trod on the clay. + +As neither in Germany nor in England any bones or teeth had been met with +in the same identical strata as the footsteps, anatomists indulged, for +several years, in various conjectures respecting the mysterious animals +from which they might have been derived. Professor Kaup suggested that the +unknown quadruped might have been allied to the _Marsupialia_; for in the +kangaroo the first toe of the fore foot is in a similar manner set +obliquely to the others, like a thumb, and the disproportion between the +fore and hind feet is also very great. But M. Link conceived that some of +the four species of animals of which the tracks had been found in Saxony +might have been gigantic _Batrachians_; and Dr. Buckland designated some of +the footsteps as those of a small web-footed animal, probably crocodilean. + +In the course of these discussions several naturalists of Liverpool, in +their report on the Storton quarries, declared their opinion that each of +the thin seams of clay in which the sandstone casts were moulded had formed +successively a surface above water, over which the _Chirotherium_ and other +animals walked, leaving impressions of their footsteps, and that each layer +had been afterwards submerged by a sinking down of the surface, so that a +new beach was formed at low water above the former, on which other tracks +were then made. The repeated occurrence of ripple-marks at various heights +and depths in the red sandstone of Cheshire had been explained in the same +manner. It was also remarked that impressions of such depth and clearness +could only have been made by animals walking on the land, as their weight +would have been insufficient to make them sink so deeply in yielding clay +under water. They must therefore have been air-breathers. + +When the inquiry had been brought to this point, the reptilian remains +discovered in the Trias, both of Germany and England, were carefully +examined by Mr. Owen. He found, after a microscopic investigation of the +teeth from the German sandstone called Keuper, and from the sandstone of +Warwick and Leamington, that neither of them could be referred to true +saurians, although they had been named _Mastodonsaurus_ and _Phytosaurus_ +by Jäger (fig. 329.). It appeared that they were of the _Batrachian_ order, +and attested the former existence of frogs of gigantic dimensions in +comparison with any now living. Both the Continental and English fossil +teeth exhibited a most complicated texture, differing from that previously +observed in any reptile, whether recent or extinct, but most nearly +analogous to the _Ichthyosaurus_. A section of one of these teeth exhibits +a series of irregular folds, resembling the labyrinthic windings of the +surface of the brain; and from this character Mr. Owen has proposed the +name _Labyrinthodon_ for the new genus. By his permission, the annexed +representation (fig. 330.) of part of one is given from his "Odontography," +plate 64. A. The entire length of this tooth is supposed to have been about +three inches and a half, and the breadth at the base one inch and a half. + +[Illustration: Fig. 329. Tooth of _Labyrinthodon_; nat. size. +Warwick sandstone.] + +[Illustration: Fig. 330. Transverse section of tooth of _Labyrinthodon +Jaegeri_, Owen (_Mastodonsaurus Jaegeri_, Meyer); nat. size, and +a segment magnified. + +_a._ Pulp cavity, from which the processes of pulp and dentine radiate.] + +When Mr. Owen had satisfied himself, from an inspection of the cranium, +jaws, and teeth, that a gigantic _Batrachian_ had existed at the period of +the Trias or Upper New Red Sandstone, he soon found, from the examination +of various bones derived from the same formation, that he could define +three species of _Labyrinthodon_, and that in this genus the hind +extremities were much larger than the anterior ones. This circumstance, +coupled with the fact of the _Labyrinthodon_ having existed at the period +when the _Chirotherian_ footsteps were made, was the first step towards the +identification of those tracks with the newly discovered _Batrachian_. It +was at the same time observed that the footmarks of _Chirotherium_ were +more like those of toads than of any other living animal; and, lastly, +that the size of the three species of _Labyrinthodon_ corresponded with the +size of three different kinds of footprints which had already been supposed +to belong to three distinct _Chirotheria_. It was moreover inferred, with +confidence, that the _Labyrinthodon_ was an _air-breathing_ reptile from +the structure of the nasal cavity, in which the posterior outlets were at +the back part of the mouth, instead of being directly under the anterior or +external nostrils. It must have respired air after the manner of saurians, +and may therefore have imprinted on the shore those footsteps, which, as we +have seen, could not have originated from an animal walking under water. + +It is true that the structure of the foot is still wanting, and that a +more connected and complete skeleton is required for demonstration; but +the circumstantial evidence above stated is strong enough to produce +the conviction that the _Chirotherium_ and _Labyrinthodon_ are one +and the same. + +In order to show the manner in which one of these formidable _Batrachians_ +may have impressed the mark of its feet upon the shore, Mr. Owen has +attempted a restoration, of which a reduced copy is annexed. + +[Illustration: Fig. 331. _Labyrinthodon pachygnathus_, Owen.] + +The only bones of this species at present known are those of the head, the +pelvis, and part of the scapula, which are shown by stronger lines in the +above figure. There is reason for believing that the head was not smooth +externally, but protected by bony scutella. + + +_Origin of Red Sandstone and Rock Salt._ + +We have seen that, in various parts of the world, red and mottled clays, +and sandstones, of several distinct geological epochs, are found associated +with salt, gypsum, magnesian limestone, or with one or all of these +substances. There is, therefore, in all likelihood, a general cause for +such a coincidence. Nevertheless, we must not forget that there are dense +masses of red and variegated sandstones and clays, thousands of feet in +thickness, and of vast horizontal extent, wholly devoid of saliferous or +gypseous matter. There are also deposits of gypsum and of muriate of soda, +as in the blue clay formation of Sicily, without any accompanying red +sandstone or red clay. + +To account for deposits of red mud and red sand, we have simply to suppose +the disintegration of ordinary crystalline or metamorphic schists. Thus, in +the eastern Grampians of Scotland, as, for example, in the north of +Forfarshire, the mountains of gneiss, mica-schist, and clay-slate, are +overspread with alluvium, derived from the disintegration of those rocks; +and the mass of detritus is stained by oxide of iron, of precisely the same +colour as the Old Red Sandstone of the adjoining Lowlands. Now this +alluvium merely requires to be swept down to the sea, or into a lake, to +form strata of red sandstone and red marl, precisely like the mass of the +"Old Red" or New Red systems of England, or those tertiary deposits of +Auvergne (see p. 182.), before described, which are in lithological +characters quite undistinguishable. The pebbles of gneiss in the Eocene red +sandstone of Auvergne point clearly to the rocks from which it has been +derived. The red colouring matter may, as in the Grampians, have been +furnished by the decomposition of hornblende, or mica, which contain oxide +of iron in large quantity. + +It is a general fact, and one not yet accounted for, that scarcely any +fossil remains are preserved in stratified rocks in which this oxide of +iron abounds; and when we find fossils in the New or Old Red Sandstone in +England, it is in the grey, and usually calcareous beds, that they occur. + +The gypsum and saline matter, occasionally interstratified with such red +clays and sandstones of various ages, primary, secondary, and tertiary, +have been thought by some geologists to be of volcanic origin. Submarine +and subaerial exhalations often occur in regions of earthquakes and +volcanos far from points of actual eruption, and charged with sulphur, +sulphuric salts, and with common salt or muriate of soda. In a word, they +are vents by which all the products which issue in a state of sublimation +from the craters of active volcanos, obtain a passage from the interior of +the earth to the surface. That such gaseous emanations and mineral springs, +impregnated with the ingredients before enumerated, and often intensely +heated, continue to flow out unaltered in composition and temperature for +ages, is well known. But before we can decide on their real instrumentality +in producing in the course of ages beds of gypsum, salt, and dolomite, we +require to know what are the chemical changes actually in progress in seas +where this volcanic agency is at work. + +Yet the origin of rock-salt is a problem of so much interest in +theoretical geology as to demand a full discussion of another hypothesis +advanced on the subject; namely, that which attributes the precipitation +of the salt to evaporation, whether of inland lakes or of lagoons +communicating with the ocean. + +At Northwich, in Cheshire, two beds of salt, in great part unmixed with +earthy matter, attain the extraordinary thickness of 90 and even 100 feet. +The upper surface of the highest bed is very uneven, forming cones and +irregular figures. Between the two masses there intervenes a bed of +indurated clay, traversed with veins of salt. The highest bed thins off +towards the south-west, losing 15 feet in thickness in the course of a +mile.[295-A] The horizontal extent of these particular masses in Cheshire +and Lancashire is not exactly known; but the area, containing saliferous +clays and sandstones, is supposed to exceed 150 miles in diameter, while +the total thickness of the trias in the same region is estimated by Mr. +Ormerod at more than 1700 feet. Ripple-marked sandstones, and the +footprints of animals, before described, are observed at so many levels +that we may safely assume the whole area to have undergone a slow and +gradual depression during the formation of the Red Sandstone. The evidence +of such a movement, wholly independent of the presence of salt itself, is +very important in reference to the theory under consideration. + +In the "Principles of Geology" (chap. 28.), I published a map, furnished to +me by the late Sir Alexander Burnes, of that singular flat region called +the Runn of Cutch, near the delta of the Indus, which is 7000 square miles +in area, or equal in extent to about one-fourth of Ireland. It is neither +land nor sea, but is dry during a part of every year, and again covered by +salt water during the monsoons. Some parts of it are liable, after long +intervals, to be overflowed by river-water. Its surface supports no grass, +but is encrusted over, here and there, by a layer of salt, about an inch in +depth, caused by the evaporation of sea-water. Certain tracts have been +converted into dry land by upheaval during earthquakes since the +commencement of the present century, and, in other directions, the +boundaries of the Runn have been enlarged by subsidence. That successive +layers of salt might be thrown down, one upon the other, over thousands of +square miles, in such a region, is undeniable. The supply of brine from the +ocean would be as inexhaustible as the supply of heat from the sun to cause +evaporation. The only assumption required to enable us to explain a great +thickness of salt in such as area is, the continuance, for an indefinite +period, of a subsiding movement, the country preserving all the time a +general approach to horizontality. Pure salt could only be formed in the +central parts of basins, where no sand could be drifted by the wind, or +sediment be brought by currents. Should the sinking of the ground be +accelerated, so as to let in the sea freely, and deepen the water, a +temporary suspension of the precipitation of salt would be the only result. +On the other hand, if the area should dry up, ripple-marked sands and the +footprints of animals might be formed, where salt had previously +accumulated. According to this view the thickness of the salt, as well as +of the accompanying beds of mud and sand, becomes a mere question of time, +or requires simply a repetition of similar operations. + +Mr. Hugh Miller, in an able discussion of this question, refers to Dr. +Frederick Parrot's account, in his journey to Ararat (1836), of the salt +lakes of Asia. In several of these lakes west of the river Manech, "the +water, during the hottest season of the year, is covered on its surface +with a crust of salt nearly an inch thick, which is collected with shovels +into boats. The crystallization of the salt is effected by rapid +evaporation from the sun's heat and the supersaturation of the water with +muriate of soda; the lake being so shallow that the little boats trail on +the bottom and leave a furrow behind them, so that the lake must be +regarded as a wide pan of enormous superficial extent, in which the brine +can easily reach the degree of concentration required." + +Another traveller, Major Harris, in his "Highlands of Ethiopia," describes +a salt lake, called the Bahr Assal, near the Abyssinian frontier, which +once formed the prolongation of the Gulf of Tadjara, but was afterwards cut +off from the gulf by a broad bar of lava or of land upraised by an +earthquake. "Fed by no rivers, and exposed in a burning climate to the +unmitigated rays of the sun, it has shrunk into an elliptical basin, seven +miles in its transverse axis, half filled with smooth water of the deepest +cærulian hue, and half with a solid sheet of glittering snow-white salt, +the offspring of evaporation." "If," says Mr. Hugh Miller, "we suppose, +instead of a barrier of lava, that sand-bars were raised by the surf on a +flat arenaceous coast during a slow and equable sinking of the surface, the +waters of the outer gulf might occasionally topple over the bar, and supply +fresh brine when the first stock had been exhausted by evaporation.[296-A] + +We may add that the permanent impregnation of the waters of a large shallow +basin with salt, beyond the proportion which is usual in the ocean, would +cause it to be uninhabitable by mollusca or fish, as is the case in the +Dead Sea, and the muriate of soda might remain in excess, even though it +were occasionally replenished by irruptions of the sea. Should the saline +deposit be eventually submerged, it might, as we have seen from the example +of the Runn of Cutch, be covered by a freshwater formation containing +fluviatile organic remains; and in this way the apparent anomaly of beds of +sea-salt and clays devoid of marine fossils, alternating with others of +freshwater origin, may be explained. + +Dr. G. Buist, in a recent communication to the Bombay Geographical Society +(vol. ix.), has asked how it happens that the Red Sea should not exceed the +open ocean in saltness, by more than 1/10th per cent. The Red Sea receives +no supply of water from any quarter save through the Straits of +Babelmandeb; and there is not a single river or rivulet flowing into it +from a circuit of 4000 miles of shore. The countries around are all +excessively sterile and arid, and composed, for the most part, of burning +deserts. From the ascertained evaporation in the sea itself, Dr. Buist +computes that nearly 8 feet of pure water must be carried off from the +whole of its surface annually, this being probably equivalent to 1/100th +part of its whole volume. The Red Sea, therefore, ought to have 1 per cent. +added annually to its saline contents; and as these constitute 4 per cent. +by weight, or 2-1/2 per cent. in volume of its entire mass, it ought, +assuming the average depth to be 800 feet, which is supposed to be far +beyond the truth, to have been converted into one solid salt formation in +less than 3000 years.[297-A] Does the Red Sea receive a supply of water +from the ocean, through the narrow Straits of Babelmandeb, sufficient to +balance the loss by evaporation? And is there an undercurrent of heavier +saline water annually flowing outwards? If not, in what manner is the +excess of salt disposed of? An investigation of this subject by our +nautical surveyors may perhaps aid the geologist in framing a true theory +of the origin of rock-salt. + + +_On the New Red Sandstone of the valley of the Connecticut River +in the United States._ + +In a depression of the granitic or hypogene rocks in the States of +Massachusetts and Connecticut, strata of red sandstone, shale, and +conglomerate are found occupying an area more than 150 miles in length from +north to south, and about 5 to 10 miles in breadth, the beds dipping to the +eastward at angles varying from 5 to 50 degrees. The extreme inclination of +50 degrees is rare, and only observed in the neighbourhood of masses of +trap which have been intruded into the red sandstone while it was forming, +or before the newer parts of the deposit had been completed. Having +examined this series of rocks in many places, I feel satisfied that they +were formed in shallow water, and for the most part near the shore, and +that some of the beds were from time to time raised above the level of the +water, and laid dry, while a newer series, composed of similar sediment, +was forming. The red flags of thin-bedded sandstone are often +ripple-marked, and exhibit on their under sides casts of cracks formed in +the underlying red and green shales. These last must have shrunk by drying +before the sand was spread over them. On some shales of the finest texture +impressions of rain drops may be seen, and casts of them in the incumbent +argillaceous sandstones. Having observed similar markings produced by +showers, of which the precise date was known, on the recent red mud of the +Bay of Fundy, and casts in relief of the same, on layers of dried mud +thrown down by subsequent tides, I feel no doubt in regard to the origin of +some of the ancient Connecticut impressions. I have also seen on the +mud-flats of the Bay of Fundy the footmarks of birds (_Tringa minuta_), +which daily run along the borders of that estuary at low water, and which I +have described in my Travels.[297-B] Similar layers of red mud, now +hardened and compressed into shale, are laid open on the banks of the +Connecticut, and retain faithfully the impressions and casts of the feet of +numerous birds and reptiles which walked over them at the time when they +were deposited, probably in the Triassic Period. + +According to Professor Hitchcock, the footprints of no less than thirty-two +species of bipeds, and twelve of quadrupeds, have been already detected in +these rocks. Thirty of these are believed to be those of birds, four of +lizards, two of chelonians, and six of batrachians. The tracks have been +found in more than twenty places, scattered through an extent of nearly 80 +miles from north to south, and they are repeated through a succession of +beds attaining at some points a thickness of more than 1000 feet, which may +have been thousands of years in forming.[298-A] + +[Illustration: Fig. 332. Footprints of a bird. Turner's Falls, Valley of +the Connecticut. (See Dr. Deane, Mem. of Amer. Acad. vol. iv. 1849.)] + +As considerable scepticism is naturally entertained in regard to the nature +of the evidence derived from footprints, it may be well to enumerate some +facts respecting them on which the faith of the geologist may rest. When I +visited the United States in 1842, more than 2000 impressions had been +observed by Professor Hitchcock, in the district alluded to, and all of +them were indented on the upper surface of the layers, while the +corresponding casts, standing out in relief, were always on the lower +surfaces or planes of the strata. If we follow a single line of marks we +find them uniform in size, and nearly uniform in distance from each other, +the toes of two successive footprints, turning alternately right and left +(see fig. 332.). Such single lines indicate a biped; and there is generally +such a deviation from a straight line, in any three successive prints, as +we remark in the tracks left by birds. There is also a striking relation +between the distance separating two footprints in one series and the size +of the impressions; in other words, an obvious proportion between the +length of the stride and the dimension of the creature which walked over +the mud. If the marks are small, they may be half an inch asunder; if +gigantic, as, for example, where the toes are 20 inches long, they are +occasionally 4 feet and a half apart. The bipedal impressions are for the +most part trifid, and show the same number of joints as exist in the feet +of living tridactylous birds. Now such birds have three phalangeal bones +for the inner toe, four for the middle and five for the outer one (see fig. +332.); but the impression of the terminal joint is that of the nail only. +The fossil footprints exhibit regularly, where the joints are seen, the +same number; and we see in each continuous line of tracks the three-jointed +and five-jointed toes placed alternately outwards, first on the one side +and then on the other. It is not often that the matrix has been fine enough +to retain impressions of the integument or skin of the foot; but in one +fine specimen found at Turner's Falls on the Connecticut, by Dr. Deane, +these markings are well preserved, and have been recognized by Mr. Owen as +resembling the skin of the ostrich, and not that of reptiles.[298-B] Much +care is required to ascertain the precise layer of a laminated rock on +which an animal has walked, because the impression usually extends +downwards through several laminæ; and if the upper layer originally trodden +upon is wanting, one or more joints, or even in some cases an entire toe, +which sank less deep into the soft ground, may disappear, and yet the +remainder of the footprint be well defined. + +The size of several of the fossil impressions of the Connecticut red +sandstone so far exceeds that of any living ostrich, that naturalists at +first were extremely adverse to the opinion of their having been made by +birds, until the bones and almost entire skeleton of the _Dinornis_ and of +other feathered giants of New Zealand were discovered. Their dimensions +have at least destroyed the force of this particular objection. The +magnitude of the impressions of the feet of a heavy animal, which has +walked on soft mud, increases for some distance below the surface +originally trodden upon. In order, therefore, to guard against +exaggeration, the casts rather than the mould are relied on. These casts +show that some of the fossil birds had feet four times as large as the +ostrich, but not perhaps larger than the _Dinornis_. + +Some of the quadrupedal footprints which accompany those of birds are +analogous to European _Chirotheria_, and with a similar disproportion +between the hind and fore feet. Others resemble that remarkable reptile, +the _Rhyncosaurus_ of the English Trias, a creature having some relation +in its osteology both to chelonians and birds. Other imprints, again, +are like those of turtles. + +Among the supposed bipedal tracks, a single distinct example only has been +observed of feet in which there are four toes directed forwards. In this +case a series of four footprints is seen, each 22 inches long and 12 wide, +with joints much resembling those in the toes of birds. Professor Agassiz +has suggested that it might have belonged to a gigantic bipedal batrachian; +but the evidence on this subject is too defective to warrant such a bold +conjecture, and if we were to give the reins to our imagination, we might +as well conceive a bird having four toes projecting forwards as a huge +two-legged frog. Nor should we forget that some quadrupeds place the hind +foot so precisely on the spot just quitted by the fore foot, as to produce +a single line of imprints like a biped. + +No bones have as yet been met with, whether of reptiles or birds, in the +rocks of the Connecticut, but there are numerous coprolites; and an +ingenious argument has been derived by Mr. Dana, from the analysis of these +bodies, and the proportion they contain of uric acid, phosphate of lime, +carbonate of lime, and organic matter, to show that, like guano, they are +the droppings of birds, rather than of reptiles.[299-A] + +Mr. Darwin, in his "Journal of a Voyage in the Beagle," informs us that the +"South American ostriches, although they live on vegetable matter, such as +roots and grass, are repeatedly seen at Bahia Blanca (lat. 39° S.), on the +coast of Buenos Ayres, coming down at low water to the extensive mud-banks +which are then dry, for the sake, as the Gauchos say, of feeding on small +fish." They readily take to the water, and have been seen at the bay of San +Blas, and at Port Valdez, in Patagonia, swimming from island to +island.[300-A] It is therefore evident, that in our times a South American +mud-bank might be trodden simultaneously by ostriches, alligators, +tortoises, and frogs; and the impressions left, in the nineteenth century, +by the feet of these various tribes of animals, would not differ from each +other more entirely than do those attributed to birds, saurians, +chelonians, and batrachians, in the rocks of the Connecticut. + +To determine the exact age of the red sandstone and shale containing these +ancient footprints in the United States, is not possible at present. No +fossil shells have yet been found in the deposit, nor plants in a +determinable state. The fossil fish are numerous and very perfect; but they +are of a peculiar type, which was originally referred to the genus +_Palæoniscus_, but has since, with propriety, been ascribed, by Sir Philip +Egerton, to a new genus. To this he has given the name of _Ischypterus_, +from the great size and strength of the fulcral rays of the dorsal fin +(from +ischys+; strength, and +pteron+, a fin). They differ from +_Palæoniscus_, as Mr. Redfield first pointed out, by having the vertebral +column prolonged to a more limited extent into the upper lobe of the tail, +or, in the language of M. Agassiz, they are less heterocercal. The teeth +also, according to Sir P. Egerton, who, in 1844, examined for me a fine +series of specimens which I procured at Durham, Connecticut, differ from +those of _Palæoniscus_ in being strong and conical. + +That the sandstones containing these fish are of older date than the +strata containing coal, before described (p. 284.) as occurring near +Richmond in Virginia, is highly probable. These were shown to be as old +at least as the oolite and lias. The higher antiquity of the Connecticut +beds cannot be proved by direct superposition, but may be presumed from +the general structure of the country. That structure proves them to be +newer than the movements to which the Appalachian or Alleghany chain +owes its flexures, and this chain includes the ancient coal formation +among its contorted rocks. The unconformable position of this _New Red_ +with ornithichnites on the edges of the inclined primary or paleozoic +rocks of the Appalachians is seen at 4. of the section, fig. 379. p. +327. The absence of fish with decidedly heterocercal tails may afford an +argument against the Permian age of the formation; and the opinion that +the red sandstone is triassic, seems, on the whole, the best that we can +embrace in the present state of our knowledge. + + +FOOTNOTES: + +[286-A] Buckland, Bridgew. Treat., vol. ii. p. 38. + +[287-A] Monog. des Bunten Sandsteins. + +[288-A] Tableau des Genres de Veg. Fos., Dict. Univ. 1849. + +[290-A] Geol. Trans., Second Series, vol. v. + +[290-B] Buckland, Proc. Geol. Soc. vol. ii. p. 439.; and Murchison and +Strickland Geol. Trans., Second Ser., vol. v. p. 347. + +[295-A] Ormerod, Quart. Geol. Journ. 1848, vol. iv. p. 277. + +[296-A] Hugh Miller, First Impressions of England, 1847, pp. 183. 214. + +[297-A] Buist, Trans. of Bombay Geograph. Soc. 1850, vol. ix. p. 38. + +[297-B] Travels in North America, vol. ii. p. 168. + +[298-A] Hitchcock, Mem. of Amer. Acad. New Ser., vol. iii. p. 129. + +[298-B] This specimen is now in Dr. Mantell's museum. + +[299-A] Amer. Journ. of Sci. vol. xlviii. p. 46. + +[300-A] Journal of Voyage of Beagle, &c. 2d edition, p. 89. 1845. + + + + +CHAPTER 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--Palæoniscus and other fish + of the marl slate--Thecodont Saurians of dolomitic conglomerate of + Bristol--Zechstein and Rothliegendes of Thuringia--Permian Flora--Its + generic affinity to the carboniferous--Psaronites or tree-ferns. + + +When the use of the term "Poikilitic" was explained in the last chapter, I +stated, that in some parts of England it is scarcely possible to separate +the red marls and sandstones so called (originally named "the New Red"), +into two distinct geological systems. Nevertheless, the progress of +investigation, and a careful comparison of English rocks between the lias +and the coal with those occupying a similar geological position in Germany +and Russia, has enabled geologists to divide the Poikilitic formation; and +has even shown that the lowermost of the two divisions is more closely +connected, by its fossil remains, with the carboniferous group than with +the trias. If, therefore, we are to draw a line between the secondary and +primary fossiliferous strata, as between the tertiary and secondary, it +must run through the middle of what was once called the "New Red," or +Poikilitic group. The inferior half of this group will rank as Primary or +Paleozoic, while its upper member will form the base of the Secondary +series. For the lower, or Magnesian Limestone division of English +geologists, Sir R. Murchison has proposed the name of Permian, from Perm, a +Russian government where these strata are more extensively developed than +elsewhere, occupying an area twice the size of France, and containing an +abundant and varied suite of fossils. + +Mr. King, in his valuable monograph, recently published, of the Permian +fossils of England, has given a table of the following six members of the +Permian system of the north of England, with what he conceives to be the +corresponding formations in Thuringia.[301-A] + + North of England. Thuringia. + + 1. Crystalline or concretionary, |1. Stinkstein. + and non-crystalline limestone. | + 2. Brecciated and pseudo-brecciated |2. Rauchwacke. + limestone. | + 3. Fossiliferous limestone. |3. Dolomit, or Upper Zechstein. + 4. Compact limestone. |4. Zechstein, or Lower Zechstein. + 5. Marl-slate. |5. Mergel-schiefer, or Kupferschiefer. + 6. Inferior sandstones of various |6. Rothliegendes. + colours. | + +I shall proceed, therefore, to treat briefly of these subdivisions, +beginning with the highest, and referring the reader, for a fuller +description of the lithological character of the whole group, as it occurs +in the north of England, to a valuable memoir by Professor Sedgwick, +published in 1835.[302-A] + +_Crystalline or concretionary limestone_ (No. 1.).--This formation is seen +upon the coast of Durham and Yorkshire, between the Wear and the Tees. +Among its characteristic fossils are _Schizodus Schlotheimi_ (fig. 333.) +and _Mytilus septifer_ (fig. 335.). + +[Illustration: Fig. 333. _Schizodus Schlotheimi_, Geinitz. Syn. _Axinus +obscurus_, Sow. Crystalline limestone, Permian.] + +[Illustration: Fig. 334. _Schizodus truncatus_, King; to show +hinge. Permian.] + +[Illustration: Fig. 335. _Mytilus septifer_, King. Syn. _Modiola +acuminata_, James Sow. Permian crystalline limestone.] + +These shells occur at Hartlepool and Sunderland, where the rock assumes an +oolitic and botryoidal character. Some of the beds in this division are +ripple-marked; and Mr. King imagines that the absence of corals and the +character of the shells indicate shallow water. In some parts of the coast +of Durham, where the rock is not crystalline, it contains as much as +forty-four per cent. of carbonate of magnesia, mixed with carbonate of +lime. In other places,--for it is extremely variable in structure,--it +consists chiefly of carbonate of lime, and has concreted into globular and +hemispherical masses, varying from the size of a marble to that of a +cannon-ball, and radiating from the centre. Occasionally earthy and +pulverulent beds pass into compact limestone or hard granular dolomite. The +stratification is very irregular, in some places well-defined, in others +obliterated by the concretionary action which has re-arranged the materials +of the rocks subsequently to their original deposition. Examples of this +are seen at Pontefract and Ripon in Yorkshire. + +_The brecciated limestone_ (No. 2.) contains no fragments of foreign +rocks, but seems composed of the breaking-up of the Permian limestone +itself, about the time of its consolidation. Some of the angular masses +in Tynemouth Cliff are 2 feet in diameter. This breccia is considered by +Professor Sedgwick as one of the forms of the preceding limestone, No. +1., rather than as regularly underlying it. The fragments are angular +and never water-worn, and appear to have been re-cemented on the spot +where they were formed. It is, therefore, suggested that they may have +been due to those internal movements of the mass which produced the +concretionary structure; but the subject is very obscure, and after +studying the phenomenon in the Marston Rocks, on the coast of Durham, I +found it impossible to form any positive opinion on the subject. The +well-known brecciated limestones of the Pyrenees appeared to me to +present the nearest analogy, but on a much smaller scale. + +_The fossiliferous limestone_ (No. 3.) is regarded by Mr. King as a +deep-water formation, from the numerous delicate corals which it includes. +One of these, _Fenestella retiformis_ (fig. 336.), is a very variable +species, and has received many different names. It sometimes attains a +large size, measuring 8 inches in width. The same zoophyte is also found +abundantly in the Permian of Germany. + +[Illustration: Fig. 336. Fenestella. + + _a._ _Fenestella retiformis_, Schlot. + Syn. _Gorgonia infundibuliformis_, Goldf.; _Retepora flustracea_, + Phillips. + _b._ Part of the same highly magnified. + +Magnesian limestone, Humbleton Hill, near Sunderland.[303-A]] + +Shells of the genera _Spirifer_ and _Productus_, which do not occur in +strata newer than the Permian, are abundant in this division of the series +in the ordinary yellow magnesian limestone. (See figs. 337, 338.) + +[Illustration: Fig. 337. _Productus calvus_, Sow. Min. Con. Syn. _Productus +horridus_, Bronn's Index, &c., King's Monogr., &c.; _Leptæna_, Dalman. + +Magnesian Limestone.] + +[Illustration: Fig. 338. _Spirifer undulatus_, Sow. Min. Con. Syn. +_Triogonotreta undulata_, King's Monogr. + +Magnesian Limestone.] + +_The compact limestone_ (No. 4.) also contains organic remains, +especially corallines, and is intimately connected with the preceding. +Beneath it lies the _marl-slate_ (No. 5.), which consists of hard, +calcareous shales, marl-slate, and thin-bedded limestones. At East +Thickley, in Durham, where it is thirty feet thick, this slate has +yielded many fine specimens of fossil fish of the genera _Palæoniscus_, +_Pygopterus_, _Coelacanthus_, and _Platysomus_, genera which are all +found in the coal-measures of the carboniferous epoch, and which +therefore, says Mr. King, probably lived at no great distance from +the shore. But the Permian species are peculiar, and, for the +most part, identical with those found in the marl-slate or +copper-slate of Thuringia. + +[Illustration: Fig. 339. Restored outline of a fish of the genus +_Palæoniscus_, Agass. _Palæothrissum_, Blainville.] + +The _Palæoniscus_ above mentioned belongs to that division of fishes +which M. Agassiz has called "Heterocercal," which have their tails +unequally bilobate, like the recent shark and sturgeon, and the +vertebral column running along the upper caudal lobe. (See fig. 340.) +The "Homocercal" fish, which comprise almost all the 8000 species at +present known in the living creation, have the tail-fin either single or +equally divided; and the vertebral column stops short, and is not +prolonged into either lobe. (See fig. 341.) + +[Illustration: Fig. 340. Shark. + +_Heterocercal._] + +[Illustration: Fig. 341. Shad. (_Clupea_, Herring tribe.) + +_Homocercal._] + +Now it is a singular fact, first pointed out by Agassiz, that the +heterocercal form, which is confined to a small number of genera in the +existing creation, is universal in the Magnesian limestone, and all the +more ancient formations. It characterizes the earlier periods of the +earth's history, when the organization of fishes made a greater approach to +that of saurian reptiles than at later epochs. In all the strata above the +Magnesian limestone the homocercal tail predominates. + +A full description has been given by Sir Philip Egerton of the species of +fish characteristic of the marl-slate in Mr. King's monograph before +referred to, where figures of the ichthyolites which are very entire and +well preserved, will be found. Even a single scale is usually so +characteristically marked as to indicate the genus, and sometimes even the +particular species. They are often scattered through the beds singly, and +maybe useful to a geologist in determining the age of the rock. + +[Illustration: Fig. 342. _Palæoniscus comtus_, Agassiz. Scale +magnified. Marl-slate.] + +[Illustration: Fig. 343. _Palæoniscus elegans_, Sedg. Under surface of +scale magnified. Marl-slate.] + +[Illustration: Fig. 344. _Palæoniscus glaphyrus_, Ag. Under surface of +scale magnified. Marl-slate.] + +[Illustration: Fig. 345. _Coelacanthus caudalis_, Egerton. Scale showing +granulated surface magnified. Marl-slate.] + +[2 Illustrations: Scales of fish. Magnesian limestone. + +Fig. 346. _Pygopterus mandibularis_, Ag. Marl-slate. + _a._ Outside of scale magnified. + _b._ Under surface of same. + +Fig. 347. _Acrolepis Sedgwickii_, Ag. Marl-slate.] + +The _inferior sandstones_ (No. 6. Tab. p. 301.), which lie beneath the +marl-slate, consist of sandstone and sand, separating the magnesian +limestone from the coal, in Yorkshire and Durham. In some instances, red +marl and gypsum have been found associated with these beds. They have been +classed with the magnesian limestone by Professor Sedgwick, as being nearly +co-extensive with it in geographical range, though their relations are very +obscure. In some regions we find it stated that the imbedded plants are all +specifically identical with those of the carboniferous series; and, if so, +they probably belong to that epoch; for the true Permian flora appears, +from the researches of MM. Murchison and de Verneuil in Russia, and of +Colonel von Gutbier in Saxony, to be, with few exceptions, distinct from +that of the coal (see p. 307.). + +_Dolomitic conglomerate of Bristol._--Near Bristol, in Somersetshire, +and in other counties bordering the Severn, the unconformable beds of +the Lower New Red, resting immediately upon the Coal, consist of a +conglomerate called "dolomitic," because the pebbles of older rocks are +cemented together by a red or yellow base of dolomite or magnesian +limestone. This conglomerate or breccia, for the imbedded fragments are +sometimes angular, occurs in patches over the whole of the downs near +Bristol, filling up the hollows and irregularities in the mountain +limestone, and being principally composed at every spot of the debris of +those rocks on which it immediately rests. At one point we find pieces +of coal shale, in another of mountain limestone, recognizable by its +peculiar shells and zoophytes. Fractured bones, also, and teeth of +saurians, are dispersed through some parts of the breccia. + +These saurians (which until the discovery of the _Archegosaurus_ in the +coal were the most ancient examples of fossil reptiles) are all +distinguished by having the teeth implanted deeply in the jaw-bone, and +in distinct sockets, instead of being soldered, as in frogs, to a simple +alveolar parapet. In the dolomitic conglomerate near Bristol the remains +of species of two distinct genera have been found, called +_Thecodontosaurus_ and _Palæosaurus_ by Dr. Riley and Mr. +Stutchbury[306-A]; the teeth of which are conical, compressed, and with +finely serrated edges (figs. 348 and 349.). + +[Illustration: Fig. 348. Tooth of _Palæosaurus_ platyodon, nat. size.] + +[Illustration: Fig. 349. Tooth of _Thecodontosaurus_, 3 times magnified.] + +In Russia, also, Thecodont saurians occur, in beds of the Permian age, of +several genera, while others named _Protorosaurus_ are met with in the +Zechstein of Thuringia. This family of reptiles is allied to the living +monitor, and its appearance in a primary or paleozoic formation, observes +Mr. Owen, is opposed to the doctrine of the progressive development of +reptiles from fish, or from simpler to more complex forms; for, if they +existed at the present day, these monitors would take rank at the head of +the Lacertian order.[306-B] + +In Russia the Permian rocks are composed of white limestone, with gypsum +and white salt; and of red and green grits, with occasionally copper ore; +also magnesian limestones, marlstones, and conglomerates. + +The country of Mansfeld, in Thuringia, may be called the classic ground of +the Lower New Red, or Magnesian Limestone, or Permian formation, on the +Continent. It consists there principally of, first, the Zechstein, +corresponding to the upper portion of our English series; and, secondly, +the marl-slate, with fish of species identical with those of the bed so +called in Durham. This slaty marlstone is richly impregnated with copper +pyrites, for which it is extensively worked. Magnesian limestone, gypsum, +and rock-salt, occur among the superior strata of this group. At its base +lies the Rothliegendes, supposed to correspond with the Inferior or Lower +New Red Sandstone above mentioned, which occupies a similar place in +England between the marl-slate and coal. Its local name of Rothliegendes, +_red-lyer_, or "Roth-todt-liegendes," _red-dead-lyer_, was given by the +workmen in the German mines from its red colour, and because the copper has +_died out_ when they reach this rock, which is not metalliferous. It is, in +fact, a great deposit of red sandstone and conglomerate, with associated +porphyry, basaltic trap, and amygdaloid. + +_Permian Flora._--We learn from the recent investigation of Colonel von +Gutbier, that in the Permian rocks of Saxony no less than sixty species of +fossil plants have been met with, forty of which have not yet been found +elsewhere. Two or three of these, as _Calamites gigas_, _Sphenopteris +erosa_, and _S. lobata_, are also met with in the government of Perm in +Russia. Seven others, and among them _Neuropteris Loshii_, _Pecopteris +arborescens_, and _P. similis_, with several species of _Walchia_ +(Lycopodites), are common to the coal-measures. + +Among the genera also enumerated by Colonel Gutbier are _Asterophyllites_ +and _Annularia_, so characteristic of the carboniferous period; also +_Lepidodendron_, which is common to the Permian of Saxony, Thuringia, and +Russia, although not abundant. _Noeggerathia_ (see fig. 350.), supposed by +A. Brongniart to be allied to _Cycas_, is another link between the Permian +and carboniferous vegetation. Coniferæ, of the Araucarian division, also +occur; but these are likewise met with both in older and newer rocks. The +plants called _Sigillaria_ and _Stigmaria_, so marked a feature in the +carboniferous period, are as yet wanting. + +[Illustration: Fig. 350. _Noeggerathia cuneifolia._ Ad. Brongniart.[307-A]] + +Among the remarkable fossils of the rothliegendes, or lowest part of the +Permian in Saxony and Bohemia, are the silicified trunks of tree-ferns +called generically _Psaronius_. Their bark was surrounded by a dense +mass of air-roots, which often constituted a great addition to the +original stem, so as to double or quadruple its diameter. The same +remark holds good in regard to certain living extra-tropical arborescent +ferns, particularly those of New Zealand. + +Psaronites are also found in the uppermost coal of Autun in France, and in +the upper coal-measures of the State of Ohio in the United States, but +specifically different from those of the rothliegendes. They serve to +connect the Permian flora with the more modern portion of the preceding or +carboniferous group. Upon the whole, it is evident that the Permian plants +approach nearer to the carboniferous ones than to the triassic; and the +same may be said of the Permian fauna. + + +FOOTNOTES: + +[301-A] Palæontographical Society, 1848, London. + +[302-A] Trans. Geol. Soc. Lond., Second Series, vol. iii. p. 37. + +[303-A] King's Monograph, pl. 2. + +[306-A] See paper by Messrs. Riley and Stutchbury, Geol. Trans., Second +Series, vol. v. p. 349., plate 29., figures 2. and 5. + +[306-B] Owen, Report on Reptiles, British Assoc., Eleventh Meeting, +1841, p. 197. + +[307-A] Murchison's Russia, vol. ii. pl. A. fig. 3. + + + + +CHAPTER 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, Sigillariæ, Stigmariæ--Coniferæ--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. + + +The next group which we meet with in the descending order is the +Carboniferous, commonly called "The Coal;" because it contains many beds +of that mineral, in a more or less pure state, interstratified with +sandstones, shales, and limestones. The coal itself, even in Great +Britain and Belgium, where it is most abundant, constitutes but an +insignificant portion of the whole mass. In the north of England, for +example, the thickness of the coal-bearing strata has been estimated at +3000 feet, while the various coal-seams, 20 or 30 in number, do not in +the aggregate exceed 60 feet.[308-A] + +The carboniferous formation comprises two very distinct members: 1st, that +usually called the Coal-measures, of mixed freshwater, terrestrial, and +marine origin, often including seams of coal; 2dly, that named in England +the Mountain or Carboniferous limestone, of purely marine origin, and +containing corals, shells, and encrinites. + +In the south-western part of our island, in Somersetshire and South Wales, +the three divisions usually spoken of by English geologists are: + + 1. Coal-measures { Strata of shale, sandstone, and grit, with + { occasional seams of coal, from 600 to 12,000 + { feet thick. + + 2. Millstone grit { A coarse quartzose sandstone passing into a + { conglomerate, sometimes used for millstones, with + { beds of shale; usually devoid of coal; + { occasionally above 600 feet thick. + + 3. Mountain or } A calcareous rock containing marine shells and + Carboniferous } corals; devoid of coal; thickness variable, + limestone } sometimes 900 feet. + +The millstone grit may be considered as one of the coal sandstones of +coarser texture than usual, with some accompanying shales, in which coal +plants are occasionally found. In the north of England some bands of +limestone, with pectens, oysters, and other marine shells, occur in this +grit, just as in the regular coal-measures, and even a few seams of coal. I +shall treat, therefore, of the whole group, as consisting of two divisions +only, the Coal-measures and Mountain Limestone. The latter is found in the +southern British coal-fields, at the base of the system, or immediately in +contact with the subjacent Old Red Sandstone; but as we proceed northwards +to Yorkshire and Northumberland it begins to alternate with true +coal-measures, the two deposits forming together a series of strata about +1000 feet in thickness. To this mixed formation succeeds the great mass of +genuine mountain limestone.[309-A] Farther north, in the Fifeshire +coal-field in Scotland, we observe a still wider departure from the type of +the south of England, or a more complete intercalation of dense masses of +marine limestones with sandstones, and shales containing coal. + + +COAL-MEASURES. + +In South Wales the coal-measures have been ascertained by actual +measurement to attain the extraordinary thickness of 12,000 feet, the beds +throughout, with the exception of the coal itself, appearing to have been +formed in water of moderate depth, during a slow but perhaps intermittent +depression of the ground, in a region to which rivers were bringing a +never-failing supply of muddy sediment and sand. The same area was +sometimes covered with vast forests, such as we see in the deltas of great +rivers in warm climates, which are liable to be submerged beneath fresh or +salt water should the ground sink vertically a few feet. + +In one section near Swansea, in South Wales, where the total thickness of +strata is 3246 feet, we learn from Sir H. De la Beche that there are ten +principal masses of sandstone. One of these is 500 feet thick, and the +whole of them make together a thickness of 2125 feet. They are separated by +masses of shale, varying in thickness from 10 to 50 feet. The intercalated +coal-beds, sixteen in number, are generally from 1 to 5 feet thick, one of +them, which has two or three layers of clay interposed, attaining 9 +feet.[309-B] At other points in the same coal-field the shales predominate +over the sandstones. The horizontal extent of some seams of coal is much +greater than that of others, but they all present one characteristic +feature, in having, each of them, what is called its _underclay_. These +underclays, co-extensive with every layer of coal, consist of arenaceous +shale, sometimes called firestone, because it can be made into bricks which +stand the fire of a furnace. They vary in thickness from 6 inches to more +than 10 feet; and Mr. Logan first announced to the scientific world in 1841 +that they were regarded by the colliers in South Wales as an essential +accompaniment of each of the one hundred seams of coal met with in their +coal-field. They are said to form the _floor_ on which the coal rests; and +some of them have a slight admixture of carbonaceous matter, while others +are quite blackened by it. + +All of them, as Mr. Logan pointed out, are characterized by inclosing a +peculiar species of fossil vegetable called _Stigmaria_, to the exclusion +of other plants. It was also observed that, while in the overlying shales +or "roof" of the coal, ferns and trunks of trees abound without any +_Stigmariæ_, and are flattened and compressed, those singular plants in the +underclays always retain their natural forms, branching freely, and sending +out their slender leaves, as they were formerly styled, through the mud in +all directions. Several species of _Stigmaria_ had long been known to +botanists, and described by them, before their position under each seam of +coal was pointed out. It was conjectured that they might be aquatic, +perhaps floating plants, which sometimes extended their branches and leaves +freely in fluid mud, and which were finally enveloped in the same mud. + + +CARBONIFEROUS FLORA. + +These statements will suffice to convince the reader that we cannot arrive +at a satisfactory theory of the origin of coal till we understand the true +nature of _Stigmaria_; and in order to explain what is now known of this +plant, and of others which have contributed by their decay to produce coal, +it will be necessary to offer a brief preliminary sketch of the whole +carboniferous flora, an assemblage of fossil plants, with which we are +better acquainted than with any other which flourished antecedently to the +tertiary epoch. It should also be remarked that Göppert has ascertained +that the remains of every family of plants scattered through the +coal-measures are sometimes met with in the pure coal itself, a fact which +adds greatly to the geological interest attached to this flora. + +_Ferns._--The number of species of carboniferous plants hitherto +described amounts, according to M. Ad. Brongniart, to about 500. These +may perhaps be a fragment only of the entire flora, but they are enough +to show that the state of the vegetable world was then extremely +different from that now established. We are struck at the first glance +with the similarity of many of the ferns to those now living, and the +dissimilarity of almost all the other fossils except the coniferæ. Among +the ferns, as in the case of _Pecopteris_ for example (fig. 351.), it is +not always easy to decide whether they should be referred to different +genera from those established for the classification of living species; +whereas, in regard to most of the other contemporary tribes, with the +exception of the coniferæ, it is often difficult to guess the family, +or even the class, to which they belong. The ferns of the carboniferous +period are generally without organs of fructification, but in some +specimens these are well preserved. In the general absence of such +characters, they have been divided into genera, distinguished chiefly +by the branching of the fronds, and the way in which the veins of +the leaves are disposed. The larger portion are supposed to have been +of the size of ordinary European ferns, but some were decidedly +arborescent, especially the group called _Caulopteris_, by Lindley, +and the _Psaronius_ of the upper or newest coal-measures, before +alluded to (p. 307.). + +[Illustration: Fig. 351. _Pecopteris lonchitica._ (Foss. Flo. 153.)] + +[Illustration: Fig. 352. Sphenopteris. (Foss. Flo. 101.) + + _a._ _Sphenopteris crenata._ + _b._ The same, magnified.] + +[Illustration: Fig. 353. _Caulopteris primæva_, Lindley.] + +All the recent tree-ferns belong to one tribe (_Polypodiaceæ_), and to a +small number only of genera in that tribe, in which the surface of the +trunk is marked with scars, or cicatrices, left after the fall of the +fronds. These scars resemble those of _Caulopteris_ (see fig. 353.). No +less than 250 ferns have already been obtained from the coal strata; and +even if we make some reduction on the ground of varieties which have been +mistaken, in the absence of their fructification, for species, still the +result is singular, because the whole of Europe affords at present no more +than 50 indigenous species. + +[3 Illustrations: Living tree-ferns of different genera. (Ad. Brong.) + +Fig. 354. Tree-fern from Isle of Bourbon. + +Fig. 355. _Cyathea glauca_, Mauritius. + +Fig. 356. Tree fern from Brazil.] + +[3 Illustrations: _Lepidodendron Sternbergii._ Coal-measures, +near Newcastle. + +Fig. 357. Branching trunk, 49 feet long, supposed to have belonged to _L. +Sternbergii_. (Foss. Flo. 203.) + +Fig. 358. Branching stem with bark and leaves of _L. Sternbergii_. +(Foss. Flo. 4.) + +Fig. 359. Portion of same nearer the root; natural size. (Ibid.)] + +_Lepidodendra._--These fossils belong to the family of _Lycopodiums_, yet +most of them grew to the size of large trees. The annexed figures represent +a large fossil _Lepidodendron_, 49 feet long, found in Jarrow Colliery, +near Newcastle, lying in shale parallel to the planes of stratification. +Fragments of others, found in the same shale, indicate, by the size of the +rhomboidal scars which cover them, a still greater magnitude. The living +club-mosses, of which there are about 200 species, are abundant in tropical +climates, where one species is sometimes met with attaining a height of 3 +feet. They usually creep on the ground, but some stand erect, as the _L. +densum_, from New Zealand (fig. 360.). + +[Illustration: Fig. 360. Lycopodium. + + _a._ _Lycopodium densum_; banks of R. Thames, New Zealand. + _b._ branch, natural size. + _c._ part of same, magnified.] + +In the carboniferous strata of Coalbrook Dale, and in many other +coal-fields, elongated cylindrical bodies, called fossil cones, named by +M. Adolphe Brongniart _Lepidostrobus_, are met with. (See fig. 361.) +They often form the nucleus of concretionary balls of clay-iron-stone, +and are well preserved, exhibiting a conical axis, around which a great +quantity of scales were compactly imbricated. The opinion of M. +Brongniart is now generally adopted, that the _Lepidostrobus_ is +the fruit of _Lepidodendron_. + +[Illustration: Fig. 361. _Lepidostrobus ornatus_, Brong.; half +nat. size. Shropshire.] + +[Illustration: Fig. 362. _Calamites cannæformis_, Schlot. (Foss. Flo. 79.) +Lower end with rootlets.] + +[Illustration: Fig. 363. _Calamites Suckowii_, Brong.; natural size. Common +in coal throughout Europe.] + +_Equisetaceæ._--To this family belong two species of the genus +_Equisetites_, allied to the living "horse-tail" which now grows in marshy +grounds. Other species, which have jointed stems, depart more widely from +_Equisetum_, but are yet of analogous organization. They differed from it +principally in being furnished with a thin bark, which is represented in +the stem of _C. Suckowii_ (fig. 363.), in which it will be seen that the +striped external pattern does not agree with that left on the stone where +the bark is stripped off; so that if the two impressions were seen +separately, they might be mistaken for two distinct species. + +The tallest living "horse-tails" are only 2 or 3 feet high in Europe, and +even in tropical climates only attain, as in the case of _Equisetum +giganteum_, discovered by Humboldt and Bonpland, in South America, a height +of about 5 feet, the stem being an inch in diameter. Several of the +Calamites of the coal acquired the height and dimensions of small trees. + +[Illustration: Fig. 364. _Asterophyllites foliosa._ (Foss. Flo. 25.) +Coal-measures, Newcastle.] + +_Asterophyllites._--In this family, M. Brongniart includes several genera, +and among them _Calamodendron_, _Asterophyllites_, and _Annularia_. The +graceful plant, represented in the annexed figure, is supposed to be the +branch of a shrub called _Calamodendron_, a new genus, divided off by +Brongniart from the _Calamites_ of former authors. Its pith and medullary +rays seem to show that it was dicotyledonous, and it appears to have been +allied, by the nature of its tissue, to the gymnogens, or, still more, to +the _Sigillaria_, which will next be mentioned. + +_Sigillaria._--A large portion of the trees of the carboniferous period +belonged to this genus, of which about thirty-five species are known. The +structure, both internal and external, was very peculiar, and, with +reference to existing types, very anomalous. They were formerly referred, +by M. Ad. Brongniart, to ferns, which they resemble in the scalariform +texture of their vessels, and, in some degree, in the form of the +cicatrices left by the base of the leafstalks which have fallen off (see +fig. 365.). But with these points of analogy to cryptogamia, they combine +an internal organization much resembling that of cycads, and some of them +are ascertained to have had long linear leaves, quite unlike those of +ferns. They grew to a great height, from 30 to 60, or even 70 feet, with +regular cylindrical stems, and without branches, although some species were +dichotomous towards the top. Their fluted trunks, from 1 to 5 feet in +diameter, appear to have decayed rapidly in the interior, so as to become +hollow, when standing; when, therefore, they were thrown prostrate on the +mud, they were squeezed down and flattened. Hence, we find the bark of the +two opposite sides (now converted into bright shining coal) to constitute +two horizontal layers, one upon the other, half an inch, or an inch, in +thickness. These same trunks, when they are placed obliquely or vertically +to the planes of stratification, retain their original rounded form, and +are uncompressed, the cylinder of bark having been filled with sand, which +now affords a cast of the interior. + +[Illustration: Fig. 365. _Sigillaria lævigata_, Brong.] + +_Stigmaria._--This fossil, the importance of which has already been pointed +out, was formerly conjectured to be an aquatic plant. It is now ascertained +to be the root of _Sigillaria_. The connection of the roots with the stem, +previously suspected, on botanical grounds, by Brongniart, was first +proved, by actual contact, in the Lancashire coal-field, by Mr. Binney. The +fact has lately been shown, even more distinctly, by Mr. Richard Brown, in +his description of the _Stigmariæ_ occurring in the underclays of the +coal-seams of the Island of Cape Breton, in Nova Scotia. + +[Illustration: Fig. 366. Stigmaria attached to a trunk of +_Sigillaria_.[315-A]] + +In a specimen of one of these, represented in the annexed figure (fig. +366.), the spread of the roots was 16 feet, and some of them sent out +rootlets, in all directions, into the surrounding clay. + +The manner of attachment of the fibres to the stem resembles that of a ball +and socket joint, the base of each rootlet being concave, and fitting on to +a tubercle (see figs. 367 and 368.). Rows of these tubercles are arranged +spirally round each root, which have always a medullary cavity and woody +texture, much resembling that of _Sigillaria_, the structure of the +vessels being, like it, scalariform. + +[Illustration: Fig. 367. Surface of another individual of same species, +showing form of tubercles. (Foss. Flo. 34.)] + +[Illustration: Fig. 368. _Stigmaria ficoides_, Brong. One fourth of +nat. size. (Foss. Flo. 32.)] + +_Conifers._--The coniferous trees of this period are referred to five +genera; the woody structure of some of them showing that they were allied +to the Araucarian division of pines, more than to any of our common +European firs. Some of their trunks exceeded 44 feet in height. + +_Endogens._--Hitherto but few monocotyledonous plants have been +discovered in the coal-strata. Most of these consist of fruits referred +by some botanists to palms. The three-sided nuts, called +_Trigonocarpum_, seven species of which are known, appear to have the +best claim to rank as palms, although M. Ad. Brongniart entertains some +doubt even as to their being monocotyledons. + + +_Exogens._ + +The entire absence, so far as our paleontological investigations have +hitherto gone, of ordinary dicotyledons or exogens in the coal measures, is +most remarkable. Hence, M. Adolphe Brongniart has called this period the +age of acrogens, in consequence of the vast preponderance of ferns and +_Lepidodendra_.[316-A] Nevertheless, a forest of the period, now under +consideration, may have borne a considerable resemblance to those woody +regions of New Zealand, in which ferns, arborescent and herbaceous, and +lycopodiums, with many coniferæ, abound. + +The comparative proportion of living ferns and _Araucariæ_, in Norfolk +Island, to all the other plants, appears to be very similar to that +formerly borne by these tribes respectively in a forest of the coal-period. + +I have already stated that Professor Göppert, after examining the fossil +vegetables of the coal-fields of Germany, has detected, in beds of pure +coal, remains of plants of every family hitherto known to occur fossil in +the coal. Many seams, he remarks, are rich in _Sigillaria_, +_Lepidodendron_, and _Stigmaria_, the latter in such abundance, as to +appear to form the bulk of the coal. In some places, almost all the plants +are calamites, in others ferns.[316-B] + +_Coal, how formed--Erect trees._--I shall now consider the manner in +which the above-mentioned plants are imbedded in the strata, and how +they may have contributed to produce coal. "Some of the plants of our +coal," says Dr. Buckland, "grew on the identical banks of sand, silt, +and mud, which, being now indurated to stone and shale, form the strata +that accompany the coal; whilst other portions of these plants have been +drifted to various distances from the swamps, savannahs, and forests +that gave them birth, particularly those that are dispersed through the +sandstones, or mixed with fishes in the shale beds." "At Balgray, three +miles north of Glasgow," says the same author, "I saw in the year 1824, +as there still may be seen, an unequivocal example of the stumps of +several stems of large trees, standing close together in their native +place, in a quarry of sandstone of the coal formation."[317-A] + +Between the years 1837 and 1840, six fossil trees were discovered in the +coal-field of Lancashire, where it is intersected by the Bolton railway. +They were all in a vertical position, with respect to the plane of the +bed, which dips about 15° to the south. The distance between the first +and the last was more than 100 feet, and the roots of all were imbedded +in a soft argillaceous shale. In the same plane with the roots is a bed +of coal, eight or ten inches thick, which has been ascertained to extend +across the railway, or to the distance of at least ten yards. Just above +the covering of the roots, yet beneath the coal seam, so large a +quantity of the _Lepidostrobus variabilis_ was discovered inclosed in +nodules of hard clay, that more than a bushel was collected from the +small openings around the base of the trees (see figure of this genus, +p. 313.). The exterior trunk of each was marked by a coating of friable +coal, varying from one quarter to three quarters of an inch in +thickness; but it crumbled away on removing the matrix. The dimensions +of one of the trees is 15-1/2 feet in circumference at the base, 7-1/2 +feet at the top, its height being 11 feet. All the trees have large +spreading roots, solid and strong, sometimes branching, and traced to a +distance of several feet, and presumed to extend much farther. Mr. +Hawkshaw, who has described these fossils, thinks that, although they +were hollow when submerged, they may have consisted originally of hard +wood throughout; for solid dicotyledonous trees, when prostrated in +tropical forests, as in Venezuela, on the shore of the Caribbean Sea, +were observed by him to be destroyed in the interior, so that little +more is left than an outer shell, consisting chiefly of the bark. This +decay, he says, goes on most rapidly in low and flat tracks, in which +there is a deep rich soil and excessive moisture, supporting tall +forest-trees and large palms, below which bamboos, canes, and minor +palms flourish luxuriantly. Such tracts, from their lowness, would be +most easily submerged, and their dense vegetation might then give rise +to a seam of coal.[317-B] + +In a deep valley near Capel-Coelbren, branching from the higher part of the +Swansea valley, four stems of upright _Sigillariæ_ were seen, in 1838, +piercing through the coal-measures of S. Wales; one of them was 2 feet in +diameter, and one 13 feet and a half high, and they were all found to +terminate downwards in a bed of coal. "They appear," says Sir H. De la +Beche, "to have constituted a portion of a subterranean forest at the epoch +when the lower carboniferous strata were formed.[318-A] + +In a colliery near Newcastle, say the authors of the Fossil Flora, a great +number of _Sigillariæ_ were placed in the rock as if they had retained the +position in which they grew. Not less than thirty, some of them 4 or 5 feet +in diameter, were visible within an area of 50 yards square, the interior +being sandstone, and the bark having been converted into coal. The roots of +one individual were found imbedded in shale; and the trunk, after +maintaining a perpendicular course and circular form for the height of +about 10 feet, was then bent over so as to become horizontal. Here it was +distended laterally, and flattened so as to be only one inch thick, the +flutings being comparatively distinct.[318-B] Such vertical stems are +familiar to our miners, under the name of coal-pipes. One of them, 72 feet +in length, was discovered, in 1829, near Gosforth, about five miles from +Newcastle, in coal-grit, the strata of which it penetrated. The exterior of +the trunk was marked at intervals with knots, indicating the points at +which branches had shot off. The wood of the interior had been converted +into carbonate of lime; and its structure was beautifully shown by cutting +transverse slices, so thin as to be transparent. (See p. 40.) + +These "coal-pipes" are much dreaded by our miners, for almost every year in +the Bristol, Newcastle, and other coal-fields, they are the cause of fatal +accidents. Each cylindrical cast of a tree, formed of solid sandstone, and +increasing gradually in size towards the base, and being without branches, +has its whole weight thrown downwards, and receives no support from the +coating of friable coal which has replaced the bark. As soon, therefore, as +the cohesion of this external layer is overcome, the heavy column falls +suddenly in a perpendicular or oblique direction from the roof of the +gallery whence coal has been extracted, wounding or killing the workman who +stands below. It is strange to reflect how many thousands of these trees +fell originally in their native forests in obedience to the law of gravity; +and how the few which continued to stand erect, obeying, after myriads of +ages, the same force, are cast down to immolate their human victims. + +It has been remarked, that if, instead of working in the dark, the miner +was accustomed to remove the upper covering of rock from each seam of coal, +and to expose to the day the soils on which ancient forests grew, the +evidence of their former growth would be obvious. Thus in South +Staffordshire a seam of coal was laid bare in the year 1844, in what is +called an open work at Parkfield Colliery, near Wolverhampton. In the +space of about a quarter of an acre the stumps of no less than 73 trees +with their roots attached appeared, as shown in the annexed plan (fig. +369.), some of them more than 8 feet in circumference. The trunks, broken +off close to the root, were lying prostrate in every direction, often +crossing each other. One of them measured 15, another 30 feet in length, +and others less. They were invariably flattened to the thickness of one or +two inches, and converted into coal. Their roots formed part of a stratum +of coal 10 inches thick, which rested on a layer of clay 2 inches thick, +below which was a second forest, resting on a 2-foot seam of coal. Five +feet below this again was a third forest with large stumps of +_Lepidodendra_, _Calamites_, and other trees. + +[Illustration: Fig. 369. Ground-plan of a fossil forest, Parkfield +Colliery, near Wolverhampton, showing the position of 73 trees in a +quarter of an acre.[319-A]] + +In the account given, in 1821, by M. Alex. Brongniart of the coal-mine +of Treuil, at St. Etienne, near Lyons, he states, that distinct +horizontal strata of micaceous sandstone are traversed by vertical +trunks of monocotyledonous vegetables, resembling bamboos or large +_Equiseta_.[319-B] Since the consolidation of the stone, there has been +here and there a sliding movement, which has broken the continuity of +the stems, throwing the upper parts of them on one side, so that they +are often not continuous with the lower. + +From these appearances it was inferred that we have here the monuments +of a submerged forest. I formerly objected to this conclusion, +suggesting that, in that case, all the roots ought to have been found at +one and the same level, and not scattered irregularly through the mass. +I also imagined that the soil to which the roots were attached should +have been different from the sandstone in which the trunks are enclosed. +Having, however, seen calamites near Pictou, in Nova Scotia, buried at +various heights in sandstone and in similar erect attitudes, I have now +little doubt that M. Brongniart's view was correct. These plants seem +to have grown on a sandy soil, liable to be flooded from time to time, +and raised by new accessions of sediment, as may happen in swamps near +the banks of a large river in its delta. Trees which delight in marshy +grounds are not injured by being buried several feet deep at their base; +and other trees are continually rising up from new soils, several feet +above the level of the original foundation of the morass. In the banks +of the Mississippi, when the water has fallen, I have seen sections of a +similar deposit in which portions of the stumps of trees with their +roots _in situ_ appeared at many different heights.[320-A] + +[Illustration: Fig. 370. Section showing the erect position of fossil trees +in coal sandstone at St. Etienne. (Alex. Brongniart.)] + +When I visited, in 1843, the quarries of Treuil above-mentioned, the fossil +trees seen in fig. 370. were removed, but I obtained proofs of other +forests of erect trees in the same coal-field. + +[Illustration: Fig. 371. Inclined position of a fossil tree, cutting +through horizontal beds of sandstone, Craigleith quarry, Edinburgh. Angle +of inclination from _a_ to _b_ 27°.] + +_Snags._--In 1830, a slanting trunk was exposed in Craigleith quarry, +near Edinburgh, the total length of which exceeded 60 feet. Its diameter +at the top was about 7 inches, and near the base it measured 5 feet in +its greater, and 2 feet in its lesser width. The bark was converted into +a thin coating of the purest and finest coal, forming a striking +contrast in colour with the white quartzose sandstone in which it lay. +The annexed figure represents a portion of this tree, about 15 feet +long, which I saw exposed in 1830, when all the strata had been removed +from one side. The beds which remained were so unaltered and undisturbed +at the point of junction, as clearly to show that they had been +tranquilly deposited round the tree, and that the tree had not +subsequently pierced through them, while they were yet in a soft state. +They were composed chiefly of siliceous sandstone, for the most part +white; and divided into laminæ so thin, that from six to fourteen of +them might be reckoned in the thickness of an inch. Some of these thin +layers were dark, and contained coaly matter; but the lowest of the +intersected beds were calcareous. The tree could not have been hollow +when imbedded, for the interior still preserved the woody texture in a +perfect state, the petrifying matter being, for the most part, +calcareous.[321-A] It is also clear, that the lapidifying matter was not +introduced laterally from the strata through which the fossil passes, as +most of these were not calcareous. It is well known that, in the +Mississippi and other great American rivers, where thousands of trees +float annually down the stream, some sink with their roots downwards, +and become fixed in the mud. Thus placed, they have been compared to a +lance in rest; and so often do they pierce through the bows of vessels +which run against them, that they render the navigation extremely +dangerous. Mr. Hugh Miller mentions four other huge trunks exposed in +quarries near Edinburgh, which lay diagonally across the strata at an +angle of about 30°, with their lower or heavier portions downwards, the +roots of all, save one, rubbed off by attrition. One of these was 60 and +another 70 feet in length, and from 4 to 6 feet in diameter. + +[Illustration: Fig. 372. Section of the cliffs of the South Joggins, +near Minudie, Nova Scotia.] + +The number of years for which the trunks of trees, when constantly +submerged, can resist decomposition, is very great; as we might suppose +from the durability of wood, in artificial piles, permanently covered by +water. Hence these fossil snags may not imply a rapid accumulation of beds +of sand, although the channel of a river or part of a lagoon is often +filled up in a very few years. + +_Nova Scotia._--One of the finest examples in the world of a succession of +fossil forests of the carboniferous period, laid open to view in a natural +section, is that seen in the lofty cliffs bordering the Chignecto Channel, +a branch of the Bay of Fundy, in Nova Scotia.[321-B] + +In the annexed section (fig. 372.), which I examined in July, 1842, the +beds from _c_ to _i_ are seen all dipping the same way, their average +inclination being at an angle of 24° S.S.W. The vertical height of the +cliffs is from 150 to 200 feet; and between _d_ and _g_, in which space I +observed seventeen trees in an upright position, or, to speak more +correctly, at right angles to the planes of stratification, I counted +nineteen seams of coal, varying in thickness from 2 inches to 4 feet. At +low tide a fine horizontal section of the same beds is exposed to view on +the beach. The thickness of the beds alluded to, between _d_ and _g_, is +about 2,500 feet, the erect trees consisting chiefly of large _Sigillariæ_, +occurring at ten distinct levels, one above the other; but Mr. Logan, who +afterwards made a more detailed survey of the same line of cliffs, found +erect trees at seventeen levels, extending through a vertical thickness of +4,515 feet of strata; and he estimated the total thickness of the +carboniferous formation, with and without coal, at no less than 14,570 +feet, every where devoid of marine organic remains.[322-A] The usual height +of the buried trees seen by me was from 6 to 8 feet; but one trunk was +about 25 feet high and 4 feet in diameter, with a considerable bulge at the +base. In no instance could I detect any trunk intersecting a layer of coal, +however thin; and most of the trees terminated downwards in seams of coal. +Some few only were based in clay and shale, none of them in sandstone. The +erect trees, therefore, appeared in general to have grown on beds of coal. +In some of the underclays I observed _Stigmaria_. + +[Illustration: Fig. 373. Fossil tree at right angles to planes of +stratification. Coal measures, Nova Scotia.] + +In regard to the plants, they belonged to the same genera, and most of them +to the same species, as those met with in the distant coal-fields of +Europe. In the sandstone, which filled their interiors, I frequently +observed fern leaves, and sometimes fragments of _Stigmaria_, which had +evidently entered together with sediment after the trunk had decayed and +become hollow, and while it was still standing under water. Thus the tree, +_a b_, fig. 373., the same which is represented at _a_, fig. 374., or in +the bed _e_ in the larger section, fig. 372., is a hollow trunk 5 feet 8 +inches in length, traversing various strata, and cut off at the top by a +layer of clay 2 feet thick on which rests a seam of coal (_b_, fig. 374.) +1 foot thick. On this coal again stood two large trees (_c_ and _d_), while +at a greater height the trees _f_ and _g_ rest upon a thin seam of coal +(_e_), and above them is an underclay, supporting the 4-foot coal. + +[Illustration: Fig. 374. Erect fossil trees. Coal-measures, Nova Scotia.] + +If we now return to the tree first mentioned (fig. 373.), we find the +diameter (_a b_) 14 inches at the top and 16 inches at the bottom, the +length of the trunk 5 feet 8 inches. The strata in the interior +consisted of a series entirely different from those on the outside. The +lowest of the three outer beds which it traversed consisted of purplish +and blue shale (_c_, fig. 373.), 2 feet thick, above which was sandstone +(_d_) 1 foot thick, and, above this, clay (_e_) 2 feet 8 inches. But, in +the interior, were nine distinct layers of different composition: at the +bottom, first, shale 4 inches, then sandstone 1 foot, then shale 4 +inches, then sandstone 4 inches, then shale 11 inches, then clay (_f_) +with nodules of ironstone 2 inches, then pure clay 2 feet, then +sandstone 3 inches, and, lastly, clay 4 inches. Owing to the outward +slope of the face of the cliff, the section (fig. 373.) was not exactly +perpendicular to the axis of the tree; and hence, probably, the apparent +sudden termination at the base without a stump and roots. + +In this example the layers of matter in the inside of the tree are more +numerous than those without; but it is more common in the coal-measures +of all countries to find a cylinder of pure sandstone,--the cast of the +interior of a tree, intersecting a great many alternating beds of shale +and sandstone, which originally enveloped the trunk as it stood erect in +the water. Such a want of correspondence in the materials outside and +inside, is just what we might expect if we reflect on the difference of +time at which the deposition of sediment will take place in the two +cases; the imbedding of the tree having gone on for many years before +its decay had made much progress. + +The high tides of the Bay of Fundy, rising more than 60 feet, are so +destructive as to undermine and sweep away continually the whole face of +the cliffs, and thus a new crop of erect trees is brought into view +every three or four years. They are known to extend over a space between +two and three miles from north to south, and more than twice that +distance from east to west, being seen in the banks of streams +intersecting the coal-field. + +In Cape Breton, Mr. Richard Brown has observed in the Sydney coal-field a +total thickness of coal-measures, without including the underlying +millstone grit, of 1843 feet, dipping at an angle of 8°. He has published +minute details of the whole series, showing at how many different levels +erect trees occur, consisting of _Sigillaria_, _Lepidodendron_, _Calamite_, +and other genera. In one place eight erect trunks, with roots and rootlets +attached to them, were seen at the same level, within a horizontal space 80 +feet in length. Beds of coal of various thickness are interstratified. Some +of the associated strata are ripple-marked, with impressions of rain-drops. +Taking into account forty-one clays filled with roots of _Stigmaria_ in +their natural position, and eighteen layers of upright trees at other +levels, there is, on the whole, clear evidence of at least fifty-nine +fossil forests, ranged one above the other, in this coal-field, in the +above-mentioned thickness of strata.[324-A] + +The fossil shells in Cape Breton and in the Nova Scotia section (fig. +372.), consisting of _Cypris_, _Unio_ (?), _Modiola_, _Microconchus +carbonarius_ (see fig. 375.), and _Spirorbis_, seem to indicate brackish +water; but we ought never to be surprised if, in pursuing the same stratum, +we come to a fresh or purely marine deposit; for this will depend upon our +taking a direction higher up or lower down the ancient river or delta +deposit. When the Purbeck beds of the Wealden were described in Chap. +XVIII., I endeavoured to explain the intimate connection of strata formed +at a river's mouth, or in the tranquil lagoons of the delta, or in the sea, +after a slight submergence of the land, with its dirt-beds. + +In the English coal-fields the same association of fresh, or rather +brackish water with marine strata, in close connection with beds of coal of +terrestrial origin, has been frequently recognized. Thus, for example, a +deposit near Shrewsbury, probably formed in brackish water, has been +described by Sir R. Murchison as the youngest member of the carboniferous +series of that district, at the point where the coal-measures are in +contact with the Permian or "Lower New Red." It consists of shales and +sandstones about 150 feet thick, with coal and traces of plants; including +a bed of limestone, varying from 2 to 9 feet in thickness, which is +cellular, and resembles some lacustrine limestones of France and Germany. +It has been traced for 30 miles in a straight line, and can be recognized +at still more distant points. The characteristic fossils are a small +bivalve, having the form of a _Cyclas_, a small _Cypris_ (fig. 376.), and +the microscopic shell of an annelid of an extinct genus called +_Microconchus_ (fig. 375.), allied to _Serpula_ or _Spirorbis_. + +In the lower coal-measures of Coalbrook Dale, the strata, according to Mr. +Prestwich, often change completely within very short distances, beds of +sandstone passing horizontally into clay, and clay into sandstone. The +coal-seams often wedge out or disappear; and sections, at places nearly +contiguous, present marked lithological distinctions. In this single field, +in which the strata are from 700 to 800 feet thick, between forty and +fifty species of terrestrial plants have been discovered, besides several +fishes and trilobites of forms distinct from those occurring in the +Silurian strata. Also upwards of forty species of mollusca, among which are +two or three referred to the freshwater genus _Unio_, and others of marine +forms, such as _Nautilus_, _Orthoceras_, _Spirifer_, and _Productus_. Mr. +Prestwich suggests that the intermixture of beds containing freshwater +shells with others full of marine remains, and the alternation of coarse +sandstone and conglomerate with beds of fine clay or shale containing the +remains of plants, may be explained by supposing the deposit of Coalbrook +Dale to have originated in a bay of the sea or estuary into which flowed a +considerable river subject to occasional freshes.[325-A] + +[2 Illustrations: Freshwater Fossils--Coal. + +Fig. 375. + + _a._ _Microconchus carbonarius_. + _b._ var. of same; nat. size, and magnified. + +Fig. 376. _Cypris inflata_, natural size, and magnified. Murchison.[325-B]] + +In the Edinburgh coal-field, at Burdiehouse, fossil fishes, mollusca, +and cypris, very similar to those in Shropshire and Staffordshire, have +been found by Dr. Hibbert.[325-C] In the coal-field also of Yorkshire +there are freshwater strata, some of which contain shells referred to +the genus _Unio_; but in the midst of the series there is one thin but +very widely spread stratum, abounding in fishes and marine shells, such +as _Ammonites Listeri_ (fig. 377.), _Orthoceras_, and _Avicula +papyracea_, Goldf. (fig. 378.)[325-D] + +[Illustration: Fig. 377. _Ammonites Listeri_, Sow.] + +[Illustration: Fig. 378. _Avicula papyracea_, Goldf. (_Pecten +papyraceus_, Sow.)] + +No similarly intercalated layer of marine shells has been noticed in the +neighbouring coal-field of Newcastle, where, as in South Wales and +Somersetshire, the marine deposits are entirely below those containing +terrestrial and freshwater remains.[326-A] + +_Clay-iron-stone._--Bands and nodules of clay-iron-stone are common in +coal-measures, and are formed, says Sir H. De la Beche, of carbonate of +iron, mingled mechanically with earthy matter, like that constituting the +shales. Mr. Hunt, of the Museum of Practical Geology, instituted a series +of experiments to illustrate the production of this substance, and found +that decomposing vegetable matter, such as would be distributed through all +coal strata, prevented the farther oxidation of the proto-salts of iron, +and converted the peroxide into protoxide by taking a portion of its oxygen +to form carbonic acid. Such carbonic acid, meeting with the protoxide of +iron in solution, would unite with it and form a carbonate of iron; and +this mingling with fine mud, when the excess of carbonic acid was removed, +might form beds or nodules of argillaceous iron-stone.[326-B] + + +FOOTNOTES: + +[308-A] Phillips; art. "Geology," Encyc. Britan. + +[309-A] Sedgwick, Geol. Trans., Second Series, vol. iv.; and Phillips, +Geol. of Yorksh. part 2. + +[309-B] Memoirs of Geol. Survey, vol. i. p. 195. + +[315-A] The trunk in this case is referred by Mr. Brown to _Lepidodendron_, +but his illustrations seem to show the usual markings assumed by +_Sigillaria_ near its base. + +[316-A] For terminology of classification of plants, see above, +note, p. 223. + +[316-B] Quart. Geol. Journ., vol. v., Mem., p. 17. + +[317-A] Anniv. Address to Geol. Soc., 1840. + +[317-B] Hawkshaw, Geol. Soc. Proceedings, Nos. 64. and 69. + +[318-A] Geol. Report on Cornwall, &c. p. 143. + +[318-B] Lindley and Hutton, Foss. Flo. part 6. p. 150. + +[319-A] See papers by Messrs. Beckett and Ick. Proceed. in Geol. Soc., +vol. iv. p. 287. + +[319-B] Annales des Mines, 1821. + +[320-A] Principles of Geol., 8th ed., p. 215. + +[321-A] See figures of texture, Witham, Foss. Veget., pl. 3. + +[321-B] See Lyell's Travels in N. America, vol. ii. p. 179. + +[322-A] Quart. Geol. Journ., vol. ii. p. 177. + +[324-A] Geol. Quart. Journ., vol. ii. p. 393.; and vol. vi. p. 115. + +[325-A] Prestwich, Geol. Trans., 2d Series, vol. v. p. 440. Murchison, +Silurian System, p. 105. + +[325-B] Silurian System, p. 84. + +[325-C] Trans. Roy. Soc. Edin. vol. xiii. Horner, Edin. New Phil. +Journ., April, 1836. + +[325-D] Phillips; art. "Geology," Encyc. Metrop., p. 590. + +[326-A] Phillips; art. "Geology," Encyc. Metrop., p. 592. + +[326-B] Memoirs of Geol. Survey, pp. 51. 255, &c. + + + + +CHAPTER 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. + + +It was stated in the last chapter that a great uniformity prevails in the +fossil plants of the coal-measures of Europe and North America; and I may +add that four-fifths of those collected in Nova Scotia have been identified +with European species. Hence the former existence at the remote period +under consideration (the carboniferous) of a continent or chain of islands +where the Atlantic now rolls its waves seems a fair inference. Nor are +there wanting other and independent proofs of such an ancient land situated +to the eastward of the present Atlantic coast of North America; for the +geologist deduces the same conclusion from the mineral composition of the +carboniferous and some older groups of rocks as they are developed on the +eastern flanks of the Alleghanies, contrasted with their character in the +low country to the westward of those mountains. + +The annexed diagram (fig. 379.) will assist the reader in understanding +the phenomena now alluded to, although I must guard him against +supposing that it is a true section. A great number of details have of +necessity been omitted, and the scale of heights and horizontal +distances are unavoidably falsified. + +[Illustration: Fig. 379. Diagram explanatory of the geological structure of +a part of the United States between the Atlantic and the Mississippi. + +Length from E. to W. 850 miles. + + Appalachian Coal Field. Alleghanies, or Appalachians. + +Same section--_continued_. + + Mississippi. Illinois Coal Field. Cincinnati. Appalachian Coal Field. + + A B. Atlantic plain. + B C. Atlantic slope. + C D. Alleghanies or Appalachian chain. + D E. Appalachian coal-field west of the mountains. + E F. Dome-shaped outcrop of strata on the Ohio, older than the coal. + F G. Illinois coal-field. + _h._ Falls and rapids of the rivers at the junction of the hypogene and + newer formations. + _i_, _k_, _l_, _m_. Parallel folds of Appalachians becoming successively + more open, and flatter in going from E. to W. + +_References to the different Formations._ + + 1. Miocene tertiary. + 2. Eocene tertiary. + 3. Cretaceous strata. + 4. Red sandstone with ornithichnites (new red or trias?) usually much + invaded by trap. + 5. Coal-measures (bituminous coal). + 5' Anthracitic coal-measures. + 5'' Carboniferous limestone of the Illinois coal-field, wanting in the + Appalachian. + 6. Old red or Devonian, Olive slate, &c. + 7. Primary fossiliferous or Silurian strata. + 8. Hypogene strata, or gneiss, mica schist, &c., with granite veins. + +_Note._ The dotted lines at _i_ and _k_ express portions of rock removed +by denudation, the amount of which may be estimated by supposing similar +lines prolonged from other points where different strata end abruptly +at the surface. + +_N.B._ The lower section at ** joins on to the upper one at *.] + +Starting from the shores of the Atlantic, on the eastern side of the +Continent, we first come to a low region (A B), which was called the +alluvial plain by the first geographers. It is occupied by tertiary and +cretaceous strata, before described (pp. 171. 206. and 224.), which are +nearly horizontal. The next belt, from B to C, consists of granitic rocks +(hypogene), chiefly gneiss and mica-schist, covered occasionally with +unconformable red sandstone, No. 4. (New Red or Trias?), remarkable for its +ornithichnites (see p. 327.). Sometimes, also, this sandstone rests on the +edges of the disturbed paleozoic rocks (as seen in the section). The region +(B C), sometimes called the "Atlantic Slope," corresponds nearly in average +width with the low and flat plain (A, B), and is characterized by hills of +moderate height, contrasting strongly, in their rounded shape and altitude, +with the long, steep, and lofty parallel ridges of the Alleghany mountains. +The outcrop of the strata in these ridges, like the two belts of hypogene +and newer rocks (A B, and B C), above alluded to, when laid down on a +geological map, exhibit long stripes of different colours, running in a +N.E. and S.W. direction, in the same way as the lias, chalk, and other +secondary formations in the middle and eastern half of England. + +The narrow and parallel zones of the Appalachians here mentioned, consist +of strata, folded into a succession of convex and concave flexures, +subsequently laid open by denudation. The component rocks are of great +thickness, all referable to the Silurian, Devonian, and Carboniferous +formations. There is no principal or central axis, as in the Pyrenees and +many other chains--no nucleus to which all the minor ridges conform; but +the chain consists of many nearly equal and parallel foldings, having what +is termed an anticlinal and synclinal arrangement (see above, p. 48.). This +system of hills extends, geologically considered, from Vermont to Alabama, +being more than 1000 miles long, from 50 to 150 miles broad, and varying in +height from 2000 to 6000 feet. Sometimes the whole assemblage of ridges +runs perfectly straight for a distance of more than 50 miles, after which +all of them wheel round together, and take a new direction, at an angle of +20 or 30 degrees to the first. + +We are indebted to the state surveyors of Virginia and Pennsylvania, Prof. +W. B. Rogers and his brother Prof. H. D. Rogers, for the important +discovery of a clue to the general law of structure prevailing throughout +this range of mountains, which, however simple it may appear when once made +out and clearly explained, might long have been overlooked; amidst so great +a mass of complicated details. It appears that the bending and fracture of +the beds is greatest on the south-eastern or Atlantic side of the chain, +and the strata become less and less disturbed as we go westward, until at +length they regain their original or horizontal position. By reference to +the section (fig. 379.), it will be seen that on the eastern side, or in +the ridges and troughs nearest the Atlantic, south-eastern dips +predominate, in consequence of the beds having been folded back upon +themselves, as in _i_, those on the north-western side of each arch having +been inverted. The next set of arches (such as _k_) are more open, each +having its western side steepest; the next (_l_) opens out still more +widely, the next (_m_) still more, and this continues until we arrive at +the low and level part of the Appalachian coal-field (D E). + +In nature or in a true section, the number of bendings or parallel folds is +so much greater that they could not be expressed in a diagram without +confusion. It is also clear that large quantities of rock have been removed +by aqueous action or denudation, as will appear if we attempt to complete +all the curves in the manner indicated by the dotted lines at _i_ and _k_. + +The movements which imparted so uniform an order of arrangement to this +vast system of rocks must have been, if not contemporaneous, at least +parts of one and the same series, depending on some common cause. Their +geological date is well defined, at least within certain limits, for +they must have taken place after the deposition of the carboniferous +strata (No. 5.), and before the formation of the red sandstone (No. 4.). +The greatest disturbing and denuding forces have evidently been exerted +on the south-eastern side of the chain; and it is here that igneous or +plutonic rocks are observed to have invaded the strata, forming dykes, +some of which run for miles in lines parallel to the main direction of +the Appalachians, or N.N.E. and S.S.W. + +The thickness of the carboniferous rocks in the region C is very great, and +diminishes rapidly as we proceed to the westward. The surveys of +Pennsylvania and Virginia show that the south-east was the quarter whence +the coarser materials of these strata were derived, so that the ancient +land lay in that direction. The conglomerate which forms the general base +of the coal-measures is 1500 feet thick in the Sharp Mountain, where I saw +it (at C) near Pottsville; whereas it has only a thickness of 500 feet, +about thirty miles to the north-west, and dwindles gradually away when +followed still farther in the same direction, till its thickness is reduced +to 30 feet.[329-A] The limestones, on the other hand, of the coal-measures, +augment as we trace them westward. Similar observations have been made in +regard to the Silurian and Devonian formations in New York; the sandstones +and all the mechanically-formed rocks thinning out as they go westward, and +the limestones thickening, as it were, at their expense. It is, therefore, +clear that the ancient land was to the east, where the Atlantic now is; the +deep sea, with its banks of coral and shells to the west, or where the +hydrographical basin of the Mississippi is now situated. + +In that region, near Pottsville, where the thickness of the coal-measures +is greatest, there are thirteen seams of anthracitic coal, several of them +more than 2 yards thick. Some of the lowest of these alternate with beds of +white grit and conglomerate of coarser grain than I ever saw elsewhere, +associated with pure coal. The pebbles of quartz are often of the size of a +hen's egg. On following these pudding-stones and grits for several miles +from Pottsville, by Tamaqua, to the Lehigh Summit Mine, in company with Mr. +H. D. Rogers, in 1841, he pointed out to me that the coarse-grained strata +and their accompanying shales gradually thin out, until seven seams of +coal, at first widely separated, are brought nearer and nearer together, +until they successively unite; so that at last they form one mass, between +40 and 50 feet thick. I saw this enormous bed of anthracitic coal quarried +in the open air at Mauch Chunk (or the Bear Mountain), the overlying +sandstone, 40 feet thick, having been removed bodily from the top of the +hill, which, to use the miner's expression, had been "scalped." The +accumulation of vegetable matter now constituting this vast bed of +anthracite, may perhaps, before it was condensed by pressure and the +discharge of its hydrogen, oxygen, and other volatile ingredients, have +been between 200 and 300 feet thick. The origin of such a vast thickness of +vegetable remains, so unmixed with earthy ingredients, can, I think, be +accounted for in no other way, than by the growth, during thousands of +years, of trees and ferns, in the manner of peat,--a theory which the +presence of the Stigmaria _in situ_ under each of the seven layers of +anthracite, fully bears out. The rival hypothesis, of the drifting of +plants into a sea or estuary, leaves the absence of sediment, or, in this +case, of sand and pebbles, wholly unexplained. + +[Illustration: Fig. 380. Cross section.] + +[Illustration: Fig. 381. Cross section.] + +But the student will naturally ask, what can have caused so many seams of +coal, after they had been persistent for miles, to come together and blend +into one single seam, and that one equal in the aggregate to the thickness +of the several separate seams? Often had the same question been put by +English miners before a satisfactory answer was given to it by the late Mr. +Bowman. The following is his solution of the problem. Let _a a'_, fig. +380., be a mass of vegetable matter, capable, when condensed, of forming a +3-foot seam of coal. It rests on the underclay _b b'_, filled with roots of +trees _in situ_, and it supports a growing forest (C D). Suppose that part +of the same forest D E had become submerged by the ground sinking down 25 +feet, so that the trees have been partly thrown down and partly remain +erect in water, slowly decaying, their stumps and the lower parts of their +trunks being enveloped in layers of sand and mud, which are gradually +filling up the lake D F. When this lake or lagoon has at length been +entirely silted up and converted into land, say, in the course of a +century, the forest C D will extend once more continuously over the whole +area C F, as in fig. 381., and another mass of vegetable matter (_g g'_), +forming 3 feet more of coal, may accumulate from C to F. We then find in +the region F, two seams of coal (_a'_ and _g'_) each 3 feet thick, and +separated by 25 feet of sandstone and shale, with erect trees based upon +the lower coal, while, between D and C, we find these two seams united into +a 2-yard coal. It may be objected that the uninterrupted growth of plants +during the interval of a century will have caused the vegetable matter in +the region C D to be thicker than the two distinct seams _a'_ and _g'_ at +F; and no doubt there would actually be a slight excess representing one +generation of trees with the remains of other plants, forming half an inch +or an inch of coal; but this would not prevent the miner from affirming +that the seam _a g_, throughout the area C D, was equal to the two seams +_a'_ and _g'_ at F. + +The reader has seen, by reference to the section (fig. 379. p. 327.), that +the strata of the Appalachian coal-field assume an horizontal position west +of the mountains. In that less elevated country, the coal-measures are +intersected by three great navigable rivers, and are capable of supplying +for ages, to the inhabitants of a densely peopled region, an inexhaustible +supply of fuel. These rivers are the Monongahela, the Alleghany, and the +Ohio, all of which lay open on their banks the level seams of coal. Looking +down the first of these at Brownsville, we have a fine view of the main +seam of bituminous coal 10 feet thick, commonly called the Pittsburg seam, +breaking out in the steep cliff at the water's edge; and I made the +accompanying sketch of its appearance from the bridge over the river (see +fig. 382.). Here the coal, 10 feet thick, is covered by carbonaceous shale +(_b_), and this again by micaceous sandstone (_c_). Horizontal galleries +may be driven everywhere at very slight expense, and so worked as to drain +themselves, while the cars, laden with coal and attached to each other, +glide down on a railway, so as to deliver their burden into barges moored +to the river's bank. The same seam is seen at a distance, on the right bank +(at _a_), and may be followed the whole way to Pittsburg, fifty miles +distant. As it is nearly horizontal, while the river descends it crops out +at a continually increasing, but never at an inconvenient, height above the +Monongahela. Below the great bed of coal at Brownsville is a fire-clay 18 +inches thick, and below this, several beds of limestone, below which again +are other coal seams. I have also shown in my sketch another layer of +workable coal (at _d d_), which breaks out on the slope of the hills at a +greater height. Here almost every proprietor can open a coal-pit on his own +land, and the stratification being very regular, he may calculate with +precision the depth at which coal may be won. + +The Appalachian coal-field, of which these strata form a part (from C +to E, section, fig. 379., p. 327.), is remarkable for its vast area; +for, according to Professor H. D. Rogers, it stretches continuously from +N.E. to S.W., for a distance of 720 miles, its greatest width being +about 180 miles. On a moderate estimate, its superficial area amounts to +63,000 square miles. + +[Illustration: Fig. 382. View of the great Coal Seam on the Monongahela at +Brownsville, Pennsylvania, U. S. + + _a._ Ten-foot seam of coal. + _b._ Black bituminous or carbonaceous shale, 10 feet thick. + _c._ Micaceous sandstone. + _d d._ Upper seam of coal, 6 feet thick.] + +This coal formation, before its original limits were reduced by +denudation, must have measured 900 miles in length, and in some places more +than 200 miles in breadth. By again referring to the section (fig. 379., p. +327.), it will be seen that the strata of coal are horizontal to the +westward of the mountains in the region D E, and become more and more +inclined and folded as we proceed eastward. Now it is invariably found, as +Professor H. D. Rogers has shown by chemical analysis, that the coal is +most bituminous towards its western limit, where it remains level and +unbroken, and that it becomes progressively debituminized as we travel +south-eastward towards the more bent and distorted rocks. Thus, on the +Ohio, the proportion of hydrogen, oxygen, and other volatile matters, +ranges from forty to fifty per cent. Eastward of this line, on the +Monongahela, it still approaches forty per cent., where the strata begin to +experience some gentle flexures. On entering the Alleghany Mountains, where +the distinct anticlinal axes begin to show themselves, but before the +dislocations are considerable, the volatile matter is generally in the +proportion of eighteen or twenty per cent. At length, when we arrive at +some insulated coal-fields (5', fig. 379.) associated with the boldest +flexures of the Appalachian chain, where the strata have been actually +turned over, as near Pottsville, we find the coal to contain only from six +to twelve per cent. of bitumen, thus becoming a genuine anthracite.[333-A] + +It appears from the researches of Liebig and other eminent chemists, that +when wood and vegetable matter are buried in the earth, exposed to +moisture, and partially or entirely excluded from the air, they decompose +slowly and evolve carbonic acid gas, thus parting with a portion of their +original oxygen. By this means, they become gradually converted into +lignite or wood-coal, which contains a larger proportion of hydrogen than +wood does. A continuance of decomposition changes this lignite into common +or bituminous coal, chiefly by the discharge of carburetted hydrogen, or +the gas by which we illuminate our streets and houses. According to +Bischoff, the inflammable gases which are always escaping from mineral +coal, and are so often the cause of fatal accidents in mines, always +contain carbonic acid, carburetted hydrogen, nitrogen, and olefiant gas. +The disengagement of all these gradually transforms ordinary or bituminous +coal into anthracite, to which the various names of splint coal, glance +coal, culm, and many others, have been given. + +We have seen that, in the Appalachian coal-field, there is an intimate +connection between the extent to which the coal has parted with its +gaseous contents, and the amount of disturbance which the strata have +undergone. The coincidence of these phenomena may be attributed partly +to the greater facility afforded for the escape of volatile matter, +where the fracturing of the rocks had produced an infinite number of +cracks and crevices, and also to the heat of the gases and water +penetrating these cracks, when the great movements took place, which +have rent and folded the Appalachian strata. It is well known that, at +the present period, thermal waters and hot vapours burst out from the +earth during earthquakes, and these would not fail to promote the +disengagement of volatile matter from the carboniferous rocks. + +_Continuity of seams of coal._--As single seams of coal are continuous over +very wide areas, it has been asked, how forests could have prevailed +uninterruptedly over such wide spaces, without being oftener flooded by +turbid rivers, or, when submerged, denuded by marine currents. It appears, +from the description of the Cape Breton coal-field, by Mr. Richard Brown, +that false stratification is common in the beds of sand, and some partial +denudation of these, at least, must often have taken place during the +accumulation of the carboniferous series. + +In the Forest of Dean, ancient river-channels are found, which pass through +beds of coal, and in which rounded pebbles of coal occur. They are of older +date than the overlying and undisturbed coal-measures. The late Mr. Buddle, +who described them to me, told me he had seen similar phenomena in the +Newcastle coal-field. Nevertheless, instances of these channels are much +more rare than we might have anticipated, especially when we remember how +often the roots of trees (_Stigmariæ_) have been torn up, and drifted in +broken fragments into the grits and sandstones. The prevalence of a +downward movement is, no doubt, the principal cause which has saved so many +extensive seams of coal from destruction by fluviatile action. + +The purity of the coal, or its non-intermixture with earthy matter, +presents another theoretical difficulty to many geologists, who are +inclined to believe that the trees and smaller plants of the +carboniferous period grew in extensive swamps, rather than on land not +liable to be inundated. It appears, however, that in the alluvial plain +and delta of the Mississippi, extensive "cypress swamps," as they are +called, densely covered with various trees, occur, into which no matter +held in mechanical suspension is ever introduced during the greatest +inundations, inasmuch as they are all surrounded by a dense marginal +belt of reeds, canes, and brushwood. Through this thick barrier the +river-water must pass, so that it is invariably well filtered before it +can reach the interior of the forest-covered area, within which, +vegetable matter is continually accumulating from the decay of trees and +semi-aquatic plants. In proof of this, I may observe, that whenever any +part of a swamp is dried up, during an unusually hot season, and the +wood set on fire, pits are burnt into the ground many feet deep, or as +far down as the fire can descend without meeting with water, and it is +then found that scarcely any residuum or earthy matter is left.[334-A] +At the bottom of these "cypress swamps" of the Mississippi, a bed of +clay is found, with roots of the tall cypress (_Taxodium distichum_), +just as the underclays of the coal are filled with _Stigmaria_. + +_Climate of Coal Period._--So long as the botanist taught that a tropical +climate was implied by the carboniferous flora, geologists might well be at +a loss to reconcile the preservation of so much vegetable matter with a +high temperature; for heat hastens the decomposition of fallen leaves and +trunks of trees, whether in the atmosphere or in water.[335-A] It is well +known that peat, so abundant in the bogs of high latitudes, ceases to grow +in the swamps of warmer regions. It seems, however, to have become a more +and more received opinion, that the coal-plants do not, on the whole, +indicate a climate resembling that now enjoyed in the equatorial zone. +Tree-ferns range as far south as the southern part of New Zealand, and +Araucarian pines occur in Norfolk Island. A great predominance of ferns and +lycopodiums indicates warmth, moisture, equability of temperature, and +freedom from frost, rather than intense heat; and we know too little of the +sigillariæ, calamites, asterophyllites, and other peculiar forms of the +carboniferous period, to be able to speculate with confidence on the kind +of climate they may have required. + +No doubt, we are entitled to presume, from the corals and cephalopoda of +the mountain limestone, that a warm temperature characterized the northern +seas in the carboniferous era; but the absence of cold may have given rise +(as at present in the seas of the Bermudas, under the influence of the gulf +stream) to a very wide geographical range of stone-building corals and +shell-bearing cuttle-fish, without its being necessary to call in the aid +of tropical heat.[335-B] + + +CARBONIFEROUS REPTILES. + +Where we have evidence in a single coal-field, as in that of Nova Scotia, +or South Wales, of fifty or even a hundred ancient forests buried one above +the other, with the roots of trees still in their original position, and +with some of the trunks still remaining erect, we are apt to wonder that +until the year 1844 no remains of contemporaneous air-breathing creatures, +except a few insects, had been discovered. No vertebrated animals more +highly organized than fish, no mammalia or birds, no saurians, frogs, +tortoises, or snakes, were yet known in rocks of such high antiquity. In +the coal-field of Coalbrook Dale mention had been made of two species of +beetles of the family _Curculionidæ_, and of a neuropterous insect +resembling the genus _Corydalis_, with another related to the +_Phasmidæ_.[335-C] In other coal-measures in Europe we find notice of a +scorpion and of a moth allied to _Tinea_, also of one air-breathing +crustacean, or land-crab. Yet Agassiz had already described in his great +work on fossil fishes more than one hundred and fifty species of +ichthyolites from the coal strata, ninety-four belonging to the families of +shark and ray, and fifty-eight to the class of ganoids. Some of these fish +are very remote in their organization from any now living, especially +those of the family called _Sauroid_ by Agassiz; as _Megalichthys_, +_Holoptychius_, and others, which are often of great size, and all +predaceous. Their osteology, says M. Agassiz, reminds us in many respects +of the skeletons of saurian reptiles, both by the close sutures of the +bones of the skull, their large conical teeth striated longitudinally (see +fig. 383.), the articulations of the spinous processes with the vertebræ, +and other characters. Yet they do not form a family intermediate between +fish and reptiles, but are true _fish_, though doubtless more highly +organized than any living fish.[336-A] + +[Illustration: Fig. 383. _Holoptychius Hibberti_, Ag. Fifeshire +coal-field; natural size.] + +The annexed figure represents a large tooth of the _Megalichthys_, +found by Mr. Horner in the Cannel coal of Fifeshire. It probably +inhabited an estuary, like many of its contemporaries, and frequented +both rivers and the sea. + +[Illustration: Fig. 384. _Archegosaurus minor_, Goldfuss. Fossil reptile +from the coal-measures, Saarbrück.] + +At length, in 1844, the first skeleton of a true reptile was announced from +the coal of Münster-Appel in Rhenish Bavaria, by H. von Meyer, under the +name of _Apateon pedestris_, the animal being supposed to be nearly related +to the salamanders. Three years later, in 1847, Prof. von Dechen found in +the coal-field of Saarbrück, at the village of Lebach, between Strasburg +and Treves, the skeletons of no less than three distinct species of +air-breathing reptiles, which were described by the late Prof. Goldfuss +under the generic name of _Archegosaurus_. The ichthyolites and plants +found in the same strata, left no doubt that these remains belonged to the +true coal period. The skulls, teeth, and the greater portions of the +skeleton, nay, even a large part of the skin, of two of these reptiles have +been faithfully preserved in the centre of spheroidal concretions of +clay-iron-stone. The largest of these lizards, _Archegosaurus Decheni_, +must have been 3 feet 6 inches long. The annexed drawing represents the +smallest of the three of the natural size. They were considered by Goldfuss +as saurians, but by Herman von Meyer as most nearly allied to the +_Labyrinthodon_, and therefore connected with the batrachians, as well as +the lizards. The remains of the extremities leave no doubt that they were +quadrupeds, "provided," says Von Meyer, "with hands and feet terminating in +distinct toes; but these limbs were weak, serving only for swimming or +creeping." The same anatomist has pointed out certain points of analogy +between their bones and those of the _Proteus anguinus_; and Mr. Owen has +observed to me that they make an approach to the _Proteus_ in the shortness +of their ribs. Two of these ancient reptiles retain a large part of the +outer skin, which consisted of long, narrow, wedge-shaped, tile-like, and +horny scales, arranged in rows (see fig. 385.). + +[Illustration: Fig. 385. Imbricated covering of skin of _Archegosaurus +medius_, Goldf.; magnified.[337-A]] + +_Cheirotherian footprints in coal measures, United States._--In 1844, +the very year when the Apateon or Salamander of the coal was first met +with in the country between the Moselle and the Rhine, Dr. King +published an account of the footprints of a large reptile discovered by +him in North America. These occur in the coal strata of Greensburg, in +Westmoreland County, Pennsylvania; and I had an opportunity of examining +them in 1846. I was at once convinced of their genuineness, and declared +my conviction on that point, on which doubts had been entertained both +in Europe and the United States. The footmarks were first observed +standing out in relief from the lower surface of slabs of sandstone, +resting on thin layers of fine unctuous clay. I brought away one of +these masses, which is represented in the accompanying drawing (fig. +386.). It displays, together with footprints, the casts of cracks (_a_, +_a'_) of various sizes. The origin of such cracks in clay, and casts of +the same, has before been explained, and referred to the drying and +shrinking of mud, and the subsequent pouring of sand into open crevices. +It will be seen that some of the cracks, as at _b_, _c_, traverse the +footprints, and produce distortion in them, as might have been expected, +for the mud must have been soft when the animal walked over it and left +the impressions; whereas, when it afterwards dried up and shrank, it +would be too hard to receive such indentations. + +No less than twenty-three footsteps were observed by Dr. King in the +same quarry before it was abandoned, the greater part of them so +arranged (see fig. 387.) on the surface of one stratum as to imply that +they were made successively by the same animal. Everywhere there was a +double row of tracks, and in each row they occur in pairs, each pair +consisting of a hind and fore foot, and each being at nearly equal +distances from the next pair. In each parallel row the toes turn the one +set to the right, the other to the left. In the European +_Cheirotherium_, before mentioned (p. 290.), both the hind and fore feet +have each five toes, and the size of the hind foot is about five times +as large as the fore foot. In the American fossil the posterior +footprint is not even twice as large as the anterior, and the number of +toes is unequal, being five in the hinder and four in the anterior foot. +In this, as in the European _Cheirotherium_, one toe stands out like a +thumb, and these thumb-like toes turn the one set to the right, and the +other to the left. The American _Cheirotherium_ was evidently a broader +animal, and belonged to a distinct genus from that of the triassic +age in Europe.[338-A] + +[Illustration: Fig. 386. _Scale one-sixth the original._ Slab of sandstone +from the coal-measures of Pennsylvania, with footprints of air-breathing +reptile and casts of cracks.] + +We may assume that the reptile which left these prints on the ancient +sands of the coal-measures was an air-breather, because its weight would +not have been sufficient under water to have made impressions so deep and +distinct. The same conclusion is also borne out by the casts of the cracks +above described, for they show that the clay had been exposed to the air +and sun, so as to have dried and shrunk. + +[Illustration: Fig. 387. Series of reptilian footprints in the coal-strata +of Westmoreland County, Pennsylvania. + +_a._ Mark of nail?] + +The geological position of the sandstone of Greensburg is perfectly clear, +being situated in the midst of the Appalachian coal-field, having the main +bed of coal, called the Pittsburg seam, above mentioned (p. 331.), 3 yards +thick, 100 feet above it, and worked in the neighbourhood, with several +other seams of coal at lower levels. The impressions of _Lepidodendron_, +_Sigillaria_, _Stigmaria_, and other characteristic carboniferous plants, +are found both above and below the level of the reptilian footsteps. + +Analogous footprints of a large reptile of still older date have since +been found (1849), by Mr. Isaac Lea, in the lowest beds of the coal +formation at Pottsville, near Philadelphia, so that we may now be said +to have the footmarks of two reptilians of the coal period, and the +skeletons of four.[340-A] + + +CARBONIFEROUS OR MOUNTAIN LIMESTONE. + +We have already seen that this rock lies sometimes entirely beneath the +coal-measures, while, in other districts, it alternates with the shales and +sandstone of the coal. In both cases it is destitute of land plants, and +usually charged with corals, which are often of large size; and several +species belong to the lamelliferous class of Lamarck, which enter largely +into the structure of coral reefs now growing. There are also a great +number of _Crinoidea_ (see fig. 388.), and a few _Echinoderms_, associated +with the zoophytes above mentioned. The _Brachiopoda_ constitute a large +proportion of the Mollusca, many species being referable to two extinct +genera, _Spirifer_ (or _Spirifera_) (fig. 389.), and _Productus_ +(_Leptæna_) (fig. 390.). + +[Illustration: Fig. 388. _Cyathocrinites planus_, Miller. +Mountain limestone.] + +[Illustration: Fig. 389. _Spirifer glaber_, Sow. Mountain limestone.] + +[Illustration: Fig. 390. _Productus Martini_, Sow. (_P. semireticulatus_, +Flem.) Mountain limestone.] + +Among the spiral univalve shells the extinct genus _Euomphalus_ (see fig. +391.) is one of the commonest fossils of the Mountain limestone. In the +interior it is often divided into chambers (see fig. 391. _d_); the septa +or partitions not being perforated, as in foraminiferous shells, or in +those having siphuncles, like the Nautilus. The animal appears, like the +recent _Bulimus decollatus_, to have retreated at different periods of its +growth, from the internal cavity previously formed, and to have closed all +communication with it by a septum. The number of chambers is irregular, and +they are generally wanting in the innermost whorl. + +[Illustration: Fig. 391. _Euomphalus pentagulatus_, Min. Con. +Mountain limestone. + +_a._ Upper side; _b._ lower, or umbilical side; _c._ view showing mouth +which is less pentagonal in older individuals; _d._ view of polished +section, showing internal chambers.] + +[Illustration: Fig. 392. Portion of _Orthoceras laterale_, +Phillips. Mountain limestone.] + +There are also many univalve and bivalve shells of existing genera in the +Mountain limestone, such as _Turritella_, _Buccinum_, _Patella_, +_Isocardia_, _Nucula_, and _Pecten_.[341-A] But the _Cephalopoda_ depart, +in general, more widely from living forms, some being generically distinct +from all those found in strata newer than the coal. In this number may be +mentioned _Orthoceras_, a siphuncled and chambered shell, like a _Nautilus_ +uncoiled and straightened. Some species of this genus are several feet long +(fig. 392.). The _Goniatite_ is another genus, nearly allied to the +_Ammonite_, from which it differs in having the lobes of the septa free +from lateral denticulations, or crenatures; so that the outline of these is +continuous and uninterrupted (see _a_, fig. 393.). Their siphon is small, +and in the form of the striæ of growth they resemble _Nautili_. Another +extinct generic form of Cephalopod, abounding in the Mountain limestone, +and not found in strata of later date, is the _Bellerophon_ (fig. 394.), of +which the shell, like the living Argonaut, was without chambers. + +[Illustration: Fig. 393. _Goniatites evolutus_, Phillips.[342-A] +Mountain limestone.] + +[Illustration: Fig. 394. _Bellerophon costatus_, Sow.[342-B] +Mountain limestone.] + + +FOOTNOTES: + +[329-A] H. D. Rogers, Trans. Assoc. Amer. Geol., 1840-42, p. 440. + +[333-A] Trans. of Ass. of Amer. Geol., p. 470. + +[334-A] Lyell's Second Visit to the U. S., vol. ii. p. 245. American +Journ. of Sci., 2d series, vol. v. p. 17. + +[335-A] Principles of Geol., p. 696. + +[335-B] For changes in climate, see Principles of Geol., chaps. +vii. and viii. + +[335-C] Geol. Trans., 2d series, vol. vi. p. 330. + +[336-A] Agassiz, Poiss. Foss., lib. 4. p. 62. and liv. 5. p. 88. + +[337-A] Goldfuss, Neue Jenaische Lit. Zeit., 1848; and Von Meyer, Quart. +Geol. Journ., vol. iv. p. 51., memoirs. + +[338-A] See Lyell's Second Visit, &c., vol. ii. p. 305. + +[340-A] These impressions, found by Mr. Lea, were imagined to be in a rock +as ancient as the old red sandstone; but, according to Mr. H. D. Rogers, +they are in the lowest part of the coal formation. + +[341-A] Phillips, Geol. of Yorksh., vol. ii. p. 208. + +[342-A] Phillips, Geol. of Yorksh., pl. 20. fig. 65. + +[342-B] Ibid., pl. 17. fig. 15. + + + + +CHAPTER 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. + + +It was stated in Chap. XXII. that the Carboniferous formation is +surmounted by one called the "New Red," and underlaid by another called +the "Old Red Sandstone."[342-C] The British strata of the last mentioned +series were first recognized in Herefordshire and Scotland as of great +thickness, and immediately subjacent to the coal; but they were in +general so barren of organic remains, that it was difficult to find +paleontological characters of sufficient importance to distinguish them +as an independent group. In Scotland, and on the borders of Wales, the +"Old Red" consists chiefly of red sandstone, conglomerate, and shale, +with few fossils; but limestones of the same age, peculiarly rich in +organic remains, were at length found in Devonshire. + +I shall first advert to the characters of the group as developed in +Herefordshire, Worcestershire, Shropshire, and South Wales. Its thickness +has been estimated at 8000 feet, and it has been subdivided into-- + + 1st. A quartzose conglomerate passing downwards into chocolate-red and + green sandstone and marl. + + 2d. Cornstone and marl--red and green argillaceous spotted marls, with + irregular courses of impure concretionary limestone, provincially + called Cornstone. + +Here, as usual, fossils are extremely rare in the clays and sandstones in +which the red oxide of iron prevails; but remains of fishes of the genera +_Cephalaspis_ and _Onchus_ have been discovered in the Cornstone. + +The whole of the northern part of Scotland, from Cape Wrath to the +southern flank of the Grampians, has been well described by Mr. Miller +as consisting of a nucleus of granite, gneiss, and other hypogene rocks, +which seem as if set in a sandstone frame.[343-A] The beds of the Old +Red Sandstone constituting this frame, may once perhaps have extended +continuously over the entire Grampians before the upheaval of that +mountain range; for one band of the sandstone follows the course of the +Moray Frith far into the interior of the great Caledonian valley; and +detached hills and island-like patches occur in several parts of the +interior, capping some of the higher summits in Sutherlandshire, and +appearing in Morayshire like oases among the granite rocks of +Strathspey. On the western coast of Ross-shire, the Old Red forms those +three immense insulated hills before described (p. 67.), where beds of +horizontal sandstone, 3000 feet high, rest unconformably on a base of +gneiss, attesting the vast denudation which has taken place. + +But in order to observe the uppermost part of the Old Red, we must travel +south of the Grampians, and examine its junction with the bottom of the +Carboniferous series in Fifeshire. This upper member may be seen in Dura +Den, south of Cupar, to consist of a belt of yellow sandstone, in which Dr. +Fleming first discovered scales of _Holoptychius_, and in which species of +fish of the genera _Pterichthys_, _Pamphractus_, and others, have been met +with. (For genus _Pterichthys_, see fig. 400. p. 345.) + +The beds next below the yellow sandstone are well seen in the large zone of +Old Red which skirts the southern flank of the Grampians from Stonehaven to +the Frith of Clyde. It there forms, together with trap, the Sidlaw Hills +and the strata of the valley of Strathmore. A section of this region has +been already given (p. 48.), extending from the foot of the Grampians in +Forfarshire to the sea at Arbroath, a distance of about 20 miles, where the +entire series of strata is several thousand feet thick, and may be divided +into three principal masses: 1st, and uppermost, red and mottled marls, +cornstone, and sandstone (Nos. 1. and 2. of the section); 2d, Conglomerate, +often of vast thickness (No. 3. ibid.); 3d, Roofing and paving stone, +highly micaceous, and containing a slight admixture of carbonate of lime +(No. 4. ibid.). In the first of these divisions, which may be considered as +succeeding the yellow sandstone of Fifeshire before mentioned, a gigantic +species of fish of the genus _Holoptychius_ has been found at Clashbinnie +near Perth. Some scales (see fig. 395.) have been seen which measured 3 +inches in length by 2-1/2 in breadth. + +At the top of the next division, or immediately under the conglomerate +(No. 3. p. 48.), there have been found in Forfarshire some remarkable +crustaceans, with several fish of the genus named by Agassiz _Cephalaspis_, +or "buckler-headed," from the extraordinary shield which covers the head +(see fig. 396.), and which has often been mistaken for that of a trilobite, +of the division _Asaphus_. + +[Illustration: Fig. 395. Scale of _Holoptychius nobilissimus_, Agas. +Clashbinnie. Nat. size.] + +Species of the same genus are considered in England as characteristic of +the second or Cornstone division (p. 343.). + +[Illustration: Fig. 396. _Cephalaspis Lyellii_, Agass. Length 6-3/4 inches. +From a specimen in my collection found at Glammiss, in Forfarshire. See +other figures, Agassiz, vol. ii. tab. 1. _a_. and 1. _b_. + + _a._ One of the peculiar scales with which the head is covered when + perfect. These scales are generally removed, as in the specimen + above figured. + _b, c._ Scales from different parts of the body and tail.] + +[Illustration: Fig. 397. _Eggs of gasteropodous mollusk?_ Lower beds of +Old Red, Ley's Mill, Forfarshire.] + +[Illustration: Fig. 398. _Fucoids and eggs of gasteropodous mollusk?_ +Lower Old Red, Fife.] + +In the same grey paving-stones and coarse roofing-slates, in which the +_Cephalaspis_ occurs, in Forfarshire and Kincardineshire, the remains of +marine plants or fucoids abound. They are frequently accompanied by groups +of hexagonal, or nearly hexagonal markings, which consist of small +flattened carbonaceous bodies, placed in a slight depression of the +sandstone or shale. (See figs. 397 and 398.) They much resemble in form the +spawn of the recent Natica (see fig. 399.), in which the eggs are arranged +in a thin layer of sand, and seem to have acquired a polygonal form by +pressing against each other. The substance of the egg, if fossilized, might +give rise to small pellicles of carbonaceous matter. + +[Illustration: Fig. 399. Fragment of spawn of British species of _Natica_.] + +These fossils I have met with, both to the north of Strathmore, in the +vertical shale beneath the conglomerate, and in the same beds in the Sidlaw +hills, at all the points where fig. 4. is introduced in the section, p. 48. + +[Illustration: Fig. 400. _Pterichthys_, Agassiz; upper side, showing mouth; +as restored by H. Miller.[345-A]] + +Beds of red shale and red sandstone, sometimes associated with +pudding-stone (older than No. 3., fig. 62. p. 48.), and destitute of +organic remains, separate, in the region of Strathmore, the above-described +fossiliferous strata from the older crystalline rocks of the Grampians. +But, in the north of Scotland, we find, at the base of the Old Red, other +grey slaty sandstones, in the counties of Banff, Nairn, Moray, Cromarty, +Caithness, and in Orkney, rich in ichthyolites of peculiar forms, belonging +to the genera _Pterichthys_ (fig. 400.), _Coccosteus_, _Diplopterus_, +_Dipterus_, _Cheiracanthus_, and others of Agassiz. + +Five species of _Pterichthys_ have been found in this lowest division of +the Old Red. The wing-like appendages, whence the genus is named, were +first supposed by Mr. Miller to be paddles, like those of the turtle; +but Agassiz regards them as weapons of defence, like the occipital +spines of the River Bull-head (_Cottus gobio_, Linn.); and considers the +tail to have been the only organ of motion. The genera _Dipterus_ and +_Diplopterus_ are so named, because their two dorsal fins are so placed +as to front the anal and ventral fins, so as to appear like two pairs of +wings. They have bony enamelled scales. + +_South Devon and Cornwall._--A great step was made in the classification of +the slaty and calciferous strata of South Devon and Cornwall in 1837, when +a large portion of the beds, previously referred to the "transition" or +most ancient fossiliferous series, were found to belong in reality to the +period of the Old Red Sandstone. For this reform we are indebted to the +labours of Professor Sedgwick and Sir R. Murchison, assisted by a +suggestion of Mr. Lonsdale, who, in 1837, after examining the South +Devonshire fossils, perceived that some of them agreed with those of the +Carboniferous group, others with those of the Silurian, while many could +not be assigned to either system, the whole taken together exhibiting a +peculiar and intermediate character. But these paleontological observations +alone would not have enabled us to assign, with accuracy, the true place in +the geological series of these slate-rocks and limestones of South Devon, +had not Messrs. Sedgwick and Murchison, in 1836 and 1837, discovered that +the culmiferous or anthracitic shales of North Devon belonged to the Coal, +and not, as preceding observers had imagined, to the transition period. + +As the strata of South Devon here alluded to are far richer in organic +remains than the red sandstones of contemporaneous date in Herefordshire +and Scotland, the new name of the "Devonian system" was proposed as a +substitute for that of Old Red Sandstone. + +The rocks of this group in South Devon consist, in great part, of green +chloritic slates, alternating with hard quartzose slates and sandstones. +Here and there calcareous slates are interstratified with blue crystalline +limestone, and in some divisions conglomerates, passing into red sandstone. + +The link supplied by the whole assemblage of imbedded fossils, connecting +as it does the paleontology of the Silurian and Carboniferous groups, is +one of the highest interest, and equally striking, whether we regard the +_genera_ of corals or of shells. The _species_ are almost all distinct. + +Among the more abundant corals, we find the genera _Favosites_ and +_Cyathophyllum_, common on the one hand to the Mountain limestone, and on +the other to the Silurian system. Some few even of the _species_ are common +to the Devonian and Silurian groups, as, for example, _Favosites +polymorpha_ (fig. 401.), very abundant in South Devon. + +[Illustration: Fig. 401. _Favosites polymorpha_, Goldf., S. Devon. +From a polished specimen. + +_a._ portion of the same, magnified to show the pores.] + +The _Cyathophyllum cæspitosum_ (fig. 402.) and _Porites pyriformis_ (fig. +424. p. 356.) are more peculiarly characteristic of the Devonian rocks. + +In regard to the shells, all the brachiopodous genera, such as +_Terebratula_, _Orthis_, _Spirifer_, _Atrypa_, and _Productus_, which are +found in the Mountain limestone, occur, together with those of the Silurian +system, except the _Pentamerus_. Some forms, however, seem exclusively +Devonian, as for example, _Calceola sandalina_ (fig. 403.) and +_Strygocephalus Burtini_ (fig. 404.), which have been met with both in the +Eifel, in Germany, and in Devonshire, in the very lowest Devonian beds. + +[Illustration: Fig. 402. Cyathophyllum. + + _a._ _Cyathophyllum cæspitosum_, Goldf., Plymouth. + _b._ a terminal star. + _c._ vertical section exhibiting transverse plates, and part of + another branch.] + +Among the peculiar lamellibranchiate bivalves, also common to Devonshire +and the Eifel, we find _Megalodon cucullatus_ (fig. 405.). Several spiral +univalves are abundant, among which are many species of _Pleurotomaria_ and +_Euomphalus_. Among the Cephalopoda we find _Bellerophon_ and _Orthoceras_, +as in the Silurian and Carboniferous groups, and _Goniatite_ and +_Cyrtoceras_, as in the Carboniferous. In some of the upper Devonian beds, +a shell, resembling a flattened _Goniatite_, occurs, called _Clymenia_, by +Munster (_Endosiphonites_, Ansted.[347-A]). + +[Illustration: Fig. 403. _Calceola sandalina_, Lam. Eifel; also South +Devon. + + _a._ both valves united. + _b._ inner side of opercular valve.] + +[Illustration: Fig. 404. _Strygocephalus Burtini_. (_Terebratula porrecta_, +Sow.) Eifel; also South Devon. + + _a._ valves united. + _b_. side view of same. + _c._ interior of larger valve, showing thick partition, and thinner one + continued from it.] + +[Illustration: Fig. 405. _Megalodon cucullatus_, Sow. Eifel; also +Bradley, S. Devon. + + _a._ the valves united. + _b._ interior of valve, showing the large cardinal tooth.] + +[Illustration: Fig. 406. _Clymenia linearis_, Munster. (_Endosiphonites +carinatus_, Ansted.) Cornwall.] + +A peculiar species of trilobite, called _Brontes flabellifer_ (fig. 407.), +is found in the Devonian strata of the Eifel and in South Devon. It should +be observed, however, that the head in the specimen here figured by +Goldfuss, the most perfect which could be obtained, is incomplete, and a +restoration has been attempted by Mr. Salter in fig. 408., from data +supplied by other species of the same genus occurring in older rocks. + +[Illustration: Fig. 407. _Brontes flabellifer_, Goldf. Eifel; +also S. Devon.] + +[Illustration: Fig. 408. Restored outline of head of +_Brontes flabellifer_.] + +For determining the true equivalents of the Devonian group in the Rhenish +provinces and adjacent parts of Germany, we are indebted to the labours of +Messrs. Sedgwick and Murchison, in 1839, from which it appears that rocks +of that age emerge from beneath the coal-field of Westphalia, and are also +found in troughs among the Silurian rocks in Nassau. Many of the +limestones, particularly those on the river Lahn, are identical, both in +structure and in coralline remains, with the beautiful marbles of +Babbacombe, Torquay, and Plymouth. + +The limestones of the Eifel, long ago celebrated for their fossils, and +which lie in a basin supported by Silurian rocks, are found to be referable +to the lower part of the Devonian system. + +In Russia, also, Messrs. Murchison and De Verneuil have shown (1840) that +the "Old Red" group occupies a wide area south from St. Petersburg. It was +formerly supposed to be the New Red Sandstone, on account of its saliferous +and gypseous beds; but it is now proved to be the Old Red by containing +ichthyolites of genera which characterize this group in the British Isles, +as, for example, _Holoptychius_, _Coccosteus_, _Diplopterus_, &c.[349-A], +associated with mollusca found in the Devonian of Western Europe. Among the +fish are also many species of sharks of the Cestraciont division, a fact +worthy of notice, because the squaloid fishes of the present day offer the +highest organization of the brain and of the generative organs, and make, +in these respects, the nearest approach to the higher vertebrate classes. + + +_Devonian Strata in the United States._ + +The position of this formation between the carboniferous rocks of +Pennsylvania and Ohio, is pointed out in the section, fig. 379. p. 327., +and it is a remark of M. de Verneuil that in no European country is +there so complete and uninterrupted a development of the Devonian system +as in North America. At the falls of the Ohio, at Louisville, in +Kentucky, there is a grand display of one of the limestones of this +period, resembling a modern coral reef. A wide extent of surface is +exposed in a series of horizontal ledges, at all seasons, when the water +is not high; and the softer parts of the stone having decomposed and +wasted away, the harder calcareous corals stand out in relief, and many +of them send out branches from their erect stems precisely as if they +were living. Among other species I observed large masses, not less than +5 feet in diameter, of _Favosites gothlandica_, with its beautiful +honeycomb structure well displayed, and, by the side of it, the +_Favistella_, combining a similar honeycombed form with the star of the +_Astrea_. There was also the cup-shaped _Cyathophyllum_, and the +delicate network of the _Fenestella_, and that elegant and well-known +European species of fossil, called "the chain coral," _Catenipora +escharoides_, with a profusion of others (see fig. 423. p. 355.). These +coralline forms were mingled with the joints, stems, and occasionally +the heads, of lily encrinites. Although hundreds of fine specimens have +been detached from these rocks, to enrich the museums of Europe and +America, another crop is constantly working its way out, under the +action of the stream, and of the sun and rain, in the warm season when +the channel is laid dry. The waters of the Ohio, when I visited the +spot in April, 1846, were more than 40 feet below their highest level, +and 20 feet above their lowest, so that large spaces of bare rock were +exposed to view.[349-B] + + +_Devonian Flora._ + +With the exception of the fucoids above mentioned (p. 344.), but little +is known with certainty of the plants of the Devonian group. Those found +in the department of La Sarthe in France, and in various parts of +Brittany, formerly referred to the Devonian era, have been shown (in +1850), by M. de Verneuil, to belong to the carboniferous series. The +same may be said of the species of _Lepidodendron_, _Knorria_, +_Calamite_, _Sagenaria_, and other genera recently figured (1850), by +Mr. F. A. Römer, from the formation called "Greywacké à Posodonomyes" in +the Hartz.[350-A] They are accompanied by _Goniatites reticulatus_ +Phillips, _G. intercostatus_ Phil., and other mountain limestone +species, and had been previously assigned to the oldest part of the +carboniferous series by Messrs. Murchison and Sedgwick. + +If hereafter we should become well acquainted with the land plants of the +Devonian era, we may confidently expect that nearly all of them will agree +generically with those of the carboniferous period, but the species will be +as different as are the Devonian vertebrate and invertebrate animals from +the fossil species of the Coal. + + +FOOTNOTES: + +[342-C] See section, fig. 318. p. 287. + +[343-A] The Old Red Sandstone, by Hugh Miller, 1841. + +[345-A] Old Red Sandstone. Plate 1. fig. 1. Mr. M.'s description of the +fish is most graphic and correct. + +[347-A] Camb. Phil. Trans., vol. vi. pl. 8. fig. 2. + +[349-A] See Proceedings of Geol. Soc., and the anniversary speech of +Dr. Buckland, P. G. S., for 1841. + +[349-B] Lyell's Second Visit to the United States, vol. ii. p. 277. + +[350-A] Memoir on the Hartz, Palæontographica of Dunker and Von Meyer, +part iii. + + + + +CHAPTER XXVII. + +SILURIAN GROUP. + + Silurian strata formerly called transition--Term + grauwacké--Subdivisions of Upper and Lower Silurian--Ludlow formation + and fossils--Wenlock formation, corals and shells--Caradoc and + Llandeilo beds--Graptolites--Lingula--Trilobites--Cystideæ--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. + + +We come next in the descending order to the most ancient of the primary +fossiliferous rocks, that series which comprises the greater part of the +strata formerly called "transition" by Werner, for reasons explained in +Chap. VIII., pp. 91 and 92. Geologists have also applied to these older +strata the general name of "grauwacké," by which the German miners +designate a particular variety of sandstone, usually an aggregate of small +fragments of quartz, flinty slate (or Lydian stone), and clay-slate +cemented together by argillaceous matter. Far too much importance has been +attached to this kind of rock, as if it belonged to a certain epoch in the +earth's history, whereas a similar sandstone or grit is found sometimes in +the Old Red, and in the Millstone Grit of the Coal, and sometimes in +certain Cretaceous and even Eocene formations in the Alps. + +The name of _Silurian_ was first proposed by Sir Roderick Murchison, for a +series of fossiliferous strata lying below the Old Red Sandstone, and +occupying that part of Wales and some contiguous counties of England, which +once constituted the kingdom of the _Silures_, a tribe of ancient Britons. +The strata have been divided into Upper and Lower Silurian, and these +again in the region alluded to admit of several well-marked subdivisions, +all of them explained in the following table. + + UPPER SILURIAN ROCKS. + + Prevailing Thickness Organic + Lithological in Feet. Remains. + characters. + + { {Finely laminated } } + {Tilestones. { reddish and }800? } + { { green sandstones } } + { { and shales. } } + 1. Ludlow { }Marine mollusca of + formation {Upper {Micaceous grey } } almost every order, + {Ludlow. { sandstone. } } the Brachiopoda most + { } } abundant. Serpula, + {Aymestry {Argillaceous } } Corals, Sauroid fish, + {limestone. { limestone. }2000 } Fuci. + { } } + {Lower {Shale, with } } + {Ludlow. { concretions of } } + { { limestone. } } + + {Wenlock }Concretionary } {Marine mollusca of + {limestone. } limestone. } { various orders as + 2. Wenlock { } }1800 { before, Crustaceans + formation. { } } { of the Trilobite + { } } { family. + {Wenlock }Argillaceous } {Oldest bones of + {shale. } shale. } { fish yet known. + + + LOWER SILURIAN ROCKS. + + {Flags of shelly } { + { limestone and } {Crinoidea, Corals, + 3. Caradoc {Caradoc { sandstone, thick }2500 { Mollusca, chiefly + formation. {sandstones. { bedded white } { Brachiopoda, + { freestone. } { Trilobites. + + 4. Llandeilo {Llandeilo }Dark coloured }1200 {Mollusca, + formation. {flags. } calcareous flags. } { Trilobites. + + +UPPER SILURIAN ROCKS. + +_Ludlow formation._--This member of the Upper Silurian group, as will +be seen by the above table, is of great thickness, and subdivided into +four parts,--the Tilestone, the Upper and Lower Ludlow, and the +intervening Aymestry limestone. Each of these may be distinguished near +the town of Ludlow, and at other places in Shropshire and Herefordshire, +by peculiar organic remains. + +1. _Tilestones._--This uppermost division was originally classed by +Sir R. Murchison with the Old Red Sandstone, because they decompose +into a red soil throughout the Silurian region. At the same time he +regarded the tilestones as a transition group forming a passage from +Silurian to Old Red. It is now ascertained that the fossils agree in +great part specifically, and in general character entirely, with those +of the succeeding formation. + +2. _Upper Ludlow._--The next division, called the Upper Ludlow, consists of +grey calcareous sandstone, decomposing into soft mud, and contains, among +other shells, the _Lingula cornea_, which is common to it and the lowest, +or tilestone beds of the Old Red. But the _Orthis orbicularis_ is peculiar +to the Upper Ludlow, and very common; and the lowest or mudstone beds, are +loaded for a thickness of 30 feet with _Terebratula navicula_ (fig. 410.), +in vast numbers. Among the cephalopodous mollusca occur the genera +_Bellerophon_ and _Orthoceras_, and among the crustacea the _Homalonotus_ +(fig. 418. p. 354.). A coral called _Favosites polymorpha_, Goldf. (fig. +401. p. 346.) is found both in this subdivision and in the Devonian system. + +[Illustration: Fig. 409. _Orthis orbicularis_, J. Sow. Delbury. +Upper Ludlow.] + +[Illustration: Fig. 410. _Terebratula navicula_, J. Sow. Aymestry +limestone; also in Upper and Lower Ludlow.] + +Among the fossil shells are species of _Leptæna_, _Orthis_, _Terebratula_, +_Avicula_, _Trochus_, _Orthoceras_, _Bellerophon_, and others.[352-A] + +Some of the Upper Ludlow sandstones are ripple-marked, thus affording +evidence of gradual deposition; and the same may be said of the +accompanying fine argillaceous shales which are of great thickness, and +have been provincially named "mudstones." In these shales many zoophytes +are found enveloped in an erect position, having evidently become fossil on +the spots where they grew at the bottom of the sea. The facility with which +these rocks, when exposed to the weather, are resolved into mud, proves +that, notwithstanding their antiquity, they are nearly in the state in +which they were first thrown down. + +The scales, spines (_ichthyodorulites_), jaws, and teeth of fish of the +genera _Onchus_, _Plectrodus_, and others of the same family, have been met +with in the Upper Ludlow rocks. + +[Illustration: Fig. 411. _Pentamerus Knightii_, Sow. Aymestry. + + _a._ view of both valves united. + _b._ longitudinal section through both valves, showing the central plate + or septum; half nat. size.] + +3. _Aymestry limestone._--The next group is a subcrystalline and +argillaceous limestone, which is in some places 50 feet thick, and +distinguished around Aymestry by the abundance of _Pentamerus Knightii_, +Sow. (fig. 411.), also found in the Lower Ludlow. This genus of +brachiopoda has only been found in the Silurian strata. The name was +derived from +pente+, _pente_, five, and +meros+, _meros_, a part, because +both valves are divided by a central septum, making four chambers, and in +one valve the septum itself contains a small chamber, making five; but +neither the structure of this shell, nor the connection of the animal with +its several parts, are as yet understood. Messrs. Murchison and De Verneuil +discovered this species dispersed in myriads through a white limestone +of upper Silurian age, on the banks of the Is, on the eastern flank of +the Urals in Russia. + +[Illustration: Fig. 412. _Lingula Lewisii_, J. Sow. Abberley Hills.] + +Three other abundant shells in the Aymestry limestone are, 1st, _Lingula +Lewisii_ (fig. 412.); 2d, _Terebratula Wilsoni_, Sow. (fig. 413.), which is +also common to the Lower Ludlow and Wenlock limestone; 3d, _Atrypa +reticularis_, Lin. (fig. 414.), which has a very wide range, being found in +every part of the Silurian system, except the Llandeilo flags. + +[Illustration: Fig. 413. _Terebratula Wilsoni_, Sow. Aymestry.] + +[Illustration: Fig. 414. _Atrypa reticularis._ Linn. Syn. _Terebratula +affinis_, Min. Con. Aymestry. + + _a._ upper valve. + _b._ lower. + _c._ anterior margin of the valves.] + +4. _Lower Ludlow shale._--A dark grey argillaceous deposit, containing, +among other fossils, the new genera of chambered shells, the _Phragmoceras_ +of Broderip, and the _Lituites_ of Breyn (see figs. 415, 416.). The latter +is partly straight and partly convoluted, nearly as in _Spirula_. + +[Illustration: Fig. 415. _Phragmoceras ventricosum_, J. Sow. (_Orthoceras +ventricosum_, Stein.) Aymestry; 1/4 nat. size.] + +[Illustration: Fig. 416. _Lituites giganteus_, J. Sow. Near Ludlow; also in +the Aymestry and Wenlock limestones; 1/4 nat. size.] + +[Illustration: Fig. 417. Fragments of Orthoceras. + + _a._ Fragment of _Orthoceras Ludense_, J. Sow. + _b._ Polished section, showing siphuncle. Ludlow.] + +The _Orthoceras Ludense_ (fig. 417.), as well as the shell last mentioned, +is peculiar to this member of the series. The _Homalonotus +delphinocephalus_ (fig. 418.) is common to this division and to the Wenlock +limestone. This crustacean belongs to a group of trilobites which has been +met with in the Silurian rocks only, and in which the tripartite character +of the dorsal crust is almost lost. + +[Illustration: Fig. 418. _Homalonotus delphinocephalus_, König.[354-A] +Dudley Castle; 1/2 nat. size.] + +A species of Graptolite, _G. Ludensis_, Murch. (fig. 419.), a form of +zoophyte which has not yet been met with in strata newer than the Silurian, +occurs in the Lower Ludlow. + +_Wenlock formation._--We next come to the Wenlock formation, which has been +divided (see Table, p. 351.) into + +1. Wenlock limestone, formerly well known to collectors by the name of the +Dudley limestone, which forms a continuous ridge, ranging for about 20 +miles from S.W. to N.E., about a mile distant from the nearly parallel +escarpment of the Aymestry limestone. The prominence of this rock in +Shropshire, like that of Aymestry, is due to its solidity, and to the +softness of the shales above and below. It is divided into large +concretional masses of pure limestone, and abounds in trilobites, among +which the prevailing species are _Phacops caudatus_ (fig. 422.) and +_Calymene Blumenbachii_, commonly called the Dudley trilobite. The latter +is often found coiled up like a wood-louse (see fig. 420.). + +[Illustration: Fig. 419. _Graptolithus Ludensis_, Murchison. Lower Ludlow.] + +[Illustration: Fig. 420. _Calymene Blumenbachii_, Brong. Wenlock, L. +Ludlow, and Aym. limest.] + +[Illustration: Fig. 421. _Leptæna depressa._ Wenlock.] + +[Illustration: Fig. 422. _Phacops caudatus_, Brong. Wenlock, Aym. +limest., and L. Ludlow.] + +_Leptæna depressa_, Sow., is common in this rock, but also ranges through +the Lower Ludlow, Wenlock shale, and Caradoc Sandstone. + +[Illustration: Fig. 423. _Catenipora escharoides._] + +Among the corals in which this formation is very rich, the _Catenipora +escharoides_, Lam. (fig. 423.), or chain coral, may be pointed out as +one very easily recognized, and widely spread in Europe, ranging +through all parts of the Silurian group, from the Aymestry limestone +to the bottom of the series. + +Another coral, the _Porites pyriformis_, is also met with in profusion; a +species common to the Devonian rocks. + +_Cystiphyllum Siluriense_ (fig. 425.) is a species peculiar to the Wenlock +limestone. This new genus, the name of which is derived from +kystis+, a +_bladder_, and +phyllon+, a _leaf_, was instituted by Mr. Lonsdale for +corals of the Silurian and Devonian groups. It is composed of small +bladder-like cells (see fig. 425. _b._). + +2. The Wenlock Shale, which exceeds 700 feet in thickness, contains +many species of brachiopoda, such as a small variety of the _Lingula +Lewisii_ (fig. 412.), and the _Atrypa reticularis_ (fig. 414.) before +mentioned, and it will be seen that several other fossils before +enumerated range into this shale. + +[Illustration: Fig. 424. _Porites pyriformis_, Ehren. Wenlock limest. and +shale. Also in Aymestry limestone, and L. Ludlow. + +_a._ Vertical section, showing transverse lamellæ.] + +[Illustration: Fig. 425. Cystiphyllum. + + _a._ _Cystiphyllum Siluriense_, Lonsd. Wenlock. + _b._ Section of portion, showing cells.] + + +LOWER SILURIAN ROCKS. + +The Lower Silurian rocks have been subdivided into two portions. + +1. The Caradoc sandstone, which abuts against the trappean chain called the +Caradoc Hills, in Shropshire. Its thickness is estimated at 2500 feet, and +the larger proportion of its fossils are specifically distinct from those +of the Upper Silurian rocks. Among them we find many trilobites and shells +of the genera _Orthoceras_, _Nautilus_, and _Bellerophon_; and among the +Brachiopoda the _Pentamerus oblongus_ and _P. lævis_ (fig. 426.), which are +very abundant and peculiar to this bed; also _Orthis grandis_ (fig. 427.), +and a fossil of well-defined form, _Tentaculites annulatus_, Schlot. (fig. +428.), which Mr. Salter has shown to be referable to the Annelids and to +the same tribe as _Serpula_. + +[Illustration: Fig. 426. _Pentamerus lævis_, Sow. Caradoc Sandstone. +Perhaps the young of _Pentamerus oblongus_. + + _a, b._ Views of the shell itself, from figures in Murchison's Sil. Syst. + _c._ Cast with portion of shell remaining, and with the hollow of the + central septum filled with spar. + _d._ Internal cast of a valve, the space once occupied by the septum + being represented by a hollow in which is seen a cast of the + chamber within the septum.] + +[Illustration: Fig. 427. Cast of _Orthis grandis_, J. Sow. Horderley; +two-thirds of nat. size.] + +[Illustration: Fig. 428. _Tentaculites scalaris_, Schlot. Eastnor Park; +nat. size, and magnified.] + +The most ancient bony remains of fish yet discovered in Great Britain are +those obtained from the Wenlock limestones; but coprolites referred to fish +occur still lower in the Silurian series in Wales. + +[Illustration: Fig. 429. _Ogygia Buchii_, Burmeister. Syn. _Asaphus +Buchii_, Brong. 1/4 nat. size. Radnorshire.] + +2. The _Llandeilo flags_, so named from a town in Caermarthenshire, form +the base of the Silurian system, consisting of dark-coloured micaceous +grit, frequently calcareous, and distinguished by containing the large +trilobites _Asaphus Buchii_ and _A. tyrannus_, Murch., both of which are +peculiar to these rocks. Several species of Graptolites (fig. 430.) +occur in these beds. + +[Illustration: Fig. 430. _a_, _b_. _Graptolithus Murchisonii_, +Beck. Llandeilo flags.] + +[Illustration: Fig. 431. _G. foliaceus_, _Murchison_. Llandeilo flags.] + +In the fine shales of this formation Graptolites are very abundant. I +collected these same bodies in great numbers in Sweden and Norway in +1835-6, both in the higher and lower shales of the Silurian system; and +was informed by Dr. Beck of Copenhagen, that they were fossil zoophytes +related to the genera _Pennatula_ and _Virgularia_, of which the living +species now inhabit mud and slimy sediment. The most eminent naturalists +still hold to this opinion. + +A species of _Lingula_ is met with in the lowest part of the Llandeilo +beds; and it is remarkable that this brachiopod is among the earliest, if +not the most ancient animal form detected in the lowest Silurian of North +America. These inhabitants of the seas, of so remote an epoch, belonged so +strictly to the living genus _Lingula_, as to demonstrate, like the +pteriform ferns of the coal, through what incalculable periods of time the +same plan and type of organization has sometimes prevailed. + +Among the forms of trilobite extremely characteristic of the Lower Silurian +throughout Europe and North America, the _Trinucleus_ may be mentioned. +This family of crustaceans appears to have swarmed in the Silurian seas, +just as crabs, shrimps, and other genera of crustaceans abound in our own. +Burmeister, in his work on the organization of trilobites, supposes them to +have swum at the surface of the water in the open sea and near coasts, +feeding on smaller marine animals, and to have had the power of rolling +themselves into a ball as a defence against injury. They underwent various +transformations analogous to those of living crustaceans. M. Barrande, +author of a work on the Silurian rocks of Bohemia, has traced the same +species from the young state just after its escape from the egg to the +adult form, through various metamorphoses, each having the appearance of a +distinct species. Yet, notwithstanding the numerous species of preceding +naturalists which he has thus succeeded in uniting into one, he announces a +forthcoming work in which descriptions and figures of 250 species of +Trilobite will be given. + +[Illustration: Fig. 432. _Trinucleus ornatus_, Burm.] + +_Cystideæ._--Among the additions which recent research has made to the +paleontology of the oldest Silurian rocks, none are more remarkable than +the radiated animals called _Cystideæ_. Their structure and relations were +first elucidated in an essay published by Von Buch at Berlin in 1845. They +are usually met with as spheroidal bodies covered with polygonal plates, +with a mouth on the upper side, and a point of attachment for a stem _b_ +(which is almost always broken off) on the lower. (See fig. 433.) They are +considered by Professor E. Forbes as intermediate between the crinoids and +echinoderms. The _Sphæronites_ here represented (fig. 433.) occurs in the +Llandeilo beds in Wales.[358-A] + +[Illustration: Fig. 433. _Sphæronites balticus_, Eichwald. (Of the +family _Cystideæ_.) + + _a._ mouth. + _b._ point of attachment of stem. + +Lower Silurian, Shole's Hook and Bala.] + +_Thickness and unconformability of Silurian strata._--According to the +observation of our government surveyors in North Wales, the Lower Silurian +strata of that region attain, in conjunction with the contemporaneous +volcanic rocks, the extraordinary thickness of 27,000 feet. One of the +groups, called the trappean, consisting of slates and associated volcanic +ash and greenstone, is 15,000 feet thick. Another series, called the Bala +group, composed of slates and grits with an impure limestone rich in +organic remains, is 9,000 feet thick.[359-A] + +Throughout North Wales the Wenlock shales rest unconformably upon the +Caradoc sandstones; and the Caradoc is in its turn unconformable to the +Llandeilo beds, showing a considerable interval of time between the +deposition of this group and that of the formations next above and below +it. The Caradoc sandstone in the neighbourhood of the Longmynd Hills in +Shropshire, appears to Professor E. Forbes to have been a deep-sea deposit +formed around the margin of high and steep land. That land consisted partly +of upraised Llandeilo flags and partly of rocks of still older date.[359-B] + +Such evidence of the successive disturbance of strata during the Silurian +period in Great Britain is what we might look for when we have discovered +the signs of so grand a series of volcanic eruptions as the contemporaneous +greenstones and tuffs of the Welsh mountains afford. + + +_Silurian Strata of the United States._ + +The position of some of these strata, where they are bent and highly +inclined in the Appalachian chain, or where they are nearly horizontal +to the west of that chain, is shown in the section, fig. 379. p. 327. +But these formations can be studied still more advantageously north of +the same line of section, in the states of New York, Ohio, and other +regions north and south of the great Canadian lakes. Here they are +found, as in Russia, in horizontal position, and are more rich in +well-preserved fossils than in almost any spot in Europe. The American +strata may readily be divided into Upper and Lower Silurian, +corresponding in age and fossils to the European divisions bearing the +same names. The subordinate members of the New York series, founded on +lithological and geographical considerations, are most useful in the +United States, but even there are only of local importance. Some few of +them, however, tally very exactly with English divisions, as for example +the limestone, over which the Niagara is precipitated at the great +cataract, which, with its underlying shales, agrees paleontologically +with the Wenlock limestone and shale of Siluria. There is also a marked +general correspondence in the succession of fossil forms, and even +species, as we trace the organic remains downwards from the highest +to the lowest beds. + +Mr. D. Sharpe, in his report on the mollusca collected by me from these +strata in North America[359-C], has concluded that the number of species +common to the Silurian rocks, on both sides of the Atlantic, is between 30 +and 40 per cent.; a result which, although no doubt liable to future +modification, when a larger comparison shall have been made, proves, +nevertheless, that many of the species had a wide geographical range. It +seems that comparatively few of the gasteropods and lamellibranchiate +bivalves of North America can be identified specifically with European +fossils, while no less than two-fifths of the brachiopoda are the same. In +explanation of these facts, it is suggested, that most of the recent +brachiopoda (especially the orthidiform ones) are inhabitants of deep +water, and may have had a wider geographical range than shells living near +shore. The predominance of bivalve mollusca of this peculiar class has +caused the Silurian period to be sometimes styled the age of brachiopods. + +_Whether the Silurian rocks are of deep-water origin._--The grounds relied +upon by Professor E. Forbes, for inferring that the larger part of the +Silurian Fauna is indicative of a sea more than 70 fathoms deep, are the +following: first, the small size of the greater number of conchifera; +secondly, the paucity of pectinibranchiata (or spiral univalves); thirdly, +the great number of floaters, such as _Bellerophon_, _Orthoceras_, &c.; +fourthly, the abundance of orthidiform brachiopoda; fifthly, the absence or +great rarity of fossil fish. + +It is doubtless true that some living _Terebratulæ_, on the coast of +Australia, inhabit shallow water; but all the known species, allied in +form to the extinct _Orthis_, inhabit the depths of the sea. It should +also be remarked that Mr. Forbes, in advocating these views, was well +aware of the existence of shores, bounding the Silurian sea in +Shropshire, and of the occurrence of littoral species of this early date +in the northern hemisphere. Such facts are not inconsistent with his +theory; for he has shown, in another work, how, on the coast of Lycia, +deep-sea strata are at present forming in the Mediterranean, in the +vicinity of high and steep land. + +Had we discovered the ancient delta of some large Silurian river, we should +doubtless have known more of the shallow, and brackish water, and +fluviatile animals, and of the terrestrial flora of the period under +consideration. To assume that there were no such deltas in the Silurian +world, would be almost as gratuitous an hypothesis, as for the inhabitants +of the coral islands of the Pacific to indulge in a similar generalization +respecting the actual condition of the globe.[360-A] + + +_Mineral Character of Silurian Strata._ + +In lithological character, the Silurian strata vary greatly when we +trace them through Europe and North America. The shales called +mudstones are as little altered from some deposits, found in recent +submarine banks, as are those of many tertiary formations. We meet +with red sandstone and red marl, with gypsum and salt, of Upper +Silurian date, in the Niagara district, which might be mistaken for +trias. The whitish granular sandstone at the base of the Silurian series +in Sweden resembles the tertiary siliceous grit of Fontainebleau. The +Calcareous Grit, oolite, and pisolite of Upper Silurian age in +Gothland, are described by Sir R. Murchison as singularly like rocks +of the oolitic period near Cheltenham; and, not to cite more examples, +the Wenlock or Dudley limestone often resembles a modern coral-reef. If, +therefore, uniformity of aspect has been thought characteristic of rocks +of this age, the idea must have arisen from the similarity of feature +acquired by strata subject to metamorphic action. This influence, seeing +that the causes of change are always shifting the theatre of their +principal development, must be multiplied throughout a wider +geographical area by time, and become more general in any given system +of rocks in proportion to their antiquity. We are now acquainted with +dense groups of Eocene slates in the Alps, which were once mistaken by +experienced geologists for Transition or Silurian formations. The error +arose from attaching too great importance to mineral character as a +test of age, for the tertiary slates in question having acquired that +crystalline texture which is in reality most prevalent in the most +ancient sedimentary formations. + + +CAMBRIAN GROUP. + +Below the Silurian strata in North Wales, and in the region of the +Cumberland lakes, there are some slaty rocks, devoid of organic remains, or +in which a few obscure traces only of fossils have been detected (for which +the names of Cambrian and Cumbrian have been proposed). Whether these will +ever be entitled by the specific distinctness of their fossils to rank as +independent groups, we have not yet sufficient data to determine. + + * * * * * + +TABULAR VIEW OF FOSSILIFEROUS STRATA, + +_Showing the Order of Superposition or Chronological Succession of the +principal European Groups_. + + + I. POST-TERTIARY. + + A. POST-PLIOCENE. + + Periods and Groups. Examples. Observations. + + 1. Recent. { Peat mosses and shell-marl, } All the imbedded shells, + { with bones of land animals, } freshwater and marine, + { human remains, and works } of living species, + { of art. } with occasional + { } human remains and + { Newer parts of modern deltas } works of art. + { and coral reefs. } + + 2. Post-Pliocene. { Clay, marl, and volcanic tuff } All the shells of living + { of Ischia, p. 113. } species. No human + { } remains or works + { Loess of the Rhine, p. 117. } of art. Bones of + { } quadrupeds, partly + { Newer part of boulder } of extinct species. + { formation, with erratics, } + { p. 124. } + + II. TERTIARY. + + B. PLIOCENE. + + 3. Newer Pliocene { Boulder formation or drift of { Three-fourths of the + or Pleistocene. { northern Europe and North { fossil shells of + { America, chaps. 11. & 12. { existing species. + { { + { Cavern deposits and osseous { A majority of the + { breccias, p. 153. { mammalia extinct; + { { but the genera + { Fluvio-marine crag of Norwich, { corresponding with + { p. 148. { those now surviving in + { { the same great + { Limestone of Girgenti, { geographical and + { in Sicily, p. 152. { zoological province, + { p. 157. + { + { During part of this + { period icebergs + { frequent in the seas + { of the northern + { hemisphere, and + { glaciers on hills + { of moderate height. + + 4. Older Pliocene. { Red and Coralline crag of { A third or more of the + { Suffolk, p. 162. { species of mollusca + { { extinct. + { Subapennine beds, p. 166. { + { Nearly, if not all, the + { mammalia extinct. + + C. MIOCENE. + + 5. Miocene. { Faluns of Touraine, p. 168. { About two-thirds of the + { { species of shells + { Part of Bordeaux beds, p. 171. { extinct. + { { + { Part of Molasse of { The recent species of + { Switzerland, p. 171. { shells often not + { found in the + { adjoining seas, but + { in warmer latitudes. + { + { All the mammalia + { extinct. + + D. EOCENE. + + 6. Upper Eocene. { Upper marine of Paris basin, } Fossil shells of the + { Fontainebleau sandstone, } Eocene period, with + { p. 175. } very few exceptions, + { } extinct. Those which + { Upper freshwater and millstone } are identified with + { of same. } living species rarely + { Kleyn Spauwen beds, p. 176. } belong to neighbouring + { } regions. + { Hermsdorf tile-clay, near } + { Berlin. } All the mammalia of + { } extinct species, and + { Mayence tertiary strata, } the greater part of + { p. 177. } them of extinct + { } genera. + { Freshwater beds of Limagne } + { d'Auvergne, p. 181. } Plants of Upper Eocene, + } indicating a south + 7. Middle Eocene. { Paris gypsum with } European or + { Paleotherium, &c., p. 191. } Mediterranean climate; + { } those of Lower Eocene, + { Freshwater and fluvio-marine } a tropical climate. + { beds of Headon Hill, Isle } + { of Wight, p. 197. } + { } + { Barton beds, Hants, p. 198. } + { } + { Calcaire Grossier, Paris, } + { p. 193. } + { } + { Bagshot and Bracklesham beds, } + { Surrey and Sussex, p. 198. } + } + 8. Lower Eocene. { London clay proper of Highgate } + { Hill and Sheppey,--Bognor } + { beds, Sussex, p. 200. } + { } + { Sables inférieurs, and lits } + { coquilliers of Paris basin, } + { p. 196. } + { } + { Mottled and plastic clays and } + { sands of the Hampshire and } + { London basins, p. 203. } + { } + { Sables inférieurs and argiles } + { plastiques of Paris basin, } + { p. 196. } + { } + { Nummulitic formation of the } + { Alps, p. 205. } + + III. SECONDARY. + + E. CRETACEOUS. + + § UPPER CRETACEOUS. + + 9. Maestricht { Yellowish white limestone of { Ammonite, Baculite, and + beds. { Maestricht, p. 209. { Belemnite, associated + { { with Cypræa, Oliva, + { Coralline limestone of Faxoe, { Mitra, Trochus, &c. + { Denmark, p. 210. { Large marine saurians. + + 10. Upper White { White chalk with flints of } Marine limestone + Chalk. { North and South } formed in part of + { Downs,--Surrey and Sussex, } decomposed corals. + { p. 211. } + + 11. Lower White { Chalk without flints, and } + Chalk. { chalk marl, ibid. } + + 12. Upper { Loose sand, with bright green } + Greensand. { particles, ibid. } + { } + { Firestone of Merstham, Kent, } + { p. 218. } + { } + { Marly stone, with layers of } + { chert, south of Isle of } + { Wight. } + + 13. Gault. { Dark blue marl at base of { Numerous extinct genera + { chalk escarpment,--Kent { of conchiferous + { and Sussex, p. 218. { cephalopoda, Hamite, + { Scaphite, Ammonite, &c. + + §§ LOWER CRETACEOUS. + + 14. Lower { Sand with green matter,--Weald } Species of shells, &c., + Greensand. { of Kent and Sussex, } nearly all distinct + { p. 219. } from those of Upper + { } Cretaceous; most of + { White, yellowish, and } the genera the same. + { ferruginous sand, with } + { concretions of limestone and } + { chert,--Atherfield, Isle } + { of Wight. } + { } + { Limestone called Kentish Rag } + + F. WEALDEN. + + 15. Weald Clay. { Clay with occasional bands of { Of freshwater origin. + { limestone,--Weald of Kent, { Shells of + { Surrey, and Sussex, p. 227. { pulmoniferous + { mollusca, and of + { Cypris. Land reptiles. + + 16. Hastings Sand. { Sand with calciferous grit and { Freshwater with + { clay,--Hastings, Sussex, { intercalated bed of + { Cuckfield, Kent, p. 229. { brackish and salt + { water origin. Shells + { of fluviatile and + { lacustrine genera. + { Reptiles of the genera + { Pterodactyle, + { Iguanodon, + { Megalosaurus, + { Plesiosaurus, Trionyx, + { and Emys. + + 17. Purbeck Beds. Limestones, calcareous slates { Chiefly freshwater, and + and marls, p. 231. { divisible into three + { groups, each + { containing distinct + { species of freshwater + { mollusca and of + { entomostraca. + { Alternations of + { deposits formed in + { fresh, brackish, and + { marine water, and of + { ancient soils formed + { on land and retaining + { roots of trees. + { Plants chiefly cycads + { and conifers, p. 231. + + G. OOLITE. + + 18. Upper Oolite. { _a._ Portland building stone, } Ammonites and Belemnites + { p. 259. } numerous. + { } + { _b._ Portland sand. } Large saurians, as + { } Pterodactyles, + { _c._ Kimmeridge clay, } Plesiosaurs, + { Dorsetshire, p. 260. } Ichthyosaurs. + } + 19. Middle Oolite. { _a._ Coral Rag, p. 260. } No cetaceans yet known, + { Calcareous freestones, } but three species of + { oolitic, often full of } terrestrial mammalia, + { corals. Oxfordshire. } p. 267, 268. + { } Preponderance of + { _b._ Oxford clay--Dark blue } ganoid fish. The + { clay,--Oxfordshire and } plants chiefly cycads, + { midland counties, p. 262. } conifers, and ferns, + } with a few palms. + 20. Lower Oolite. { _a._ Cornbrash and forest } + { marble, Wiltshire, p. 263. } + { } + { _b._ Great oolite and } + { Stonesfield slate,--Bath, } + { Bradford, Stonesfield near } + { Woodstock, Oxfordshire, } + { p. 266. } + { } + { _c._ Fuller's earth,--Clay } + { containing fuller's earth } + { near Bath, p. 272. } + { } + { _d._ Inferior oolite, } + { calcareous freestone, and } + { yellow sands,--Cotteswold } + { Hills, Dundry Hill, near } + { Bristol, p. 272. } + + H. LIAS. + + 21. Lias. { Argillaceous limestone, marl { Mollusca, reptiles, + { and clay,--Lyme Regis, { and fish of genera + { Dorsetshire, p. 273. { analogous to the + { oolitic. + + I. TRIAS. + + 22. Upper Trias. { Keuper of Germany, or } Batrachian reptiles, + { variegated marls--Red, grey, } _e.g._ Labyrinthodon, + { green, blue, and white marls } Rhyncosaurus, &c. + { and sandstones with } Cephalopoda: + { gypsum--Würtemberg, bone-bed } Ceratites. No + { of Axmouth, Dorset, p. 289. } Belemnites. Plants: + } Ferns, Cycads, + } Conifers. + + 23. Middle Trias { Compact greyish limestone } With Equisetites + or { with beds of dolomite and } and Calamite. + Muschelkalk. { gypsum,--North of Germany, } + { p. 287. Wanting in } + { England. } + + 24. Lower Trias. { Variegated or Bunter sandstone } Plants different for + { of Germans--Red and white } the most part from + { spotted sandstone with } those of the Upper + { gypsum and rock-salt, P. 288 } Trias. + { } + { Part of New Red sandstone of } + { of Cheshire with rock-salt, } + { p. 294. } + + IV. PRIMARY. + + K. PERMIAN. + + 25. Upper Permian. { Yellow magnesian limestone, } Organic remains, both + { Yorkshire and Durham, } animal and vegetable, + { P. 301. } more allied to primary + { } than to secondary + { Zechstein of Thuringia, Upper } periods. + { part of Permian beds, } + { Russia. } + + 26. Lower Permian. { _a._ Marl slate of Durham and } Thecodont saurians. + { Thuringia. } Heterocercal fish of + { } genus Palæoniscus, &c. + { _b._ Lower New Red sandstone } + { of north of England and } + { Rothliegendes of Germany. } + { } + { _a._ and _b._ Lower part of } + { Permian beds, Russia, } + { p. 301. } + + L. CARBONIFEROUS. + + 27. Coal measures. { _a._ Strata of sandstone and } Great thickness of + { shale, with beds of } strata of + { coal,--S. Wales and } fluvio-marine origin, + { Northumberland, p. 309. } with beds of coal of + { } vegetable origin, + { _b._ Millstone grit,--S. } based on soils + { Wales, Bristol coal-field, } retaining the roots + { Yorkshire, p. 308. } of trees. + } + } Oldest of known reptiles + } or Archegosaurus. + } Sauroid fish. + + 28. Mountain { Carboniferous or mountain { Brachiopoda of genus + limestone. { limestone, with marine { Productus. + { shells and corals. { + { { Cephalopoda of genera + { Mendip Hills, and many parts { Cyrtoceras, Goniatite, + { of Ireland, p. 340. { Orthoceras. + { + { Crustaceans of the + { genus Phillipsia. + { + { Crinoideans abundant. + + M. DEVONIAN. + + 29. Upper { _a._ Yellow sandstone of Dura } Tribe of fish with hard + Devonian. { Den, Fife. } coverings like + { } chelonians, + { _b._ Red sandstone and marl } Pterichthys, + { with cornstone of } Pamphractus, &c.; + { Herefordshire and } also of genera + { Forfarshire. } Cephalaspis, + { } Holoptichius, &c. + { Paving and roofing-stone, } + { Forfarshire. } No reptiles yet known. + { } + { Upper part of Devonian beds } + { of South Devon. } + + 30. Lower { Grey sandstone with } Fish, partly of same + Devonian. { Ichthyolites,--Caithness, } genera, but of + { Cromarty, and Orkney, Lower } distinct species from + { part of Devonian beds of } those in Upper + { South Devon, and green } Devonian; also + { chloritic slates of } Osteolepis, + { Cornwall, limestone of } Coccosteus, + { Gerolstein, Eifel. } Glyptolepis, + } Dipterus, &c. + + N. SILURIAN. + + 31. Upper { _a._ Tilestone of Brecon and { Oldest of fossil fish + Silurian. { Caermarthen. { yet discovered. + { { + { _b._ Limestone and shale, { Trilobites and + { Ludlow, Shropshire. { Graptolites abundant. + { { + { _c._ Wenlock or Dudley { Brachiopoda very + { limestone. { numerous. + { + { Cephalopoda: + { Bellerophon, + { Orthoceras. + + { Same genera of + 32. Lower { _a._ Caradoc sandstone, Caer { invertebrate animals + Silurian. { Caradoc, Shropshire. { as in Upper Silurian, + { { but species chiefly + { _b._ Llandeilo flags, { distinct. Trinucleus + { calcareous flags and { caractaci, Cystideæ, + { schists,--Builth, { p. 358. + { Radnorshire, Llandeilo, { + { Caermarthenshire. { No land plants yet + { known. + { + { Footprints of tortoise, + { see note, p. 360. + + +FOOTNOTES: + +[352-A] Murchison, Silurian System, p. 198, 199. + +[354-A] Silurian System, pl. 7. bis. fig. 1. b. + +[358-A] Quart. Geol. Journ., vol. ii. p. 11.; and Memoirs of Geol. Survey, +vol. ii. p. 518. + +[359-A] Quart. Geol. Journ., vol. iv. p. 300. + +[359-B] Ibid., 299. + +[359-C] Ibid., 145. + +[360-A] Since this was written, Mr. Logan has discovered chelonian +footprints in the lowest fossiliferous beds of the Silurian series, near +Montreal, in Canada. Professor Owen inclines to refer them to the genus +_Emys_.--_Quart. Journ. G. S._, vol. vii. p. lxxvi. + + + + +CHAPTER 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. + + +The aqueous or fossiliferous rocks having now been described, we have next +to examine those which may be called volcanic, in the most extended sense +of that term. Suppose _a a_ in the annexed diagram, to represent the +crystalline formations, such as the granitic and metamorphic; _b b_ the +fossiliferous strata; and _c c_ the volcanic rocks. These last are +sometimes found, as was explained in the first chapter, breaking through +_a_ and _b_, sometimes overlying both, and occasionally alternating with +the strata _b b_. They also are seen, in some instances, to pass insensibly +into the unstratified division of _a_, or the Plutonic rocks. + +[Illustration: Fig. 434. Cross section. + + _a._ Hypogene formations, stratified and unstratified. + _b._ Aqueous formations. + _c._ Volcanic rocks.] + +When geologists first began to examine attentively the structure of the +northern and western parts of Europe, they were almost entirely ignorant of +the phenomena of existing volcanos. They also found certain rocks, for the +most part without stratification, and of a peculiar mineral composition, to +which they gave different names, such as basalt, greenstone, porphyry, and +amygdaloid. All these, which were recognized as belonging to one family, +were called "trap" by Bergmann, from _trappa_, Swedish for a flight of +steps--a name since adopted very generally into the nomenclature of the +science; for it was observed that many rocks of this class occurred in +great tabular masses of unequal extent, so as to form a succession of +terraces or steps on the sides of hills. This configuration appears to be +derived from two causes. First, the abrupt original terminations of sheets +of melted matter, which have spread, whether on the land or bottom of the +sea, over a level surface. For we know, in the case of lava flowing from a +volcano, that a stream, when it has ceased to flow, and grown solid, very +commonly ends in a steep slope, as at _a_, fig. 435. But, secondly, the +step-like appearance arises more frequently from the mode in which +horizontal masses of igneous rock, such as _b c_, intercalated between +aqueous strata, have, subsequently to their origin, been exposed, at +different heights, by denudation. Such an outline, it is true, is not +peculiar to trap rocks; great beds of limestone, and other hard kinds of +stone, often presenting similar terraces and precipices: but these are +usually on a smaller scale, or less numerous, than the volcanic _steps_, or +form less decided features in the landscape, as being less distinct in +structure and composition from the associated rocks. + +[Illustration: Fig. 435. Step-like appearance of trap.] + +Although the characters of trap rocks are greatly diversified, the +beginner will easily learn to distinguish them as a class from the +aqueous formations. Sometimes they present themselves, as already +stated, in tabular masses, which are not divided into strata: sometimes +in shapeless lumps and irregular cones, forming chains of small hills. +Often they are seen in dikes and wall-like masses, intersecting +fossiliferous beds. The rock is occasionally found divided into columns, +often decomposing into balls of various sizes, from a few inches to +several feet in diameter. The decomposing surface very commonly assumes +a coating of a rusty iron colour, from the oxidation of ferruginous +matter, so abundant in the traps in which augite or hornblende occur; +or, in the felspathic varieties of trap, it acquires a white opaque +coating, from the bleaching of the mineral called felspar. On examining +any of these volcanic rocks, where they have not suffered +disintegration, we rarely fail to detect a crystalline arrangement in +one or more of the component minerals. Sometimes the texture of the mass +is cellular or porous, or we perceive that it has once been full of +pores and cells, which have afterwards become filled with carbonate of +lime, or other infiltrated mineral. + +Most of the volcanic rocks produce a fertile soil by their disintegration. +It seems that their component ingredients, silica, alumina, lime, potash, +iron, and the rest, are in proportions well fitted for vegetation. As they +do not effervesce with acids, a deficiency of calcareous matter might at +first be suspected; but although _the carbonate_ of lime is rare, except in +the nodules of amygdaloids, yet it will be seen that lime sometimes enters +largely into the composition of augite and hornblende. (See Table, p. 377.) + +_Cones and Craters._--In regions where the eruption of volcanic matter +has taken place in the open air, and where the surface has never since +been subjected to great aqueous denudation, cones and craters constitute +the most striking peculiarity of this class of formations. Many hundreds +of these cones are seen in central France, in the ancient provinces of +Auvergne, Velay, and Vivarais, where they observe, for the most part, a +linear arrangement, and form chains of hills. Although none of the +eruptions have happened within the historical era, the streams of lava +may still be traced distinctly descending from many of the craters, and +following the lowest levels of the existing valleys. The origin of the +cone and crater-shaped hill is well understood, the growth of many +having been watched during volcanic eruptions. A chasm or fissure first +opens in the earth, from which great volumes of steam and other gases +are evolved. The explosions are so violent as to hurl up into the air +fragments of broken stone, parts of which are shivered into minute +atoms. At the same time melted stone or _lava_ usually ascends through +the chimney or vent by which the gases make their escape. Although +extremely heavy, this lava is forced up by the expansive power of +entangled gaseous fluids, chiefly steam or aqueous vapour, exactly in +the same manner as water is made to boil over the edge of a vessel when +steam has been generated at the bottom by heat. Large quantities of the +lava are also shot up into the air, where it separates into fragments, +and acquires a spongy texture by the sudden enlargement of the included +gases, and thus forms _scoriæ_, other portions being reduced to an +impalpable powder or dust. The showering down of the various ejected +materials round the orifice of eruption gives rise to a conical mound, +in which the successive envelopes of sand and scoriæ form layers, +dipping on all sides from a central axis. In the mean time a hollow, +called a _crater_, has been kept open in the middle of the mound by the +continued passage upwards of steam and other gaseous fluids. The lava +sometimes flows over the edge of the crater, and thus thickens and +strengthens the sides of the cone; but sometimes it breaks it down on +one side, and often it flows out from a fissure at the base of the +hill (see fig. 436.).[368-A] + +[Illustration: Fig. 436. Part of the chain of extinct volcanos called the +Monts Dome, Auvergne. (Scrope.)] + +_Composition and nomenclature._--Before speaking of the connection between +the products of modern volcanos and the rocks usually styled trappean, and +before describing the external forms of both, and the manner and position +in which they occur in the earth's crust, it will be desirable to treat of +their mineral composition and names. The varieties most frequently spoken +of are basalt, greenstone, syenitic greenstone, clinkstone, claystone, and +trachyte; while those founded chiefly on peculiarities of texture, are +porphyry, amygdaloid, lava, tuff, scoriæ, and pumice. It may be stated +generally, that all these are mainly composed of two minerals, or families +of simple minerals, _felspar_ and _hornblende_; some almost entirely of +hornblende, others of felspar. + +These two minerals may be regarded as two groups, rather than species. +Felspar, for example, may be, first, common felspar, that is to say, +potash-felspar, in which the alkali is potash (see table, p. 377.); or, +secondly, albite, that is to say, soda-felspar, where the alkali is soda +instead of potash; or, thirdly, Labrador-felspar (Labradorite), which +differs not only in its iridescent hues, but also in its angle of +fracture or cleavage, and its composition. We also read much of two +other kinds, called glassy felspar and compact felspar, which, however, +cannot rank as varieties of equal importance, for both the albitic and +common felspar appear sometimes in transparent or _glassy_ crystals; and +as to compact felspar, it is a compound of a less definite nature, +sometimes containing both soda and potash; and which might be called a +felspathic paste, being the residuary matter after portions of the +original matrix have crystallized. + +The other group, or _hornblende_, consists principally of two varieties; +first, hornblende, and, secondly, augite, which were once regarded as very +distinct, although now some eminent mineralogists are in doubt whether they +are not one and the same mineral, differing only as one crystalline form of +native sulphur differs from another. + +The history of the changes of opinion on this point is curious and +instructive. Werner first distinguished augite from hornblende; and his +proposal to separate them obtained afterwards the sanction of Haüy, +Mohs, and other celebrated mineralogists. It was agreed that the form of +the crystals of the two species were different, and their structure, as +shown by _cleavage_, that is to say, by breaking or cleaving the mineral +with a chisel, or a blow of the hammer, in the direction in which it +yields most readily. It was also found by analysis that augite usually +contained more lime, less alumina, and no fluoric acid; which last, +though not always found in hornblende, often enters into its composition +in minute quantity. In addition to these characters, it was remarked as +a geological fact, that augite and hornblende are very rarely associated +together in the same rock; and that when this happened, as in some lavas +of modern date, the hornblende occurs in the mass of the rock, where +crystallization may have taken place more slowly, while the augite +merely lines cavities where the crystals may have been produced rapidly. +It was also remarked, that in the crystalline slags of furnaces, augitic +forms were frequent, the hornblendic entirely absent; hence it was +conjectured that hornblende might be the result of slow, and augite of +rapid cooling. This view was confirmed by the fact, that Mitscherlich +and Berthier were able to make augite artificially, but could never +succeed in forming hornblende. Lastly, Gustavus Rose fused a mass of +hornblende in a porcelain furnace, and found that it did not, on +cooling, assume its previous shape, but invariably took that of +augite. The same mineralogist observed certain crystals in rocks from +Siberia which presented a hornblende _cleavage_, while they had the +external form of augite. + +If, from these data, it is inferred that the same substance may assume +the crystalline forms of hornblende or augite indifferently, according +to the more or less rapid cooling of the melted mass, it is +nevertheless certain that the variety commonly called augite, and +recognized by a peculiar crystalline form, has usually more lime in it, +and less alumina, than that called hornblende, although the quantities +of these elements do not seem to be always the same. Unquestionably the +facts and experiments above mentioned show the very near affinity of +hornblende and augite; but even the convertibility of one into the other +by melting and recrystallizing, does not perhaps demonstrate their +absolute identity. For there is often some portion of the materials in +a crystal which are not in perfect chemical combination with the rest. +Carbonate of lime, for example, sometimes carries with it a considerable +quantity of silex into its own form of crystal, the silex being +mechanically mixed as sand, and yet not preventing the carbonate of +lime from assuming the form proper to it. This is an extreme case, +but in many others some one or more of the ingredients in a crystal +may be excluded from perfect chemical union; and, after fusion, when +the mass recrystallizes, the same elements may combine perfectly or +in new proportions, and thus a new mineral may be produced. Or some +one of the gaseous elements of the atmosphere, the oxygen for example, +may, when the melted matter reconsolidates, combine with some one of +the component elements. + +The different quantity of the impurities or refuse above alluded to, which +may occur in all but the most transparent and perfect crystals, may partly +explain the discordant results at which experienced chemists have arrived +in their analysis of the same mineral. For the reader will find that a +mineral determined to be the same by its physical characters, crystalline +form, and optical properties, has often been declared by skilful analyzers +to be composed of distinct elements. (See the table at p. 377.) This +disagreement seemed at first subversive of the atomic theory, or the +doctrine that there is a fixed and constant relation between the +crystalline form and structure of a mineral, and its chemical composition. +The apparent anomaly, however, which threatened to throw the whole science +of mineralogy into confusion, was in a great degree reconciled to fixed +principles by the discoveries of Professor Mitscherlich at Berlin, who +ascertained that the composition of the minerals which had appeared so +variable, was governed by a general law, to which he gave the name of +_isomorphism_ (from +isos+, _isos_, equal, and +morphê+, _morphe_, form). +According to this law, the ingredients of a given species of mineral are +not absolutely fixed as to their kind and quality; but one ingredient may +be replaced by an equivalent portion of some analogous ingredient. Thus, in +augite, the lime may be in part replaced by portions of protoxide of iron, +or of manganese, while the form of the crystal, and the angle of its +cleavage planes, remain the same. These vicarious substitutions, however, +of particular elements cannot exceed certain defined limits. + +Having been led into this digression on the recent progress of mineralogy, +I may here observe that the geological student must endeavour as soon as +possible to familiarize himself with the characters of five at least of +the most abundant simple minerals of which rocks are composed. These are, +felspar, quartz, mica, hornblende, and carbonate of lime. This knowledge +cannot be acquired from books, but requires personal inspection, and the +aid of a teacher. It is well to accustom the eye to know the appearance of +rocks under the lens. To learn to distinguish felspar from quartz is the +most important step to be first aimed at. In general we may know the +felspar because it can be scratched with the point of a knife, whereas the +quartz, from its extreme hardness, receives no impression. But when these +two minerals occur in a granular and uncrystallized state, the young +geologist must not be discouraged if, after considerable practice, he often +fails to distinguish them by the eye alone. If the felspar is in crystals, +it is easily recognized by its cleavage: but when in grains the blow-pipe +must be used, for the edges of the grains can be rounded in the flame, +whereas those of _quartz_ are infusible. If the geologist is desirous of +distinguishing the three varieties of felspar above enumerated, or +hornblende from augite, it will often be necessary to use the reflecting +goniometer as a test of the angle of cleavage, and shape of the crystal. +The use of this instrument will not be found difficult. + +The external characters and composition of the felspars are extremely +different from those of augite or hornblende; so that the volcanic rocks in +which either of these minerals decidedly predominates, are easily +recognized. But there are mixtures of the two elements in every possible +proportion, the mass being sometimes exclusively composed of felspar, at +other times solely of augite, or, again, of both in equal quantities. +Occasionally, the two extremes, and all the intermediate gradations, may be +detected in one continuous mass. Nevertheless there are certain varieties +or compounds which prevail so largely in nature, and preserve so much +uniformity of aspect and composition, that it is useful in geology to +regard them as distinct rocks, and to assign names to them, such as basalt, +greenstone, trachyte, and others, already mentioned. + +_Basalt._--As an example of rocks in which augite greatly prevails, basalt +may first be mentioned. Although we are more familiar with this term than +with that of any other kind of trap, it is difficult to define it, the name +having been used so vaguely. It has been very generally applied to any trap +rock of a black, bluish, or leaden-grey colour, having a uniform and +compact texture. Most strictly, it consists of an intimate mixture of +augite, felspar, and iron, to which a mineral of an olive green colour, +called olivine, is often superadded, in distinct grains or nodular masses. +The iron is usually magnetic, and is often accompanied by another metal, +titanium. Augite is the predominant mineral, the felspar being in much +smaller proportions. There is no doubt that many of the fine-grained and +dark-coloured trap rocks, called basalt, contained hornblende in the place +of augite; but this will be deemed of small importance after the remarks +above made. Other minerals are occasionally found in basalt; and this rock +may pass insensibly into almost every variety of trap, especially into +greenstone, clinkstone, and wacké, which will be presently described. + +_Greenstone_, or _Dolerite_, is usually defined as a granular rock, the +constituent parts of which are hornblende and imperfectly crystallized +felspar; the felspar being more abundant than in basalt; and the grains or +crystals of the two minerals more distinct from each other. This name may +also be extended to those rocks in which augite is substituted for +hornblende (the dolorite of some authors), or to those in which albite +replaces common felspar, forming the rock sometimes called Andesite. + +_Syenitic greenstone._--The highly crystalline compounds of the same two +minerals, felspar and hornblende, having a granitiform texture, and +with occasionally some quartz accompanying, may be called Syenitic +greenstone, a rock which frequently passes into ordinary trap, and +as frequently into granite. + +_Trachyte._--A porphyritic rock of a whitish or greyish colour, composed +principally of glassy felspar, with crystals of the same, generally with +some hornblende and some titaniferous iron. In composition it is extremely +different from basalt, this being a felspathic, as the other is an augitic, +rock. It has a peculiar rough feel, whence the name +trachys+, _trachus_, +rough. Some varieties of trachyte contain crystals of quartz. + +[Illustration: Fig. 437. Porphyry. + +White crystals of felspar in a dark base of hornblende and felspar.] + +_Porphyry_ is merely a certain form of rock, very characteristic of the +volcanic formations. When distinct crystals of one or more minerals are +scattered through an earthy or compact base, the rock is termed a porphyry +(see fig. 437.). Thus trachyte is porphyritic; for in it, as in many modern +lavas, there are crystals of felspar; but in some porphyries the crystals +are of augite, olivine, or other minerals. If the base be greenstone, +basalt, or pitchstone, the rock may be denominated greenstone-porphyry, +pitchstone-porphyry, and so forth. + +_Amygdaloid._--This is also another form of igneous rock, admitting of +every variety of composition. It comprehends any rock in which round or +almond-shaped nodules of some mineral, such as agate, calcedony, calcareous +spar, or zeolite, are scattered through a base of wacké, basalt, +greenstone, or other kind of trap. It derives its name from the Greek word +_amygdala_, an almond. The origin of this structure cannot be doubted, for +we may trace the process of its formation in modern lavas. Small pores or +cells are caused by bubbles of steam and gas confined in the melted matter. +After or during consolidation, these empty spaces are gradually filled up +by matter separating from the mass, or infiltered by water permeating the +rock. As these bubbles have been sometimes lengthened by the flow of the +lava before it finally cooled, the contents of such cavities have the form +of almonds. In some of the amygdaloidal traps of Scotland, where the +nodules have decomposed, the empty cells are seen to have a glazed or +vitreous coating, and in this respect exactly resemble scoriaceous lavas, +or the slags of furnaces. + +[Illustration: Fig. 438. Scoriaceous lava in part converted into +an amygdaloid. + +Montagne de la Veille, Department of Puy de Dome, France.] + +The annexed figure represents a fragment of stone taken from the upper part +of a sheet of basaltic lava in Auvergne. One half is scoriaceous, the pores +being perfectly empty; the other part is amygdaloidal, the pores or cells +being mostly filled up with carbonate of lime, forming white kernels. + +_Scoriæ_ and _Pumice_ may next be mentioned as porous rocks, produced by +the action of gases on materials melted by volcanic heat. _Scoriæ_ are +usually of a reddish-brown and black colour, and are the cinders and slags +of basaltic or augitic lavas. _Pumice_ is a light, spongy, fibrous +substance, produced by the action of gases on trachytic and other lavas; +the relation, however, of its origin to the composition of lava is not yet +well understood. Von Buch says that it never occurs where only +Labrador-felspar is present. + +_Lava._--This term has a somewhat vague signification, having been applied +to all melted matter observed to flow in streams from volcanic vents. When +this matter consolidates in the open air, the upper part is usually +scoriaceous, and the mass becomes more and more stony as we descend, or in +proportion as it has consolidated more slowly and under greater pressure. +At the bottom, however, of a stream of lava, a small portion of scoriaceous +rock very frequently occurs, formed by the first thin sheet of liquid +matter, which often precedes the main current, or in consequence of the +contact with water in or upon the damp soil. + +The more compact lavas are often porphyritic, but even the scoriaceous part +sometimes contains imperfect crystals, which have been derived from some +older rocks, in which the crystals pre-existed, but were not melted, as +being more infusible in their nature. + +Although melted matter rising in a crater, and even that which enters rents +on the side of a crater, is called lava, yet this term belongs more +properly to that which has flowed either in the open air or on the bed of a +lake or sea. If the same fluid has not reached the surface, but has been +merely injected into fissures below ground, it is called trap. + +There is every variety of composition in lavas; some are trachytic, as in +the Peak of Teneriffe; a great number are basaltic, as in Vesuvius and +Auvergne; others are andesitic, as those of Chili; some of the most modern +in Vesuvius consist of green augite, and many of those of Etna of augite +and Labrador-felspar.[374-A] + +_Trap tuff, volcanic tuff._--Small angular fragments of the scoriæ and +pumice, above mentioned, and the dust of the same, produced by volcanic +explosions, form the tuffs which abound in all regions of active +volcanos, where showers of these materials, together with small pieces +of other rocks ejected from the crater, fall down upon the land or into +the sea. Here they often become mingled with shells, and are stratified. +Such tuffs are sometimes bound together by a calcareous cement, and form +a stone susceptible of a beautiful polish. But even when little or no +lime is present, there is a great tendency in the materials of ordinary +tuffs to cohere together. + +Besides the peculiarity of their composition, some tuffs, or _volcanic +grits_, as they have been termed, differ from ordinary sandstones by +the angularity of their grains. When the fragments are coarse, the +rock is styled a volcanic _breccia_. _Tufaceous conglomerates_ result +from the intermixture of rolled fragments or pebbles of volcanic and +other rocks with tuff. + +According to Mr. Scrope, the Italian geologists confine the term +_tuff_, or tufa, to felspathose mixtures, and those composed +principally of pumice, using the term _peperino_ for the basaltic +tuffs.[374-B] The peperinos thus distinguished are usually brown, +and the tuffs grey or white. + +We meet occasionally with extremely compact beds of volcanic materials, +interstratified with fossiliferous rocks. These may sometimes be tuffs, +although their density or compactness is such as to cause them to resemble +many of those kinds of trap which are found in ordinary dikes. The +chocolate-coloured mud, which was poured for weeks out of the crater of +Graham's Island, in the Mediterranean, in 1831, must, when unmixed with +other materials, have constituted a stone heavier than granite. Each cubic +inch of the impalpable powder which has fallen for days through the +atmosphere, during some modern eruptions, has been found to weigh, without +being compressed, as much as ordinary trap rocks, and to be often identical +with these in mineral composition. + +The fusibility of the igneous rocks generally exceeds that of other rocks, +for there is much alkaline matter and lime in their composition, which +serves as a flux to the large quantity of silica, which would be otherwise +so refractory an ingredient. + +It is remarkable that, notwithstanding the abundance of this silica, +quartz, that is, crystalline silica, is usually wanting in the volcanic +rocks, or is present only as an occasional mineral, like mica. The elements +of mica, as of quartz, occur in lava and trap; but the circumstances under +which these rocks are formed are evidently unfavourable to the development +of mica and quartz, minerals so characteristic of the hypogene formations. + +It would be tedious to enumerate all the varieties of trap and lava which +have been regarded by different observers as sufficiently abundant to +deserve distinct names, especially as each investigator is too apt to +exaggerate the importance of local varieties which happen to prevail in +districts best known to him. It will be useful, however, to subjoin here, +in the form of a glossary, an alphabetical list of the names and synonyms +most commonly in use, with brief explanations, to which I have added a +table of the analysis of the simple minerals most abundant in the volcanic +and hypogene rocks. + + +_Explanation of the names, synonyms, and mineral composition of the more +abundant volcanic rocks._ + +AMPHIBOLITE. _See_ Hornblende rock, amphibole being Haüy's name +for hornblende. + +AMYGDALOID. A particular form of volcanic rock; _see_ p. 372. + +AUGITE ROCK. A kind of basalt or greenstone, composed wholly or principally +of granular augite. (_Leonhard's Mineralreich_, 2d edition, p. 85.) + +AUGITIC-PORPHYRY. Crystals of Labrador-felspar and of augite, in a green or +dark grey base. (_Rose_, _Ann. des Mines_, tom. 8. p. 22. 1835.) + +BASALT. Chiefly augite--an intimate mixture of augite and felspar with +magnetic iron, olivine, &c. _See_ p. 371. The yellowish green mineral +called olivine, can easily be distinguished from yellowish felspar by +its infusibility, and having no cleavage. The edges turn brown in the +flame of the blow-pipe. + +BASANITE. Name given by Alex. Brongniart to a rock, having a base of +basalt, with more or less distinct crystals of augite disseminated +through it. + +CLAYSTONE and CLAYSTONE-PORPHYRY. An earthy and compact stone, usually of a +purplish colour, like an indurated clay; passes into hornstone; generally +contains scattered crystals of felspar and sometimes of quartz. + +CLINKSTONE. _Syn._ Phonolite, fissile Petrosilex; a greenish or greyish +rock, having a tendency to divide into slabs and columns; hard, with clean +fracture, ringing under the hammer; principally composed of compact +felspar, and, according to Gmelin, of felspar and mesotype. (_Leonhard_, +_Mineralreich_, p. 102.) A rock much resembling clinkstone, and called by +some Petrosilex, contains a considerable percentage of quartz and felspar. +As both trachyte and basalt pass into clinkstone, the rock so called must +be very various in composition. + +COMPACT FELSPAR, which has also been called Petrosilex; the rock so called +includes the hornstone of some mineralogists, is allied to clinkstone, but +is harder, more compact, and translucent. It is a varying rock, of which +the chemical composition is not well defined, and is perhaps the same as +that of clay. (_MacCulloch's Classification of Rocks_, p. 481.) Dr. +MacCulloch says, that it contains both potash and soda. + +CORNEAN. A variety of claystone allied to hornstone. A fine homogeneous +paste, supposed to consist of an aggregate of felspar, quartz, and +hornblende, with occasionally epidote, and perhaps chlorite; it passes into +compact felspar and hornstone. (_De la Beche_, _Geol. Trans._ second +series, vol. 2. p. 3.) + +DIALLAGE ROCK. _Syn_. Euphotide, Gabbro, and some Ophiolites. Compounded of +felspar and diallage, sometimes with the addition of serpentine, or mica, +or quartz. (_MacCulloch. ibid_. p. 648.) + +DIORITE. A kind of greenstone, which see. Components, felspar and +hornblende in grains. According to _Rose_, _Ann. des Mines_, tom. 8. p. 4., +_diorite_ consists of albite and hornblende. + +DIORITIC-PORPHYRY. A porphyritic greenstone, composed of crystals of albite +and hornblende, in a greenish or blackish base. (_Rose_, _ibid._ p. 10.) + +DOLERITE. Formerly defined as a synonym of greenstone, which see. But, +according to Rose (_ibid._ p. 32.), its composition is black augite and +Labrador-felspar; according to Leonhard (_Mineralreich_, &c. p. 77.), +augite, Labrador-felspar, and magnetic iron. + +DOMITE. An earthy _trachyte_, found in the Puy de Dome, in Auvergne. + +EUPHOTIDE. A mixture of grains of Labrador-felspar and diallage. (_Rose_, +_ibid._ p. 19.) According to some, this rock is defined to be a mixture of +augite or hornblende, and saussurite, a mineral allied to jade. (_Allan's +Mineralogy_, p. 158.) _See_ Diallage rock. + +FELSPAR-PORPHYRY. _Syn._ Hornstone-porphyry; a base of felspar, +with crystals of felspar, and crystals and grains of quartz. _See_ +also Hornstone. + +GABBRO, _see_ Diallage rock. + +GREENSTONE. _Syn._ Dolerite and diorite; components, hornblende and +felspar, or augite and felspar in grains. See above, p. 372. + +GREYSTONE. (Graustein of Werner.) Lead grey and greenish rock, composed of +felspar and augite, the felspar being more than seventy-five per cent. +(_Scrope_, _Journ. of Sci._ No. 42. p. 221.) Greystone lavas are +intermediate in composition between basaltic and trachytic lavas. + +HORNBLENDE ROCK. A greenstone, composed principally of granular hornblende, +or augite. (_Leonhard_, _Mineralreich_, &c., p. 85.) + +HORNSTONE, HORNSTONE-PORPHYRY. A kind of felspar porphyry (_Leonhard_, +_ibid._), with a base of hornstone, a mineral approaching near to flint, +differing from compact felspar in being infusible. + +HYPERSTHENE ROCK, a mixture of grains of Labrador-felspar and hypersthene +(_Rose_, _Ann. des Mines_, tom. 8. p. 13.), having the structure of syenite +or granite; abundant among the traps of Skye. Some geologists consider it a +greenstone, in which hypersthene replaces hornblende. + +LATERITE. A red jaspery rock, composed of silicate of alumina and oxide of +iron. Abundant in the Deccan, in India; and referred to the trap formation; +from Later, a brick or tile. + +MELAPHYRE. A variety of black porphyry, the base being black augite with +crystals of felspar; from +melas+, _melas_, black. + +OBSIDIAN. Vitreous lava like melted glass, nearly allied to pitchstone. + +OPHIOLITE, sometimes same as Diallage rocks (_Leonhard_, p. 77.); sometimes +a kind of serpentine. + +OPHITE. A green porphyritic rock composed chiefly of hornblende, with +crystals of that mineral in a base of the same, mixed with some felspar. +It passes into serpentine by a mixture of talc. (_Burat's d'Aubuisson_, +tom. ii. p. 63.) + +PEARLSTONE. A volcanic rock, having the lustre of mother of pearl; +usually having a nodular structure; intimately related to obsidian, +but less glassy. + +PEPERINO. A form of volcanic tuff, composed of basaltic scoriæ. +_See_ p. 374. + +PETROSILEX. _See_ Clinkstone and Compact Felspar. + +PHONOLITE. _Syn._ of Clinkstone, which see. + +PITCHSTONE. Vitreous lava, less glassy than obsidian; a blackish green rock +resembling glass, having a resinous lustre and appearance of pitch; +composition various, usually felspar and augite; passes into basalt; occurs +in veins, and in Arran forms a dike thirty feet wide, cutting through +sandstone; forms the outer walls of some basaltic dikes. + +PORPHYRY. Any rock in which detached crystals of felspar, or of one or more +minerals, are diffused through a base. _See_ p. 372. + +POZZOLANA. A kind of tuff. _See_ p. 36. + +PUMICE. A light, spongy, fibrous form of trachyte. _See_ p. 373. + +PYROXENIC-PORPHYRY, same as augitic-porphyry, pyroxene being Haüy's +name for augite. + +SCORIÆ. _Syn._ volcanic cinders; reddish brown or black porous form of +lava. _See_ p. 373. + +SERPENTINE. A greenish rock, in which there is much magnesia; usually +contains diallage, which is nearly allied to the simple mineral called +serpentine. Occurs sometimes, though rarely, in dikes, altering the +contiguous strata; is indifferently a member of the trappean or +hypogene series. + +SYENITIC-GREENSTONE; composition, crystals or grains of felspar and +hornblende. _See_ p. 372. + +TEPHRINE, synonymous with lava. Name proposed by Alex. Brongniart. + +TOADSTONE. A local name in Derbyshire for a kind of wacké, which see. + +TRACHYTE. Chiefly composed of glassy felspar, with crystals of glassy +felspar. _See_ p. 372. + +TRAP TUFF. _See_ p. 374. + +TRASS. A kind of tuff or mud poured out by lake craters during eruptions; +common in the Eifel, in Germany. + +TUFACEOUS CONGLOMERATE. _See_ p. 374. + +TUFF. _Syn._ Trap-tuff, volcanic tuff. _See_ p. 374. + +VITREOUS LAVA. _See_ Pitchstone and Obsidian. + +VOLCANIC TUFF. _See_ p. 374. + +WACKÉ. A soft and earthy variety of trap, having an argillaceous aspect. It +resembles indurated clay, and when scratched exhibits a shining streak. + +WHINSTONE. A Scotch provincial term for greenstone and other hard +trap rocks. + + +ANALYSIS OF MINERALS MOST ABUNDANT IN THE VOLCANIC AND HYPOGENE ROCKS. + + Silica. Alu- Mag- Lime. Pot- Soda. Iron. Manga- Remain- + mina. nesia. ash. Oxide. nese. der. + + Actinolite 64· -- 22· -- -- -- 3· a 43·05 C. + (Bergman) trace + + Albite (Rose) 68·84 20·53 -- a trace -- 9·12 -- -- -- + --(mean of 4 69·45 19·44 0·13 0·22 -- 9·95 a -- -- + analyses) trace + + Augite (Rose) 53·36 -- 4·99 22·19 -- -- 17·38 0·09 -- + --(mean of 4 53·57 1· 11·26 20·9 -- -- 10·75 0·67 -- + analyses) + + Carbonate of -- -- -- 56·33 -- -- -- -- -- + Lime (Biot) + + Chiastolite 68·49 30·17 4·12 -- -- -- 2·7 -- 0·27 W. + (Landgrabe) + + Chlorite 26· 18·5 8· -- -- 2· 43· -- -- + (Vauquelin) + --(mean of 3 27·43 17·9 14·56 0·50 1·56 -- 30·63 -- 6·92 W. + analyses) + + Diallage 60· -- 27·5 -- -- -- 10·5 -- -- + (Klaproth) + --(mean of 3 43·33 2·2 26·41 5·58 -- -- 11·53 -- 8·54 W. + analyses) + + Epidote 37· 21· -- 15· -- -- 24· 1·5 -- + (Vauquelin) + + Felspar, 62·83 17·02 -- 3· - 13· -- 1· -- -- + common (Vauq.) + --(Rose) 66·75 17·5 -- 1·25 12· -- 0·75 -- -- + --(mean of 7 64·04 18·94 -- 0·76 13·66 -- 0·74 -- -- + analyses) + + Garnet 35·75 27·25 -- -- -- -- 36· 0·25 -- + (Klaproth) + --(Phillips) 43· 16· -- 20· -- -- 16· -- -- + + Hornblende 42· 12· 2·25 11· a -- 30· 0·25 -- + (Klap.) trace + --(Bonsdorff.) 45·69 12·18 18·79 13·85 -- -- 7·32 0·22 1·5 F. + + Hypersthene 54·25 2·25 14· 1·5 -- -- 24·5 a 1· W. + (Klaproth) trace + + Labrador- 55·75 26·5 -- 11· -- 4· 1·25 -- 0·5 W. + felspar (Klap.) + + Leucite 53·75 24·62 -- -- 21·35 -- -- -- -- + (Klap.) + + Mesotype 54·64 19·70 -- 1·61 -- 15·09 -- -- 9·83 W. + (Gehlen) + + Mica 42·5 11·5 9· -- 10· -- 22· 2· -- + (Klaproth) + --(Vauquelin) 50· 35· -- 1·33 -- -- 7· -- -- + --(mean of 3 45·83 22·58 -- -- 11·08 -- 14· 1·45 -- + analyses) + + Olivine 50· -- 38·5 -- -- -- 12· -- -- + (Klaproth) + + Schorl or 35·48 34·75 4·68 -- 0·48 1·75 17·44 1·89 4·02 B. + Tourmaline (Gmelin) + --(mean of 6 36·03 35·82 4·44 0·28 0·71 1·96 13·71 1·62 -- + analyses) + + Serpentine 43·07 0·25 40·37 0·5 -- -- 1·17 -- 12·45 W. + (Hisinger) + --(mean of 5 37·29 4·97 36·8 2·89 -- -- 3·14 -- 12·77 W. + analyses) + + Steatite 64· -- 22· -- -- -- 3· -- 5· W. + (Vauquelin) + --(mean of 3 48·3 6·18 26·65 -- -- -- 2· -- 9·5 W. + anal. by Klap.) + + Talc. 61·75 -- 30·5 -- 2·75 -- 2·5 -- -- + (Klaproth) + +In the last column of the above Table, the letters B. C. F. W. represent +Boracic acid, Carbonic acid, Fluoric acid, and Water. + + +FOOTNOTES: + +[368-A] For a description and theory of active volcanos, see Principles of +Geology, chaps. xxiv. to xxvii. + +[374-A] G. Rose, Ann. des Mines, tom. viii. p. 32. + +[374-B] Geol. Trans. vol. ii. p. 211. 2d series. + + + + +CHAPTER XXIX. + +VOLCANIC ROCKS--_continued_. + + Trap dikes--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 corresponds + generally to ancient trap--Structure and physical features of Palma + and some other extinct volcanos. + + +Having in the last chapter spoken of the composition and mineral characters +of volcanic rocks, I shall next describe the manner and position in which +they occur in the earth's crust, and their external forms. Now the leading +varieties, such as basalt, greenstone, trachyte, porphyry, and the rest, +are found sometimes in dikes penetrating stratified and unstratified +formations, sometimes in shapeless masses protruding through or overlying +them, or in horizontal sheets intercalated between strata. + +_Volcanic dikes._--Fissures have already been spoken of as occurring in +all kinds of rocks, some a few feet, others many yards in width, and +often filled up with earth or angular pieces of stone, or with sand and +pebbles. Instead of such materials, suppose a quantity of melted stone +to be driven or injected into an open rent, and there consolidated, we +have then a tabular mass resembling a wall, and called a trap dike. It +is not uncommon to find such dikes passing through strata of soft +materials, such as tuff or shale, which, being more perishable than the +trap, are often washed away by the sea, rivers, or rain, in which case +the dike stands prominently out in the face of precipices, or on the +level surface of a country. (See the annexed figure.[378-A]) + +[Illustration: Fig. 439. Dike in inland valley, near the Brazen +Head, Madeira.] + +In the islands of Arran, Skye, and other parts of Scotland, where +sandstone, conglomerate, and other hard rocks are traversed by dikes of +trap, the converse of the above phenomenon is seen. The dike having +decomposed more rapidly than the containing rock, has once more left open +the original fissure, often for a distance of many yards inland from the +sea-coast, as represented in the annexed view (fig. 440.). In these +instances, the greenstone of the dike is usually more tough and hard than +the sandstone; but chemical action, and chiefly the oxidation of the iron, +has given rise to the more rapid decay. + +[Illustration: Fig. 440. Fissures left vacant by decomposed trap. +Strathaird, Skye. (MacCulloch.)] + +There is yet another case, by no means uncommon in Arran and other parts of +Scotland, where the strata in contact with the dike, and for a certain +distance from it, have been hardened, so as to resist the action of the +weather more than the dike itself, or the surrounding rocks. When this +happens, two parallel walls of indurated strata are seen protruding above +the general level of the country, and following the course of the dike. + +[Illustration: Fig. 441. Trap veins in Airdnamurchan.] + +As fissures sometimes send off branches, or divide into two or more +fissures of equal size, so also we find trap dikes bifurcating and +ramifying, and sometimes they are so tortuous as to be called veins, though +this is more common in granite than in trap. The accompanying sketch (fig. +441.) by Dr. MacCulloch represents part of a sea-cliff in Argyleshire, +where an overlying mass of trap, _b_, sends out some veins which terminate +downwards. Another trap vein, _a a_, cuts through both the limestone, _c_, +and the trap, _b_. + +In fig. 442., a ground plan is given of a ramifying dike of greenstone, +which I observed cutting through sandstone on the beach near Kildonan +Castle, in Arran. The larger branch varies from 5 to 7 feet in width, which +will afford a scale of measurement for the whole. + +[Illustration: Fig. 442. Ground plan of greenstone dike traversing +sandstone. Arran.] + +In the Hebrides and other countries, the same masses of trap which +occupy the surface of the country far and wide, concealing the subjacent +stratified rocks, are seen also in the sea cliffs, prolonged downwards +in veins or dikes, which probably unite with other masses of igneous +rock at a greater depth. The largest of the dikes represented in the +annexed diagram, and which are seen in part of the coast of Skye, is no +less than 100 feet in width. + +[Illustration: Fig. 443. Trap dividing and covering sandstone near +Suishnish in Skye. (MacCulloch.)] + +Every variety of trap-rock is sometimes found in these dikes, as basalt, +greenstone, felspar-porphyry, and more rarely trachyte. The amygdaloidal +traps also occur, and even tuff and breccia, for the materials of these +last may be washed down into open fissures at the bottom of the sea, or +during eruptions on the land may be showered into them from the air. + +Some dikes of trap may be followed for leagues uninterruptedly in nearly a +straight direction, as in the north of England, showing that the fissures +which they fill must have been of extraordinary length. + +_Dikes more crystalline in the centre._--In many cases trap at the edges or +sides of a dike is less crystalline or more earthy than in the centre, in +consequence of the melted matter having cooled more rapidly by coming in +contact with the cold sides of the fissure; whereas, in the centre, the +matter of the dike being kept long in a fluid or soft state, the crystals +are slowly formed. In the ancient part of Vesuvius, called Somma, a thin +band of half-vitreous lava is found at the edge of some dikes. At the +junction of greenstone dikes with limestone, a _sahlband_, or selvage, of +serpentine is occasionally observed. + +[Illustration: Fig. 444. Syenitic greenstone dike of Næsodden, Christiania. + +_b._ imbedded fragment of crystalline schist surrounded by a band +of greenstone.] + +On the left shore of the fiord of Christiania, in Norway, I examined, in +company with Professor Keilhau, a remarkable dike of syenitic greenstone, +which is traced through Silurian strata, until at length, in the promontory +of Næsodden, it enters mica-schist. Fig. 444. represents a ground plan, +where the dike appears 8 paces in width. In the middle it is highly +crystalline and granitiform, of a purplish colour, and containing a few +crystals of mica, and strongly contrasted with the whitish mica-schist, +between which and the syenitic rock there is usually on each side a +distinct black band, 18 inches wide, of dark greenstone. When first seen, +these bands have the appearance of two accompanying dikes; yet they are, in +fact, only the different form which the syenitic materials have assumed +where near to or in contact with the mica-schist. At one point, _a_, one +of the sahlbands terminates for a space; but near this there is a large +detached block, _b_, having a gneiss-like structure, consisting of +hornblende and felspar, which is included in the midst of the dike. Round +this a smaller encircling zone is seen, of dark basalt, or fine-grained +greenstone, nearly corresponding to the larger ones which border the dike, +but only 1 inch wide. + +It seems, therefore, evident that the fragment, _b_, has acted on the +matter of the dike, probably by causing it to cool more rapidly, in the +same manner as the walls of the fissure have acted on a larger scale. The +facts, also, illustrate the facility with which a granitiform syenite may +pass into ordinary rocks of the volcanic family. + +[Illustration: Fig. 445. Greenstone dike, with fragments of gneiss. +Sorgenfri, Christiania.] + +The fact above alluded to, of a foreign fragment, such as _b_, fig. +444., included in the midst of the trap, as if torn off from some +subjacent rock or the walls of a fissure, is by no means uncommon. A +fine example is seen in another dike of greenstone, 10 feet wide, in the +northern suburbs of Christiania, in Norway, of which the annexed figure +is a ground plan. The dike passes through shale, known by its fossils to +belong to the Silurian series. In the black base of greenstone are +angular and roundish pieces of gneiss, some white, others of a light +flesh-colour, some without lamination, like granite, others with laminæ, +which, by their various and often opposite directions, show that they +have been scattered at random through the matrix. These imbedded pieces +of gneiss measure from 1 to about 8 inches in diameter. + +_Rocks altered by volcanic dikes._--After these remarks on the form and +composition of dikes themselves, I shall describe the alterations which +they sometimes produce in the rocks in contact with them. The changes are +usually such as the intense heat of melted matter and the entangled gases +might be expected to cause. + +_Plas-Newydd._--A striking example, near Plas-Newydd, in Anglesea, has +been described by Professor Henslow.[381-A] The dike is 134 feet wide, +and consists of a rock which is a compound of felspar and augite +(dolerite of some authors). Strata of shale and argillaceous limestone, +through which it cuts perpendicularly, are altered to a distance of 30, +or even, in some places, to 35 feet from the edge of the dike. The +shale, as it approaches the trap, becomes gradually more compact, and is +most indurated where nearest the junction. Here it loses part of its +schistose structure, but the separation into parallel layers is still +discernible. In several places the shale is converted into hard +porcellanous jasper. In the most hardened part of the mass the fossil +shells, principally _Producti_, are nearly obliterated; yet even here +their impressions may frequently be traced. The argillaceous limestone +undergoes analogous mutations, losing its earthy texture as it +approaches the dike, and becoming granular and crystalline. But the most +extraordinary phenomenon is the appearance in the shale of numerous +crystals of analcime and garnet, which are distinctly confined to those +portions of the rock affected by the dike.[382-A] Some garnets contain +as much as 20 per cent. of lime, which they may have derived from the +decomposition of the fossil shells or Producti. The same mineral has +been observed, under very analogous circumstances, in High Teesdale, +by Professor Sedgwick, where it also occurs in shale and limestone, +altered by basalt.[382-B] + +_Antrim._--In several parts of the county of Antrim, in the north of +Ireland, chalk with flints is traversed by basaltic dikes. The chalk is +there converted into granular marble near the basalt, the change sometimes +extending 8 or 10 feet from the wall of the dike, being greatest near the +point of contact, and thence gradually decreasing till it becomes +evanescent. "The extreme effect," says Dr. Berger, "presents a dark brown +crystalline limestone, the crystals running in flakes as large as those of +coarse primitive (_metamorphic_) limestone; the next state is saccharine, +then fine grained and arenaceous; a compact variety, having a porcellanous +aspect and a bluish-grey colour, succeeds: this, towards the outer edge, +becomes yellowish-white, and insensibly graduates into the unaltered chalk. +The flints in the altered chalk usually assume a grey yellowish +colour."[382-C] All traces of organic remains are effaced in that part of +the limestone which is most crystalline. + +[Illustration: Fig. 446. Basaltic dikes in chalk in island of +Rathlin, Antrim. Ground plan, as seen on the beach. (Conybeare +and Buckland.[382-D])] + +The annexed drawing (fig. 446.) represents three basaltic dikes +traversing the chalk, all within the distance of 90 feet. The chalk +contiguous to the two outer dikes is converted into a finely granular +marble, _m m_, as are the whole of the masses between the outer dikes +and the central one. The entire contrast in the composition and colour +of the intrusive and invaded rocks, in these cases, renders the +phenomena peculiarly clear and interesting. + +Another of the dikes of the north-east of Ireland has converted a mass +of red sandstone into hornstone.[382-E] By another, the slate clay of +the coal measures has been indurated, and has assumed the character of +flinty slate[383-A]; and in another place the slate clay of the lias +has been changed into flinty slate, which still retains numerous +impressions of ammonites.[383-B] + +It might have been anticipated that beds of coal would, from their +combustible nature, be effected in an extraordinary degree by the +contact of melted rock. Accordingly, one of the greenstone dikes of +Antrim, on passing through a bed of coal, reduces it to a cinder for the +space of 9 feet on each side.[383-C] + +At Cockfield Fell, in the north of England, a similar change is observed. +Specimens taken at the distance of about 30 yards from the trap are not +distinguishable from ordinary pit coal; those nearer the dike are like +cinders, and have all the character of coke; while those close to it are +converted into a substance resembling soot.[383-D] + +As examples might be multiplied without end, I shall merely select one +or two others, and then conclude. The rock of Stirling Castle is a +calcareous sandstone, fractured and forcibly displaced by a mass of +greenstone which has evidently invaded the strata in a melted state. The +sandstone has been indurated, and has assumed a texture approaching to +hornstone near the junction. In Arthur's Seat and Salisbury Craig, near +Edinburgh, a sandstone which comes in contact with greenstone is +converted into a jaspideous rock.[383-E] + +The secondary sandstones in Skye are converted into solid quartz in +several places, where they come in contact with veins or masses of trap; +and a bed of quartz, says Dr. MacCulloch, found near a mass of trap, +among the coal strata of Fife, was in all probability a stratum of +ordinary sandstone, having been subsequently indurated and turned into +quartzite by the action of heat.[383-F] + +But although strata in the neighbourhood of dikes are thus altered in a +variety of cases, shale being turned into flinty slate or jasper, limestone +into crystalline marble, sandstone into quartz, coal into coke, and the +fossil remains of all such strata wholly and in part obliterated, it is by +no means uncommon to meet with the same rocks, even in the same districts, +absolutely unchanged in the proximity of volcanic dikes. + +This great inequality in the effects of the igneous rocks may often arise +from an original difference in their temperature, and in that of the +entangled gases, such as is ascertained to prevail in different lavas, or +in the same lava near its source and at a distance from it. The power also +of the invaded rocks to conduct heat may vary, according to their +composition, structure, and the fractures which they may have experienced, +and perhaps, also, according to the quantity of water (so capable of being +heated) which they contain. It must happen in some cases that the component +materials are mixed in such proportions as prepare them readily to enter +into chemical union, and form new minerals; while in other cases the mass +may be more homogeneous, or the proportions less adapted for such union. + +We must also take into consideration, that one fissure may be simply filled +with lava, which may begin to cool from the first; whereas in other cases +the fissure may give passage to a current of melted matter, which may +ascend for days or months, feeding streams which are overflowing the +country above, or are ejected in the shape of scoriæ from some crater. If +the walls of a rent, moreover, are heated by hot vapour before the lava +rises, as we know may happen on the flanks of a volcano, the additional +caloric supplied by the dike and its gases will act more powerfully. + +[Illustration: Fig. 447. Trap interposed between displaced beds +of limestone and shale, at White Force, High Teesdale, Durham. +(Sedgwick.[384-A])] + +_Intrusion of trap between strata._--In proof of the mechanical force +which the fluid trap has sometimes exerted on the rocks into which it +has intruded itself, I may refer to the Whin-Sill, where a mass of +basalt, from 60 to 80 feet in height, represented by _a_, fig. 447., is +in part wedged in between the rocks of limestone, _b_, and shale, _c_, +which have been separated from the great mass of limestone and shale, +_d_, with which they were united. + +The shale in this place is indurated; and the limestone, which at a +distance from the trap is blue, and contains fossil corals, is here +converted into granular marble without fossils. + +Masses of trap are not unfrequently met with intercalated between strata, +and maintaining their parallelism to the planes of stratification +throughout large areas. They must in some places have forced their way +laterally between the divisions of the strata, a direction in which there +would be the least resistance to an advancing fluid, if no vertical rents +communicated with the surface, and a powerful hydrostatic pressure was +caused by gases propelling the lava upwards. + +_Columnar and globular structure._--One of the characteristic forms of +volcanic rocks, especially of basalt, is the columnar, where large masses +are divided into regular prisms, sometimes easily separable, but in other +cases adhering firmly together. The columns vary in the number of angles, +from three to twelve; but they have most commonly from five to seven sides. +They are often divided transversely, at nearly equal distances, like the +joints in a vertebral column, as in the Giant's Causeway, in Ireland. They +vary exceedingly in respect to length and diameter. Dr. MacCulloch +mentions some in Skye which are about 400 feet long; others, in Morven, not +exceeding an inch. In regard to diameter, those of Ailsa measure 9 feet, +and those of Morven an inch or less.[385-A] They are usually straight, but +sometimes curved; and examples of both these occur in the island of Staffa. +In a horizontal bed or sheet of trap the columns are vertical; in a +vertical dike they are horizontal. Among other examples of the +last-mentioned phenomenon is the mass of basalt, called the Chimney, in St. +Helena (see fig. 448.), a pile of hexagonal prisms, 64 feet high, evidently +the remainder of a narrow dike, the walls of rock which the dike originally +traversed having been removed down to the level of the sea. In fig. 449. a +small portion of this dike is represented on a less reduced scale.[385-B] + +[Illustration: Fig. 448. Volcanic dike composed of horizontal prisms. +St. Helena.] + +[Illustration: Fig. 449. Small portion of the dyke in Fig. 448.] + +[Illustration: Fig. 450. Lava of La Coupe d'Ayzac, near Antraigue, in the +province of Ardèche.] + +It being assumed that columnar trap has consolidated from a fluid state, +the prisms are said to be always at right angles to the _cooling surfaces_. +If these surfaces, therefore, instead of being either perpendicular, or +horizontal, are curved, the columns ought to be inclined at every angle to +the horizon; and there is a beautiful exemplification of this phenomenon in +one of the valleys of the Vivarais, a mountainous district in the South of +France, where, in the midst of a region of gneiss, a geologist encounters +unexpectedly several volcanic cones of loose sand and scoriæ. From the +crater of one of these cones called La Coupe d'Ayzac, a stream of lava +descends and occupies the bottom of a narrow valley, except at those points +where the river Volant, or the torrents which join it, have cut away +portions of the solid lava. The accompanying sketch (fig. 450.) represents +the remnant of the lava at one of the points where a lateral torrent joins +the main valley of the Volant. It is clear that the lava once filled the +whole valley up to the dotted line _d a_; but the river has gradually swept +away all below that line, while the tributary torrent has laid open a +transverse section; by which we perceive, in the first place, that the lava +is composed, as usual in this country, of three parts: the uppermost, at +_a_, being scoriaceous; the second, _b_, presenting irregular prisms; and +the third, _c_, with regular columns, which are vertical on the banks of +the Volant, where they rest on a horizontal base of gneiss, but which are +inclined at an angle of 45° at _g_, and then horizontal at _f_, their +position having been every where determined, according to the law before +mentioned, by the concave form of the original valley. + +[Illustration: Fig 451. Columnar basalt in the Vicentin. (Fortis.)] + +In the annexed figure (451.) a view is given of some of the inclined and +curved columns which present themselves on the sides of the valleys in the +hilly region north of Vicenza, in Italy, and at the foot of the higher +Alps.[386-A] Unlike those of the Vivarais, last mentioned, the basalt of +this country was evidently submarine, and the present valleys have since +been hollowed out by denudation. + +The columnar structure is by no means peculiar to the trap rocks in which +hornblende or augite predominate; it is also observed in clinkstone, +trachyte, and other felspathic rocks of the igneous class, although in +these it is rarely exhibited in such regular polygonal forms. + +[Illustration: Fig. 452. Basaltic pillars of the Käsegrotte, +Bertrich-Baden, half way between Treves and Coblentz. Height of +grotto, from 7 to 8 feet.] + +It has been already stated that basaltic columns are often divided by +cross joints. Sometimes each segment, instead of an angular, assumes a +spheroidal form, so that a pillar is made up of a pile of balls, usually +flattened, as in the Cheese-grotto at Bertrich-Baden, in the Eifel, near +the Moselle (fig. 452.). The basalt, there, is part of a small stream of +lava, from 30 to 40 feet thick, which has proceeded from one of several +volcanic craters, still extant, on the neighbouring heights. The position +of the lava bordering the river in this valley might be represented by a +section like that already given at fig. 450. p. 385., if we merely supposed +inclined strata of slate and the argillaceous sandstone called greywacké to +be substituted for gneiss. + +In some masses of decomposing greenstone, basalt, and other trap rocks, the +globular structure is so conspicuous that the rock has the appearance of a +heap of large cannon balls. + +[Illustration: Fig. 453. Globiform pitchstone. Chiaja di Luna, Isle +of Ponza. (Scrope.)] + +A striking example of this structure occurs in a resinous trachyte or +pitchstone-porphyry in one of the Ponza islands, which rise from the +Mediterranean, off the coast of Terracina and Gaeta. The globes vary from a +few inches to three feet in diameter, and are of an ellipsoidal form (see +fig. 453.). The whole rock is in a state of decomposition, "and when the +balls," says Mr. Scrope, "have been exposed a short time to the weather, +they scale off at a touch into numerous concentric coats, like those of a +bulbous root, inclosing a compact nucleus. The laminæ of this nucleus have +not been so much loosened by decomposition; but the application of a ruder +blow will produce a still further exfoliation."[387-A] + +A fissile texture is occasionally assumed by clinkstone and other trap +rocks, so that they have been used for roofing houses. Sometimes the +prismatic and slaty structure is found in the same mass. The causes which +give rise to such arrangements are very obscure, but are supposed to be +connected with changes of temperature during the cooling of the mass, as +will be pointed out in the sequel. (See Chaps. XXXV. and XXXVI.) + + +_Relation of Trappean Rocks to the products of active Volcanos._ + +When we reflect on the changes above described in the strata near their +contact with trap dikes, and consider how great is the analogy in +composition and structure of the rocks called trappean and the lavas of +active volcanos, it seems difficult at first to understand how so much +doubt could have prevailed for half a century as to whether trap was of +igneous or aqueous origin. To a certain extent, however, there was a real +distinction between the trappean formations and those to which the term +volcanic was almost exclusively confined. The trappean rocks first studied +in the north of Germany, and in Norway, France, Scotland, and other +countries, were either such as had been formed entirely under deep water, +or had been injected into fissures and intruded between strata, and which +had never flowed out in the air, or over the bottom of a shallow sea. When +these products, therefore, of submarine or subterranean igneous action were +contrasted with loose cones of scoriæ, tuff, and lava, or with narrow +streams of lava in great part scoriaceous and porous, such as were observed +to have proceeded from Vesuvius and Etna, the resemblance seemed remote and +equivocal. It was, in truth, like comparing the roots of a tree with its +leaves and branches, which, although they belong to the same plant, differ +in form, texture, colour, mode of growth, and position. The external cone, +with its loose ashes and porous lava, may be likened to the light foliage +and branches, and the rocks concealed far below, to the roots. But it is +not enough to say of the volcano, + + "quantum vertice in auras + Ætherias, tantum radice in Tartara tendit," + +for its roots do literally reach downwards to Tartarus, or to the regions +of subterranean fire; and what is concealed far below, is probably always +more important in volume and extent than what is visible above ground. + +[Illustration: Fig. 454. Strata intersected by a trap dike, and +covered with alluvium.] + +We have already stated how frequently dense masses of strata have been +removed by denudation from wide areas (see Chap. VI.); and this fact +prepares us to expect a similar destruction of whatever may once have +formed the uppermost part of ancient submarine or subaerial volcanos, more +especially as those superficial parts are always of the lightest and most +perishable materials. The abrupt manner in which dikes of trap usually +terminate at the surface (see fig. 454.), and the water-worn pebbles of +trap in the alluvium which covers the dike, prove incontestably that +whatever was uppermost in these formations has been swept away. It is easy, +therefore, to conceive that what is gone in regions of trap may have +corresponded to what is now visible in active volcanos. + +It will be seen in the following chapters, that in the earth's crust there +are volcanic tuffs of all ages, containing marine shells, which bear +witness to eruptions at many successive geological periods. These tuffs, +and the associated trappean rocks, must not be compared to lava and scoriæ +which had cooled in the open air. Their counterparts must be sought in the +products of modern submarine volcanic eruptions. If it be objected that we +have no opportunity of studying these last, it may be answered, that +subterranean movements have caused, almost everywhere in regions of active +volcanos, great changes in the relative level of land and sea, in times +comparatively modern, so as to expose to view the effects of volcanic +operations at the bottom of the sea. + +Thus, for example, the recent examination of the igneous rocks of Sicily, +especially those of the Val di Noto, has proved that all the more ordinary +varieties of European trap have been there produced under the waters of the +sea, at a modern period; that is to say, since the Mediterranean has been +inhabited by a great proportion of the existing species of testacea. + +These igneous rocks of the Val di Noto, and the more ancient trappean rocks +of Scotland and other countries, differ from subaerial volcanic formations +in being more compact and heavy, and in forming sometimes extensive sheets +of matter intercalated between marine strata, and sometimes stratified +conglomerates, of which the rounded pebbles are all trap. They differ also +in the absence of regular cones and craters, and in the want of conformity +of the lava to the lowest levels of existing valleys. + +It is highly probable, however, that insular cones did exist in some +parts of the Val di Noto: and that they were removed by the waves, in +the same manner as the cone of Graham island, in the Mediterranean, was +swept away in 1831, and that of Nyöe, off Iceland, in 1783.[389-A] All +that would remain in such cases, after the bed of the sea has been +upheaved and laid dry, would be dikes and shapeless masses of igneous +rock, cutting through sheets of lava which may have spread over the +level bottom of the sea, and strata of tuff, formed of materials first +scattered far and wide by the winds and waves, and then deposited. Trap +conglomerates also, to which the action of the waves must give rise +during the denudation of such volcanic islands, will emerge from the +deep whenever the bottom of the sea becomes land. + +The proportion of volcanic matter which is originally submarine must +always be very great, as those volcanic vents which are not entirely +beneath the sea, are almost all of them in islands, or, if on +continents, near the shore. This may explain why extended sheets of trap +so often occur, instead of narrow threads, like lava streams. For, a +multitude of causes tend, near the land, to reduce the bottom of the sea +to a nearly uniform level,--the sediment of rivers,--materials +transported by the waves and currents of the sea from wasting +cliffs,--showers of sand and scoriæ ejected by volcanos, and scattered +by the wind and waves. When, therefore, lava is poured out on such a +surface, it will spread far and wide in every direction in a liquid +sheet, which may afterwards, when raised up, form the tabular capping +of the land. + +As to the absence of porosity in the trappean formations, the appearances +are in a great degree deceptive, for all amygdaloids are, as already +explained, porous rocks, into the cells of which mineral matter, such as +silex, carbonate of lime, and other ingredients, have been subsequently +introduced (see p. 373.); sometimes, perhaps, by secretion during the +cooling and consolidation of lavas. + +In the Little Cumbray, one of the Western Islands, near Arran, the +amygdaloid sometimes contains elongated cavities filled with brown spar; +and when the nodules have been washed out, the interior of the cavities is +glazed with the vitreous varnish so characteristic of the pores of slaggy +lavas. Even in some parts of this rock which are excluded from air and +water, the cells are empty, and seem to have always remained in this state, +and are therefore undistinguishable from some modern lavas.[390-A] + +Dr. MacCulloch, after examining with great attention these and the other +igneous rocks of Scotland, observes, "that it is a mere dispute about +terms, to refuse to the ancient eruptions of trap the name of submarine +volcanos; for they are such in every essential point, although they no +longer eject fire and smoke."[390-B] The same author also considers it not +improbable that some of the volcanic rocks of the same country may have +been poured out in the open air.[390-C] + +Although the principal component minerals of subaerial lavas are the same +as those of intrusive trap, and both the columnar and globular structure +are common to both, there are, nevertheless, some volcanic rocks which +never occur as lava, such as greenstone, clinkstone, the more crystalline +porphyries, and those traps in which quartz and mica appear as constituent +parts. In short, the intrusive trap rocks, forming the intermediate step +between lava and the plutonic rocks, depart in their characters from lava +in proportion as they approximate to granite. + +These views respecting the relations of the volcanic and trap rocks will be +better understood when the reader has studied, in the 33d chapter, what is +said of the plutonic formations. + + +FORM, STRUCTURE, AND ORIGIN OF VOLCANIC MOUNTAINS. + +The origin of volcanic cones with crater-shaped summits has been alluded to +in the last chapter (p. 368.), and more fully explained in the "Principles +of Geology" (chaps. xxiv. to xxvii.), where Vesuvius, Etna, Santorin, and +Barren Island were described. The more ancient portions of those mountains +or islands, formed long before the times of history, exhibit the same +external features and internal structure which belong to most of the +extinct volcanos of still higher antiquity. + +The island of Palma, for example, one of the Canaries, offers an excellent +illustration of what, in common with many others, I regard as the ruins of +a large dome-shaped mass formed by a series of eruptions proceeding from a +crater at the summit, this crater having been since replaced by a larger +cavity, the origin of which has afforded geologists an ample field for +discussion and speculation. + +[Illustration: Fig. 455. View of the Isle of Palma, and of the entrance +into the central cavity or Caldera. From Von Buch's "Canary Islands."] + +[Illustration: Fig. 456. Map of the Caldera of Palma and the great +ravine, called "Barranco de las Angustias." From Survey of Capt. +Vidal, R.N., 1837.] + +Von Buch, in his excellent account of the Canaries, has given us a +graphic picture of this island, which consists chiefly of a single +mountain (fig. 455.). This mountain has the general form of a great +truncated cone, with a huge and deep cavity in the middle, about six +miles in diameter, called by the inhabitants "the Caldera," or cauldron. +The range of precipices surrounding the Caldera are no less than 4000 +feet in their average height; at one point, where they are highest, they +are 7730 feet above the level of the sea. The external flanks of the +cone incline gently in every direction towards the base of the island, +and are in part cultivated; but the walls and bottom of the Caldera +present on all sides rugged and uncultivated rocks, almost completely +devoid of vegetation. So steep are these walls, that there is no part by +which they can be descended, and the only entrance is by a great ravine, +or Barranco, as it is called (see _b b'_, map, fig. 456.), which extends +from the sea to the interior of the great cavity, and by its jagged, +broken, and precipitous sides, exhibits to the geologist a transverse +section of the rocks of which the whole mountain is composed. By this +means, we learn that the cone is made up of a great number of sloping +beds, which dip outwards in every direction from the centre of the void +space, or from the hollow axis of the cone. The beds consist chiefly of +sheets of basalt, alternating with conglomerates; the materials of the +latter being in part rounded, as if rolled by water in motion. The +inclination of all the beds corresponds to that of the external slope of +the island, being greatest towards the Caldera, and least steep when +they are nearest the sea. There are a great number of tortuous veins, +and many dikes of lava or trap, chiefly basaltic, and most of them +vertical, which cut through the sloping beds laid open to view in the +great gorge or Barranco. These dikes and veins are more and more +abundant as we approach the Caldera, being therefore most numerous where +the slope of the beds is greatest. + +Assuming the cone to be a pile of volcanic materials ejected by a long +succession of eruptions (a point on which all geologists are agreed), we +have to account for the Caldera and the great Barranco. I conceive that the +cone itself may be explained, in accordance with what we know of the +ordinary growth of volcanos[392-A], by supposing most of the eruptions to +have taken place from one or more central vents, at or near the summit of +the cone, before it was truncated. From this culminating point, sheets of +lava flowed down one after the other, and showers of ashes or ejected +stones. The volcano may, in the earlier stages of its growth, have been in +great part submerged, like Stromboli, in the sea; and, therefore, some of +the fragments of rock cast out of its crater may not only have been rolled +by torrents sweeping down the mountain's side, but have also been rounded +by the waves of the sea, as we see happen on the beach near Catania, on +which the modern lavas of Etna are broken up. The increased number of +dykes, as we approach the axis of the cone, agrees well with the hypothesis +of the eruptions having been most frequent towards the centre. + +There are three known causes or modes of operation, which may have +conduced towards the vast size of the Caldera. First, the summit of a +conical mountain may have fallen in, as happened in the case of +Capacurcu, one of the Andes, according to tradition, in the year 1462, +and of many other volcanic mountains.[393-A] Sections seem wanting, to +supply us with all the data required for judging fairly of the +tenability of this hypothesis. It appears, however, from Captain Vidal's +survey (see fig. 456.), that a hill of considerable height rises up from +the bottom of the Caldera, the structure of which, if it be any where +laid open, might doubtless throw much light on this subject. Secondly, +an original crater may have been enlarged by a vast gaseous explosion, +never followed by any subsequent eruption. A serious objection to this +theory arises from our not finding that the exterior of the cone +supports a mass of ruins, such as ought to cover it, had so enormous a +volume of matter, partly made up of the solid contents of the dikes, +been blown out into the air. In that case, an extensive bed of angular +fragments of stone, and of fine dust, might be looked for, enveloping +the entire exterior of the mountain up to the very rim of the Caldera, +and ought nowhere to be intersected by a dike. The absence of such a +formation has induced Von Buch to suppose that the missing portion +of the cone was engulphed. It should, however, be remembered, that +in existing volcanos, large craters, two or three miles in diameter, +are sometimes formed by explosions, or by the discharge of great +volumes of steam. + +There is yet another cause to which the extraordinary dimensions of the +Caldera may, in part at least, be owing; namely, aqueous denudation. Von +Buch has observed, that the existence of a single deep ravine, like the +Great Barranco, is a phenomenon common to many extinct volcanos, as well as +to some active ones. Now, it will be seen by Captain Vidal's map (fig. 456. +p. 391.), that the sea-cliff at Point Juan Graje, 780 feet high, now +constituting the coast at the entrance of the great ravine, is continuous +with an inland cliff which bounds the same ravine on its north-western +side. No one will dispute that the precipice, at the base of which the +waves are now beating, owes its origin to the undermining power of the sea. +It is natural, therefore, to attribute the extension of the same cliff to +the former action of the waves, exerted at a time when the relative level +of the island and the ocean were different from what they are now. But if +the waves and tides had power to remove the rocks once filling a great +gorge which is 7 miles long, and, in its upper part, 2000 feet deep, can we +doubt that the same power may have cleared out much of the solid mass now +missing in the Great Caldera? + +The theory advanced to account for the configuration of Palma, commonly +called the "elevation crater theory," is this. All the alternating masses +of basalt and conglomerate, intersected in the Barranco, or abruptly cut +off in the escarpment or walls of the Caldera, were at first disposed in +horizontal masses on the level floor of the ocean, and traversed, when in +that position, by all the basaltic dikes which now cut through them. At +length they were suddenly uplifted by the explosive force of elastic +vapours, which raised the mass bodily, so as to tilt the beds on all sides +away from the centre of elevation, causing at the same time an opening at +the culminating point. Besides many other objections which may be urged +against this hypothesis, it leaves unexplained the unbroken continuity of +the rim of the Caldera, which is uninterrupted in all places save +one[394-A], namely, that where the great gorge or Barranco occurs. + +As a more natural way of explaining the phenomenon, the following series of +events may be imagined. The principal vent, from which a large part of the +materials of the cone were poured or thrown out, was left empty after the +last escape of vapour, when the volcano became extinct. We learn from Mr. +Dana's valuable work on the geology of the United States' Exploring +Expedition, published in 1849, that two of the principal volcanos of the +Sandwich Islands, Mounts Loa and Kea in Owyhee, are huge flattened volcanic +cones, 15,000 feet high (see fig. 457.), each equalling two and a half +Etnas in their dimensions. + +[Illustration: Fig. 457. Mount Loa, in the Sandwich Islands. (Dana) + + _a._ Crater at the summit. + _b._ The lateral crater of Kilauea. + +The dotted lines indicate a supposed column of solid rock caused by the +lava consolidating after eruptions.] + +From the summits of these lofty though featureless hills, and from vents +not far below their summits, successive streams of lava, often 2 miles or +more in width, and sometimes 26 miles long, have flowed. They have been +poured out one after the other, some of them in recent times, in every +direction from the apex of the cone, down slopes varying on an average from +4 degrees to 8 degrees; but at some places considerably steeper.[394-B] +Sometimes deep rents open on the sides of these cones, which are filled by +streams of lava passing over them, the liquid matter in such cases probably +uniting in the fissure with other lava melted in subterranean reservoirs +below, and thus explaining the origin of one great class of lateral dikes, +on Etna, Palma, and other cones. + +If the flattened domes, such as those here alluded to in the Sandwich +Islands, instead of being inland, and above water, were situated in +mid-ocean, like the Island of St. Paul, and for the most part submerged +(see figs. 458, 459, 460.), and if a gradual upheaval of such a dome should +then take place, the denuding power of the sea could scarcely fail to play +an important part in modifying the form of the volcanic mountain as it +rose. The crater will almost invariably have one side much lower than all +the others, namely, that side towards which the prevailing winds never +blow, and to which, therefore, showers of dust and scoriæ are rarely +carried during eruptions. There will also be one point on this windward or +lowest side more depressed than all the rest, by which the sea may enter as +often as the tide rises, or as often as the wind blows from that quarter. +For the same reason that the sea continues to keep open a single entrance +into the lagoon of an atoll or annular coral reef, it will not allow this +passage into the crater to be stopped up, but scour it out, at low tide, or +as often as the wind changes. The channel, therefore, will always be +deepened in proportion as the island rises above the level of the sea, at +the rate perhaps of a few feet or yards in a century. + +[Illustration: Fig. 458. Map of the Island of St. Paul, in the Indian +Ocean, lat. 38° 44´ S., long. 77° 37´ E., surveyed by Capt. Blackwood, +R.N., 1842.] + +[Illustration: Fig. 459. View of the Crater of the Island of St. Paul.] + +The island of St. Paul may perhaps be motionless; but if, like many +other parts of the earth's crust, it should begin to undergo a gradual +upheaval, or if, as has happened to the shores of the Bay of Baiæ, its +level should oscillate, with a tendency upon the whole to increased +elevation, the same power which has cut away part of the cone, and +caused the cliffs now seen on the north-east side of the island, would +have power to undermine the walls of the crater, and enlarge its +diameter, keeping open the channel, by which it enters into it. This +ravine might be excavated to the depth of 180 feet (the present depth of +the crater), and its length might be extended to many miles according to +the size of the submerged part of the cone. The crater is only a mile in +diameter, and the surrounding cliffs, where loftiest, only 800 feet +high, so that the size of this cone and crater is insignificant when +compared to those in the Sandwich Islands, and I have merely selected +it because it affords an example of a class of insular volcanos, into +the craters of which the sea now enters by a single passage. The crater +of Vesuvius in 1822 was 2000 feet deep; and if it were a half submerged +cone, like St. Paul, the excavating power of the ocean might in +conjunction with gaseous explosions and co-operating with a gradual +upheaving force, give rise to a caldera on as grand a scale as +that exhibited by Palma. + +[Illustration: Fig. 460. Side view of the Island of St. Paul (N.E. side). +Nine-pin rocks two miles distant. (Captain Blackwood.)] + +If, after the geographical changes above supposed, the volcanic fires +long dormant should recover their energy, they might, as in the case of +Teneriffe, Vesuvius, Santorin, and Barren Island, discharge from the old +central vent, long sealed up at the bottom of the caldera, new floods of +lava and clouds of elastic vapours. Should this happen, a new cone will +be built up in the middle of the cavity or circular bay, formed, partly +by explosion, partly perhaps by engulphment, and partly by aqueous +denudation. In the island of Palma this last phase of volcanic activity +has never occurred; but the subterranean heat is still in full operation +beneath the Canary Islands, so that we know not what future changes it +may be destined to undergo. + + +FOOTNOTES: + +[378-A] I have been favoured with this drawing by Captain B. Hall. + +[381-A] Cambridge Transactions, vol. i. p. 402. + +[382-A] Cambridge Trans., vol. i. p. 410. + +[382-B] Ibid. vol. ii. p. 175. + +[382-C] Dr. Berger, Geol. Trans., 1st series, vol. iii. p. 172. + +[382-D] Geol. Trans., 1st series, vol. iii. p. 210. and plate 10. + +[382-E] Ibid. p. 201. + +[383-A] Geol. Trans., 1st series, vol. iii. p. 205. + +[383-B] Ibid. p. 213.; and Playfair, Illust. of Hutt. Theory, p. 253. + +[383-C] Geol. Trans., 1st series, vol. iii. p. 206. + +[383-D] Sedgwick, Camb. Trans. vol. ii. p. 37. + +[383-E] Illust. of Hutt. Theory, § 253. and 261. Dr. MacCulloch, Geol. +Trans., 1st series, vol. ii. p. 305. + +[383-F] Syst. of Geol. vol. i. p. 206. + +[384-A] Camb. Trans. vol. ii. p. 180. + +[385-A] MacCul. Syst. of Geol. vol. ii. p. 137. + +[385-B] Seale's Geognosy of St. Helena, plate 9. + +[386-A] Fortis. Mém. sur l'Hist. Nat. de l'Italie, tom. i. p. 233. plate 7. + +[387-A] Scrope, Geol. Trans. vol. ii. p. 205. 2d series. + +[389-A] See Princ. of Geol., _Index_, "Graham Island," "Nyöe," +"Conglomerates, volcanic," &c. + +[390-A] MacCulloch, West. Isl., vol. ii. p. 487. + +[390-B] Syst. of Geol., vol. ii. p. 114. + +[390-C] Ibid. + +[392-A] See Principles, chaps. xxiv-xxvii. + +[393-A] See Principles, chaps. xxvi. and xxx.; 8th ed. p. 397-475. + +[394-A] See Principles of Geol. ch. xxiv. (8th ed. p. 355.). + +[394-B] See Lyell on Craters of Denudation, Quart. Geol. Journ. +vol. vi. p. 232. + + + + +CHAPTER 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. + + +Having referred the sedimentary strata to a long succession of geological +periods, we have next to consider how far the volcanic formations can be +classed in a similar chronological order. The tests of relative age in this +class of rocks are four:--1st, superposition and intrusion, with or without +alteration of the rocks in contact; 2d, organic remains; 3d, mineral +character; 4th, included fragments of older rocks. + +[Illustration: Fig. 461. Cross section.] + +_Tests by superposition, &c._--If a volcanic rock rests upon an aqueous +deposit, the former must be the newest of the two, but the like rule does +not hold good where the aqueous formation rests upon the volcanic, for +melted matter, rising from below, may penetrate a sedimentary mass without +reaching the surface, or may be forced in conformably between two strata, +as _b_ at D in the annexed figure (fig. 461.), after which it may cool down +and consolidate. Superposition, therefore, is not of the same value as a +test of age in the unstratified volcanic rocks as in fossiliferous +formations. We can only rely implicitly on this test where the volcanic +rocks are contemporaneous, not where they are intrusive. Now they are said +to be contemporaneous if produced by volcanic action, which was going on +simultaneously with the deposition of the strata with which they are +associated. Thus in the section at D (fig. 461.), we may perhaps ascertain +that the trap _b_ flowed over the fossiliferous bed _c_, and that, after +its consolidation, _a_ was deposited upon it, _a_ and _c_ both belonging to +the same geological period. But if the stratum _a_ be altered by _b_ at the +point of contact, we must then conclude the trap to have been intrusive, or +if, in pursuing _b_ for some distance, we find at length that it cuts +through the stratum _a_, and then overlies it as at E. + +We may, however, be easily deceived in supposing a volcanic rock to be +intrusive, when in reality it is contemporaneous; for a sheet of lava, as +it spreads over the bottom of the sea, cannot rest everywhere upon the +same stratum, either because these have been denuded, or because, if newly +thrown down, they thin out in certain places, thus allowing the lava to +cross their edges. Besides, the heavy igneous fluid will often, as it moves +along, cut a channel into beds of soft mud and sand. Suppose the submarine +lava F to have come in contact in this manner with the strata _a_, _b_, +_c_, and that after its consolidation, the strata _d_, _e_, are thrown down +in a nearly horizontal position, yet so as to lie unconformably to F, the +appearance of subsequent intrusion will here be complete, although the trap +is in fact contemporaneous. We must not, therefore, hastily infer that the +rock F is intrusive, unless we find the strata _d_ or _e_ to have been +altered at their junction, as if by heat. + +[Illustration: Fig. 462. Cross section.] + +When trap dikes were described in the preceding chapter, they were shown to +be more modern than all the strata which they traverse. A basaltic dike at +Quarrington Hill, near Durham, passes through coal-measures, the strata of +which are inclined, and shifted so that those on the north side of the dike +are 24 feet above the level of the corresponding beds on the south side +(see section, fig. 463.). But the horizontal beds of overlying Red +Sandstone and Magnesian Limestone are not cut through by the dike. Now here +the coal-measures were not only deposited, but had subsequently been +disturbed, fissured, and shifted, before the fluid trap now forming the +dike was introduced into a rent. It is also clear that some of the upper +edges of the coal strata, together with the upper part of the dike, had +been subsequently removed by denudation before the lower New Red Sandstone +and Magnesian Limestone were superimposed. Even in this case, however, +although the date of the volcanic eruption is brought within narrow limits, +it cannot be defined with precision; it may have happened either at the +close of the Carboniferous period, or early in that of the Lower New Red +Sandstone, or between these two periods, when the state of the animate +creation and the physical geography of Europe were gradually changing from +the type of the Carboniferous era to that of the Permian. + +[Illustration: Fig. 463. Section at Quarrington Hill, east of +Durham. (Sedgwick.) + + _a._ Magnesian Limestone (Permian). + _b._ Lower New Red Sandstone. + _c._ Coal strata.] + +The test of age by superposition is strictly applicable to all stratified +volcanic tuffs, according to the rules already explained in the case of +other sedimentary deposits. (See p. 96.) + +_Test of age by organic remains._--We have seen how, in the vicinity of +active volcanos, scoriæ, pumice, fine sand, and fragments of rock are +thrown up into the air, and then showered down upon the land, or into +neighbouring lakes or seas. In the tuffs so formed shells, corals, or +any other durable organic bodies which may happen to be strewed over the +bottom of a lake or sea will be imbedded, and thus continue as permanent +memorials of the geological period when the volcanic eruption occurred. +Tufaceous strata thus formed in the neighbourhood of Vesuvius, Etna, +Stromboli, and other volcanos now active in islands or near the sea, may +give information of the relative age of these tuffs at some remote +future period when the fires of these mountains are extinguished. By +such evidence we can distinctly establish the coincidence in age of +volcanic rocks, and the different primary, secondary, and tertiary +fossiliferous strata already considered. + +The tuffs now alluded to are not exclusively marine, but include, in some +places, freshwater shells; in others, the bones of terrestrial quadrupeds. +The diversity of organic remains in formations of this nature is perfectly +intelligible, if we reflect on the wide dispersion of ejected matter during +late eruptions, such as that of the volcano of Coseguina, in the province +of Nicaragua, January 19. 1835. Hot cinders and fine scoriæ were then cast +up to a vast height, and covered the ground as they fell to the depth of +more than 10 feet, and for a distance of 8 leagues from the crater in a +southerly direction. Birds, cattle, and wild animals were scorched to death +in great numbers, and buried in these ashes. Some volcanic dust fell at +Chiapa, upwards of 1200 miles to windward of the volcano, a striking proof +of a counter current in the upper region of the atmosphere; and some on +Jamaica, about 700 miles distant to the north-east. In the sea, also, at +the distance of 1100 miles from the point of eruption, Captain Eden of the +Conway sailed 40 miles through floating pumice, among which were some +pieces of considerable size.[399-A] + +_Test of age by mineral composition._--As sediment of homogeneous +composition, when discharged from the mouth of a large river, is often +deposited simultaneously over a wide space, so a particular kind of lava, +flowing from a crater during one eruption, may spread over an extensive +area; as in Iceland in 1783, when the melted matter, pouring from Skaptar +Jokul, flowed in streams in opposite directions, and caused a continuous +mass, the extreme points of which were 90 miles distant from each other. +This enormous current of lava varied in thickness from 100 feet to 600 +feet, and in breadth from that of a narrow river gorge to 15 miles.[399-B] +Now, if such a mass should afterwards be divided into separate fragments by +denudation, we might still perhaps identify the detached portions by their +similarity in mineral composition. Nevertheless, this test will not always +avail the geologist; for, although there is usually a prevailing character +in lava emitted during the same eruption, and even in the successive +currents flowing from the same volcano, still, in many cases, the different +parts even of one lava-stream, or, as before stated, of one continuous mass +of trap, vary so much in mineral composition and texture as to render these +characters of minor importance when compared to their value in the +chronology of the fossiliferous rocks. + +It will, however, be seen in the description which follows, of the European +trap rocks of different ages, that they had often a peculiar lithological +character, resembling the differences before remarked as existing between +the modern lavas of Vesuvius, Etna, and Chili. (See p. 378.) + +It has been remarked that in Auvergne, the Eifel, and other countries where +trachyte and basalt are both present, the trachytic rocks are for the most +part older than the basaltic. These rocks do, indeed, sometimes alternate +partially, as in the volcano of Mont Dor, in Auvergne; but the great mass +of trachyte occupies in general an inferior position, and is cut through +and overflowed by basalt. It can by no means be inferred that trachyte +predominated greatly at one period of the earth's history and basalt at +another, for we know that trachytic lavas have been formed at many +successive periods, and are still emitted from many active craters; but it +seems that in each region, where a long series of eruptions have occurred, +the more felspathic lavas have been first emitted, and the escape of the +more augitic kinds has followed. The hypothesis suggested by Mr. Scrope +may, perhaps, afford a solution of this problem. The minerals, he observes, +which abound in basalt are of greater specific gravity than those composing +the felspathic lavas; thus, for example, hornblende, augite, and olivine +are each more than three times the weight of water; whereas common felspar, +albite, and Labrador felspar, have each scarcely more than 2-1/2 times the +specific gravity of water; and the difference is increased in consequence +of there being much more iron in a metallic state in basalt and greenstone +than in trachyte and other felspathic lavas and traps. If, therefore, a +large quantity of rock be melted up in the bowels of the earth by volcanic +heat, the denser ingredients of the boiling fluid may sink to the bottom, +and the lighter remaining above would in that case be first propelled +upwards to the surface by the expansive power of gases. Those materials, +therefore, which occupied the lowest place in the subterranean reservoir +will always be emitted last, and take the uppermost place on the exterior +of the earth's crust. + +_Test by included fragments._--We may sometimes discover the relative age +of two trap rocks, or of an aqueous deposit and the trap on which it rests, +by finding fragments of one included in the other, in cases such as those +before alluded to, where the evidence of superposition alone would be +insufficient. It is also not uncommon to find conglomerates almost +exclusively composed of rolled pebbles of trap, associated with stratified +rocks in the neighbourhood of masses of intrusive trap. If the pebbles +agree generally in mineral character with the latter, we are then enabled +to determine the age of the intrusive rock by knowing that of the +fossiliferous strata associated with the conglomerate. The origin of such +conglomerates is explained by observing the shingle beaches composed of +trap pebbles in modern volcanic islands, or at the base of Etna. + +_Post-Pliocene Period (including the Recent)._--I shall now select examples +of contemporaneous volcanic rocks of successive geological periods, to show +that igneous causes have been in activity in all past ages of the world, +and that they have been ever shifting the places where they have broken out +at the earth's surface. + +One portion of the lavas, tuffs, and trap dikes of Etna, Vesuvius, and +the Island of Ischia, has been produced within the historical era; +another, and a far more considerable part, originated at times +immediately antecedent, when the waters of the Mediterranean were +already inhabited by the existing species of testacea. The southern and +eastern flanks of Etna are skirted by a fringe of alternating +sedimentary and volcanic deposits, of submarine origin, as at Adernò, +Trezza, and other places. Of sixty-five species of fossil shells which I +procured in 1828 from this formation, near Trezza, it was impossible to +distinguish any from species now living in the neighbouring sea. + +[Illustration: Fig. 464. View of the Isle of Cyclops in the Bay +of Trezza.[401-A]] + +The Cyclopian Islands, called by the Sicilians Dei Faraglioni, in the +sea cliffs of which these beds of clay, tuff, and associated lava are +laid open to view, are situated in the Bay of Trezza, and may be +regarded as the extremity of a promontory severed from the main land. +Here numerous proofs are seen of submarine eruptions, by which the +argillaceous and sandy strata were invaded and cut through, and +tufaceous breccias formed. Inclosed in these breccias are many angular +and hardened fragments of laminated clay in different states of +alteration by heat, and intermixed with volcanic sands. + +The loftiest of the Cyclopian islets, or rather rocks, is about 200 feet in +height, the summit being formed of a mass of stratified clay, the laminæ of +which are occasionally subdivided by thin arenaceous layers. These strata +dip to the N.W., and rest on a mass of columnar lava (see fig. 464.) in +which the tops of the pillars are weathered, and so rounded as to be often +hemispherical. In some places in the adjoining and largest islet of the +group, which lies to the north-eastward of that represented in the drawing +(fig. 464.), the overlying clay has been greatly altered, and hardened by +the igneous rock, and occasionally contorted in the most extraordinary +manner; yet the lamination has not been obliterated, but, on the contrary, +rendered much more conspicuous, by the indurating process. + +[Illustration: Fig. 465. Contortions of strata in the largest of +the Cyclopian Islands.] + +In the annexed woodcut (fig. 465.) I have represented a portion of the +altered rock, a few feet square, where the alternating thin laminæ of sand +and clay have put on the appearance which we often observe in some of the +most contorted of the metamorphic schists. + +A great fissure, running from east to west, nearly divides this larger +island into two parts, and lays open its internal structure. In the section +thus exhibited, a dike of lava is seen, first cutting through an older mass +of lava, and then penetrating the superincumbent tertiary strata. In one +place the lava ramifies and terminates in thin veins, from a few feet to a +few inches in thickness. (See fig. 466.) + +The arenaceous laminæ are much hardened at the point of contact, and the +clays are converted into siliceous schist. In this island the altered rocks +assume a honeycombed structure on their weathered surface, singularly +contrasted with the smooth and even outline which the same beds present in +their usual soft and yielding state. + +The pores of the lava are sometimes coated, or entirely filled, with +carbonate of lime, and with a zeolite resembling analcime, which has been +called cyclopite. The latter mineral has also been found in small fissures +traversing the altered marl, showing that the same cause which introduced +the minerals into the cavities of the lava, whether we suppose sublimation +or aqueous infiltration, conveyed it also into the open rents of the +contiguous sedimentary strata. + +[Illustration: Fig. 466. Post-Pliocene strata invaded by lava, Isle of +Cyclops (horizontal section). + + _a._ Lava. + _b._ Laminated clay and sand. + _c._ The same altered.] + +_Post-Pliocene formations near Naples._--I have traced in the "Principles +of Geology" the history of the changes which the volcanic region of +Campania is known to have undergone during the last 2000 years. The +aggregate effect of igneous operations during that period is far from +insignificant, comprising as it does the formation of the modern cone of +Vesuvius since the year 79, and the production of several minor cones in +Ischia, together with that of Monte Nuovo in the year 1538. Lava-currents +have also flowed upon the land and along the bottom of the sea--volcanic +sand, pumice, and scoriæ have been showered down so abundantly, that whole +cities were buried--tracts of the sea have been filled up or converted into +shoals--and tufaceous sediment has been transported by rivers and +land-floods to the sea. There are also proofs, during the same recent +period, of a permanent alteration of the relative levels of the land and +sea in several places, and of the same tract having, near Puzzuoli, been +alternately upheaved and depressed to the amount of more than 20 feet. In +connection with these convulsions, there are found, on the shores of the +Bay of Baiæ, recent tufaceous strata, filled with articles fabricated by +the hands of man, and mingled with marine shells. + +It was also stated in this work (p. 113.), that when we examine this same +region, it is found to consist largely of tufaceous strata, of a date +anterior to human history or tradition, which are of such thickness as to +constitute hills from 500 to more than 2000 feet in height. These +post-pliocene strata, containing recent marine shells, alternate with +distinct currents and sheets of lava which were of contemporaneous origin; +and we find that in Vesuvius itself, the ancient cone called Somma is of +far greater volume than the modern cone, and is intersected by a far +greater number of dikes. In contrasting this ancient part of the mountain +with that of modern date, one principal point of difference is observed; +namely, the greater frequency in the older cone of fragments of altered +sedimentary rocks ejected during eruptions. We may easily conceive that the +first explosions would act with the greatest violence, rending and +shattering whatever solid masses obstructed the escape of lava and the +accompanying gases, so that great heaps of ejected pieces of rock would +naturally occur in the tufaceous breccias formed by the earliest eruptions. +But when a passage had once been opened, and an habitual vent established, +the materials thrown out would consist of liquid lava, which would take the +form of sand and scoriæ, or of angular fragments of such solid lavas as may +have choked up the vent. + +Among the fragments which abound in the tufaceous breccias of Somma, none +are more common than a saccharoid dolomite, supposed to have been derived +from an ordinary limestone altered by heat and volcanic vapours. + +Carbonate of lime enters into the composition of so many of the simple +minerals found in Somma, that M. Mitscherlich, with much probability, +ascribes their great variety to the action of the volcanic heat on +subjacent masses of limestone. + +_Dikes of Somma._--The dikes seen in the great escarpment which Somma +presents towards the modern cone of Vesuvius are very numerous. They are +for the most part vertical, and traverse at right angles the beds of +lava, scoriæ, volcanic breccia, and sand, of which the ancient cone is +composed. They project in relief several inches, or sometimes feet, from +the face of the cliff, being extremely compact, and less destructible +than the intersected tuffs and porous lavas. In vertical extent they +vary from a few yards to 500 feet, and in breadth from 1 to 12 feet. +Many of them cut all the inclined beds in the escarpment of Somma from +top to bottom, others stop short before they ascend above half way, and +a few terminate at both ends, either in a point or abruptly. In mineral +composition they scarcely differ from the lavas of Somma, the rock +consisting of a base of leucite and augite, through which large crystals +of augite and some of leucite are scattered.[404-A] Examples are not +rare of one dike cutting through another, and in one instance a shift or +fault is seen at the point of intersection. + +In some cases, however, the rents seem to have been filled laterally, when +the walls of the crater had been broken by star-shaped cracks, as seen in +the accompanying woodcut (fig. 467.). But the shape of these rents is an +exception to the general rule; for nothing is more remarkable than the +usual parallelism of the opposite sides of the dikes, which correspond +almost as regularly as the two opposite faces of a wall of masonry. This +character appears at first the more inexplicable, when we consider how +jagged and uneven are the rents caused by earthquakes in masses of +heterogeneous composition, like those composing the cone of Somma. In +explanation of this phenomenon, M. Necker refers us to Sir W. Hamilton's +account of an eruption of Vesuvius in the year 1779, who records the +following facts:--"The lavas, when they either boiled over the crater, or +broke out from the conical parts of the volcano, constantly formed channels +as regular as if they had been cut by art down the steep part of the +mountain; and, whilst in a state of perfect fusion, continued their course +in those channels, which were sometimes full to the brim, and at other +times more or less so, according to the quantity of matter in motion. + +[Illustration: Fig. 467. Dikes or veins at the Punta del Nasone on +Somma. (Necker.[405-A])] + +"These channels, upon examination after an eruption, I have found to be in +general from two to five or six feet wide, and seven or eight feet deep. +They were often hid from the sight by a quantity of scoriæ that had formed +a crust over them; and the lava, having been conveyed in a covered way for +some yards, came out fresh again into an open channel. After an eruption, I +have walked in some of those subterraneous or covered galleries, which were +exceedingly curious, the sides, top, and bottom _being worn perfectly +smooth and even_ in most parts, by the violence of the currents of the +red-hot lavas which they had conveyed for many weeks successively."[405-B] + +Now, the walls of a vertical fissure, through which lava has ascended in +its way to a volcanic vent, must have been exposed to the same erosion as +the sides of the channels before adverted to. The prolonged and uniform +friction of the heavy fluid, as it is forced and made to flow upwards, +cannot fail to wear and smooth down the surfaces on which it rubs, and the +intense heat must melt all such masses as project and obstruct the passage +of the incandescent fluid. + +The texture of the Vesuvian dikes is different at the edges and in the +middle. Towards the centre, observes M. Necker, the rock is larger +grained, the component elements being in a far more crystalline state; +while at the edge the lava is sometimes vitreous, and always finer +grained. A thin parting band, approaching in its character to +pitchstone, occasionally intervenes, on the contact of the vertical dike +and intersected beds. M. Necker mentions one of these at the place +called Primo Monte, in the Atrio del Cavallo; and when on Somma, in +1828, I saw three or four others in different parts of the great +escarpment. These phenomena are in perfect harmony with the results of +the experiments of Sir James Hall and Mr. Gregory Watt, which have shown +that a glassy texture is the effect of sudden cooling, and that, on the +contrary, a crystalline grain is produced where fused minerals are +allowed to consolidate slowly and tranquilly under high pressure. + +It is evident that the central portion of the lava in a fissure would, +during consolidation, part with its heat more slowly than the sides, +although the contrast of circumstances would not be so great as when we +compare the lava at the bottom and at the surface of a current flowing in +the open air. In this case the uppermost part, where it has been in contact +with the atmosphere, and where refrigeration has been most rapid, is always +found to consist of scoriform, vitreous, and porous lava; while at a +greater depth the mass assumes a more lithoidal structure, and then becomes +more and more stony as we descend, until at length we are able to recognize +with a magnifying glass the simple minerals of which the rock is composed. +On penetrating still deeper, we can detect the constituent parts by the +naked eye, and in the Vesuvian currents distinct crystals of augite and +leucite become apparent. + +The same phenomenon, observes M. Necker, may readily be exhibited on a +smaller scale, if we detach a piece of liquid lava from a moving current. +The fragment cools instantly, and we find the surface covered with a +vitreous coat; while the interior, although extremely fine-grained, has a +more stony appearance. + +It must, however, be observed, that although the lateral portions of the +dikes are finer grained than the central, yet the vitreous parting layer +before alluded to is rare in Vesuvius. This may, perhaps, be accounted for, +as the above-mentioned author suggests, by the great heat which the walls +of a fissure may acquire before the fluid mass begins to consolidate, in +which case the lava, even at the sides, would cool very slowly. Some +fissures, also, may be filled from above, as frequently happens in the +volcanos of the Sandwich Islands, according to the observations of Mr. +Dana; and in this case the refrigeration at the sides would be more rapid +than when the melted matter flowed upwards from the volcanic foci, in an +intensely heated state. Mr. Darwin informs me that in St. Helena almost +every dike has a vitreous selvage. + +The rock composing the dikes both in the modern and ancient part of +Vesuvius is far more compact than that of ordinary lava, for the pressure +of a column of melted matter in a fissure greatly exceeds that in an +ordinary stream of lava; and pressure checks the expansion of those gases +which give rise to vesicles in lava. + +There is a tendency in almost all the Vesuvian dikes to divide into +horizontal prisms, a phenomenon in accordance with the formation of +vertical columns in horizontal beds of lava; for in both cases the +divisions which give rise to the prismatic structure are at right angles to +the cooling surfaces. + +_Newer Pliocene Period--Val di Noto._--I have already alluded (see p. 150.) +to the igneous rocks which are associated with a great marine formation of +limestone, sand, and marl, in the southern part of Sicily, as at Vizzini +and other places. In this formation, which was shown to belong to the Newer +Pliocene period, large beds of oysters and corals repose upon lava, and are +unaltered at the point of contact. In other places we find dikes of igneous +rock intersecting the fossiliferous beds, and converting the clays into +siliceous schist, the laminæ being contorted and shivered into innumerable +fragments at the junction, as near the town of Vizzini. + +The volcanic formations of the Val di Noto usually consist of the most +ordinary variety of basalt, with or without olivine. The rock is sometimes +compact, often very vesicular. The vesicles are occasionally empty, both in +dikes and currents, and are in some localities filled with calcareous spar, +arragonite, and zeolites. The structure is, in some places, spheroidal; in +others, though rarely, columnar. I found dikes of amygdaloid, wacké, and +prismatic basalt, intersecting the limestone at the bottom of the hollow +called Gozzo degli Martiri, below Melilli. + +[2 Illustrations: Fig. 468. Fig. 469. Ground-plan of dikes near Palagonia. + + _a._ Lava. + _b._ Peperino, consisting of volcanic sand, mixed with fragments of lava + and limestone.] + +_Dikes._--Dikes of vesicular and amygdaloidal lava are also seen traversing +marine tuff or peperino, west of Palagonia, some of the pores of the lava +being empty, while others are filled with carbonate of lime. In such cases, +we may suppose the peperino to have resulted from showers of volcanic sand +and scoriæ, together with fragments of limestone, thrown out by a submarine +explosion, similar to that which gave rise to Graham Island in 1831. When +the mass was, to a certain degree, consolidated, it may have been rent +open, so that the lava ascended through fissures, the walls of which were +perfectly even and parallel. After the melted matter that filled the rent +in fig. 468. had cooled down, it must have been fractured and shifted +horizontally by a lateral movement. + +In the second figure (fig. 469.), the lava has more the appearance of a +vein which forced its way through the peperino. It is highly probable that +similar appearances would be seen, if we could examine the floor of the sea +in that part of the Mediterranean where the waves have recently washed away +the new volcanic island; for when a superincumbent mass of ejected +fragments has been removed by denudation, we may expect to see sections of +dikes traversing tuff, or, in other words, sections of the channels of +communication by which the subterranean lavas reached the surface. + + +FOOTNOTES: + +[399-A] Caldcleugh, Phil. Trans. 1836. p. 27., and Official Documents +of Nicaragua. + +[399-B] See Principles, _Index_, "Skaptar Jokul." + +[401-A] This view of the Isle of Cyclops is from an original drawing by my +friend the late Captain Basil Hall, R. N. + +[404-A] Consult the valuable memoir of M. L. A. Necker, Mém. de la Soc. de +Phys. et d'Hist. Nat. de Génève, tom. ii. part i. Nov. 1822. + +[405-A] From a drawing of M. Necker, in Mém. above cited. + +[405-B] Phil. Trans., vol. lxx., 1780. + + + + +CHAPTER 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. + + +_Older Pliocene period--Tuscany._--In Tuscany, as at Radicofani, Viterbo, +and Aquapendente, and in the Campagna di Roma, submarine volcanic tuffs are +interstratified with the Older Pliocene strata of the Subapennine hills, in +such a manner as to leave no doubt that they were the products of eruptions +which occurred when the shelly marls and sands of the Subapennine hills +were in the course of deposition. + +_Catalonia._--Geologists are far from being able, as yet, to assign to +each of the volcanic groups scattered over Europe a precise +chronological place in the tertiary series; but I shall describe here, +as probably referable to some part of the Pliocene period, a district of +extinct volcanos near Olot, in the north of Spain, which is little +known, and which I visited in the summer of 1830. + +The whole extent of country occupied by volcanic products in Catalonia is +not more than fifteen geographical miles from north to south, and about six +from east to west. The vents of eruption range entirely within a narrow +band running north and south; and the branches, which are represented as +extending eastward in the map, are formed simply of two lava-streams--those +of Castell Follit and Cellent. + +[Illustration: Fig. 470. Volcanic district of Catalonia.] + +Dr. Maclure, the American geologist, was the first who made known the +existence of these volcanos[409-A]; and, according to his description, the +volcanic region extended over twenty square leagues, from Amer to Massanet. +I searched in vain in the environs of Massanet, in the Pyrenees, for traces +of a lava-current; and I can say, with confidence, that the adjoining map +gives a correct view of the true area of the volcanic action. + +_Geological structure of the district._--The eruptions have burst entirely +through fossiliferous rocks, composed in great part of grey and greenish +sandstone and conglomerate, with some thick beds of nummulitic limestone. +The conglomerate contains pebbles of quartz, limestone, and Lydian stone. +This system of rocks is very extensively spread throughout Catalonia; one +of its members being a red sandstone, to which the celebrated salt-rock of +Cardona, usually considered as of the cretaceous era, is subordinate. + +Near Amer, in the Valley of the Ter, on the southern borders of the region +delineated in the map, primary rocks are seen, consisting of gneiss, +mica-schist, and clay-slate. They run in a line nearly parallel to the +Pyrenees, and throw off the fossiliferous strata from their flanks, causing +them to dip to the north and north-west. This dip, which is towards the +Pyrenees, is connected with a distinct axis of elevation, and prevails +through the whole area described in the map, the inclination of the beds +being sometimes at an angle of between 40 and 50 degrees. + +It is evident that the physical geography of the country has undergone +no material change since the commencement of the era of the volcanic +eruptions, except such as has resulted from the introduction of new +hills of scoriæ, and currents of lava upon the surface. If the lavas +could be remelted and poured out again from their respective craters, +they would descend the same valleys in which they are now seen, and +re-occupy the spaces which they at present fill. The only difference in +the external configuration of the fresh lavas would consist in this, +that they would nowhere be intersected by ravines, or exhibit marks of +erosion by running water. + +_Volcanic cones and lavas._--There are about fourteen distinct cones with +craters in this part of Spain, besides several points whence lavas may have +issued; all of them arranged along a narrow line running north and south, +as will be seen in the map. The greatest number of perfect cones are in the +immediate neighbourhood of Olot, some of which (Nos. 2, 3. and 5.) are +represented in the annexed woodcut; and the level plain on which that town +stands has clearly been produced by the flowing down of many lava-streams +from those hills into the bottom of a valley, probably once of considerable +depth, like those of the surrounding country. + +[Illustration: Fig. 471. View of the Volcanos around Olot in Catalonia.] + +In this drawing an attempt is made to represent, by the shading of the +landscape, the different geological formations of which the country is +composed.[410-A] The white line of mountains (No. 1.) in the distance is +the Pyrenees, which are to the north of the spectator, and consist of +hypogene and ancient fossiliferous rocks. In front of these are the +fossiliferous formations (No. 4.) which are in shade. The hills 2, 3. 5. +are volcanic cones, and the rest of the ground on which the sunshine falls +is strewed over with volcanic ashes and lava. + +The Fluvia, which flows near the town of Olot, has cut to the depth of only +40 feet through the lavas of the plain before mentioned. The bed of the +river is hard basalt; and at the bridge of Santa Madalena are seen two +distinct lava-currents, one above the other, separated by a horizontal bed +of scoriæ 8 feet thick. + +In one place, to the south of Olot, the even surface of the plain is +broken by a mound of lava, called the "Bosque de Tosca," the upper part +of which is scoriaceous, and covered with enormous heaps of fragments of +basalt, more or less porous. Between the numerous hummocks thus formed +are deep cavities, having the appearance of small craters. The whole +precisely resembles some of the modern currents of Etna, or that of +Côme, near Clermont; the last of which, like the Bosque de Tosca, +supports only a scanty vegetation. + +Most of the Catalonian volcanos are as entire as those in the neighbourhood +of Naples, or on the flanks of Etna. One of these, called Montsacopa (No. +3. fig. 471.), is of a very regular form, and has a circular depression or +crater at the summit. It is chiefly made up of red scoriæ, +undistinguishable from that of the minor cones of Etna. The neighbouring +hills of Olivet (No. 2.) and Garrinada (No. 5.) are of similar composition +and shape. The largest crater of the whole district occurs farther to the +east of Olot, and is called Santa Margarita. It is 455 feet deep, and about +a mile in circumference. Like Astroni, near Naples, it is richly covered +with wood, wherein game of various kinds abounds. + +[Illustration: Fig. 472. Cross section. + + _a._ Secondary conglomerate. + _b._ Thin seams of volcanic sand and scoriæ.] + +Although the volcanos of Catalonia have broken out through sandstone, +shale, and limestone, as have those of the Eifel, in Germany, to be +described in the sequel, there is a remarkable difference in the nature of +the ejections composing the cones in these two regions. In the Eifel, the +quantity of pieces of sandstone and shale thrown out from the vents is +often so immense as far to exceed in volume the scoriæ, pumice, and lava; +but I sought in vain in the cones near Olot for a single fragment of any +extraneous rock; and Don Francisco Bolos, an eminent botanist of Olot, +informed me that he had never been able to detect any. Volcanic sand and +ashes are not confined to the cones, but have been sometimes scattered by +the wind over the country, and drifted into narrow valleys, as is seen +between Olot and Cellent, where the annexed section (fig. 472.) is exposed. +The light cindery volcanic matter rests in thin regular layers, just as it +alighted on the slope formed by the solid conglomerate. No flood could have +passed through the valley since the scoriæ fell, or these would have been +for the most part removed. + +[Illustration: Fig. 473. Section above the bridge of Cellent. + + _a._ Scoriaceous lava. + _b._ Schistose basalt. + _c._ Columnar basalt. + _d._ Scoria, vegetable soil, and alluvium. + _e._ Nummulitic limestone. + _.f_ Micaceous grey sandstone.] + +The currents of lava in Catalonia, like those of Auvergne, the Vivarais, +Iceland, and all mountainous countries, are of considerable depth in narrow +defiles, but spread out into comparatively thin sheets in places where the +valleys widen. If a river has flowed on nearly level ground, as in the +great plain near Olot, the water has only excavated a channel of slight +depth; but where the declivity is great, the stream has cut a deep section, +sometimes by penetrating directly through the central part of a +lava-current, but more frequently by passing between the lava and the +secondary rock which bounds the valley. Thus, in the accompanying section, +at the bridge of Cellent, six miles east of Olot, we see the lava on one +side of the small stream; while the inclined stratified rocks constitute +the channel and opposite bank. The upper part of the lava at that place, as +is usual in the currents of Etna and Vesuvius, is scoriaceous; farther down +it becomes less porous, and assumes a spheroidal structure; still lower it +divides in horizontal plates, each about 2 inches in thickness, and is more +compact. Lastly, at the bottom is a mass of prismatic basalt about 5 feet +thick. The vertical columns often rest immediately on the subjacent +secondary rocks; but there is sometimes an intervention of such sand and +scoriæ as cover the country during volcanic eruptions, and which when +unprotected, as here, by superincumbent lava, is washed away from the +surface of the land. Sometimes, the bed _d_ contains a few pebbles and +angular fragments of rock; in other places fine earth, which may have +constituted an ancient vegetable soil. + +In several localities, beds of sand and ashes are interposed between the +lava and subjacent stratified rock, as may be seen if we follow the course +of the lava-current which descends from Las Planas towards Amer, and stops +two miles short of that town. The river there has often cut through the +lava, and through 18 feet of underlying limestone. Occasionally an +alluvium, several feet thick, is interspersed between the igneous and +marine formation; and it is interesting to remark that in this, as in other +beds of pebbles occupying a similar position, there are no rounded +fragments of lava; whereas in the most modern gravel-beds of rivers of this +country, volcanic pebbles are abundant. + +The deepest excavation made by a river through lava, which I observed in +this part of Spain, is that seen in the bottom of a valley near San Feliu +de Palleróls, opposite the Castell de Stolles. The lava there has filled up +the bottom of a valley, and a narrow ravine has been cut through it to the +depth of 100 feet. In the lower part the lava has a columnar structure. A +great number of ages were probably required for the erosion of so deep a +ravine; but we have no reason to infer that this current is of higher +antiquity than those of the plain near Olot. The fall of the ground, and +consequent velocity of the stream, being in this case greater, a more +considerable volume of rock may have been removed in the same time. + +[Illustration: Fig. 474. Section at Castell Follit. + + A. Church and town of Castell Follit, overlooking precipices of basalt. + B. Small island, on each side of which branches of the river Teronel flow + to meet the Fluvia. + _c._ Precipice of basaltic lava, chiefly columnar, about 130 feet + in height. + _d._ Ancient alluvium, underlying the lava-current. + _e._ Inclined strata of secondary sandstone.] + +I shall describe one more section to elucidate the phenomena of this +district. A lava-stream, flowing from a ridge of hills on the east of +Olot, descends a considerable slope, until it reaches the valley of the +river Fluvia. Here, for the first time, it comes in contact with running +water, which has removed a portion, and laid open its internal structure +in a precipice about 130 feet in height, at the edge of which stands the +town of Castell Follit. + +By the junction of the rivers Fluvia and Teronel, the mass of lava has been +cut away on two sides; and the insular rock B (fig. 474.) has been left, +which was probably never so high as the cliff A, as it may have constituted +the lower part of the sloping side of the original current. + +From an examination of the vertical cliffs, it appears that the upper part +of the lava on which the town is built is scoriaceous, passing downwards +into a spheroidal basalt; some of the huge spheroids being no less than 6 +feet in diameter. Below this is a more compact basalt, with crystals of +olivine. There are in all five distinct ranges of basalt, the uppermost +spheroidal, and the rest prismatic, separated by thinner beds not columnar, +and some of which are schistose. These were probably formed by successive +flows of lava, whether during the same eruption or at different periods. +The whole mass rests on alluvium, ten or twelve feet in thickness, composed +of pebbles of limestone and quartz, but without any intermixture of igneous +rocks; in which circumstance alone it appears to differ from the modern +gravel of the Fluvia. + +_Bufadors._--The volcanic rocks near Olot have often a cavernous structure, +like some of the lavas of Etna; and in many parts of the hill of Batet, in +the environs of the town, the sound returned by the earth, when struck, is +like that of an archway. At the base of the same hill are the mouths of +several subterranean caverns, about twelve in number, which are called in +the country "bufadors," from which a current of cold air issues during +summer, but which in winter is said to be scarcely perceptible. I visited +one of these bufadors in the beginning of August, 1830, when the heat of +the season was unusually intense, and found a cold wind blowing from it, +which may easily be explained; for as the external air, when rarefied by +heat, ascends, the pressure of the colder and heavier air of the caverns in +the interior of the mountain causes it to rush out to supply its place. + +In regard to the age of these Spanish volcanos, attempts have been made +to prove, that in this country, as well as in Auvergne and the Eifel, +the earliest inhabitants were eye-witnesses to the volcanic action. In +the year 1421, it is said, when Olot was destroyed by an earthquake, an +eruption broke out near Amer, and consumed the town. The researches of +Don Francisco Bolos have, I think, shown, in the most satisfactory +manner, that there is no good historical foundation for the latter part +of this story; and any geologist who has visited Amer must be convinced +that there never was any eruption on that spot. It is true that, in the +year above mentioned, the whole of Olot, with the exception of a single +house, was cast down by an earthquake; one of those shocks which, at +distant intervals during the last five centuries, have shaken the +Pyrenees, and particularly the country between Perpignan and Olot, where +the movements, at the period alluded to, were most violent. + +The annihilation of the town may, perhaps, have been due to the cavernous +nature of the subjacent rocks; for Catalonia is beyond the line of those +European earthquakes which have, within the period of history, destroyed +towns throughout extensive areas. + +As we have no historical records, then, to guide us in regard to the +extinct volcanos, we must appeal to geological monuments. The annexed +diagram will present to the reader, in a synoptical form, the results +obtained from numerous sections. + +The more modern alluvium (_d_) is partial, and has been formed by the +action of rivers and floods upon the lava; whereas the older gravel (_b_) +was strewed over the country before the volcanic eruptions. In neither have +any organic remains been discovered; so that we can merely affirm, as yet, +that the volcanos broke out after the elevation of some of the newest rocks +of the nummulitic (Eocene?) series of Catalonia, and before the formation +of an alluvium (_d_) of unknown date. The integrity of the cones merely +shows that the country has not been agitated by violent earthquakes, or +subjected to the action of any great transient flood since their origin. + +[Illustration: Fig. 475. Superposition of rocks in the volcanic +district of Catalonia. + + _a._ Sandstone and nummulitic limestone. + _b._ Older alluvium without volcanic pebbles. + _c._ Cones of scoriæ and lava. + _d._ Newer alluvium.] + +East of Olot, on the Catalonian coast, marine tertiary strata occur, which, +near Barcelona, attain the height of about 500 feet. From the shells which +I collected, these strata appear to correspond in age with the Subapennine +beds; and it is not improbable that their upheaval from beneath the sea +took place during the period of volcanic eruption round Olot. In that case +these eruptions may have occurred at the close of the Older Pliocene era, +but perhaps subsequently, for their age is at present quite uncertain. + +_Miocene period--Volcanic rocks of the Eifel._--The chronological relations +of the volcanic rocks of the Lower Rhine and the Eifel are also involved in +a considerable degree of ambiguity; but we know that some portion of them +were coeval with the deposition of a tertiary formation, called +"Brown-Coal" by the Germans, which probably belongs to the Miocene, if not +referable to the Upper Eocene, epoch. + +This Brown-Coal is seen on both sides of the Rhine, in the neighbourhood +of Bonn, resting unconformably on highly inclined and vertical strata of +Silurian and Devonian rocks. Its position, and the space occupied by the +volcanic rocks, both of the Westerwald and Eifel, will be seen by +referring to the map in the next page (fig. 476.), for which I am +indebted to Mr. Horner, whose residence in the country has enabled him +to verify the maps of MM. Noeggerath and Von Oeynhausen, from which that +now given has been principally compiled. + +The Brown-Coal formation consists of beds of loose sand, sandstone, and +conglomerate, clay with nodules of clay-ironstone, and occasionally silex. +Layers of light brown, and sometimes black lignite, are interstratified +with the clays and sands, and often irregularly diffused through them. They +contain numerous impressions of leaves and stems of trees, and are +extensively worked for fuel, whence the name of the formation. + +[Illustration: Fig. 476. Map of the volcanic region of the Upper +and Lower Eifel. + + ____1____2____3____4____5 English Miles. + + Volcanic District {A. of the Upper Eifel. + {B. of the Lower Eifel. + Trachyte. + Points of eruption, with craters and scoriæ. + Basalt. + Brown-coal. + +_N.B._ The country in that part of the map which is left blank is composed +of inclined Silurian and Devonian rocks.] + +In several places, layers of trachytic tuff are interstratified, and in +these tuffs are leaves of plants identical with those found in the +brown-coal, showing that, during the period of the accumulation of the +latter, some volcanic products were ejected. + +The varieties of wood in the lignite are said to belong entirely to +dicotyledonous trees; but among the impressions of leaves, collected by Mr. +Horner, some were referred by Mr. Lindley to a palm, perhaps of the genus +_Chamærops_, and others resembled the _Cinnamomum dulce_, and _Podocarpus +macrophylla_, which would also indicate a warm climate.[416-A] + +The other organic remains of the brown-coal are principally fishes; they +are found in a bituminous shale, called paper-coal, from being divisible +into extremely thin leaves. The individuals are very numerous; but they +appear to belong to about five species, which M. Agassiz informs me are all +extinct, and hitherto peculiar to this brown-coal. They belong to the +freshwater genera _Leuciscus_, _Aspius_, and _Perca_. The remains of frogs +also, of an extinct species, have been discovered in the paper-coal; and a +complete series may be seen in the museum at Bonn, from the most imperfect +state of the tadpole to that of the full-grown animal. With these a +salamander, scarcely distinguishable from the recent species, has been +found, and several remains of insects. + +The brown-coal was evidently a freshwater formation; but fossil shells have +been scarcely ever found in it; although near Marienforst, in the vicinity +of Bonn, large blocks have been met with of a white opaque chert, +containing numerous casts of freshwater shells, which appear to belong to +_Planorbis rotundatus_ and _Limnea longiscata_, two species common both to +the Middle and Upper Eocene periods. It is very probable that the +brown-coal may be connected in age with those fluvio-marine formations +which are found in higher parts of the valley of the Rhine, as at Mayence +before mentioned (p. 177.). + +A vast deposit of gravel, chiefly composed of pebbles of white quartz, but +containing also a few fragments of other rocks, lies over the brown-coal +formation, forming sometimes only a thin covering, at others attaining a +thickness of more than 100 feet. This gravel is very distinct in character +from that now forming the bed of the Rhine. It is called "Kiesel gerolle" +by the Germans, often reaches great elevations, and is covered in several +places with volcanic ejections. It is evident that the country has +undergone great changes in its physical geography since this gravel was +formed; for its position has scarcely any relation to the existing drainage +of the country, and all the more modern volcanic rocks of the same region +are posterior to it in date. + +Some of the newest beds of volcanic sand, pumice, and scoriæ are +interstratified near Andernach and elsewhere with the loam called loess, +which was before described as being full of land and freshwater shells of +recent species, and referable to the Post-Pliocene period. I have before +hinted (see p. 118.) that this intercalation of volcanic matter between +beds of loess may possibly be explained without supposing the last +eruptions of the Lower Eifel to have taken place so recently as the era of +the deposition of the loess; but farther researches should be directed to +the investigation of this curious point. + +The igneous rocks of the Westerwald, and of the mountains called the +Siebengebirge, consist partly of basaltic and partly of trachytic lavas, +the latter being in general the more ancient of the two. There are many +varieties of trachyte, some of which are highly crystalline, resembling a +coarse-grained granite, with large separate crystals of felspar. Trachytic +tuff is also very abundant. These formations, some of which were certainly +contemporaneous with the origin of the brown-coal, were the first of a long +series of eruptions, the more recent of which happened when the country +had acquired nearly all its present geographical features. + +_Newer volcanos of the Eifel.--Lake-craters._--As I recognized in the +more modern volcanos of the Eifel characters distinct from any +previously observed by me in those of France, Italy, or Spain, I shall +briefly describe them. The fundamental rocks of the district are grey +and red sandstones and shales, with some associated limestones, replete +with fossils of the Devonian or Old Red Sandstone group. The volcanos +broke out in the midst of these inclined strata, and when the present +systems of hills and valleys had already been formed. The eruptions +occurred sometimes at the bottom of deep valleys, sometimes on the +summit of hills, and frequently on intervening platforms. In travelling +through this district we often fall upon them most unexpectedly, and may +find ourselves on the very edge of a crater before we had been led to +suspect that we were approaching the site of any igneous outburst. Thus, +for example, on arriving at the village of Gemund, immediately south of +Daun, we leave the stream, which flows at the bottom of a deep valley in +which strata of sandstone and shale crop out. We then climb a steep +hill, on the surface of which we see the edges of the same strata +dipping inwards towards the mountain. When we have ascended to a +considerable height, we see fragments of scoriæ sparingly scattered over +the surface; till, at length, on reaching the summit, we find ourselves +suddenly on the edge of a _tarn_, or deep circular lake-basin. + +[Illustration: Fig. 477. The Gemunder Maar.] + +[Illustration: Fig. 478. Cross section. + + _a._ Village of Gemund. + _b._ Gemunder Maar. + _c._ Weinfelder Maar. + _d._ Schalkenmehren Maar.] + +This, which is called the Gemunder Maar, is the first of three lakes which +are in immediate contact, the same ridge forming the barrier of two +neighbouring cavities (see fig. 477.). On viewing the first of these, we +recognize the ordinary form of a crater, for which we have been prepared +by the occurrence of scoriæ scattered over the surface of the soil. But on +examining the walls of the crater we find precipices of sandstone and shale +which exhibit no signs of the action of heat; and we look in vain for those +beds of lava and scoriæ, dipping in opposite directions on every side, +which we have been accustomed to consider as characteristic of volcanic +craters. As we proceed, however, to the opposite side of the lake, and +afterwards visit the craters _c_ and _d_ (fig. 478.), we find a +considerable quantity of scoriæ and some lava, and see the whole surface of +the soil sparkling with volcanic sand, and strewed with ejected fragments +of half-fused shale, which preserves its laminated texture in the interior, +while it has a vitrified or scoriform coating. + +A few miles to the south of the lakes above mentioned occurs the Pulvermaar +of Gillenfeld, an oval lake of very regular form, and surrounded by an +unbroken ridge of fragmentary materials, consisting of ejected shale and +sandstone, and preserving a uniform height of about 150 feet above the +water. The side slope in the interior is at an angle of about 45 degrees; +on the exterior, of 35 degrees. Volcanic substances are intermixed very +sparingly with the ejections, which in this place entirely conceal from +view the stratified rocks of the country.[419-A] + +[Illustration: Fig. 479. Outline of Mosenberg, Upper Eifel.] + +The Meerfelder Maar is a cavity of far greater size and depth, hollowed +out of similar strata; the sides presenting some abrupt sections of +inclined secondary rocks, which in other places are buried under vast +heaps of pulverized shale. I could discover no scoriæ amongst the +ejected materials, but balls of olivine and other volcanic substances +are mentioned as having been found.[419-B] This cavity, which we must +suppose to have discharged an immense volume of gas, is nearly a mile in +diameter, and is said to be more than one hundred fathoms deep. In the +neighbourhood is a mountain called the Mosenberg, which consists of red +sandstone and shale in its lower parts, but supports on its summit a +triple volcanic cone, while a distinct current of lava is seen +descending the flanks of the mountain. The edge of the crater of the +largest cone reminded me much of the form and characters of that of +Vesuvius; but I was much struck with the precipitous and almost +overhanging wall or parapet which the scoriæ presented towards the +exterior, as at _a b_ (fig. 479.); which I can only explain by supposing +that fragments of red-hot lava, as they fell round the vent, were +cemented together into one compact mass, in consequence of continuing to +be in a half-melted state. + +If we pass from the Upper to the Lower Eifel, from A to B (see map, p. +416.), we find the celebrated lake-crater of Laach, which has a greater +resemblance than any of those before mentioned to the Lago di Bolsena, and +others in Italy--being surrounded by a ridge of gently sloping hills, +composed of loose tuffs, scoriæ, and blocks of a variety of lavas. + +One of the most interesting volcanos on the left bank of the Rhine is +called the Roderberg. It forms a circular crater nearly a quarter of a mile +in diameter, and 100 feet deep, now covered with fields of corn. The highly +inclined strata of ancient sandstone and shale rise even to the rim of one +side of the crater; but they are overspread by quartzose gravel, and this +again is covered by volcanic scoriæ and tufaceous sand. The opposite wall +of the crater is composed of cinders and scorified rock, like that at the +summit of Vesuvius. It is quite evident that the eruption in this case +burst through the sandstone and alluvium which immediately overlies it; and +I observed some of the quartz pebbles mixed with scoriæ on the flanks of +the mountain, as if they had been cast up into the air, and had fallen +again with the volcanic ashes. I have already observed, that a large part +of this crater has been filled up with loess (p. 118.). + +The most striking peculiarity of a great many of the craters above +described, is the absence of any signs of alteration or torrefaction in +their walls, when these are composed of regular strata of ancient sandstone +and shale. It is evident that the summits of hills formed of the +above-mentioned stratified rocks have, in some cases, been carried away by +gaseous explosions, while at the same time no lava, and often a very small +quantity only of scoriæ, has escaped from the newly formed cavity. There +is, indeed, no feature in the Eifel volcanos more worthy of note, than the +proofs they afford of very copious aëriform discharges, unaccompanied by +the pouring out of melted matter, except, here and there, in very +insignificant volume. I know of no other extinct volcanos where gaseous +explosions of such magnitude have been attended by the emission of so small +a quantity of lava. Yet I looked in vain in the Eifel for any appearances +which could lend support to the hypothesis, that the sudden rushing out of +such enormous volumes of gas had ever lifted up the stratified rocks +immediately around the vent, so as to form conical masses, having their +strata dipping outwards on all sides from a central axis, as is assumed in +the theory of elevation craters, alluded to at the end of Chap. XXIX. + +_Trass._--In the Lower Eifel, eruptions of trachytic lava preceded the +emission of currents of basalt, and immense quantities of pumice were +thrown out wherever trachyte issued. The tufaceous alluvium called +_trass_, which has covered large areas in this region and choked up some +valleys now partially re-excavated, is unstratified. Its base consists +almost entirely of pumice, in which are included fragments of basalt and +other lavas, pieces of burnt shale, slate, and sandstone, and numerous +trunks and branches of trees. If this trass was formed during the period +of volcanic eruptions it may perhaps have originated in the manner of +the moya of the Andes. + +We may easily conceive that a similar mass might now be produced, if a +copious evolution of gases should occur in one of the lake basins. The +water might remain for weeks in a state of violent ebullition, until it +became of the consistency of mud, just as the sea continued to be charged +with red mud round Graham's Island, in the Mediterranean, in the year 1831. +If a breach should then be made in the side of the cone, the flood would +sweep away great heaps of ejected fragments of shale and sandstone, which +would be borne down into the adjoining valleys. Forests might be torn up by +such a flood, and thus the occurrence of the numerous trunks of trees +dispersed irregularly through the trass, can be explained. + +_Hungary._--M. Beudant, in his elaborate work on Hungary, describes five +distinct groups of volcanic rocks, which although nowhere of great +extent, form striking features in the physical geography of that +country, rising as they do abruptly from extensive plains composed of +tertiary strata. They may have constituted islands in the ancient sea, +as Santorin and Milo now do in the Grecian Archipelago; and M. Beudant +has remarked that the mineral products of the last-mentioned islands +resemble remarkably those of the Hungarian extinct volcanos, where many +of the same minerals as opal, calcedony, resinous silex (_silex +resinite_), pearlite, obsidian, and pitchstone abound. + +The Hungarian lavas are chiefly felspathic, consisting of different +varieties of trachyte; many are cellular, and used as millstones; some so +porous and even scoriform as to resemble those which have issued in the +open air. Pumice occurs in great quantity; and there are conglomerates, or +rather breccias, wherein fragments of trachyte are bound together by +pumiceous tuff, or sometimes by silex. + +It is probable that these rocks were permeated by the waters of hot +springs, impregnated, like the Geysers, with silica; or in some instances, +perhaps, by aqueous vapours, which, like those of Lancerote, may have +precipitated hydrate of silica. + +By the influence of such springs or vapours the trunks and branches of +trees washed down during floods, and buried in tuffs on the flanks of the +mountains, are supposed to have become silicified. It is scarcely possible, +says M. Beudant, to dig into any of the pumiceous deposits of these +mountains without meeting with opalized wood, and sometimes entire +silicified trunks of trees of great size and weight. + +It appears from the species of shells collected principally by M. Boué, +and examined by M. Deshayes, that the fossil remains imbedded in the +volcanic tuffs, and in strata alternating with them in Hungary, are of +the Miocene type, and not identical, as was formerly supposed, with the +fossils of the Paris basin. + + +FOOTNOTES: + +[409-A] Maclure, Journ. de Phys., vol. lxvi. p. 219., 1808; cited by +Daubeny, Description of Volcanos, p. 24. + +[410-A] This view is taken from a sketch which I made on the spot in 1830. + +[416-A] Trans. of Geol. Soc., 2d series, vol. v. + +[419-A] Scrope, Edin. Journ. of Sci., June, 1826, p. 145. + +[419-B] Hibbert, Extinct Volcanos of the Rhine, p. 24. + + + + +CHAPTER 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 + Côme--Puy de Pariou--Cones not denuded by general flood--Velay--Bones + of quadrupeds buried in scoriæ--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. + + +_Tertiary Volcanic Rocks.--Auvergne._--The extinct volcanos of Auvergne +and Cantal in Central France seem to have commenced their eruptions in +the Upper Eocene period, but to have been most active during the Miocene +and Pliocene eras. I have already alluded to the grand succession of +events, of which there is evidence in Auvergne since the last retreat of +the sea (see p. 178.). + +The earliest monuments of the tertiary period in that region are +lacustrine deposits of great thickness (2. fig. 480. p. 424.), in the +lowest conglomerates of which are rounded pebbles of quartz, +mica-schist, granite, and other non-volcanic rocks, without the +slightest intermixture of igneous products. To these conglomerates +succeed argillaceous and calcareous marls and limestones (3. fig. 480.) +containing Upper Eocene shells and bones of mammalia, the higher beds +of which sometimes alternate with volcanic tuff of contemporaneous +origin. After the filling up or drainage of the ancient lakes, huge +piles of trachytic and basaltic rocks, with volcanic breccias, +accumulated to a thickness of several thousand feet, and were +superimposed upon granite, or the contiguous lacustrine strata. The +greater portion of these igneous rocks appear to have originated during +the Miocene and Pliocene periods; and extinct quadrupeds of those eras, +belonging to the genera Mastodon, Rhinoceros, and others, were buried +in ashes and beds of alluvial sand and gravel, which owe their +preservation to overspreading sheets of lava. + +In Auvergne the most ancient and conspicuous of the volcanic masses is +Mont Dor, which rests immediately on the granitic rocks standing apart +from the freshwater strata.[422-A] This great mountain rises suddenly to +the height of several thousand feet above the surrounding platform, and +retains the shape of a flattened and somewhat irregular cone, all the +sides sloping more or less rapidly, until their inclination is gradually +lost in the high plain around. This cone is composed of layers of +scoriæ, pumice-stones, and their fine detritus, with interposed beds of +trachyte and basalt, which descend often in uninterrupted sheets, till +they reach and spread themselves round the base of the mountain.[423-A] +Conglomerates, also, composed of angular and rounded fragments of +igneous rocks, are observed to alternate with the above; and the various +masses are seen to dip off from the central axis, and to lie parallel to +the sloping flanks of the mountain. + +The summit of Mont Dor terminates in seven or eight rocky peaks, where no +regular crater can now be traced, but where we may easily imagine one to +have existed, which may have been shattered by earthquakes, and have +suffered degradation by aqueous agents. Originally, perhaps, like the +highest crater of Etna, it may have formed an insignificant feature in the +great pile, and may frequently have been destroyed and renovated. + +According to some geologists, this mountain, as well as Vesuvius, Etna, and +all large volcanos, has derived its dome-like form not from the +preponderance of eruptions from one or more central points, but from the +upheaval of horizontal beds of lava and scoriæ. I have explained my reasons +for objecting to this view at the close of Chap. XXIX., when speaking of +Palma, and in the Principles of Geology.[423-B] The average inclination of +the dome-shaped mass of Mont Dor is 8° 6', whereas in Mounts Loa and Kea, +before mentioned, in the Sandwich Islands (see fig. 457. p. 394.), the +flanks of which have been raised by recent lavas, we find from Mr. Dana's +description that the one has a slope of 6° 30', the other of 7° 46'. We +may, therefore, reasonably question whether there is any absolute necessity +for supposing that the basaltic currents of the ancient French volcano were +at first more horizontal than they are now. Nevertheless it is highly +probable that during the long series of eruptions required to give rise to +so vast a pile of volcanic matter, which is thickest at the summit or +centre of the dome, some dislocation and upheaval took place; and during +the distension of the mass, beds of lava and scoriæ may, in some places, +have acquired a greater, in others a less inclination, than that which at +first belonged to them. + +Respecting the age of the great mass of Mont Dor, we cannot come at present +to any positive decision, because no organic remains have yet been found in +the tuffs, except impressions of the leaves of trees of species not yet +determined. We may certainly conclude, that the earliest eruptions were +posterior in origin to those grits, and conglomerates of the freshwater +formation of the Limagne, which contain no pebbles of volcanic rocks; +while, on the other hand, some eruptions took place before the great lakes +were drained; and others occurred after the desiccation of those lakes, and +when deep valleys had already been excavated through freshwater strata. + +In the annexed section, I have endeavoured to explain the geological +structure of a portion of Auvergne, which I re-examined in 1843.[423-C] It +may convey some idea to the reader of the long and complicated series of +events, which have occurred in that country, since the first lacustrine +strata (No. 2.) were deposited on the granite (No. 1.). The changes of +which we have evidence are the more striking, because they imply great +denudation, without there being any proofs of the intervention of the sea +during the whole period. It will be seen that the upper freshwater beds +(No. 3.), once formed in a lake, must have suffered great destruction +before the excavation of the valleys of the Couze and Allier had begun. In +these freshwater beds, Upper Eocene fossils, as described in Chap. XV., +have been found. The basaltic dike 4' is one of many examples of the +intrusion of volcanic matter through the Eocene freshwater beds, and may +have been of Upper Eocene or Miocene date, giving rise, when it reached the +surface and overflowed, to such platforms of basalt, as often cap the +tertiary hills in Auvergne, and one of which (4) is seen on Mont Perrier. + +[Illustration: Fig. 480. Section from the valley of the Couze at Nechers, +through Mont Perrier and Issoire to the Valley of the Allier, and the Tour +de Boulade, Auvergne. + + 10. Lava-current of Tartaret near its termination at Nechers. + 9. Bone-bed, red sandy clay under the lava of Tartaret. + 8. Bone-bed of the Tour de Boulade. + 7. Alluvium newer than No. 6. + 6. Alluvium with bones of hippopotamus. + 5 _c._ Trachytic breccia resembling 5 _a._ + 5 _b._ Upper bone-bed of Perrier, gravel, &c. + 5 _a._ Pumiceous breccia and conglomerate, angular masses of trachyte, + quartz, pebbles, &c. + 5. Lower bone-bed of Perrier, ochreous sand and gravel. + 4 _a._ Basaltic dyke. + 4. Basaltic platform. + 3. Upper freshwater beds, limestone, marl, gypsum, &c. + 2. Lower freshwater formation, red clay, green sand, &c. + 1. Granite.] + +It not unfrequently happens that beds of gravel containing bones of extinct +mammalia are detected under these very ancient sheets of basalt, as between +No. 4. and the freshwater strata, No. 3., at A, from which it is clear that +the surface of 3 formed at that period the lowest level at which the waters +then draining the country flowed. Next in age to this basaltic platform +comes a patch of ochreous sand and gravel (No. 5.), containing many bones +of quadrupeds. Upon this rests a pumiceous breccia and conglomerate, with +angular masses of trachyte, and some quartz pebbles. This deposit is +followed by 5 _b_, which is similar to 5, and 5 _c_ similar to the +trachytic breccia 5 _a_. These two breccias are supposed, from their +similarity to others found on Mount Dor, to have descended from the flanks +of that mountain during eruptions; and the interstratified alluvial +deposits contain the remains of mastodon, rhinoceros, tapir, deer, beaver, +and quadrupeds of other genera referable to about forty species, all of +which are extinct. I formerly supposed them to belong to the same era as +the Miocene faluns of Touraine; but, whether they may not rather be +ascribed to the older Pliocene epoch is a question which farther inquiries +and comparisons must determine. + +Whatever be their date in the tertiary series, they are quadrupeds which +inhabited the country when the formations 5 and 5 _c_ originated. +Probably they were drowned during floods, such as rush down the flanks +of volcanos during eruptions, when great bodies of steam are emitted +from the crater, or when, as we have seen, both on Etna and in Iceland +in modern times, large masses of snow are suddenly melted by lava, +causing a deluge of water to bear down fragments of igneous rocks mixed +with mud, to the valleys and plains below. + +It will be seen that the valley of the Issoire, down which these ancient +inundations swept, was first excavated at the expense of the formations +2, 3, and 4, and then filled up by the masses 5 and 5 _c_, after which +it was re-excavated before the more modern alluviums (Nos. 6. and 7.) +were formed. In these again other fossil mammalia of distinct species +have been detected by M. Bravard, the bones of an hippopotamus having +been found among the rest. + +At length, when the valley of the Allier was eroded at Issoire down to its +lowest level, a talus of angular fragments of basalt and freshwater +limestone (No. 8.) was formed, called the bone-bed of the Tour de Boulade, +from which a great many other mammalia have been collected by MM. Bravard +and Pomel. In this assemblage the _Elephas primigenius_, _Rhinoceros +tichorinus_, _Deer_ (including rein-deer), _Equus_, _Bos_, _Antelope_, +_Felis_, and _Canis_, were included. Even this deposit seems hardly to be +the newest in the neighbourhood, for if we cross from the town of Issoire +(see fig. 480.) over Mont Perrier to the adjoining valley of the Couze, we +find another bone-bed (No. 9.), overlaid by a current of lava (No. 10.). + +The history of this lava-current, which terminates a few hundred yards +below the point No. 10., in the suburbs of the village of Nechers, is +interesting. It forms a long narrow stripe more than 13 miles in length, at +the bottom of the valley of the Couze, which flows out of a lake at the +foot of Mont Dor. This lake is caused by a barrier thrown across the +ancient channel of the Couze, consisting partly of the volcanic cone called +the Puy de Tartaret, formed of loose scoriæ, from the base of which has +issued the lava-current before mentioned. The materials of the dam which +blocked up the river, and caused the Lac de Chambon, are also, in part, +derived from a land-slip which may have happened at the time of the great +eruption which formed the cone. + +This cone of Tartaret affords an impressive monument of the very different +dates at which the igneous eruptions of Auvergne have happened; for it was +evidently thrown up at the bottom of the existing valley, which is bounded +by lofty precipices composed of sheets of ancient columnar trachyte and +basalt, which once flowed at very high levels from Mont Dor.[425-A] + +When we follow the course of the river Couze, from its source in the +lake of Chambon, to the termination of the lava-current at Nechers, a +distance of thirteen miles, we find that the torrent has in most places +cut a deep channel through the lava, the lower portion of which is +columnar. In some narrow gorges it has even had power to remove the +entire mass of basaltic rock, though the work of erosion must have been +very slow, as the basalt is tough and hard, and one column after another +must have been undermined and reduced to pebbles, and then to sand. +During the time required for this operation, the perishable cone of +Tartaret, composed of sand and ashes, has stood uninjured, proving that +no great flood or deluge can have passed over this region in the +interval between the eruption of Tartaret and our own times. + +If we now return to the section (fig. 480.), I may observe that the +lava-current of Tartaret, which has diminished greatly in height and +volume near its termination, presents here a steep and perpendicular +face 25 feet in height towards the river. Beneath it is the alluvium No. +9., consisting of a red sandy clay, which must have covered the bottom +of the valley when the current of melted rock flowed down. The bones +found in this alluvium, which I obtained myself, consisted of a species +of field-mouse, _Arvicola_, and the molar tooth of an extinct horse, +_Equus fossilis_. The other species, obtained from the same bed, are +referable to the genera _Sus_, _Bos_, _Cervus_, _Felis_, _Canis_, +_Martes_, _Talpa_, _Sorex_, _Lepus_, _Sciurus_, _Mus_, and _Lagomys_, in +all no less than forty-three species, all closely allied to recent +animals, yet nearly all of them, according to M. Bravard, showing some +points of difference, like those which Mr. Owen discovered in the case +of the horse above alluded to. The bones, also, of a frog, snake, and +lizard, and of several birds, were associated with the fossils before +enumerated, and several recent land shells, such as _Cyclostoma +elegans_, _Helix hortensis_, _H. nemoralis_, _H. lapicida_, and +_Clausilia rugosa_. If the animals were drowned by floods, which +accompanied the eruptions of the Puy de Tartaret, they would give an +exceedingly modern geological date to that event, which must, in that +case, have belonged to the Newer-Pliocene, or, perhaps, the +Post-Pliocene period. That the current, which has issued from the Puy de +Tartaret, may nevertheless be very ancient in reference to the events of +human history, we may conclude, not only from the divergence of the +mammiferous fauna from that of our day, but from the fact that a Roman +bridge of such form and construction as continued in use down to the +fifth century, but which may be older, is now seen at a place about a +mile and a half from St. Nectaire. This ancient bridge spans the river +Couze with two arches, each about 14 feet wide. These arches spring from +the lava of Tartaret, on both banks, showing that a ravine precisely +like that now existing, had already been excavated by the river through +that lava thirteen or fourteen centuries ago. + +In Central France there are several hundred minor cones, like that of +Tartaret, a great number of which, like Monte Nuovo, near Naples, may have +been principally due to a single eruption. Most of these cones range in a +linear direction from Auvergne to the Vivarais, and they were faithfully +described so early as the year 1802, by M. de Montlosier. They have given +rise chiefly to currents of basaltic lava. Those of Auvergne called the +Monts Dome, placed on a granitic platform, form an irregular ridge (see +fig. 436.), about 18 miles in length, and 2 in breadth. They are usually +truncated at the summit, where the crater is often preserved entire, the +lava having issued from the base of the hill. But frequently the crater is +broken down on one side, where the lava has flowed out. The hills are +composed of loose scoriæ, blocks of lava, lapilli, and pozzuolana, with +fragments of trachyte and granite. + +_Puy de Côme._--The Puy de Côme and its lava-current, near Clermont, may +be mentioned as one of these minor volcanos. This conical hill rises +from the granitic platform, at an angle of about 40°, to the height of +more than 900 feet. Its summit presents two distinct craters, one of +them with a vertical depth of 250 feet. A stream of lava takes its rise +at the western base of the hill, instead of issuing from either crater, +and descends the granitic slope towards the present site of the town of +Pont Gibaud. Thence it pours in a broad sheet down a steep declivity +into the valley of the Sioule, filling the ancient river-channel for the +distance of more than a mile. The Sioule, thus dispossessed of its bed, +has worked out a fresh one between the lava and the granite of its +western bank; and the excavation has disclosed, in one spot, a wall of +columnar basalt about 50 feet high.[427-A] + +The excavation of the ravine is still in progress, every winter some +columns of basalt being undermined and carried down the channel of the +river, and in the course of a few miles rolled to sand and pebbles. +Meanwhile the cone of Côme remains stationary, its loose materials being +protected by a dense vegetation, and the hill standing on a ridge not +commanded by any higher ground whence floods of rain-water may descend. + +_Puy Rouge._--At another point, farther down the course of the Sioule, +we find a second illustration of the same phenomenon in the Puy Rouge, a +conical hill to the north of the village of Pranal. The cone is composed +entirely of red and black scoriæ, tuff, and volcanic bombs. On its +western side there is a worn-down crater, whence a powerful stream of +lava has issued, and flowed into the valley of the Sioule. The river has +since excavated a ravine through the lava and subjacent gneiss, to the +depth of 400 feet. + +On the upper part of the precipice forming the left side of this ravine, +we see a great mass of black and red scoriaceous lava; below this a thin +bed of gravel, evidently an ancient river-bed, now at an elevation of 50 +feet above the channel of the Sioule. The gravel again rests upon +gneiss, which has been eroded to the depth of 50 feet. It is quite +evident in this case, that, while the basalt was gradually undermined +and carried away by the force of running water, the cone whence the lava +issued escaped destruction, because it stood upon a platform of gneiss +several hundred feet above the level of the valley in which the force of +running water was exerted. + +_Puy de Pariou._--The brim of the crater of the Puy de Pariou, near +Clermont, is so sharp, and has been so little blunted by time, that it +scarcely affords room to stand upon. This and other cones in an equally +remarkable state of integrity have stood, I conceive uninjured, not _in +spite_ of their loose porous nature, as might at first be naturally +supposed, but in consequence of it. No rills can collect where all the rain +is instantly absorbed by the sand and scoriæ, as is remarkably the case on +Etna; and nothing but a waterspout breaking directly upon the Puy de Pariou +could carry away a portion of the hill, so long as it is not rent or +engulphed by earthquakes. + +Hence it is conceivable that even those cones which have the freshest +aspect, and most perfect shape, may lay claim to very high antiquity. Dr. +Daubeny has justly observed, that had any of these volcanos been in a state +of activity in the age of Julius Cæsar, that general, who encamped upon the +plains of Auvergne, and laid siege to its principal city (Gergovia, near +Clermont), could hardly have failed to notice them. Had there been any +record of their eruptions in the time of Pliny or Sidonius Apollinaris, the +one would scarcely have omitted to make mention of it in his Natural +History, nor the other to introduce some allusion to it among the +descriptions of this his native province. This poet's residence was on the +borders of the Lake Aidat, which owed its very existence to the damming up +of a river by one of the most modern lava-currents.[428-A] + +_Velay._--The observations of M. Bertrand de Doue have not yet established +that any of the most ancient volcanos of Velay were in action during the +Eocene period. There are beds of gravel in Velay, as in Auvergne, covered +by lava at different heights above the channels of the existing rivers. In +the highest and most ancient of these alluviums the pebbles are exclusively +of granitic rocks; but in the newer, which are found at lower levels, and +which originated when the valleys had been cut to a greater depth, an +intermixture of volcanic rocks has been observed. + +At St. Privat d'Allier a bed of volcanic scoriæ and tuff was discovered +by Dr. Hibbert, inclosed between two sheets of basaltic lava; and in +this tuff were found the bones of several quadrupeds, some of them +adhering to masses of slaggy lava. Among other animals were _Rhinoceros +leptorhinus_, _Hyæna spelæa_, and a species allied to the spotted hyæna +of the Cape, together with four undetermined species of deer.[428-B] The +manner of the occurrence of these bones reminds us of the published +accounts of an eruption of Coseguina, 1835, in Central America (see p. +399.), during which hot cinders and scoriæ fell and scorched to death +great numbers of wild and domestic animals and birds. + +_Plomb du Cantal._--In regard to the age of the igneous rocks of the +Cantal, we can at present merely affirm, that they overlie the Eocene +lacustrine strata of that country (see Map, p. 179.). They form a great +dome-shaped mass, having an average slope of only 4°, which has evidently +been accumulated, like the cone of Etna, during a long series of eruptions. +It is composed of trachytic, phonolitic, and basaltic lavas, tuffs, and +conglomerates, or breccias, forming a mountain several thousand feet in +height. Dikes also of phonolite, trachyte, and basalt are numerous, +especially in the neighbourhood of the large cavity, probably once a +crater, around which the loftiest summits of the Cantal are ranged +circularly, few of them, except the Plomb du Cantal, rising far above the +border or ridge of this supposed crater. A pyramidal hill, called the Puy +Griou, occupies the middle of the cavity.[429-A] It is clear that the +volcano of the Cantal broke out precisely on the site of the lacustrine +deposit before described (p. 188.), which had accumulated in a depression +of a tract composed of micaceous schist. In the breccias, even to the very +summit of the mountain, we find ejected masses of the freshwater beds, and +sometimes fragments of flint, containing Eocene shells. Valleys radiate in +all directions from the central heights of the mountain, increasing in size +as they recede from those heights. Those of the Cer and Jourdanne, which +are more than 20 miles in length, are of great depth, and lay open the +geological structure of the mountain. No alternation of lavas with +undisturbed Eocene strata has been observed, nor any tuffs containing +freshwater shells, although some of these tuffs include fossil remains of +terrestrial plants, said to imply several distinct restorations of the +vegetation of the mountain in the intervals between great eruptions. On the +northern side of the Plomb du Cantal, at La Vissiere, near Murat, is a +spot, pointed out on the Map (p. 179.), where freshwater limestone and marl +are seen covered by a thickness of about 800 feet of volcanic rock. Shifts +are here seen in the strata of limestone and marl.[429-B] + +_Eocene period._--In treating of the lacustrine deposits of Central +France, in the fifteenth chapter, it was stated that, in the arenaceous +and pebbly group of the lacustrine basins of Auvergne, Cantal, and +Velay, no volcanic pebbles had ever been detected, although massive +piles of igneous rocks are now found in the immediate vicinity. As this +observation has been confirmed by minute research, we are warranted in +inferring that the volcanic eruptions had not commenced when the older +subdivisions of the freshwater groups originated. + +In Cantal and Velay no decisive proofs have yet been brought to light that +any of the igneous outbursts happened during the deposition of the +freshwater strata; but there can be no doubt that in Auvergne some volcanic +explosions took place before the drainage of the lakes, and at a time when +the Upper Eocene species of animals and plants still flourished. Thus, for +example, at Pont du Chateau, near Clermont, a section is seen in a +precipice on the right bank of the river Allier, in which beds of volcanic +tuff alternate with a freshwater limestone, which is in some places pure, +but in others spotted with fragments of volcanic matter, as if it were +deposited while showers of sand and scoriæ were projected from a +neighbouring vent.[430-A] + +Another example occurs in the Puy de Marmont, near Veyres, where a +freshwater marl alternates with volcanic tuff containing Eocene shells. The +tuff or breccia in this locality is precisely such as is known to result +from volcanic ashes falling into water, and subsiding together with ejected +fragments of marl and other stratified rocks. These tuffs and marls are +highly inclined, and traversed by a thick vein of basalt, which, as it +rises in the hill, divides into two branches. + +_Gergovia._--The hill of Gergovia, near Clermont, affords a third example. +I agree with MM. Dufrénoy and Jobert that there is no alternation here of a +contemporaneous sheet of lava with freshwater strata, in the manner +supposed by some other observers[430-B]; but the position and contents of +some of the associated tuffs, prove them to have been derived from volcanic +eruptions which occurred during the deposition of the lacustrine strata. + +[Illustration: Fig. 481. Hill of Gergovia.] + +The bottom of the hill consists of slightly inclined beds of white and +greenish marls, more than 300 feet in thickness, intersected by a dike of +basalt, which may be studied in the ravine above the village of Merdogne. +The dike here cuts through the marly strata at a considerable angle, +producing, in general, great alteration and confusion in them for some +distance from the point of contact. Above the white and green marls, a +series of beds of limestone and marl, containing freshwater shells, are +seen to alternate with volcanic tuff. In the lowest part of this division, +beds of pure marl alternate with compact fissile tuff, resembling some of +the subaqueous tuffs of Italy and Sicily called _peperinos_. Occasionally +fragments of scoriæ are visible in this rock. Still higher is seen another +group of some thickness, consisting exclusively of tuff, upon which lie +other marly strata intermixed with volcanic matter. Among the species of +fossil shells which I found in these strata were _Melania inquinata_, a +_Unio_, and a _Melanopsis_, but they were not sufficient to enable me to +determine with precision the age of the formation. + +There are many points in Auvergne where igneous rocks have been forced by +subsequent injection through clays and marly limestones, in such a manner +that the whole has become blended in one confused and brecciated mass, +between which and the basalt there is sometimes no very distinct line of +demarcation. In the cavities of such mixed rocks we often find calcedony, +and crystals of mesotype, stilbite, and arragonite. To formations of this +class may belong some of the breccias immediately adjoining the dike in the +hill of Gergovia; but it cannot be contended that the volcanic sand and +scoriæ interstratified with the marls and limestones in the upper part of +that hill were introduced, like the dike, subsequently, by intrusion from +below. They must have been thrown down like sediment from water, and can +only have resulted from igneous action, which was going on +contemporaneously with the deposition of the lacustrine strata. + +The reader will bear in mind that this conclusion agrees well with the +proofs, adverted to in the fifteenth chapter, of the abundance of silex, +travertin, and gypsum precipitated when the upper lacustrine strata were +formed; for these rocks are such as the waters of mineral and thermal +springs might generate. + +_Cretaceous period._--Although we have no proof of volcanic rocks +erupted in England during the deposition of the chalk and greensand, it +would be an error to suppose that no theatres of igneous action existed +in the cretaceous period. M. Virlet, in his account of the geology of +the Morea, p. 205., has clearly shown that certain traps in Greece, +called by him ophiolites, are of this date; as those, for example, which +alternate conformably with cretaceous limestone and greensand between +Kastri and Damala in the Morea. They consist in great part of diallage +rocks and serpentine, and of an amygdaloid with calcareous kernels, +and a base of serpentine. + +In certain parts of the Morea, the age of these volcanic rocks is +established by the following proofs: first, the lithographic limestones +of the Cretaceous era are cut through by trap, and then a conglomerate +occurs, at Nauplia and other places, containing in its calcareous cement +many well-known fossils of the chalk and greensand, together with +pebbles formed of rolled pieces of the same ophiolite, which appear in +the dikes above alluded to. + +_Period of Oolite and Lias._--Although the green and serpentinous trap +rocks of the Morea belong chiefly to the Cretaceous era, as before +mentioned, yet it seems that some eruptions of similar rocks began during +the Oolitic period[431-A]; and it is probable, that a large part of the +trappean masses, called ophiolites in the Apennines, and associated with +the limestone of that chain, are of corresponding age. + +That part of the volcanic rocks of the Hebrides, in our own country, +originated contemporaneously with the Oolite which they traverse and +overlie, has been ascertained by Prof. E. Forbes, in 1850. + +_Trap of the New Red Sandstone period._--In the southern part of +Devonshire, trappean rocks are associated with New Red Sandstone, and, +according to Sir H. De la Beche, have not been intruded subsequently into +the sandstone, but were produced by contemporaneous volcanic action. Some +beds of grit, mingled with ordinary red marl, resemble sands ejected from a +crater; and in the stratified conglomerates occurring near Tiverton are +many angular fragments of trap porphyry, some of them one or two tons in +weight, intermingled with pebbles of other rocks. These angular fragments +were probably thrown out from volcanic vents, and fell upon sedimentary +matter then in the course of deposition.[432-A] + +_Carboniferous period._--Two classes of contemporaneous trap rocks have +been ascertained by Dr. Fleming to occur in the coal-field of the Forth in +Scotland. The newest of these, connected with the higher series of +coal-measures, is well exhibited along the shores of the Forth, in +Fifeshire, where they consist of basalt with olivine, amygdaloid, +greenstone, wacké, and tuff. They appear to have been erupted while the +sedimentary strata were in a horizontal position, and to have suffered the +same dislocations which those strata have subsequently undergone. In the +volcanic tuffs of this age are found not only fragments of limestone, +shale, flinty slate, and sandstone, but also pieces of coal. + +The other or older class of carboniferous traps are traced along the +south margin of Stratheden, and constitute a ridge parallel with the +Ochils, and extending from Stirling to near St. Andrews. They consist +almost exclusively of greenstone, becoming, in a few instances, earthy +and amygdaloidal. They are regularly interstratified with the sandstone, +shale, and ironstone of the lower Coal-measures, and, on the East +Lomond, with Mountain Limestone. + +I examined these trap rocks in 1838, in the cliffs south of St. Andrews, +where they consist in great part of stratified tuffs, which are curved, +vertical, and contorted, like the associated coal-measures. In the tuff I +found fragments of carboniferous shale and limestone, and intersecting +veins of greenstone. At one spot, about two miles from St. Andrews, the +encroachment of the sea on the cliffs has isolated several masses of trap, +one of which (fig. 482.) is aptly called the "rock and spindle,"[432-B] for +it consists of a pinnacle of tuff, which may be compared to a distaff, and +near the base is a mass of columnar greenstone, in which the pillars +radiate from a centre, and appear at a distance like the spokes of a wheel. +The largest diameter of this wheel is about twelve feet, and the polygonal +terminations of the columns are seen round the circumference (or tire, as +it were, of the wheel), as in the accompanying figure. I conceive this mass +to be the extremity of a string or vein of greenstone, which penetrated the +tuff. The prisms point in every direction, because they were surrounded on +all sides by cooling surfaces, to which they always, arrange themselves at +right angles, as before explained (p. 385.). + +[Illustration: Fig. 482. Rock and Spindle, St. Andrews. + + _a._ Unstratified tuff. + _b._ Columnar greenstone. + _c._ Stratified tuff.] + +[Illustration: Fig. 483. Columns of Greenstone, seen endwise.] + +A trap dike was pointed out to me by Dr. Fleming, in the parish of Flisk, +in the northern part of Fifeshire, which cuts through the grey sandstone +and shale, forming the lowest part of the Old Red Sandstone. It may be +traced for many miles, passing through the amygdaloidal and other traps of +the hill called Normans Law. In its course it affords a good +exemplification of the passage from the trappean into the plutonic, or +highly crystalline texture. Professor Gustavus Rose, to whom I submitted +specimens of this dike, finds the rock, which he calls dolerite, to consist +of greenish black augite and Labrador felspar, the latter being the most +abundant ingredient. A small quantity of magnetic iron, perhaps +titaniferous, is also present. The result of this analysis is interesting, +because both the ancient and modern lavas of Etna consist in like manner of +augite, Labradorite, and titaniferous iron. + +_Trap of the Old Red sandstone period._--By referring to the section +explanatory of the structure of Forfarshire, already given (p. 48.), the +reader will perceive that beds of conglomerate, No. 3., occur in the +middle of the Old Red sandstone system, 1, 2, 3, 4. The pebbles in these +conglomerates are sometimes composed of granitic and quartz rocks, +sometimes exclusively of different varieties of trap, which, although +purposely omitted in the above section, are often found either intruding +themselves in amorphous masses and dikes into the old fossiliferous +tilestones, No. 4., or alternating with them in conformable beds. All +the different divisions of the red sandstone, 1, 2, 3, 4, are +occasionally intersected by dikes, but they are very rare in Nos. 1. and +2., the upper members of the group consisting of red shale and red +sandstone. These phenomena, which occur at the foot of the Grampians, +are repeated in the Sidlaw Hills; and it appears that in this part of +Scotland, volcanic eruptions were most frequent in the earlier part of +the Old Red sandstone period. + +The trap rocks alluded to consist chiefly of felspathic porphyry and +amygdaloid, the kernels of the latter being sometimes calcareous, often +calcedonic, and forming beautiful agates. We meet also with claystone, +clinkstone, greenstone, compact felspar, and tuff. Some of these rocks +flowed as lavas over the bottom of the sea, and enveloped quartz pebbles +which were lying there, so as to form conglomerates with a base of +greenstone, as is seen in Lumley Den, in the Sidlaw Hills. On either +side of the axis of this chain of hills (see section, p. 48.), the beds +of massive trap, and the tuffs composed of volcanic sand and ashes, +dip regularly to the south-east or north-west, conformably with the +shales and sandstones. + +_Silurian period._--It appears from the investigations of Sir R. +Murchison in Shropshire, that when the lower Silurian strata of that +county were accumulating, there were frequent volcanic eruptions beneath +the sea; and the ashes and scoriæ then ejected gave rise to a peculiar +kind of tufaceous sandstone or grit, dissimilar to the other rocks of +the Silurian series, and only observable in places where syenitic and +other trap rocks protrude. These tuffs occur on the flanks of the Wrekin +and Caer Caradoc, and contain Silurian fossils, such as casts of +encrinites, trilobites, and mollusca. Although fossiliferous, the stone +resembles a sandy claystone of the trap family.[435-A] + +Thin layers of trap, only a few inches thick, alternate, in some parts of +Shropshire and Montgomeryshire, with sedimentary strata of the lower +Silurian system. This trap consists of slaty porphyry and granular felspar +rock, the beds being traversed by joints like those in the associated +sandstone, limestone, and shale, and having the same strike and dip.[435-B] + +In Radnorshire there is an example of twelve bands of stratified trap, +alternating with Silurian schists and flagstones, in a thickness of 350 +feet. The bedded traps consist of felspar-porphyry, clinkstone, and other +varieties; and the interposed Llandeilo flags are of sandstone and shale, +with trilobites and graptolites.[435-C] + +The vast thickness of contemporaneous trappean rocks of lower Silurian +date in North Wales, explored by our government surveyors, has been +already alluded to.[435-D] + +_Cambrian volcanic rocks._--Professor Sedgwick, in his account of the +geology of Cumberland, has described various trap rocks which accompany the +green slates of the Cambrian system, beneath all the rocks containing +organic remains. Different felspathic and porphyritic rocks and greenstones +occur, not only in dikes, but in conformable beds; and there is +occasionally a passage from these igneous rocks to some of the green +quartzose slates. Professor Sedgwick supposes these porphyries to have +originated contemporaneously with the stratified chloritic slates, the +materials of the slates having been supplied, in part at least, by +submarine eruptions oftentimes repeated.[435-E] + + +FOOTNOTES: + +[422-A] See the map, p. 179. + +[423-A] Scrope's Central France, p. 98. + +[423-B] See chaps. xxiv., xxv., and xxvi., 7th and 8th editions. + +[423-C] See Quarterly Geol. Journ., vol. ii. p. 77. + +[425-A] For a view of Puy de Tartaret and Mont Dor, see Scrope's Volcanos +of Central France. + +[427-A] Scrope's Central France, p. 60., and plate. + +[428-A] Daubeny on Volcanos, p. 14. + +[428-B] Edin. Journ. of Sci., No. iv. N. S. p. 276. Figures of some +of these remains are given by M. Bertrand de Doue, Ann. De la Soc. +d'Agricult. de Puy, 1828. + +[429-A] Mém. de la Soc. Géol. de France, tom. i. p. 175. + +[429-B] See Lyell and Murchison, Ann. de Sci. Nat., Oct. 1829. + +[430-A] See Scrope's Central France, p. 21. + +[430-B] Ibid, p. 7. + +[431-A] Boblaye and Virlet, Morea, p. 23. + +[432-A] De la Beche, Geol. Proceedings, No. 41. p. 196. + +[432-B] "The rock," as English readers of Burn's poems may remember, is a +Scotch term for distaff. + +[435-A] Murchison, Silurian System, &c. p. 230. + +[435-B] Ibid., p. 272. + +[435-C] Ibid., p. 325. + +[435-D] Chap. XXVII. p. 356. + +[435-E] Geol. Trans., 2d series, vol. iv. p. 55. + + + + +CHAPTER 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. + + +The plutonic rocks may be treated of next in order, as they are most nearly +allied to the volcanic class already considered. I have described, in the +first chapter, these plutonic rocks as the unstratified division of the +crystalline or hypogene formations, and have stated that 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. From these and other +peculiarities it has been inferred, that the granites have been formed at +considerable depths in the earth, and have cooled and crystallized slowly +under great pressure, where the contained gases could not expand. The +volcanic rocks, on the contrary, although they also have risen up from +below, have cooled from a melted state more rapidly upon or near the +surface. From this hypothesis of the great depth at which the granites +originated, has been derived the name of "Plutonic rocks." The beginner +will easily conceive that the influence of subterranean heat may extend +downwards from the crater of every active volcano to a great depth below, +perhaps several miles or leagues, and the effects which are produced deep +in the bowels of the earth may, or rather must be, distinct; so that +volcanic and plutonic rocks, each different in texture, and sometimes even +in composition, may originate simultaneously, the one at the surface, the +other far beneath it. + +By some writers, all the rocks now under consideration have been +comprehended under the name of granite, which is, then, understood to +embrace a large family of crystalline and compound rocks, usually found +underlying all other formations; whereas we have seen that trap very +commonly overlies strata of different ages. Granite often preserves a very +uniform character throughout a wide range of territory, forming hills of a +peculiar rounded form, usually clad with a scanty vegetation. The surface +of the rock is for the most part in a crumbling state, and the hills are +often surmounted by piles of stones like the remains of a stratified mass, +as in the annexed figure, and sometimes like heaps of boulders, for which +they have been mistaken. The exterior of these stones, originally +quadrangular, acquires a rounded form by the action of air and water, for +the edges and angles waste away more rapidly than the sides. A similar +spherical structure has already been described as characteristic of basalt +and other volcanic formations, and it must be referred to analogous causes, +as yet but imperfectly understood. + +[Illustration: Fig. 484. Mass of granite near the Sharp Tor, Cornwall.] + +Although it is the general peculiarity of granite to assume no definite +shapes, it is nevertheless occasionally subdivided by fissures, so as to +assume a cuboidal, and even a columnar, structure. Examples of these +appearances may be seen near the Land's End, in Cornwall. (See figure.) + +[Illustration: Fig. 485. Granite having a cuboidal and rude columnar +structure, Land's End, Cornwall.] + +The plutonic formations also agree with the volcanic, in having veins or +ramifications proceeding from central masses into the adjoining rocks, and +causing alterations in these last, which will be presently described. They +also resemble trap in containing no organic remains; but they differ in +being more uniform in texture, whole mountain masses of indefinite extent +appearing to have originated under conditions precisely similar. They also +differ in never being scoriaceous or amygdaloidal, and never forming a +porphyry with an uncrystalline base, or alternating with tuffs. Nor do they +form conglomerates, although there is sometimes an insensible passage from +a fine to a coarse-grained granite, and occasionally patches of a fine +texture are imbedded in a coarser variety. + +[Illustration: Fig. 486. Gneiss. (See description, p. 464.)] + +Felspar, quartz, and mica are usually considered as the minerals essential +to granite, the felspar being most abundant in quantity, and the proportion +of quartz exceeding that of mica. These minerals are united in what is +termed a confused crystallization; that is to say, there is no regular +arrangement of the crystals in granite, as in gneiss (see fig. 486.), +except in the variety termed graphic granite, which occurs mostly in +granitic veins. This variety is a compound of felspar and quartz, so +arranged as to produce an imperfect laminar structure. The crystals of +felspar appear to have been first formed, leaving between them the space +now occupied by the darker-coloured quartz. This mineral, when a section is +made at right angles to the alternate plates of felspar and quartz, +presents broken lines, which have been compared to Hebrew characters. + +[2 Illustrations: Graphic granite. + +Fig. 487. Section parallel to the laminæ. + +Fig. 488. Section transverse to the laminæ.] + +As a general rule, quartz, in a compact or amorphous state, forms a +vitreous mass, serving as the base in which felspar and mica have +crystallized; for although these minerals are much more fusible than silex, +they have often imprinted their shapes upon the quartz. This fact, +apparently so paradoxical, has given rise to much ingenious speculation. We +should naturally have anticipated that, during the cooling of the mass, the +flinty portion would be the first to consolidate; and that the different +varieties of felspar, as well as garnets and tourmalines, being more easily +liquefied by heat, would be the last. Precisely the reverse has taken place +in the passage of most granitic aggregates from a fluid to a solid state, +crystals of the more fusible minerals being found enveloped in hard, +transparent, glassy quartz, which has often taken very faithful casts of +each, so as to preserve even the microscopically minute striations on the +surface of prisms of tourmaline. Various explanations of this phenomenon +have been proposed by MM. de Beaumont, Fournet, and Durocher. They refer to +M. Gaudin's experiments on the fusion of quartz, which show that silex, as +it cools, has the property of remaining in a viscous state, whereas alumina +never does. This "gelatinous flint" is supposed to retain a considerable +degree of plasticity long after the granitic mixture has acquired a low +temperature; and M. E. de Beaumont suggests, that electric action may +prolong the duration of the viscosity of silex. Occasionally, however, we +find the quartz and felspar mutually imprinting their forms on each other, +affording evidence of the simultaneous crystallization of both.[439-A] + +[Illustration: Fig. 489. Porphyritic granite. Land's End, Cornwall.] + +_Porphyritic granite._--This name has been sometimes given to that variety +in which large crystals of felspar, sometimes more than 3 inches in length, +are scattered through an ordinary base of granite. An example of this +texture may be seen in the granite of the Land's End, in Cornwall (fig. +489.). The two larger prismatic crystals in this drawing represent felspar, +smaller crystals of which are also seen, similar in form, scattered through +the base. In this base also appear black specks of mica, the crystals of +which have a more or less perfect hexagonal outline. The remainder of the +mass is quartz, the translucency of which is strongly contrasted to the +opaqueness of the white felspar and black mica. But neither the +transparency of the quartz, nor the silvery lustre of the mica, can be +expressed in the engraving. + +The uniform mineral character of large masses of granite seems to indicate +that large quantities of the component elements were thoroughly mixed up +together, and then crystallized under precisely similar conditions. There +are, however, many accidental, or "occasional," minerals, as they are +termed, which belong to granite. Among these black schorl or tourmaline, +actinolite, zircon, garnet, and fluor spar, are not uncommon; but they are +too sparingly dispersed to modify the general aspect of the rock. They +show, nevertheless, that the ingredients were not everywhere exactly the +same; and a still greater variation may be traced in the ever-varying +proportions of the felspar, quartz, and mica. + +_Syenite._--When hornblende is the substitute for mica, which is very +commonly the case, the rock becomes Syenite: so called from the celebrated +ancient quarries of Syene in Egypt. It has all the appearance of ordinary +granite, except when mineralogically examined in hand specimens, and is +fully entitled to rank as a geological member of the same plutonic family +as granite. Syenite, however, after maintaining the granitic character +throughout extensive regions, is not uncommonly found to lose its quartz, +and to pass insensibly into syenitic greenstone, a rock of the trap family. +Werner considered syenite as a binary compound of felspar and hornblende, +and regarded quartz as merely one of its occasional minerals. + +_Syenitic-granite._--The quadruple compound of quartz, felspar, mica, and +hornblende, may be so termed. This rock occurs in Scotland and in Guernsey. + +_Talcose granite_, or Protogine of the French, is a mixture of felspar, +quartz, and talc. It abounds in the Alps, and in some parts of Cornwall, +producing by its decomposition the china clay, more than 12,000 tons of +which are annually exported from that country for the potteries.[440-A] + +_Schorl rock, and schorly granite._--The former of these is an aggregate +of schorl, or tourmaline, and quartz. When felspar and mica are also +present, it may be called schorly granite. This kind of granite is +comparatively rare. + +_Eurite._--A rock in which all the ingredients of granite are blended into +a finely granular mass. Crystals of quartz and mica are sometimes scattered +through the base of Eurite. + +_Pegmatite._--A name given by French writers to a variety of granite; a +granular mixture of quartz and felspar; frequent in granite veins; passes +into graphic granite. + +All these granites pass into certain kinds of trap, a circumstance which +affords one of many arguments in favour of what is now the prevailing +opinion, that the granites are also of igneous origin. The contrast of the +most crystalline form of granite, to that of the most common and earthy +trap, is undoubtedly great; but each member of the volcanic class is +capable of becoming porphyritic, and the base of the porphyry may be more +and more crystalline, until the mass passes to the kind of granite most +nearly allied in mineral composition. + +The minerals which constitute alike the granitic and volcanic rocks +consist, almost exclusively, of seven elements, namely, silica, alumina, +magnesia, lime, soda, potash, and iron; and these may sometimes exist in +about the same proportions in a porous lava, a compact trap, or a +crystalline granite. It may perhaps be found, on farther examination--for +on this subject we have yet much to learn--that the presence of these +elements in certain proportions is more favourable than in others to their +assuming a crystalline or true granitic structure; but it is also +ascertained by experiment, that the same materials may, under different +circumstances, form very different rocks. The same lava, for example, may +be glassy, or scoriaceous, or stony, or porphyritic, according to the more +or less rapid rate at which it cools; and some trachytes and +syenitic-greenstones may doubtless form granite and syenite, if the +crystallization take place slowly. + +It has also been suggested that the peculiar nature and structure of +granite may be due to its retaining in it that water which is seen to +escape from lavas when they cool slowly, and consolidate in the atmosphere. +Boutigny's experiments have shown that melted matter, at a white heat, +requires to have its temperature lowered before it can vapourize water; and +such discoveries, if they fail to explain the manner in which granites have +been formed, serve at least to remind us of the entire distinctness of the +conditions under which plutonic and volcanic rocks must be produced.[441-A] + +It would be easy to multiply examples and authorities to prove the +gradation of the granitic into the trap rocks. On the western side of the +fiord of Christiania, in Norway, there is a large district of trap, chiefly +greenstone-porphyry, and syenitic-greenstone, resting on fossiliferous +strata. To this, on its southern limit, succeeds a region equally extensive +of syenite, the passage from the volcanic to the plutonic rock being so +gradual that it is impossible to draw a line of demarcation between them. + +"The ordinary granite of Aberdeenshire," says Dr. MacCulloch, "is the usual +ternary compound of quartz, felspar, and mica; but sometimes hornblende is +substituted for the mica. But in many places a variety occurs which is +composed simply of felspar and hornblende; and in examining more minutely +this duplicate compound, it is observed in some places to assume a fine +grain, and at length to become undistinguishable from the greenstones of +the trap family. It also passes in the same uninterrupted manner into a +basalt, and at length into a soft claystone, with a schistose tendency on +exposure, in no respect differing from those of the trap islands of the +western coast."[441-B] The same author mentions, that in Shetland, a +granite composed of hornblende, mica, felspar, and quartz, graduates in an +equally perfect manner into basalt.[441-C] + +In Hungary there are varieties of trachyte, which, geologically speaking, +are of modern origin, in which crystals, not only of mica, but of quartz, +are common, together with felspar and hornblende. It is easy to conceive +how such volcanic masses may, at a certain depth from the surface, pass +downwards into granite. + +[2 Illustrations: Fig. 490. Fig. 491. + +Junction of granite and argillaceous schist in Glen Tilt. +(MacCulloch.)[442-A]] + +I have already hinted at the close analogy in the forms of certain granitic +and trappean veins; and it will be found that strata penetrated by plutonic +rocks have suffered changes very similar to those exhibited near the +contact of volcanic dikes. Thus, in Glen Tilt, in Scotland, alternating +strata of limestone and argillaceous schist come in contact with a mass of +granite. The contact does not take place as might have been looked for, if +the granite had been formed there before the strata were deposited, in +which case the section would have appeared as in fig. 490.; but the union +is as represented in fig. 491., the undulating outline of the granite +intersecting different strata, and occasionally intruding itself in +tortuous veins into the beds of clay-slate and limestone, from which it +differs so remarkably in composition. The limestone is sometimes changed in +character by the proximity of the granitic mass or its veins, and acquires +a more compact texture, like that of hornstone or chert, with a splintery +fracture, effervescing feebly with acids. + +The annexed diagram (fig. 492.) represents another junction, in the same +district, where the granite sends forth so many veins as to reticulate the +limestone and schist, the veins diminishing towards their termination to +the thickness of a leaf of paper or a thread. In some places fragments of +granite appear entangled, as it were, in the limestone, and are not visibly +connected with any larger mass; while sometimes, on the other hand, a lump +of the limestone is found in the midst of the granite. The ordinary colour +of the limestone of Glen Tilt is lead blue, and its texture large-grained +and highly crystalline; but where it approximates to the granite, +particularly where it is penetrated by the smaller veins, the crystalline +texture disappears, and it assumes an appearance exactly resembling that of +hornstone. The associated argillaceous schist often passes into hornblende +slate, where it approaches very near to the granite.[442-B] + +[Illustration: Fig. 492. Junction of granite and limestone in Glen +Tilt. (MacCulloch.) + + _a._ Granite. _b._ Limestone. + _c._ Blue argillaceous schist.] + +The conversion of the limestone in these and many other instances into a +siliceous rock, effervescing slowly with acids, would be difficult of +explanation, were it not ascertained that such limestones are always +impure, containing grains of quartz, mica, or felspar disseminated +through them. The elements of these minerals, when the rock has been +subjected to great heat, may have been fused, and so spread more +uniformly through the whole mass. + +[Illustration: Fig. 493. Granite veins traversing clay slate. Table +Mountain, Cape of Good Hope.[443-A]] + +In the plutonic, as in the volcanic rocks, there is every gradation from +a tortuous vein to the most regular form of a dike, such as intersect +the tuffs and lavas of Vesuvius and Etna. Dikes of granite may be seen, +among other places, on the southern flank of Mount Battock, one of the +Grampians, the opposite walls sometimes preserving an exact parallelism +for a considerable distance. + +As a general rule, however, granite veins in all quarters of the globe are +more sinuous in their course than those of trap. They present similar +shapes at the most northern point of Scotland, and the southernmost +extremity of Africa, as the annexed drawings will show. + +It is not uncommon for one set of granite veins to intersect another; and +sometimes there are three sets, as in the environs of Heidelberg, where the +granite on the banks of the river Necker is seen to consist of three +varieties, differing in colour, grain, and various peculiarities of mineral +composition. One of these, which is evidently the second in age, is seen to +cut through an older granite; and another, still newer, traverses both the +second and the first. + +In Shetland there are two kinds of granite. One of them, composed of +hornblende, mica, felspar, and quartz, is of a dark colour, and is seen +underlying gneiss. The other is a red granite, which penetrates the dark +variety everywhere in veins.[444-A] + +[Illustration: Fig. 494. Granite veins traversing gneiss, Cape Wrath. +(MacCulloch.)[444-B]] + +[Illustration: Fig. 495. Granite veins traversing gneiss at Cape Wrath, in +Scotland. (MacCulloch.)] + +The accompanying sketches will explain the manner in which granite veins +often ramify and cut each other (figs. 494. and 495.). They represent +the manner in which the gneiss at Cape Wrath, in Sutherlandshire, is +intersected by veins. Their light colour, strongly contrasted with that +of the hornblende-schist, here associated with the gneiss, renders them +very conspicuous. + +Granite very generally assumes a finer grain, and undergoes a change in +mineral composition, in the veins which it sends into contiguous rocks. +Thus, according to Professor Sedgwick, the main body of the Cornish granite +is an aggregate of mica, quartz, and felspar; but the veins are sometimes +without mica, being a granular aggregate of quartz and felspar. In other +varieties quartz prevails to the almost entire exclusion both of felspar +and mica; in others, the mica and quartz both disappear, and the vein is +simply composed of white granular felspar.[444-C] + +Fig. 496. is a sketch of a group of granite veins in Cornwall, given by +Messrs. Von Oeynhausen and Von Dechen.[445-A] The main body of the granite +here is of a porphyritic appearance, with large crystals of felspar; but in +the veins it is fine-grained, and without these large crystals. The general +height of the veins is from 16 to 20 feet, but some are much higher. + +[Illustration: Fig. 496. Granite veins passing through hornblende slate, +Carnsilver Cove, Cornwall.] + +In the Valorsine, a valley not far from Mont Blanc in Switzerland, an +ordinary granite, consisting of felspar, quartz, and mica, sends forth +veins into a talcose gneiss (or stratified protogine), and in some places +lateral ramifications are thrown off from the principal veins at right +angles (see fig. 497.), the veins, especially the minute ones, being finer +grained than the granite in mass. + +[Illustration: Fig. 497. Veins of granite in talcose gneiss. +(L. A. Necker.)] + +It is here remarked, that the schist and granite, as they approach, seem to +exercise a reciprocal influence on each other, for both undergo a +modification of mineral character. The granite, still remaining +unstratified, becomes charged with green particles; and the talcose gneiss +assumes a granitiform structure without losing its stratification.[445-B] + +Professor Keilhau drew my attention to several localities in the country +near Christiania, where the mineral character of gneiss appears to have +been affected by a granite of much newer origin, for some distance from the +point of contact. The gneiss, without losing its laminated structure, seems +to have become charged with a larger quantity of felspar, and that of a +redder colour, than the felspar usually belonging to the gneiss of Norway. + +Granite, syenite, and those porphyries which have a granitiform +structure, in short all plutonic rocks, are frequently observed to +contain metals, at or near their junction with stratified formations. On +the other hand, the veins which traverse stratified rocks are, as a +general law, more metalliferous near such junctions than in other +positions. Hence it has been inferred that these metals may have been +spread in a gaseous form through the fused mass, and that the contact of +another rock, in a different state of temperature, or sometimes the +existence of rents in other rocks in the vicinity, may have caused the +sublimation of the metals.[446-A] + +There are many instances, as at Markerud, near Christiania, in Norway, +where the strike of the beds has not been deranged throughout a large area +by the intrusion of granite, both in large masses and in veins. This fact +is considered by some geologists to militate against the theory of the +forcible injection of granite in a fluid state. But it may be stated in +reply, that ramifying dikes of trap, which almost all now admit to have +been once fluid, pass through the same fossiliferous strata, near +Christiania, without deranging their strike or dip.[446-B] + +[Illustration: Fig. 498. General view of junction of granite and schist of +the Valorsine. (L. A. Necker.)] + +The real or apparent isolation of large or small masses of granite detached +from the main body, as at _a b_, fig. 498., and above, fig. 492., and _a_, +fig. 497., has been thought by some writers to be irreconcilable with the +doctrine usually taught respecting veins; but many of them may, in fact, be +sections of root-shaped prolongations of granite; while, in other cases, +they may in reality be detached portions of rock having the plutonic +structure. For there may have been spots in the midst of the invaded +strata, in which there was an assemblage of materials more fusible than the +rest, or more fitted to combine readily into some form of granite. + +Veins of pure quartz are often found in granite, as in many stratified +rocks, but they are not traceable, like veins of granite or trap, to +large bodies of rock of similar composition. They appear to have been +cracks, into which siliceous matter was infiltered. Such segregation, as +it is called, can sometimes be shown to have clearly taken place long +subsequently to the original consolidation of the containing rock. Thus, +for example, in the gneiss of Tronstad Strand, near Drammen, in Norway, +the annexed section is seen on the beach. It appears that the +alternating strata of whitish granitiform gneiss, and black +hornblende-schist, were first cut through by a greenstone dike, about +2-1/2 feet wide; then the crack _a b_ passed through all these rocks, +and was filled up with quartz. The opposite walls of the vein are in +some parts incrusted with transparent crystals of quartz, the middle of +the vein being filled up with common opaque white quartz. + +[Illustration: Fig. 499. _a, b._ Quartz vein passing through gneiss and +greenstone, Tronstad Strand, near Christiania.] + +[Illustration: Fig. 500. Euritic porphyry alternating with primary +fossiliferous strata, near Christiania.] + +We have seen that the volcanic formations have been called overlying, +because they not only penetrate others, but spread over them. Mr. Necker +has proposed to call the granites the underlying igneous rocks, and the +distinction here indicated is highly characteristic. It was indeed +supposed by some of the earlier observers, that the granite of +Christiania, in Norway, was intercalated in mountain masses between the +primary or paleozoic strata of that country, so as to overlie +fossiliferous shale and limestone. But although the granite sends veins +into these fossiliferous rocks, and is decidedly posterior in origin, +its actual superposition in mass has been disproved by Professor +Keilhau, whose observations on this controverted point I had +opportunities in 1837 of verifying. There are, however, on a smaller +scale, certain beds of euritic porphyry, some a few feet, others many +yards in thickness, which pass into granite, and deserve perhaps to be +classed as plutonic rather than trappean rocks, which may truly be +described as interposed conformably between fossiliferous strata, as the +porphyries (_a c_, fig. 500.), which divide the bituminous shales and +argillaceous limestones, _f f_. But some of these same porphyries are +partially unconformable, as _b_, and may lead us to suspect that the +others also, notwithstanding their appearance of interstratification, +have been forcibly injected. Some of the porphyritic rocks above +mentioned are highly quartzose, others very felspathic. In proportion as +the masses are more voluminous, they become more granitic in their +texture, less conformable, and even begin to send forth veins into +contiguous strata. In a word, we have here a beautiful illustration of +the intermediate gradations between volcanic and plutonic rocks, not +only in their mineralogical composition and structure, but also in their +relations of position to associated formations. If the term overlying +can in this instance be applied to a plutonic rock, it is only in +proportion as that rock begins to acquire a trappean aspect. + +It has been already hinted that the heat, which in every active volcano +extends downwards to indefinite depths, must produce simultaneously very +different effects near the surface, and far below it; and we cannot suppose +that rocks resulting from the crystallizing of fused matter under a +pressure of several thousand feet, much less miles, of the earth's crust +can resemble those formed at or near the surface. Hence the production at +great depths of a class of rocks analogous to the volcanic, and yet +differing in many particulars, might almost have been predicted, even had +we no plutonic formations to account for. How well these agree, both in +their positive and negative characters, with the theory of their deep +subterranean origin, the student will be able to judge by considering the +descriptions already given. + +It has, however, been objected, that if the granitic and volcanic rocks +were simply different parts of one great series, we ought to find in +mountain chains volcanic dikes passing upwards into lava, and downwards +into granite. But we may answer, that our vertical sections are usually of +small extent; and if we find in certain places a transition from trap to +porous lava, and in others a passage from granite to trap, it is as much as +could be expected of this evidence. + +The prodigious extent of denudation which has been already demonstrated to +have occurred at former periods, will reconcile the student to the belief +that crystalline rocks of high antiquity, although deep in the earth's +crust when originally formed, may have become uncovered and exposed at the +surface. Their actual elevation above the sea may be referred to the same +causes to which we have attributed the upheaval of marine strata, even to +the summits of some mountain chains. But to these and other topics, I shall +revert when speaking, in the next chapter, of the relative ages of +different masses of granite. + + +FOOTNOTES: + +[439-A] Bulletin, 2d sèrie, iv. 1304.; and Archiac, Hist. des Progrès +de Geol., i. 38. + +[440-A] Boase on Primary Geology, p. 16. + +[441-A] Bulletin, vol. iv., 2d ser., pp. 1318. and 1320. + +[441-B] Syst. of Geol., vol. i. p. 157. + +[441-C] Ibid., p. 158. + +[442-A] Geol. Trans., 1st series, vol. iii. pl. 21. + +[442-B] MacCulloch, Geol. Trans., vol. iii. p. 259. + +[443-A] Capt. B. Hall, Trans. Roy. Soc. Edin., vol. vii. + +[444-A] MacCulloch, Syst. of Geol., vol. i. p. 58. + +[444-B] Western Islands, pl. 31. + +[444-C] On Geol. of Cornwall, Camb. Trans. vol. i. p. 124. + +[445-A] Phil. Mag. and Annals, No. 27. new series, March, 1829. + +[445-B] Necker, sur la Val. de Valorsine, Mém. de la Soc. de Phys. de +Génève, 1828. I visited, in 1832, the spot referred to in fig. 497. + +[446-A] Necker, Proceedings of Geol. Soc., No. 26. p. 392. + +[446-B] See Keilhau's Gæa Norvegica; Christiania, 1838. + + + + +CHAPTER 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. + + +When we adopt the igneous theory of granite, as explained in the last +chapter, and believe that different plutonic rocks have originated at +successive periods beneath the surface of the planet, we must be prepared +to encounter greater difficulty in ascertaining the precise age of such +rocks, than in the case of volcanic and fossiliferous formations. We must +bear in mind, that the evidence of the age of each contemporaneous volcanic +rock was derived, either from lavas poured out upon the ancient surface, +whether in the sea or in the atmosphere, or from tuffs and conglomerates, +also deposited at the surface, and either containing organic remains +themselves, or intercalated between strata containing fossils. But all +these tests fail when we endeavour to fix the chronology of a rock which +has crystallized from a state of fusion in the bowels of the earth. In that +case, we are reduced to the following tests; 1st, relative position; 2dly, +intrusion, and alteration of the rocks in contact; 3dly, mineral +characters; 4thly, included fragments. + +_Test of age by relative position._--Unaltered fossiliferous strata of +every age are met with reposing immediately on plutonic rocks; as at +Christiania, in Norway, where the Newer Pliocene deposits rest on granite; +in Auvergne, where the freshwater Eocene strata, and at Heidelberg, on the +Rhine, where the New Red sandstone, occupy a similar place. In all these, +and similar instances, inferiority in position is connected with the +superior antiquity of granite. The crystalline rock was solid before the +sedimentary beds were superimposed, and the latter usually contain in them +rounded pebbles of the subjacent granite. + +_Test by intrusion and alteration._--But when plutonic rocks send veins +into strata, and alter them near the point of contact, in the manner before +described (p. 442.), it is clear that, like intrusive traps, they are newer +than the strata which they invade and alter. Examples of the application of +this test will be given in the sequel. + +_Test by mineral composition._--Notwithstanding a general uniformity in the +aspect of plutonic rocks, we have seen in the last chapter that there are +many varieties, such as Syenite, Talcose granite, and others. One of these +varieties is sometimes found exclusively prevailing throughout an extensive +region, where it preserves a homogeneous character; so that having +ascertained its relative age in one place, we can easily recognize its +identity in others, and thus determine from a single section the +chronological relations of large mountain masses. Having observed, for +example, that the syenitic granite of Norway, in which the mineral called +zircon abounds, has altered the Silurian strata wherever it is in contact, +we do not hesitate to refer other masses of the same zircon-syenite in the +south of Norway to the same era. + +Some have imagined that the age of different granites might, to a great +extent, be determined by their mineral characters alone; syenite, for +instance, or granite with hornblende, being more modern than common or +micaceous granite. But modern investigations have proved these +generalizations to have been premature. The syenitic granite of Norway +already alluded to may be of the same age as the Silurian strata, which it +traverses and alters, or may belong to the Old Red sandstone period; +whereas the granite of Dartmoor, although consisting of mica, quartz, and +felspar, is newer than the coal. (See p. 456.) + +_Test by included fragments._--This criterion can rarely be of much +importance, because the fragments involved in granite are usually so +much altered, that they cannot be referred with certainty to the rocks +whence they were derived. In the White Mountains, in North America, +according to Professor Hubbard, a granite vein traversing granite, +contains fragments of slate and trap, which must have fallen into the +fissure when the fused materials of the vein were injected from +below[450-A], and thus the granite is shown to be newer than certain +superficial slaty and trappean formations. + +_Recent and Pliocene plutonic rocks, why invisible._--The explanation +already given in the 29th and in the last chapter, of the probable relation +of the plutonic to the volcanic formations, will naturally lead the reader +to infer, that rocks of the one class can never be produced at or near the +surface without some members of the other being formed below +simultaneously, or soon afterwards. It is not uncommon for lava-streams to +require more than ten years to cool in the open air; and where they are of +great depth, a much longer period. The melted matter poured from Jorullo, +in Mexico, in the year 1759, which accumulated in some places to the height +of 550 feet, was found to retain a high temperature half a century after +the eruption.[450-B] We may conceive, therefore, that great masses of +subterranean lava may remain in a red-hot or incandescent state in the +volcanic foci for immense periods, and the process of refrigeration may be +extremely gradual. Sometimes, indeed, this process may be retarded for an +indefinite period, by the accession of fresh supplies of heat; for we find +that the lava in the crater of Stromboli, one of the Lipari Islands, has +been in a state of constant ebullition for the last two thousand years; and +we may suppose this fluid mass to communicate with some caldron or +reservoir of fused matter below. In the Isle of Bourbon, also, where there +has been an emission of lava once in every two years for a long period, the +lava below can scarcely fail to have been permanently in a state of +liquefaction. If then it be a reasonable conjecture, that about 2000 +volcanic eruptions occur in the course of every century, either above the +waters of the sea or beneath them[451-A], it will follow, that the quantity +of plutonic rock generated, or in progress during the Recent epoch, must +already have been considerable. + +But as the plutonic rocks originate at some depth in the earth's crust, +they can only be rendered accessible to human observation, by subsequent +upheaval and denudation. Between the period when a plutonic rock +crystallizes in the subterranean regions, and the era of its protrusion at +any single point of the surface, one or two geological periods must usually +intervene. Hence, we must not expect to find the Recent or Newer Pliocene +granites laid open to view, unless we are prepared to assume that +sufficient time has elapsed since the commencement of the Newer Pliocene +period for great upheaval and denudation. A plutonic rock, therefore, must, +in general, be of considerable antiquity relatively to the fossiliferous +and volcanic formations, before it becomes extensively visible. As we know +that the upheaval of land has been sometimes accompanied in South America +by volcanic eruptions and the emission of lava, we may conceive the more +ancient plutonic rocks to be forced upwards to the surface by the newer +rocks of the same class formed successively below,--subterposition in the +plutonic, like superposition in the sedimentary rocks, being usually +characteristic of a newer origin. + +In the accompanying diagram (fig. 501.), an attempt is made to show the +inverted order in which sedimentary and plutonic formations may occur +in the earth's crust. + +The oldest plutonic rock, No. I., has been upheaved at successive periods +until it has become exposed to view in a mountain-chain. This protrusion of +No. I. has been caused by the igneous agency which produced the newer +plutonic rocks Nos. II. III. and IV. Part of the primary fossiliferous +strata, No. 1., have also been raised to the surface by the same gradual +process. It will be observed that the Recent _strata_ No. 4., and the +Recent _granite_ or plutonic rock No. IV., are the most remote from each +other in position, although of contemporaneous date. According to this +hypothesis, the convulsions of many periods will be required before +_Recent_ granite will be upraised so as to form the highest ridges and +central axes of mountain-chains. During that time the _Recent_ strata No. +4. might be covered by a great many newer sedimentary formations. + +[Illustration: Fig. 501. Diagram showing the relative position which the +plutonic and sedimentary formations of different ages may occupy. + + I. Primary plutonic. 4. Recent strata. + II. Secondary plutonic. 3. Tertiary strata. + III. Tertiary plutonic. 2. Secondary strata. + IV. Recent plutonic. 1. Primary fossiliferous strata. + +The metamorphic rocks are not indicated in this diagram; but the student +will infer, from what has been said in Chap. XXXII., that some portions +of the stratified formations Nos. 1. and 2. invaded by granite will +have become metamorphic.] + +_Eocene granite and plutonic rocks._--In a former part of this volume (p. +205.), the great nummulitic formation of the Alps and Pyrenees was +referred to the Eocene period, and it follows that those vast movements +which have raised fossiliferous rocks from the level of the sea to the +height of more than 10,000 feet above its level have taken place since the +commencement of the tertiary epoch. Here, therefore, if anywhere, we might +expect to find hypogene formations of Eocene date breaking out in the +central axis or most disturbed region of the loftiest chain in Europe. +Accordingly, in the Swiss Alps, even the _flysch_, or upper portion of the +nummulitic series, has been occasionally invaded by plutonic rocks, and +converted into crystalline schists of the hypogene class. There can be +little doubt that even the talcose granite of Mont Blanc itself has been in +a fused or pasty state since the _flysch_ was deposited at the bottom of +the sea; and the question as to its age is not so much whether it be a +secondary or tertiary granite, as whether it should be assigned to the +Eocene or Miocene epoch. + +Great upheaving movements have been experienced in the region of the +Andes, during the Post-Pliocene period. In some part, therefore, of this +chain, we may expect to discover tertiary plutonic rocks laid open to +view. What we already know of the structure of the Chilian Andes seems +to realize this expectation. In a transverse section, examined by Mr. +Darwin, between Valparaiso and Mendoza, the Cordillera was found to +consist of two separate and parallel chains, formed of sedimentary rocks +of different ages, the strata in both resting on plutonic rocks, by +which they have been altered. In the western or oldest range, called the +Peuquenes, are black calcareous clay-slates, rising to the height of +nearly 14,000 feet above the sea, in which are shells of the genera +_Gryphæa_, _Turritella_, _Terebratula_, and _Ammonite_. These rocks are +supposed to be of the age of the central parts of the secondary series +of Europe. They are penetrated and altered by dikes and mountain masses +of a plutonic rock, which has the texture of ordinary granite, but +rarely contains quartz, being a compound of albite and hornblende. + +The second or eastern chain consists chiefly of sandstones and +conglomerates, of vast thickness, the materials of which are derived +from the ruins of the western chain. The pebbles of the conglomerates +are, for the most part, rounded fragments of the fossiliferous slates +before mentioned. The resemblance of the whole series to certain +tertiary deposits on the shores of the Pacific, not only in mineral +character, but in the imbedded lignite and silicified woods, leads to +the conjecture that they also are tertiary. Yet these strata are not +only associated with trap rocks and volcanic tuffs, but are also altered +by a granite consisting of quartz, felspar, and talc. They are +traversed, moreover, by dikes of the same granite, and by numerous veins +of iron, copper, arsenic, silver, and gold; all of which can be traced +to the underlying granite.[453-A] We have, therefore, strong ground to +presume that the plutonic rock, here exposed on a large scale in the +Chilian Andes, is of later date than certain tertiary formations. + +But the theory adopted in this work of the subterranean origin of the +hypogene formations would be untenable, if the supposed fact here alluded +to, of the appearance of tertiary granite at the surface was not a rare +exception to the general rule. A considerable lapse of time must intervene +between the formation in the nether regions of plutonic and metamorphic +rocks, and their emergence at the surface. For a long series of +subterranean movements must occur before such rocks can be uplifted into +the atmosphere or the ocean; and, before they can be rendered visible to +man, some strata which previously covered them must usually have been +stripped off by denudation. + +We know that in the Bay of Baiæ, in 1538, in Cutch in 1819, and on several +occasions in Peru and Chili, since the commencement of the present century, +the permanent upheaval or subsidence of land has been accompanied by the +simultaneous emission of lava at one or more points in the same volcanic +region. From these and other examples it may be inferred that the rising or +sinking of the earth's crust, operations by which sea is converted into +land, and land into sea, are a part only of the consequences of +subterranean igneous action. It can scarcely be doubted that this action +consists, in a great degree, of the baking, and occasionally the +liquefaction, of rocks, causing them to assume, in some cases a larger, in +others a smaller volume than before the application of heat. It consists +also in the generation of gases, and their expansion by heat, and the +injection of liquid matter into rents formed in superincumbent rocks. The +prodigious scale on which these subterranean causes have operated in Sicily +since the deposition of the Newer Pliocene strata will be appreciated, when +we remember that throughout half the surface of that island such strata are +met with, raised to the height of from 50 to that of 2000 and even 3000 +feet above the level of the sea. In the same island also the older rocks +which are contiguous to these marine tertiary strata must have undergone, +within the same period, a similar amount of upheaval. + +The like observations may be extended to nearly the whole of Europe, for, +since the commencement of the Eocene period, the entire European area, +including some of the central and very lofty portions of the Alps +themselves, as I have elsewhere shown[454-A], has, with the exception of a +few districts, emerged from the deep to its present altitude; and even +those tracts, which were already dry land before the Eocene era, have +almost everywhere acquired additional height. A large amount of subsidence +has also occurred during the same period, so that the extent of the +subterranean spaces which have either become the receptacles of sunken +fragments of the earth's crust, or have been rendered capable of supporting +other fragments at a much greater height than before, must be so great that +they probably equal, if not exceed in volume, the entire continent of +Europe. We are entitled, therefore, to ask what amount of change of +equivalent importance can be proved to have occurred in the earth's crust +within an equal quantity of time anterior to the Eocene epoch. They who +contend for the more intense energy of subterranean causes in the remoter +eras of the earth's history, may find it more difficult to give an answer +to this question than they anticipated. + +The principal effect of volcanic action in the nether regions, during +the tertiary period, seems to have consisted in the upheaval to the +surface of hypogene formations of an age anterior to the carboniferous. +The repetition of another series of movements, of equal violence, might +upraise the plutonic and metamorphic rocks of many secondary periods; +and if the same force should still continue to act, the next convulsions +might bring up to the day the _tertiary_ and _recent_ hypogene rocks. In +the course of such changes many of the existing sedimentary strata would +suffer greatly by denudation, others might assume a metamorphic +structure, or become melted down into plutonic and volcanic rocks. +Meanwhile the deposition of a vast thickness of new strata would not +fail to take place during the upheaval and partial destruction of the +older rocks. But I must refer the reader to the last chapter but one of +this volume for a fuller explanation of these views. + +[Illustration: Fig. 502. Block section.] + +_Cretaceous period._--It will be shown in the next chapter that chalk, as +well as lias, has been altered by granite in the eastern Pyrenees. Whether +such granite be cretaceous or tertiary cannot easily be decided. Suppose +_b, c, d_, to be three members of the Cretaceous series, the lowest of +which, _b_, has been altered by the granite A, the modifying influence not +having extended so far as _c_, or having but slightly affected its lowest +beds. Now it can rarely be possible for the geologist to decide whether the +beds d existed at the time of the intrusion of A, and alteration of _b_ and +_c_, or whether they were subsequently thrown down upon _c_. + +As some Cretaceous rocks, however, have been raised to the height of more +than 9000 feet in the Pyrenees, we must not assume that plutonic formations +of the same age may not have been brought up and exposed by denudation, at +the height of 2000 or 3000 feet on the flanks of that chain. + +_Period of Oolite and Lias._--In the department of the Hautes Alpes, in +France, near Vizille, M. Elie de Beaumont traced a black argillaceous +limestone, charged with belemnites, to within a few yards of a mass of +granite. Here the limestone begins to put on a granular texture, but is +extremely fine-grained. When nearer the junction it becomes grey, and +has a saccharoid structure. In another locality, near Champoleon, a +granite composed of quartz, black mica, and rose-coloured felspar, is +observed partly to overlie the secondary rocks, producing an alteration +which extends for about 30 feet downwards, diminishing in the beds which +lie farthest from the granite. (See fig. 503.) In the altered mass the +argillaceous beds are hardened, the limestone is saccharoid, the grits +quartzose, and in the midst of them is a thin layer of an imperfect +granite. It is also an important circumstance that near the point of +contact, both the granite and the secondary rocks become metalliferous, +and contain nests and small veins of blende, galena, iron, and copper +pyrites. The stratified rocks become harder and more crystalline, but +the granite, on the contrary, softer and less perfectly crystallized +near the junction.[456-A] + +[Illustration: Fig. 503. Junction of granite with Jurassic or Oolite strata +in the Alps, near Champoleon.] + +Although the granite is incumbent in the above section (fig. 503.), we +cannot assume that it overflowed the strata, for the disturbances of the +rocks are so great in this part of the Alps that they seldom retain the +position which they must originally have occupied. + +A considerable mass of syenite, in the Isle of Skye, is described by Dr. +MacCulloch as intersecting limestone and shale, which are of the age of the +lias.[456-B] The limestone, which, at a greater distance from the granite, +contains shells, exhibits no traces of them near its junction, where it has +been converted into a pure crystalline marble.[456-C] + +At Predazzo, in the Tyrol, secondary strata, some of which are limestones +of the Oolitic period, have been traversed and altered by plutonic rocks, +one portion of which is an augitic porphyry, which passes insensibly into +granite. The limestone is changed into granular marble, with a band of +serpentine at the junction.[456-D] + +_Carboniferous period._--The granite of Dartmoor, in Devonshire, was +formerly supposed to be one of the most ancient of the plutonic rocks, but +is now ascertained to be posterior in date to the culm-measures of that +county, which, from their position, and as containing true coal-plants, are +regarded by Professor Sedgwick and Sir R. Murchison as members of the true +carboniferous series. This granite, like the syenitic granite of +Christiania, has broken through the stratified formations without much +changing their strike. Hence, on the north-west side of Dartmoor, the +successive members of the culm-measures abut against the granite, and +become metamorphic as they approach. These strata are also penetrated by +granite veins, and plutonic dikes, called "elvans."[457-A] The granite of +Cornwall is probably of the same date, and, therefore, as modern as the +Carboniferous strata, if not much newer. + +_Silurian period._--It has long been known that the granite near +Christiania, in Norway, is of newer origin than the Silurian strata +of that region. Von Buch first announced, in 1813, the discovery +of its posteriority in date to limestones containing orthocerata +and trilobites. The proofs consist in the penetration of granite +veins into the shale and limestone, and the alteration of the strata, +for a considerable distance from the point of contact, both of these +veins and the central mass from which they emanate. (See p. 447.) +Von Buch supposed that the plutonic rock alternated with the +fossiliferous strata, and that large masses of granite were sometimes +incumbent upon the strata; but this idea was erroneous, and arose from +the fact that the beds of shale and limestone often dip towards the +granite up to the point of contact, appearing as if they would pass +under it in mass, as at _a_, fig. 504., and then again on the opposite +side of the same mountain, as at _b_, dip away from the same granite. +When the junctions, however, are carefully examined, it is found that +the plutonic rock intrudes itself in veins, and nowhere covers the +fossiliferous strata in large overlying masses, as is so commonly the +case with trappean formations.[457-B] + +[Illustration: Fig. 504. Cross section.] + +Now this granite, which is more modern than the Silurian strata of Norway, +also sends veins in the same country into an ancient formation of gneiss; +and the relations of the plutonic rock and the gneiss, at their junction, +are full of interest when we duly consider the wide difference of epoch +which must have separated their origin. + +[Illustration: Fig. 505. Granite sending veins into Silurian strata and +Gneiss,--Christiania, Norway.] + +The length of this interval of time is attested by the following +facts:--The fossiliferous, or Silurian beds, rest unconformably upon the +truncated edges of the gneiss, the inclined strata of which had been +disturbed and denuded before the sedimentary beds were superimposed (see +fig. 505.). The signs of denudation are twofold; first, the surface of the +gneiss is seen occasionally, on the removal of the newer beds, containing +organic remains, to be worn and smoothed; secondly, pebbles of gneiss have +been found in some of the transition strata. Between the origin, therefore, +of the gneiss and the granite there intervened, first, the period when the +strata of gneiss were inclined; secondly, the period when they were +denuded; thirdly, the period of the deposition of the transition deposits. +Yet the granite produced, after this long interval, is often so intimately +blended with the ancient gneiss, at the point of junction, that it is +impossible to draw any other than an arbitrary line of separation between +them; and where this is not the case, tortuous veins of granite pass freely +through gneiss, ending sometimes in threads, as if the older rock had +offered no resistance to their passage. It seems necessary, therefore, to +conceive that the gneiss was softened and more or less melted when +penetrated by the granite. But had such junctions alone been visible, and +had we not learnt, from other sections, how long a period elapsed between +the consolidation of the gneiss and the injection of this granite, we might +have suspected that the gneiss was scarcely solidified, or had not yet +assumed its complete metamorphic character, when invaded by the plutonic +rock. From this example we may learn how impossible it is to conjecture +whether certain granites in Scotland, and other countries, which send veins +into gneiss and other metamorphic rocks, are primary, or whether they may +not belong to some secondary or tertiary period. + +_Oldest granites._--It is not half a century since the doctrine was very +general that all granitic rocks were _primitive_, that is to say, that they +originated before the deposition of the first sedimentary strata, and +before the creation of organic beings (see above, p. 9.). But so greatly +are our views now changed, that we find it no easy task to point out a +single mass of granite demonstrably more ancient than all the known +fossiliferous deposits. Could we discover some Lower Cambrian strata +resting immediately on granite, there being no alterations at the point of +contact, nor any intersecting granitic veins, we might then affirm the +plutonic rock to have originated before the oldest known fossiliferous +strata. Still it would be presumptuous to suppose that when a small part +only of the globe has been investigated, we are acquainted with the oldest +fossiliferous strata in the crust of our planet. Even when these are found, +we cannot assume that there never were any antecedent strata containing +organic remains, which may have become metamorphic. If we find pebbles of +granite in a conglomerate of the Lower Cambrian system, we may then feel +assured that the parent granite was formed before the Lower Cambrian +formation. But if the incumbent strata be merely Silurian or Upper +Cambrian, the fundamental granite, although of high antiquity, may be +posterior in date to _known_ fossiliferous formations. + +_Protrusion of solid granite._--In part of Sutherlandshire, near Brora, +common granite, composed of felspar, quartz, and mica, is in immediate +contact with Oolitic strata, and has clearly been elevated to the surface +at a period subsequent to the deposition of those strata.[459-A] Professor +Sedgwick and Sir R. Murchison conceive that this granite has been upheaved +in a solid form; and that in breaking through the submarine deposits, with +which it was not perhaps originally in contact, it has fractured them so as +to form a breccia along the line of junction. This breccia consists of +fragments of shale, sandstone, and limestone, with fossils of the oolite, +all united together by a calcareous cement. The secondary strata, at some +distance from the granite, are but slightly disturbed, but in proportion to +their proximity the amount of dislocation becomes greater. + +If we admit that solid hypogene rocks, whether stratified or +unstratified, have in such cases been driven upwards so as to pierce +through yielding sedimentary deposits, we shall be enabled to account +for many geological appearances otherwise inexplicable. Thus, for +example, at Weinböhla and Hohnstein, near Meissen, in Saxony, a mass of +granite has been observed covering strata of the Cretaceous and Oolitic +periods for the space of between 300 and 400 yards square. It appears +clearly from a recent Memoir of Dr. B. Cotta on this subject[459-B], +that the granite was thrust into its actual position when solid. There +are no intersecting veins at the junction--no alteration as if by heat, +but evident signs of rubbing, and a breccia in some places, in which +pieces of granite are mingled with broken fragments of the secondary +rocks. As the granite overhangs both the lias and chalk, so the lias is +in some places bent over strata of the cretaceous era. + +_Relative age of the granites of Arran._--In this island, the largest in +the Firth of Clyde, being twenty miles in length from north to south, the +four great classes of rocks, the fossiliferous, volcanic, plutonic, and +metamorphic, are all conspicuously displayed within a very small area, and +with their peculiar characters strongly contrasted. In the north of the +island the granite rises to the height of nearly 3000 feet above the sea, +terminating in mountainous peaks. (See section, fig. 506.) On the flanks of +the same mountains are chloritic-schists, blue roofing-slate, and other +rocks of the metamorphic order (No. 1.), into which the granite (No. 2.) +sends veins. This granite, therefore, is newer than the hypogene schists +(No. 1.), which it penetrates. + +These schists are highly inclined. Upon them rest beds of conglomerate +and sandstone (No. 3.), which are referable to the Old Red formation, to +which succeed various shales and limestones (No. 4.) containing the +fossils of the Carboniferous period, upon which are other strata of +sandstone and conglomerate (upper part of No. 4.), in which no fossils +have been met with, which it is conjectured may belong to the New Red +sandstone period. All the preceding formations are cut through by the +volcanic rocks (No. 5.), which consist of greenstone, basalt, +pitchstone, claystone-porphyry, and other varieties. These appear either +in the form of dikes, or in dense masses from 50 to 700 feet in +thickness, overlying the strata (No. 4.). They sometimes pass into +syenite of so crystalline a form, that it may rank as a plutonic +formation; and in one region, at Ploverfield, in Glen Cloy, a +fine-grained granite (6. _a_) is seen associated with the trap +formation, and sending veins into the sandstone or into the upper strata +of No. 4. This interesting discovery of granite in the southern region +of Arran, at a point where it is separated from the northern mass of +granite by a great thickness of secondary strata and overlying trap, was +made by Mr. L. A. Necker of Geneva, during his survey of Arran in 1839. +We also learn from the recent investigations of Prof. A. C. Ramsay, that +a similar fine-grained granite (No. 6. _b_) appears in the interior of +the northern granitic district, forming the nucleus of it, and sending +veins into the older coarse-grained granite (No. 2.). The trap dikes +which penetrate the older granite are cut off, according to Mr. Ramsay, +at the junction of the fine grained. + +It is not improbable that the granite (No. 6. _b_) may be of the same age +as that of Ploverfield (No. 6. _a_), and this again may belong to the same +geological epoch as the trap formations (No. 5.). If there be any +difference of date, it would seem that the fine-grained granite must be +newer than the trappean rocks. But, on the other hand, the coarser granite +(No. 2.) may be the oldest rock in Arran, with the exception of the +hypogene slates (No. 1.), into which it sends veins. + +[Illustration: Fig. 506. General Section of Arran from north to south. + + 1. Metamorphic or Hypogene schists, the oldest formations in Arran. + 2. Coarse-grained granite sending veins into the schists, No. 1. + 3. Old Red Sandstone and Conglomerate containing pebbles exclusively + derived from the rocks, No 1., without any intermixture of granitic + fragments. + 4. Carboniferous strata and red sandstone (New Red?). + 5. Trap, overlying and in dikes, passing occasionally into Syenites of + the Plutonic class. + 6. _a._ Fine-grained granite, associated with the overlying trap, No. 5. + 6. _b._ Similar fine-grained granite, sending veins into the older + granite, No. 2., and cutting off the trappean dikes, _c_, _d_.[461-A]] + +An objection may perhaps at first be started to this conclusion, derived +from the curious and striking fact, the importance of which was first +emphatically pointed out by Dr. MacCulloch, that no pebbles of granite +occur in the conglomerates of the red sandstone in Arran, although these +conglomerates are several hundred feet in thickness, and lie at the foot of +lofty granite mountains, which tower above them. As a general rule, all +such aggregates of pebbles and sand are mainly composed of the wreck of +pre-existing rocks occurring in the immediate vicinity. The total absence +therefore of granitic pebbles has justly been a theme of wonder to those +geologists who have successively visited Arran, and they have carefully +searched there, as I have done myself, to find an exception, but in vain. +The rounded masses consist exclusively of quartz, chlorite-schist, and +other members of the metamorphic series; nor in the newer conglomerates of +No. 4. have any granitic fragments been discovered. Are we then entitled to +affirm that the coarse-grained granite (No. 2.), like the fine-grained +variety (No. 6. _a_), is more modern than all the other rocks of the +island? This we cannot assume at present, but we may confidently infer that +when the various beds of sandstone and conglomerate were formed, no granite +had reached the surface, or had been exposed to denudation in Arran. It is +clear that the crystalline schists were ground into sand and shingle when +the strata No. 3. were deposited, and at that time the waves had never +acted upon the granite, which now sends its veins into the schist. May we +then conclude, that the schists suffered denudation before they were +invaded by granite? This opinion, although not inadmissible, is by no +means fully borne out by the evidence. For at the time when the Old Red +sandstone originated, the metamorphic strata may have formed islands in the +sea, as in fig. 507., over which the breakers rolled, or from which +torrents and rivers descended, carrying down gravel and sand. The plutonic +rock or granite (B) may even then have been previously injected at a +certain depth below, and yet may never have been exposed to denudation. + +[Illustration: Fig. 507. Cross section.] + +As to the time and manner of the subsequent protrusion of the +coarse-grained granite (No. 2.), this rock may have been thrust up +bodily, in a solid form, during that long series of igneous operations +which produced the trappean and plutonic formations (Nos. 5., 6. +_a_, and 6. _b_). + +We have shown that these eruptions, whatever their date, were posterior to +the deposition of all the fossiliferous strata of Arran. We can also prove +that subsequently both the granitic and trappean rocks underwent great +aqueous denudation, which they probably suffered during their emergence +from the sea. The fact is demonstrated by the abrupt truncation of numerous +dikes, such as those at _c_, _d_, _e_, which are cut off on the surface of +the granite and trap. The overlying trap also ceases very abruptly on +approaching the boundary of the great hypogene region, and terminates in a +steep escarpment facing towards it as at _f_, fig. 506. When in its +original fluid state it could not have come thus suddenly to an end, but +must have filled up the hollow now separating it from the hypogene rocks, +had such a hollow then existed. This necessity of supposing that both the +trap and the conglomerate once extended farther, and that veins such as +_c_, _d_, fig. 506., were once prolonged farther upwards, prepares us to +believe that the whole of the northern granite may at one time have been +covered by newer formations, under the pressure of which, before its +protrusion, it assumed its highly crystalline texture. + +The theory of the protrusion in a solid form of the northern nucleus of +granite is confirmed by the manner in which the hypogene slates (No. 1.) +and the beds of conglomerate (No. 3.) dip away from it on all sides. In +some places indeed the slates are inclined towards the granite, but this +exception might have been looked for, because these hypogene strata have +undergone disturbances at more than one geological epoch, and may at some +points, perhaps, have their original order of position inverted. The high +inclination, therefore, and the quâquâversal dip of the beds around the +borders of the granitic boss, and the comparative horizontality of the +fossiliferous strata in the southern part of the island, are facts which +all accord with the hypothesis of a great amount of movement at that point +where the granite is supposed to have been thrust up bodily, and where we +may conceive it to have been distended laterally by the repeated injection +of fresh supplies of melted materials.[463-A] + + +FOOTNOTES: + +[450-A] Silliman's Journ., No. 69. p. 123. + +[450-B] See "Principles," _Index_, "Jorullo." + +[451-A] "Principles," _Index_, "Volcanic Eruptions." + +[453-A] Darwin, pp. 390. 406.; second edition, p. 319. + +[454-A] See map of Europe and explanation, in Principles, book i. + +[456-A] Elie de Beaumont, sur les Montagnes de l'Oisans, &c. Mém. de la +Soc. d'Hist. Nat. de Paris, tom. v. + +[456-B] See Murchison, Geol. Trans., 2d series, vol. ii. part ii. +pp. 311-321. + +[456-C] Western Islands, vol. i. p. 330. plate 18., figs. 3, 4. + +[456-D] Von Buch, Annales de Chimie, &c. + +[457-A] Proceedings of Geol. Soc., vol. ii. p. 562. + +[457-B] See the Gæa Norvegica and other works of Keilhau, with whom I +examined this country. + +[459-A] Murchison, Geol. Trans., 2d series, vol. ii. p. 307. + +[459-B] Geognostische Wanderungen, Leipzig, 1838. + +[461-A] In the above section I have attempted to represent the new +discoveries made since 1839, by Mr. Necker and Mr. A. C. Ramsay, in regard +to the plutonic formations, 6. _a_, and 6. _b_. + +[463-A] For the geology of Arran consult the works of Drs. Hutton and +MacCulloch, the Memoirs of Messrs. Von Dechen and Oeynhausen, that of +Professor Sedgwick and Sir R. Murchison (Geol. Trans. 2d series), Mr. L. +A. Necker's Memoir, read to the Royal Soc. of Edin. 20th April, 1840, +and Mr. Ramsay's Geol. of Arran, 1841. I examined myself a large part +of Arran in 1836. + + + + +CHAPTER 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. + + +We have now considered three distinct classes of rocks: first, the aqueous, +or fossiliferous; secondly, the volcanic; and, thirdly, the plutonic, or +granitic; and we have now, lastly, to examine those crystalline (or +hypogene) strata to which the name of _metamorphic_ has been assigned. The +last-mentioned term expresses, as before explained, a theoretical opinion +that such strata, after having been deposited from water, acquired, by the +influence of heat and other causes, a highly crystalline texture. They who +still question this opinion may call the rocks under consideration the +stratified hypogene, or schistose hypogene formations. + +These rocks, when in their most characteristic or normal state, are +wholly devoid of organic remains, and contain no distinct fragments of +other rocks, whether rounded or angular. They sometimes break out in the +central parts of narrow mountain chains, but in other cases extend over +areas of vast dimensions, occupying, for example, nearly the whole of +Norway and Sweden, where, as in Brazil, they appear alike in the lower +and higher grounds. In Great Britain, those members of the series which +approach most nearly to granite in their composition, as gneiss, +mica-schist, and hornblende-schist, are confined to the country north of +the rivers Forth and Clyde. + +Many attempts have been made to trace a general order of succession or +superposition in the members of this family; gneiss, for example, having +been often supposed to hold invariably a lower geological position than +mica-schist. But although such an order may prevail throughout limited +districts, it is by no means universal, nor even general, throughout the +globe. To this subject, however, I shall again revert, in the last +chapter of this volume, when the chronological relations of the +metamorphic rocks are pointed out. + +The following may be enumerated as the principal members of the metamorphic +class:--gneiss, mica-schist, hornblende-schist, clay-slate, +chlorite-schist, hypogene or metamorphic limestone, and certain kinds of +quartz-rock or quartzite. + +[Illustration: Fig. 508. Fragment of gneiss, natural size; section at right +angles to planes of stratification.] + +_Gneiss._--The first of these, gneiss, may be called stratified granite, +being formed of the same materials as granite, namely, felspar, quartz, +and mica. In the specimen here figured, the white layers consist almost +exclusively of granular felspar, with here and there a speck of mica and +grain of quartz. The dark layers are composed of grey quartz and black +mica, with occasionally a grain of felspar intermixed. The rock splits +most easily in the plane of these darker layers, and the surface thus +exposed is almost entirely covered with shining spangles of mica. The +accompanying quartz, however, greatly predominates in quantity, but the +most ready cleavage is determined by the abundance of mica in certain +parts of the dark layer. + +Instead of these thin laminæ, gneiss is sometimes simply divided into thick +beds, in which the mica has only a slight degree of parallelism to the +planes of stratification. + +The term "gneiss," however, in geology is commonly used in a wider sense, +to designate a formation in which the above-mentioned rock prevails, but +with which any one of the other metamorphic rocks, and more especially +hornblende-schist, may alternate. These other members of the metamorphic +series are, in this case, considered as subordinate to the true gneiss. + +The different varieties of rock allied to gneiss, into which felspar +enters as an essential ingredient, will be understood by referring to +what was said of granite. Thus, for example, hornblende may be +superadded to mica, quartz, and felspar, forming a syenitic gneiss; or +talc may be substituted for mica, constituting talcose gneiss, a rock +composed of felspar, quartz, and talc, in distinct crystals or grains +(stratified protogine of the French). + +_Hornblende-schist_ is usually black, and composed principally of +hornblende, with a variable quantity of felspar, and sometimes grains of +quartz. When the hornblende and felspar are nearly in equal quantities, +and the rock is not slaty, it corresponds in character with the +greenstones of the trap family, and has been called "primitive +greenstone." It may be termed hornblende rock. Some of these hornblendic +masses may really have been volcanic rocks, which have since assumed a +more crystalline or metamorphic texture. + +_Mica-schist_, or _Micaceous schist_, is, next to gneiss, one of the most +abundant rocks of the metamorphic series. It is slaty, essentially composed +of mica and quartz, the mica sometimes appearing to constitute the whole +mass. Beds of pure quartz also occur in this formation. In some districts, +garnets in regular twelve-sided crystals form an integrant part of +mica-schist. This rock passes by insensible gradations into clay-slate. + +_Clay-slate_, or _Argillaceous schist_.--This rock resembles an +indurated clay or shale, is for the most part extremely fissile, often +affording good roofing slate. It may consist of the ingredients of +gneiss, or of an extremely fine mixture of mica and quartz, or talc and +quartz. Occasionally it derives a shining and silky lustre from the +minute particles of mica or talc which it contains. It varies from +greenish or bluish-grey to a lead colour. It may be said of this, more +than of any other schist, that it is common to the metamorphic and +fossiliferous series, for some clay-slates taken from each division +would not be distinguishable by mineralogical characters. + +_Quartzite_, or _Quartz rock_, is an aggregate of grains of quartz, +which are either in minute crystals, or in many cases slightly rounded, +occurring in regular strata, associated with gneiss or other metamorphic +rocks. Compact quartz, like that so frequently found in veins, is also +found together with granular quartzite. Both of these alternate with +gneiss or mica-schist, or pass into those rocks by the addition of mica, +or of felspar and mica. + +_Chlorite-schist_ is a green slaty rock, in which chlorite is abundant +in foliated plates, usually blended with minute grains of quartz, or +sometimes with felspar or mica. Often associated with, and graduating +into, gneiss and clay-slate. + +_Hypogene_, or _Metamorphic limestone_.--This rock, commonly called +_primary limestone_, is sometimes a thick bedded white crystalline +granular marble used in sculpture; but more frequently it occurs in thin +beds, forming a foliated schist much resembling in colour and appearance +certain varieties of gneiss and mica-schist. It alternates with both +these rocks, and in like manner with argillaceous schist. It then +usually contains some crystals of mica, and occasionally quartz, +felspar, hornblende, and talc. This member of the metamorphic series +enters sparingly into the structure of the hypogene districts of Norway, +Sweden, and Scotland, but is largely developed in the Alps. + + +Before offering any farther observations on the probable origin of the +metamorphic rocks, I subjoin, in the form of a glossary, a brief +explanation of some of the principal varieties and their synonymies. + +ACTINOLITE-SCHIST. A slaty foliated rock, composed chiefly of actinolite, +(an emerald-green mineral, allied to hornblende,) with some admixture of +felspar, or quartz, or mica. + +AMPELITE. Aluminous slate (Brongniart); occurs both in the metamorphic +and fossiliferous series. + +AMPHIBOLITE. Hornblende rock, which see. + +ARGILLACEOUS-SCHIST, or CLAY-SLATE. _See_ p. 465. + +ARKOSE. Term used by Brongniart for granular Quartzite, which see. + +CHIASTOLITE-SLATE scarcely differs from clay-slate, but includes numerous +crystals of Chiastolite; in considerable thickness in Cumberland. +Chiastolite occurs in long slender rhomboidal crystals. For composition, +see Table, p. 377. + +CHLORITE-SCHIST. A green slaty rock, in which chlorite, a green scaly +mineral, is abundant. _See_ p. 465. + +CLAY-SLATE, or ARGILLACEOUS-SCHIST. _See_ p. 465. + +EURITE and EURITIC PORPHYRY. A base of compact felspar, with grains of +laminar felspar, and often mica and other minerals disseminated +(Brongniart). M. D'Aubuisson regards eurite as an extremely fine-grained +granite, in which felspar predominates, the whole forming an apparently +homogeneous rock. Eurite has been already mentioned as a plutonic rock, but +occurs also in beds subordinate to gneiss or mica-slate. + +GNEISS. A stratified or laminated rock, same composition as granite. +_See_ p. 464. + +HORNBLENDE ROCK, or AMPHIBOLITE. Composed of hornblende and felspar. +The same composition as hornblende-schist, stratified, but not fissile. +_See_ p. 376. + +HORNBLENDE-SCHIST, or SLATE. Composed chiefly of hornblende, with +occasionally some felspar. _See_ p. 464. + +HORNBLENDIC or SYENITIC-GNEISS. Composed of felspar, quartz, +and hornblende. + +HYPOGENE LIMESTONE. _See_ p. 465. + +MARBLE. _See_ p. 465. + +MICA-SCHIST, or MICACEOUS-SCHIST. A slaty rock, composed of mica and quartz +in variable proportions. _See_ p. 465. + +MICA-SLATE. _See_ MICA-SCHIST, p. 465. + +PHYLLADE. D'Aubuisson's term for clay-slate, from +phullas+, +a heap of leaves. + +PRIMARY LIMESTONE. _See_ HYPOGENE LIMESTONE, p. 465. + +PROTOGINE. _See_ TALCOSE-GNEISS, p. 464.; when unstratified +it is Talcose-granite. + +QUARTZ ROCK, or QUARTZITE. A stratified rock; an aggregate of grains of +quartz. _See_ p. 465. + +SERPENTINE occurs in both divisions of the hypogene series, as a stratified +or unstratified rock; contains much magnesia; is chiefly composed of the +mineral called serpentine, mixed with diallage, talc, and steatite. The +pure varieties of this rock, called noble serpentine, consist of a hydrated +silicate of magnesia, generally of a greenish colour: this base is commonly +mixed with oxide of iron. + +TALCOSE-GNEISS. Same composition as talcose-granite or protogine, but +either stratified or laminated. _See_ p. 464. + +TALCOSE-SCHIST consists chiefly of talc, or of talc and quartz, or of talc +and felspar, and has a texture something like that of clay-slate. + +WHITESTONE. Same as Eurite. + + +_Origin of the Metamorphic Strata._ + +Having said thus much of the mineral composition of the metamorphic +rocks, I may combine what remains to be said of their structure and +history with an account of the opinions entertained of their probable +origin. At the same time, it may be well to forewarn the reader that we +are here entering upon ground of controversy, and soon reach the limits +where positive induction ends, and beyond which we can only indulge in +speculations. It was once a favourite doctrine, and is still maintained +by many, that these rocks owe their crystalline texture, their want of +all signs of a mechanical origin, or of fossil contents, to a peculiar +and nascent condition of the planet at the period of their formation. +The arguments in refutation of this hypothesis will be more fully +considered when I show, in the last chapter of this volume, to how many +different ages the metamorphic formations are referable, and how gneiss, +mica-schist, clay-slate, and hypogene limestone (that of Carrara for +example), have been formed, not only since the first introduction of +organic beings into this planet, but even long after many distinct races +of plants and animals had passed away in succession. + +The doctrine respecting the crystalline strata, implied in the name +metamorphic, may properly be treated of in this place; and we must first +inquire whether these rocks are really entitled to be called stratified +in the strict sense of having been originally deposited as sediment from +water. The general adoption by geologists of the term stratified, as +applied to these rocks, sufficiently attests their division into beds +very analogous, at least in form, to ordinary fossiliferous strata. This +resemblance is by no means confined to the existence in both of an +occasional slaty structure, but extends to every kind of arrangement +which is compatible with the absence of fossils, and of sand, pebbles, +ripple-mark, and other characters which the metamorphic theory supposes +to have been obliterated by plutonic action. Thus, for example, we +behold alike in the crystalline and fossiliferous formations an +alternation of beds varying greatly in composition, colour, and +thickness. We observe, for instance, gneiss alternating with layers of +black hornblende-schist, or with granular quartz, or limestone; and the +interchange of these different strata may be repeated for an indefinite +number of times. In the like manner, mica-schist alternates with +chlorite-schist, and with granular limestone in thin layers. + +As in fossiliferous formations strata of pure siliceous sand alternate with +micaceous sand and with layers of clay, so in the crystalline or +metamorphic rocks we have beds of pure quartzite alternating with +mica-schist and clay-slate. As in the secondary and tertiary series we meet +with limestone alternating again and again with micaceous or argillaceous +sand, so we find in the hypogene, gneiss and mica-schist alternating with +pure and impure granular limestones. + +It has also been shown that the ripple-mark is very commonly repeated +throughout a considerable thickness of fossiliferous strata; so in +mica-schist and gneiss, there is sometimes an undulation of the laminæ on a +minute scale, which may, perhaps, be a modification of similar inequalities +in the original deposit. + +In the crystalline formations also, as in many of the sedimentary before +described, single strata are sometimes made up of laminæ placed diagonally, +such laminæ not being regularly parallel to the planes of cleavage. + +[Illustration: Fig. 509. Lamination of clay-slate, Montagne de Seguinat, +near Gavarnie, in the Pyrenees.] + +This disposition of the layers is illustrated in the accompanying diagram, +in which I have represented carefully the stratification of a coarse +argillaceous schist, which I examined in the Pyrenees, part of which +approaches in character to a green and blue roofing slate, while part is +extremely quartzose, the whole mass passing downwards into micaceous +schist. The vertical section here exhibited is about 3 feet in height, and +the layers are sometimes so thin that fifty may be counted in the thickness +of an inch. Some of them consist of pure quartz. + +The inference drawn from the phenomena above described in favour of the +aqueous origin of clay-slate and other crystalline strata, is greatly +strengthened by the fact that many of these metamorphic rocks occasionally +alternate with, and sometimes pass by intermediate gradations into, rocks +of a decidedly mechanical origin, and exhibiting traces of organic remains. +The fossiliferous formations, moreover, into which this passage is +effected, are by no means invariably of the same age nor of the highest +antiquity, as will be afterwards explained. + +_Stratification of the metamorphic rocks distinct from cleavage._--The beds +into which gneiss, mica-schist, and hypogene limestone divide, exhibit most +commonly, like ordinary strata, a want of perfect geometrical parallelism. +For this reason, therefore, in addition to the alternate recurrence of +layers of distinct materials, the stratified arrangement of the crystalline +rocks cannot be explained away by supposing it to be simply a divisional +structure like that to which we owe some of the slates used for writing and +roofing. _Slaty cleavage_, as it has been called, has in many cases been +produced by the regular deposition of thin plates of fine sediment one upon +another; but there are many instances where it is decidedly unconnected +with such a mode of origin, and where it is not even confined to the +aqueous formations. Some kinds of trap, for example, as clinkstone, split +into laminæ, and are used for roofing. + +There are, says Professor Sedgwick, three distinct forms of structure +exhibited in certain rocks throughout large districts: viz.--First, +stratification; secondly, joints; and thirdly, slaty cleavage; the two +last having no connection with true bedding, and having been superinduced +by causes absolutely independent of gravitation. All these different +structures must have different names, even though there be some cases where +it is impossible, after carefully studying the appearances, to decide upon +the class to which they belong.[469-A] + +_Joints._--Now, in regard to the second of these forms of structure or +joints, they are natural fissures which often traverse rocks in straight +and well-determined lines. They afford to the quarryman, as Sir R. +Murchison observes, when speaking of the phenomena, as exhibited in +Shropshire and the neighbouring counties, the greatest aid in the +extraction of blocks of stone; and, if a sufficient number cross each +other, the whole mass of rock is split into symmetrical blocks.[469-B] The +faces of the joints are for the most part smoother and more regular than +the surfaces of true strata. The joints are straight-cut chinks, often +slightly open, often passing, not only through layers of successive +deposition, but also through balls of limestone or other matter which have +been formed by concretionary action, since the original accumulation of the +strata. Such joints, therefore, must often have resulted from one of the +last changes superinduced upon sedimentary deposits.[469-C] + +In the annexed diagram the flat surfaces of rock A, B, C, represent exposed +faces of joints, to which the walls of other joints, J J, are parallel. S S +are the lines of stratification; D D are lines of slaty cleavage, which +intersect the rock at a considerable angle to the planes of stratification. + +[Illustration: Fig. 510. Stratification, joints, and cleavage.] + +Joints, according to Professor Sedgwick, are distinguishable from lines of +slaty cleavage in this, that the rock intervening between two joints has no +tendency to cleave in a direction parallel to the planes of the joints, +whereas a rock is capable of indefinite subdivision in the direction of its +slaty cleavage. In some cases where the strata are curved, the planes of +cleavage are still perfectly parallel. This has been observed in the slate +rocks of part of Wales (see fig. 511.), which consist of a hard greenish +slate. The true bedding is there indicated by a number of parallel stripes, +some of a lighter and some of a darker colour than the general mass. Such +stripes are found to be parallel to the true planes of stratification, +wherever these are manifested by ripple-mark, or by beds containing +peculiar organic remains. Some of the contorted strata are of a coarse +mechanical structure, alternating with fine-grained crystalline chloritic +slates, in which case the same slaty cleavage extends through the coarser +and finer beds, though it is brought out in greater perfection in +proportion as the materials of the rock are fine and homogeneous. It is +only when these are very coarse that the cleavage planes entirely vanish. +These planes are usually inclined at a very considerable angle to the +planes of the strata. In the Welsh chains, for example, the average angle +is as much as from 30° to 40°. Sometimes the cleavage planes dip towards +the same point of the compass as those of stratification, but more +frequently to opposite points. It may be stated as a general rule, that +when beds of coarser materials alternate with those composed of finer +particles, the slaty cleavage is either entirely confined to the +fine-grained rock, or is very imperfectly exhibited in that of coarser +texture. This rule holds, whether the cleavage is parallel to the planes of +stratification or not. + +[Illustration: Fig. 511. Parallel planes of cleavage intersecting +curved strata. (Sedgwick.)] + +In the Swiss and Savoy Alps, as Mr. Bakewell has remarked, enormous masses +of limestone are cut through so regularly by nearly vertical partings, and +these are often so much more conspicuous than the seams of stratification, +that an inexperienced observer will almost inevitably confound them, and +suppose the strata to be perpendicular in places where in fact they are +almost horizontal.[470-A] + +Now these joints are supposed to be analogous to those partings which have +been already observed to separate volcanic and plutonic rocks into cuboidal +and prismatic masses. On a small scale we see clay and starch when dry +split into similar shapes, which is often caused by simple contraction, +whether the shrinking be due to the evaporation of water, or to a change of +temperature. It is well known that many sandstones and other rocks expand +by the application of moderate degrees of heat, and then contract again on +cooling; and there can be no doubt that large portions of the earth's crust +have, in the course of past ages, been subjected again and again to very +different degrees of heat and cold. These alternations of temperature have +probably contributed largely to the production of joints in rocks. + +In some countries, as in Saxony, where masses of basalt rest on sandstone, +the aqueous rock has for the distance of several feet from the point of +junction assumed a columnar structure similar to that of the trap. In like +manner some hearthstones, after exposure to the heat of a furnace without +being melted, have become prismatic. Certain crystals also acquire by the +application of heat a new internal arrangement, so as to break in a new +direction, their external form remaining unaltered. + +Sir R. Murchison observes, that in referring both joints and slaty cleavage +to crystalline action, we are borne out by a well-known analogy in which +crystallization has in like manner given rise to two distinct kinds of +structure in the same body. Thus, for example, in a six-sided prism of +quartz, the planes of cleavage are distinct from those of the prism. It is +impossible to cleave the crystals parallel to the plane of the prism, just +as slaty rocks cannot be cleaved parallel to the joints; but the quartz +crystal, like the older schists, may be cleaved _ad infinitum_ in the +direction of the cleavage planes.[471-A] + +It seems, therefore, that the fissures called joints may have been the +result of different causes, as of some modification of crystalline action, +or simple contraction during consolidation, or during a change of +temperature. And there are cases where joints may have been due to +mechanical violence, and the strain exerted on strata during their +upheaval, or when they have sunk down below their former level. Professor +Phillips has suggested that the previous existence of divisional planes may +often have determined, and must greatly have modified, the lines and points +of fracture caused in rocks by those forces to which they owe their +elevation or dislocations. These lines and points being those of least +resistance, cannot fail to have influenced the direction in which the solid +mass would give way on the application of external force. + +Professor Phillips has also remarked that in some slaty rocks the form of +the outline of fossil shells and trilobites has been much changed by +distortion, which has taken place in a longitudinal, transverse, or oblique +direction. This change, he adds, seems to be the result of a "creeping +movement" of the particles of the rock along the planes of cleavage, its +direction being always uniform over the same tract of country, and its +amount in space being sometimes measurable, and being as much as a quarter +or even half an inch. The hard shells are not affected, but only those +which are thin.[471-B] Mr. D. Sharpe, following up the same line of +inquiry, came to the conclusion, that the present distorted forms of the +shells in certain British slate rocks may be accounted for by supposing +that the rocks in which they are imbedded have undergone compression in a +direction perpendicular to the planes of cleavage, and a corresponding +expansion in the direction of the dip of the cleavage.[471-C] + +Mr. Darwin infers from his observations, that in South America the +strike of the cleavage planes is very uniform over wide regions, and +that it corresponds with the strike of the planes of foliation in the +gneiss and mica-schists of the same parts of Chili, Tierra del Fuego, +&c. The explanation which he suggests, is based upon a combination of +mechanical and crystalline forces. The planes, he says, of cleavage, and +even the foliation of mica-schist and gneiss, may be intimately +connected with the planes of different tension to which the area was +long subjected, _after_ the main fissures or axis of upheavement had +been formed, but _before_ the final consolidation of the mass and the +total cessation of all molecular movement.[472-A] + +I have already stated that some extremely fine slates are perfectly +parallel to the planes of stratification, as those of the Niesen, for +example, near the Lake of Thun, in Switzerland, which contain fucoids, and +are no doubt due to successive aqueous deposition. Even where the slates +are oblique to the general planes of the strata, it by no means follows as +a matter of course that they have been caused by crystalline action, for +they may be the result of that diagonal lamination which I have before +described (p. 17.). In this case, however, there is usually much +irregularity, whereas cleavage planes oblique to the true stratification, +which are referred to a crystalline action, are often perfectly +symmetrical, and observe a strict geometrical parallelism, even when the +strata are contorted, as already described (p. 470.). + +Professor Sedgwick, speaking of the planes of slaty cleavage, where they +are decidedly distinct from those of sedimentary deposition, declares +his opinion that no retreat of parts, no contraction in the dimensions +of rocks in passing to a solid state, can account for the phenomenon. It +must be referred to crystalline or polar forces acting simultaneously, +and somewhat uniformly, in given directions, on large masses having +a homogeneous composition. + +Sir John Herschel, in allusion to slaty cleavage, has suggested, "that +if rocks have been so heated as to allow a commencement of +crystallization; that is to say, if they have been heated to a point +at which the particles can begin to move amongst themselves, or at +least on their own axes, some general law must then determine the +position in which these particles will rest on cooling. Probably that +position will have some relation to the direction in which the heat +escapes. Now, when all, or a majority of particles of the same nature, +have a general tendency to one position, that must of course determine +a cleavage plane. Thus we see the infinitesimal crystals of fresh +precipitated sulphate of barytes, and some other such bodies, arrange +themselves alike in the fluid in which they float; so as, when stirred, +all to glance with one light, and give the appearance of silky +filaments. Some sorts of soap, in which insoluble margarates[472-B] +exist, exhibit the same phenomenon when mixed with water; and what +occurs in our experiments on a minute scale may occur in nature on +a great one."[472-C] + + +FOOTNOTES: + +[469-A] Geol. Trans., 2d series, vol. iii. p. 480. + +[469-B] The Silurian System of Rocks, as developed in Salop, Hereford, +&c., p. 245. + +[469-C] Ibid., p. 246. + +[470-A] Introduction to Geology, chap. iv. + +[471-A] Silurian System of Rocks, &c., p. 246. + +[471-B] Report, Brit. Ass., Cork, 1843, p. 60. + +[471-C] Quart. Geol. Journ., vol. iii. p. 87. 1847. + +[472-A] Geol. Obs. on S. America, 1846, p. 168. + +[472-B] Margaric acid is an oleaginous acid, formed from different animal +and vegetable fatty substances. A margarate is a compound of this acid with +soda, potash, or some other base, and is so named from its pearly lustre. + +[472-C] Letter to the author, dated Cape of Good Hope, Feb. 20. 1836. + + + + +CHAPTER 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. + + +It has been seen that geologists have been very generally led to infer, +from the phenomena of joints and slaty cleavage, that mountain masses, of +which the sedimentary origin is unquestionable, have been acted upon +simultaneously by vast crystalline forces. That the structure of +fossiliferous strata has often been modified by some general cause since +their original deposition, and even subsequently to their consolidation and +dislocation, is undeniable. These facts prepare us to believe that still +greater changes may have been worked out by a greater intensity, or more +prolonged development of the same agency, combined, perhaps, with other +causes. Now we have seen that, near the immediate contact of granitic veins +and volcanic dikes, very extraordinary alterations in rocks have taken +place, more especially in the neighbourhood of granite. It will be useful +here to add other illustrations, showing that a texture undistinguishable +from that which characterizes the more crystalline metamorphic formations, +has actually been superinduced in strata once fossiliferous. + +In the southern extremity of Norway there is a large district, on the west +side of the fiord of Christiania, in which granite or syenite protrudes in +mountain masses through fossiliferous strata, and usually sends veins into +them at the point of contact. The stratified rocks, replete with shells and +zoophytes, consist chiefly of shale, limestone, and some sandstone, and all +these are invariably altered near the granite for a distance of from 50 to +400 yards. The aluminous shales are hardened and have become flinty. +Sometimes they resemble jasper. Ribboned jasper is produced by the +hardening of alternate layers of green and chocolate-coloured schist, each +stripe faithfully representing the original lines of stratification. Nearer +the granite the schist often contains crystals of hornblende, which are +even met with in some places for a distance of several hundred yards from +the junction; and this black hornblende is so abundant that eminent +geologists, when passing through the country, have confounded it with the +ancient hornblende-schist, subordinate to the great gneiss formation of +Norway. Frequently, between the granite and the hornblende slate, above +mentioned, grains of mica and crystalline felspar appear in the schist, so +that rocks resembling gneiss and mica-schist are produced. Fossils can +rarely be detected in these schists, and they are more completely effaced +in proportion to the more crystalline texture of the beds, and their +vicinity to the granite. In some places the siliceous matter of the schist +becomes a granular quartz; and when hornblende and mica are added, the +altered rock loses its stratification, and passes into a kind of granite. +The limestone, which at points remote from the granite is of an earthy +texture, blue colour, and often abounds in corals, becomes a white granular +marble near the granite, sometimes siliceous, the granular structure +extending occasionally upwards of 400 yards from the junction; and the +corals being for the most part obliterated, though sometimes preserved, +even in the white marble. Both the altered limestone and hardened slate +contain garnets in many places, also ores of iron, lead, and copper, with +some silver. These alterations occur equally, whether the granite invades +the strata in a line parallel to the general strike of the fossiliferous +beds, or in a line at right angles to their strike, as will be seen by the +accompanying ground plan.[474-A] + +[Illustration: Fig. 512. Altered zone of fossiliferous slate and limestone +near granite. Christiania. + +_The arrows indicate the dip, and the straight lines the strike, +of the beds._] + +The indurated and ribboned schists above mentioned bear a strong +resemblance to certain shales of the coal found at Russell's Hall, near +Dudley, where coal-mines have been on fire for ages. Beds of shale of +considerable thickness, lying over the burning coal, have been baked and +hardened so as to acquire a flinty fracture, the layers being alternately +green and brick-coloured. + +The granite of Cornwall, in like manner, sends forth veins into a coarse +argillaceous-schist, provincially termed killas. This killas is converted +into hornblende-schist near the contact with the veins. These appearances +are well seen at the junction of the granite and killas, in St. Michael's +Mount, a small island nearly 300 feet high, situated in the bay, at a +distance of about three miles from Penzance. + +The granite of Dartmoor, in Devonshire, says Sir H. De la Beche, has +intruded itself into the slate and slaty sandstone called greywacké, +twisting and contorting the strata, and sending veins into them. Hence some +of the slate rocks have become "micaceous; others more indurated, and with +the characters of mica-slate and gneiss; while others again appear +converted into a hard-zoned rock strongly impregnated with felspar."[475-A] + +We learn from the investigations of M. Dufrénoy, that in the eastern +Pyrenees there are mountain masses of granite posterior in date to the +formations called lias and chalk of that district, and that these +fossiliferous rocks are greatly altered in texture, and often charged with +iron-ore, in the neighbourhood of the granite. Thus in the environs of St. +Martin, near St. Paul de Fénouillet, the chalky limestone becomes more +crystalline and saccharoid as it approaches the granite, and loses all +trace of the fossils which it previously contained in abundance. At some +points, also, it becomes dolomitic, and filled with small veins of +carbonate of iron, and spots of red iron-ore. At Rancié the lias nearest +the granite is not only filled with iron-ore, but charged with pyrites, +tremolite, garnet, and a new mineral somewhat allied to felspar, called, +from the place in the Pyrenees where it occurs, "couzeranite." + +Now the alterations above described as superinduced in rocks by volcanic +dikes and granite veins, prove incontestably that powers exist in nature +capable of transforming fossiliferous into crystalline strata--powers +capable of generating in them a new mineral character, similar, nay, often +absolutely identical, with that of gneiss, mica-schist, and other +stratified members of the hypogene series. The precise nature of these +altering causes, which may provisionally be termed plutonic, is in a great +degree obscure and doubtful; but their reality is no less clear, and we +must suppose the influence of heat to be in some way connected with the +transmutation, if, for reasons before explained, we concede the igneous +origin of granite. + +The experiments of Gregory Watt, in fusing rocks in the laboratory, and +allowing them to consolidate by slow cooling, prove distinctly that a rock +need not be perfectly melted in order that a re-arrangement of its +component particles should take place, and a partial crystallization +ensue.[475-B] We may easily suppose, therefore, that all traces of shells +and other organic remains may be destroyed; and that new chemical +combinations may arise, without the mass being so fused as that the lines +of stratification should be wholly obliterated. + +We must not, however, imagine that heat alone, such as may be applied to +a stone in the open air, can constitute all that is comprised in +plutonic action. We know that volcanos in eruption not only emit fluid +lava, but give off steam and other heated gases, which rush out in +enormous volume, for days, weeks, or years continuously, and are even +disengaged from lava during its consolidation. When the materials of +granite, therefore, came in contact with the fossiliferous stratum in +the bowels of the earth under great pressure, the contained gases might +be unable to escape; yet when brought into contact with rocks, might +pass through their pores with greater facility than water is known to do +(p. 35.). These aëriform fluids, such as sulphuretted hydrogen, muriatic +acid, and carbonic acid, issue in many places from rents in rocks, which +they have discoloured and corroded, softening some and hardening others. +If the rocks are charged with water, they would pass through more +readily; for, according to the experiments of Henry, water, under an +hydrostatic pressure of 96 feet, will absorb three times as much +carbonic acid gas as it can under the ordinary pressure of the +atmosphere. Although this increased power of absorption would be +diminished, in consequence of the higher temperature found to exist as +we descend in the earth, yet Professor Bischoff has shown that the heat +by no means augments in such a proportion as to counteract the effect of +augmented pressure.[476-A] There are other gases, as well as the +carbonic acid, which water absorbs, and more rapidly in proportion to +the amount of pressure. Now even the most compact rocks may be regarded, +before they have been exposed to the air and dried, in the light of +sponges filled with water; and it is conceivable that heated gases +brought into contact with them, at great depths, may be absorbed +readily, and transfused through their pores. Although the gaseous matter +first observed would soon be condensed, and part with its heat, yet the +continual arrival of fresh supplies from below might, in the course of +ages, cause the temperature of the water, and with it that of the +containing rock, to be materially raised. + +M. Fournet, in his description of the metalliferous gneiss near Clermont, +in Auvergne, states that all the minute fissures of the rock are quite +saturated with free carbonic acid gas, which rises plentifully from the +soil there and in many parts of the surrounding country. The various +elements of the gneiss, with the exception of the quartz, are all softened; +and new combinations of the acid, with lime, iron, and manganese, are +continually in progress.[476-B] + +Another illustration of the power of subterranean gases is afforded by the +stufas of St. Calogero, situated in the largest of the Lipari Islands. +Here, according to the description published by Hoffmann, horizontal strata +of tuff, extending for 4 miles along the coast, and forming cliffs more +than 200 feet high, have been discoloured in various places, and strangely +altered by the "all-penetrating vapours." Dark clays have become yellow, or +often snow-white; or have assumed a chequered or brecciated appearance, +being crossed with ferruginous red stripes. In some places the fumaroles +have been found by analysis to consist partly of sublimations of oxide of +iron; but it also appears that veins of chalcedony and opal, and others of +fibrous gypsum, have resulted from these volcanic exhalations.[476-C] + +The reader may also refer to M. Virlet's account of the corrosion of hard, +flinty, and jaspideous rocks near Corinth, by the prolonged agency of +subterranean gases[477-A]; and to Dr. Daubeny's description of the +decomposition of trachytic rocks in the Solfatara, near Naples, by +sulphuretted hydrogen and muriatic acid gases.[477-B] + +Although in all these instances we can only study the phenomena as +exhibited at the surface, it is clear that the gaseous fluids must have +made their way through the whole thickness of porous or fissured rocks, +which intervene between the subterranean reservoirs of gas and the external +air. The extent, therefore, of the earth's crust, which the vapours have +permeated and are now permeating, may be thousands of fathoms in thickness, +and their heating and modifying influence may be spread throughout the +whole of this solid mass. + +We learn from Professor Bischoff that the steam of a hot spring at +Aix-la-Chapelle, although its temperature is only from 133° to 167° F., has +converted the surface of some blocks of black marble into a doughy mass. He +conceives, therefore, that steam in the bowels of the earth having a +temperature equal or even greater than the melting point of lava, and +having an elasticity of which even Papin's digester can give but a faint +idea, may convert rocks into liquid matter.[477-C] + +The above observations are calculated to meet some of the objections which +have been urged against the metamorphic theory on the ground of the small +power of rocks to conduct heat; for it is well known that rocks, when dry +and in the air, differ remarkably from metals in this respect. It has been +asked how the changes which extend merely for a few feet from the contact +of a dike could have penetrated through mountain masses of crystalline +strata several miles in thickness. Now it has been stated that the plutonic +influence of the syenite of Norway has sometimes altered fossiliferous +strata for a distance of a quarter of a mile, both in the direction of +their dip and of their strike. (See fig. 512. p. 474.) This is undoubtedly +an extreme case; but is it not far more philosophical to suppose that this +influence may, under favourable circumstances, affect denser masses, than +to invent an entirely new cause to account for effects merely differing in +quantity, and not in kind? The metamorphic theory does not require us to +affirm that some contiguous mass of granite has been the altering power; +but merely that an action, existing in the interior of the earth at an +unknown depth, whether thermal, electrical, or other, analogous to that +exerted near intruding masses of granite, has, in the course of vast and +indefinite periods, and when rising perhaps from a large heated surface, +reduced strata thousands of yards thick to a state of semi-fusion, so that +on cooling they have become crystalline, like gneiss. Granite may have been +another result of the same action in a higher state of intensity, by which +a thorough fusion has been produced; and in this manner the passage from +granite into gneiss may be explained. + +Some geologists are of opinion, that the alternate layers of mica and +quartz, or mica and felspar, or lime and felspar, are so much more +distinct, in certain metamorphic rocks, than the ingredients composing +alternate layers in many sedimentary deposits, that the similar particles +must be supposed to have exerted a molecular attraction for each other, and +to have thus congregated together in layers more distinct in mineral +composition than before they were crystallized. + +In considering, then, the various data already enumerated, the forms of +stratification in metamorphic rocks, their passage on the one hand into the +fossiliferous, and on the other into the plutonic formations, and the +conversions which can be ascertained to have occurred in the vicinity of +granite, we may conclude that gneiss and mica-schist may be nothing more +than altered micaceous and argillaceous sandstones that granular quartz may +have been derived from siliceous sandstone, and compact quartz from the +same materials. Clay-slate may be altered shale, and granular marble may +have originated in the form of ordinary limestone, replete with shells and +corals, which have since been obliterated; and, lastly, calcareous sands +and marls may have been changed into impure crystalline limestones. + +"Hornblende-schist," says Dr. MacCulloch, "may at first have been mere +clay; for clay or shale is found altered by trap into Lydian stone, a +substance differing from hornblende-schist almost solely in compactness and +uniformity of texture."[478-A] "In Shetland," remarks the same author, +"argillaceous-schist (or clay-slate), when in contact with granite, is +sometimes converted into hornblende-schist, the schist becoming first +siliceous, and ultimately, at the contact, hornblende-schist."[478-B] + +The anthracite and plumbago associated with hypogene rocks may have been +coal; for not only is coal converted into anthracite in the vicinity of +some trap dikes, but we have seen that a like change has taken place +generally even far from the contact of igneous rocks, in the disturbed +region of the Appalachians.[478-C] At Worcester, in the state of +Massachusetts, 45 miles due west of Boston, a bed of plumbago and impure +anthracite occurs, interstratified with mica-schist. It is about 2 feet in +thickness, and has been made use of both as fuel, and in the manufacture of +lead pencils. At the distance of 30 miles from the plumbago, there occurs, +on the borders of Rhode Island, an impure anthracite in slates, containing +impressions of coal-plants of the genera _Pecopteris_, _Neuropteris_, +_Calamites_, &c. This anthracite is intermediate in character between that +of Pennsylvania and the plumbago of Worcester, in which last the gaseous or +volatile matter (hydrogen, oxygen, and nitrogen) is to the carbon only in +the proportion of 3 per cent. After traversing the country in various +directions, I came to the conclusion that the carboniferous shales or +slates with anthracite and plants, which in Rhode Island often pass into +mica-schist, have at Worcester assumed a perfectly crystalline and +metamorphic texture; the anthracite having been nearly transmuted into that +state of pure carbon which is called plumbago or graphite.[479-A] + +The total absence of any trace of fossils has inclined many geologists to +attribute the origin of crystalline strata to a period antecedent to the +existence of organic beings. Admitting, they say, the obliteration, in some +cases, of fossils by plutonic action, we might still expect that traces of +them would oftener occur in certain ancient systems of slate, in which, as +in Cumberland, some conglomerates occur. But in urging this argument, it +seems to have been forgotten that there are stratified formations of +enormous thickness, and of various ages, and some of them very modern, all +formed after the earth had become the abode of living creatures, which are, +nevertheless, in certain districts, entirely destitute of all vestiges of +organic bodies. In some, the traces of fossils may have been effaced by +water and acids, at many successive periods; and it is clear, that, the +older the stratum, the greater is the chance of its being +non-fossiliferous, even if it has escaped all metamorphic action. + +It has been also objected to the metamorphic theory, that the chemical +composition of the secondary strata differs essentially from that of the +crystalline schists, into which they are supposed to be convertible.[479-B] +The "primary" schists, it is said, usually contain a considerable +proportion of potash or of soda, which the secondary clays, shales, and +slates do not, these last being the result of the decomposition of +felspathic rocks, from which the alkaline matter has been abstracted during +the process of decomposition. But this reasoning proceeds on insufficient +and apparently mistaken data; for a large portion of what is usually called +clay, marl, shale, and slate does actually contain a certain, and often a +considerable, proportion of alkali; so that it is difficult, in many +countries, to obtain clay or shale sufficiently free from alkaline +ingredients to allow of their being burnt into bricks or used for pottery. + +Thus the argillaceous shales and slates of the Old Red sandstone, in +Forfarshire and other parts of Scotland, are so much charged with +alkali, derived from triturated felspar, that, instead of hardening when +exposed to fire, they sometimes melt into a glass. They contain no lime, +but appear to consist of extremely minute grains of the various +ingredients of granite, which are distinctly visible in the +coarser-grained varieties, and in almost all the interposed sandstones. +These laminated clays and shales might certainly, if crystallized, +resemble in composition many of the primary strata. + +There is also potash in fossil vegetable remains, and soda in the salts by +which strata are sometimes so largely impregnated, as in Patagonia. + +Another objection has been derived from the alternation of highly +crystalline strata with others having a less crystalline texture. The heat, +it is said, in its ascent from below, must have traversed the less altered +schists before it reached a higher and more crystalline bed. In answer to +this, it may be observed, that if a number of strata differing greatly in +composition from each other be subjected to equal quantities of heat, there +is every probability that some will be more fusible than others. Some, for +example, will contain soda, potash, lime, or some other ingredient capable +of acting as a flux; while others may be destitute of the same elements, +and so refractory as to be very slightly affected by a degree of heat +capable of reducing others to semi-fusion. Nor should it be forgotten that, +as a general rule, the less crystalline rocks do really occur in the upper, +and the more crystalline in the lower part of each metamorphic series. + +There are geologists, however, of high authority, who admit the +metamorphic origin of gneiss and mica-schist even on a grand scale in +some mountain-chains, and who nevertheless believe that gneiss has in +some instances been an eruptive rock, deriving its lamination from +motion when in a fluid or viscous state. Mr. Scrope, in his description +of the Ponza Islands, ascribes "the zoned structure of the Hungarian +perlite (a semi-vitreous trachyte) to its having subsided, in obedience +to the impulse of its own gravity, down a slightly inclined plane, while +possessed of an imperfect fluidity. In the islands of Ponza and +Palmarola, the direction of the zones is more frequently vertical than +horizontal, because the mass was impelled from below upwards."[480-A] In +like manner, Mr. Darwin attributes the lamination and fissile structure +of volcanic rocks of the trachytic series, including some obsidians in +Ascension, Mexico, and elsewhere, to their having moved when liquid in +the direction of the laminæ. The zones consist sometimes of layers of +air-cells drawn out and lengthened in the supposed direction of the +moving mass. He compares this division into parallel zones, thus caused +by the stretching of a pasty mass as it flowed slowly onwards, to the +zoned or ribboned structure of ice, which Professor James Forbes has +so ably explained, showing that it is due to the fissuring of a +viscous body in motion.[480-B] Mr. Darwin also imagines the lamination +or _foliation_, as he terms it, of gneiss and mica-schist in South +America to be the extreme result of that process of which cleavage +is the first effect.[480-C] + +M. Elie de Beaumont, while he regards the greater part of the gneiss and +mica-schist of the Alps as sedimentary strata altered by plutonic action, +still conceives that some of the Alpine gneiss may have been erupted, or, +in other words, may be granite drawn out into parallel laminæ in the manner +of trachyte as above alluded to.[480-D] + +Opinions such as these, and others which might be cited, prove the +difficulty of arriving at clear theoretical views on this subject. I may +also add another difficulty. In many extensive regions experienced +geologists have been at a loss to decide which of two sets of divisional +planes were referable to cleavage and which to stratification; and that, +too, where the rocks are of undisputed aqueous origin. After much doubt, +they have sometimes discovered that they had at first mistaken the lines of +cleavage for those of deposition, because the former were by far the most +marked of the two. Now if such slaty masses should become highly +crystalline, and be converted into gneiss, hornblende-schist, or any other +member of the hypogene class, the cleavage planes would be more likely to +remain visible than those of stratification. + +But although the cause last-mentioned may, in some instances, be a "vera +causa," as applied to gneiss and mica-schist, I believe it to be an +exception to the general rule. Nor would it, I conceive, produce that kind +of irregular parallelism in the laminæ which belongs to so many of the +hypogene rocks of the Grampians, Pyrenees, and the White mountains of North +America, where I have chiefly studied them. + +But it will be impossible for the reader duly to appreciate the propriety +of the term metamorphic, as applied to the strata formerly called +primitive, until I have shown, in the next chapter, at how many distinct +periods these crystalline strata have been formed. + + +FOOTNOTES: + +[474-A] Keilhau, Gæa Norvegica, pp. 61-63. + +[475-A] Geol. Manual, p. 479. + +[475-B] Phil. Trans., 1804. + +[476-A] Poggendorf's Annalen, No. xvi., 2d series, vol. iii. + +[476-B] See Principles, _Index_, "Carbonated Springs," &c. + +[476-C] Hoffmann's Liparischen Inseln, p. 38. Leipzig, 1832. + +[477-A] See Princ. of Geol.; and Bulletin de la Soc. Géol. de France, +tom. ii. p. 230. + +[477-B] See Princ. of Geol.; and Daubeny's Volcanos, p. 167. + +[477-C] Jam. Ed. New Phil. Journ., No. 51. p. 43. + +[478-A] Syst. of Geol., vol. i. p. 210. + +[478-B] Ibid., p. 211. + +[478-C] See above, pp. 327, 333. + +[479-A] See Lyell, Quart. Geol. Journ., vol. i. p. 199. + +[479-B] Dr. Boase, Primary Geology, p. 319. + +[480-A] Geol. Trans., 2d series, vol. ii. p. 227. + +[480-B] Darwin, Volcanic Islands, pp. 69, 70. + +[480-C] Geol. Obs. in S. America, p. 167. See also above, p. 471. + +[480-D] Bulletin, vol. iv. p. 1301. + + + + +CHAPTER 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. + + +According to the theory adopted in the last chapter, the age of each set of +metamorphic strata is twofold--they have been deposited at one period, they +have become crystalline at another. We can rarely hope to define with +exactness the date of both these periods, the fossils having been destroyed +by plutonic action, and the mineral characters being the same, whatever the +age. Superposition itself is an ambiguous test, especially when we desire +to determine the period of crystallization. Suppose, for example, we are +convinced that certain metamorphic strata in the Alps, which are covered by +cretaceous beds, are altered lias; this lias may have assumed its +crystalline texture in the cretaceous or in some tertiary period, the +Eocene for example. If in the latter, it should be called Eocene when +regarded as a metamorphic rock, although it be liassic when considered in +reference to the era of its deposition. According to this view, the +superposition of chalk does not prevent the subjacent _metamorphic_ rock +from being Eocene. If, however, in the progress of science, we should +succeed in ascertaining the twofold chronological relations of the +metamorphic formations, it might be useful to adopt a twofold terminology. +We might call the strata above alluded to Liassic-Eocene, or +Liassic-Cretaceous strata of the Hypogene class; the first term referring +to the era of deposition, the second to that of crystallization. + +When discussing the ages of the plutonic rocks, we have seen that examples +occur of various primary, secondary, and tertiary deposits converted into +metamorphic strata, near their contact with granite. There can be no doubt +in these cases that strata, once composed of mud, sand, and gravel, or of +clay, marl, and shelly limestone, have for the distance of several yards, +and in some instances several hundred feet, been turned into gneiss, +mica-schist, hornblende-schist, chlorite-schist, quartz rock, statuary +marble, and the rest. (See the two preceding Chapters.) + +But when the metamorphic action has operated on a grander scale, it tends +entirely to destroy all monuments of the date of its development. It may be +easy to prove the identity of two different parts of the same stratum; one, +where the rock has been in contact with a volcanic or plutonic mass, and +has been changed into marble or hornblende-schist, and another not far +distant, where the same bed remains unaltered and fossiliferous; but when +we have to compare two portions of a mountain chain--the one metamorphic, +and the other unaltered--all the labour and skill of the most practised +observers are required. I shall mention one or two examples of alteration +on a grand scale, in order to explain to the student the kind of reasoning +by which we are led to infer that dense masses of fossiliferous strata have +been converted into crystalline rocks. + +_Northern Apennines--Carrara._--The celebrated marble of Carrara, used in +sculpture, was once regarded as a type of primitive limestone. It abounds +in the mountains of Massa Carrara, or the "Apuan Alps," as they have been +called, the highest peaks of which are nearly 6000 feet high. Its great +antiquity was inferred from its mineral texture, from the absence of +fossils, and its passage downwards into talc-schist and garnetiferous +mica-schist; these rocks again graduating downwards into gneiss, which is +penetrated, at Forno, by granite veins. Now the researches of MM. Savi, +Boué, Pareto, Guidoni, De la Beche, Hoffmann, and Pilla, have demonstrated +that this marble, once supposed to be formed before the existence of +organic beings, is, in fact, an altered limestone of the Oolitic period, +and the underlying crystalline schists are secondary sandstones and shales, +modified by plutonic action. In order to establish these conclusions it was +first pointed out, that the calcareous rocks bordering the Gulf of Spezia, +and abounding in Oolitic fossils, assume a texture like that of Carrara +marble, in proportion as they are more and more invaded by certain trappean +and plutonic rocks, such as diorite, euphotide, serpentine, and granite, +occurring in the same country. + +It was then observed that, in places where the secondary formations are +unaltered, the uppermost consist of common Apennine limestone with +nodules of flint, below which are shales, and at the base of all, +argillaceous and siliceous sandstones. In the limestone, fossils are +frequent, but very rare in the underlying shale and sandstone. Then a +gradation was traced laterally from these rocks into another and +corresponding series, which is completely metamorphic; for at the top of +this we find a white granular marble, wholly devoid of fossils, and +almost without stratification, in which there are no nodules of flint, +but in its place siliceous matter disseminated through the mass in the +form of prisms of quartz. Below this, and in place of the shales, are +talc-schists, jasper, and hornstone; and at the bottom, instead of the +siliceous and argillaceous sandstones, are quartzite and gneiss.[483-A] +Had these secondary strata of the Apennines undergone universally as +great an amount of transmutation, it would have been impossible to form +a conjecture respecting their true age; and then, according to the +common method of geological classification, they would have ranked as +primary rocks. In that case the date of their origin would have been +thrown back to an era antecedent to the deposition of the Lower Silurian +or Cambrian strata, although in reality they were formed in the Oolitic +period, and altered at some subsequent and perhaps much later epoch. + +_Alps of Switzerland._--In the Alps, analogous conclusions have been drawn +respecting the alteration of strata on a still more extended scale. In the +eastern part of that chain, some of the primary fossiliferous strata, as +well as the older secondary formations, together with the oolitic and +cretaceous rocks, are distinctly recognizable. Tertiary deposits also +appear in a less elevated position on the flanks of the Eastern Alps; but +in the Central or Swiss Alps, the primary fossiliferous and older secondary +formations disappear, and the Cretaceous, Oolitic, Liassic, and at some +points even the Eocene strata, graduate insensibly into metamorphic rocks, +consisting of granular limestone, talc-schist, talcose-gneiss, micaceous +schist, and other varieties. In regard to the age of this vast assemblage +of crystalline strata, we can merely affirm that some of the upper portions +are altered newer secondary, and some of them even Eocene deposits; but we +cannot avoid suspecting that the disappearance both of the older secondary +and primary fossiliferous rocks may be owing to their having been all +converted in this region into crystalline schist. + +It is difficult to convey to those who have never visited the Alps a just +idea of the various proofs which concur to produce this conviction. In the +first place, there are certain regions where Oolitic, Cretaceous, and +Eocene strata have been turned into granular marble, gneiss, and other +metamorphic schists, near their contact with granite. This fact shows +undeniably that plutonic causes continued to be in operation in the Alps +down to a late period, even after the deposition of some of the nummulitic +or older Eocene formations. Having established this point, we are the more +willing to believe that many inferior fossiliferous rocks, probably exposed +for longer periods to a similar action, may have become metamorphic to a +still greater extent. + +We also discover in parts of the Swiss Alps dense masses of secondary +and even tertiary strata, which have assumed that semi-crystalline +texture which Werner called transition, and which naturally led his +followers, who attached great importance to mineral characters taken +alone, to class them as transition formations, or as groups older than +the lowest secondary rocks. (See p. 92.) Now, it is probable that these +strata have been affected, although in a less intense degree, by that +same plutonic action which has entirely altered and rendered metamorphic +so many of the subjacent formations; for in the Alps, this action has by +no means been confined to the immediate vicinity of granite. Granite, +indeed, and other plutonic rocks, rarely make their appearance at the +surface, notwithstanding the deep ravines which lay open to view the +internal structure of these mountains. That they exist below at no great +depth we cannot doubt, and we have already seen (p. 445.) that at some +points, as in the Valorsine, near Mont Blanc, granite and granitic veins +are observable, piercing through talcose gneiss, which passes insensibly +upwards into secondary strata. + +It is certainly in the Alps of Switzerland and Savoy, more than in any +other district in Europe, that the geologist is prepared to meet with the +signs of an intense development of plutonic action; for here we find the +most stupendous monuments of mechanical violence, by which strata thousands +of feet thick have been bent, folded, and overturned. (See p. 58.) It is +here that marine secondary formations of a comparatively modern date, such +as the Oolitic and Cretaceous, have been upheaved to the height of 12,000, +and some Eocene strata to elevations of 10,000 feet above the level of the +sea; and even deposits of the Miocene era have been raised 4000 or 5000 +feet, so as to rival in height the loftiest mountains in Great Britain. + +If the reader will consult the works of many eminent geologists who have +explored the Alps, especially those of MM. De Beaumont, Studer, Necker, +Boué, and Murchison, he will learn that they all share, more or less fully, +in the opinions above expressed. It has, indeed, been stated by MM. Studer +and Hugi, that there are complete alternations on a large scale of +secondary strata, containing fossils, with gneiss and other rocks, of a +perfectly metamorphic structure. I have visited some of the most remarkable +localities referred to by these authors; but although agreeing with them +that there are passages from the fossiliferous to the metamorphic series +far from the contact of granite or other plutonic rocks, I was unable to +convince myself that the distinct alternations of highly crystalline, with +unaltered strata above alluded to, might not admit of a different +explanation. In one of the sections described by M. Studer in the highest +of the Bernese Alps, namely in the Roththal, a valley bordering the line of +perpetual snow on the northern side of the Jungfrau, there occurs a mass +of gneiss 1000 feet thick, and 15,000 feet long, which I examined, not only +resting upon, but also again covered by strata containing oolitic fossils. +These anomalous appearances may partly be explained by supposing great +solid wedges of intrusive gneiss to have been forced in laterally between +strata to which I found them to be in many sections unconformable. The +superposition, also, of the gneiss to the oolite may, in some cases, be due +to a reversal of the original position of the beds in a region where the +convulsions have been on so stupendous a scale. + +On the Sattel also, at the base of the Gestellihorn, above Enzen, in the +valley of Urbach, near Meyringen, some of the intercalations of gneiss +between fossiliferous strata may, I conceive, be ascribed to mechanical +derangement. Almost any hypothesis of repeated changes of position may be +resorted to in a region of such extraordinary confusion. The secondary +strata may first have been vertical, and then certain portions may have +become metamorphic (the plutonic influence ascending from below), while +intervening strata remained unchanged. The whole series of beds may then +again have been thrown into a nearly horizontal position, giving rise to +the superposition of crystalline upon fossiliferous formations. + +It was remarked, in Chap. XXXIV., that as the hypogene rocks, both +stratified and unstratified, crystallize originally at a certain depth +beneath the surface, they must always, before they are upraised and exposed +at the surface, be of considerable antiquity, relatively to a large portion +of the fossiliferous and volcanic rocks. They may be forming at all +periods; but before any of them can become visible, they must be raised +above the level of the sea, and some of the rocks which previously +concealed them must have been removed by denudation. + +_Order of succession in metamorphic rocks._--There is no universal and +invariable order of superposition in metamorphic rocks, although a +particular arrangement may prevail throughout countries of great extent, +for the same reason that it is traceable in those sedimentary formations +from which crystalline strata are derived. Thus, for example, we have seen +that in the Apennines, near Carrara, the descending series, where it is +metamorphic, consists of, 1st, saccharine marble; 2dly, talcose-schist; and +3dly, of quartz-rock and gneiss; where unaltered, of, 1st, fossiliferous +limestone; 2dly, shale; and 3dly, sandstone. + +But if we investigate different mountain chains, we find gneiss, +mica-schist, hornblende-schist, chlorite-schist, hypogene, limestone, +and other rocks, succeeding each other, and alternating with each other, +in every possible order. It is, indeed, more common to meet with some +variety of clay-slate forming the uppermost member of a metamorphic +series than any other rock; but this fact by no means implies, as some +have imagined, that all clay-slates were formed at the close of an +imaginary period, when the deposition of the crystalline strata gave way +to that of ordinary sedimentary deposits. Such clay-slates, in fact, are +variable in composition, and sometimes alternate with fossiliferous +strata, so that they may be said to belong almost equally to the +sedimentary and metamorphic order of rocks. It is probable that had they +been subjected to more intense plutonic action, they would have been +transformed into hornblende-schist, foliated chlorite-schist, scaly +talcose-schist, mica-schist, or other more perfectly crystalline rocks, +such as are usually associated with gneiss. + +_Uniformity of mineral character in Hypogene rocks._--Humboldt has +emphatically remarked, that when we pass to another hemisphere, we see +new forms of animals and plants, and even new constellations in the +heavens; but in the rocks we still recognize our old acquaintances,--the +same granite, the same gneiss, the same micaceous schist, quartz-rock, +and the rest. It is certainly true that there is a great and striking +general resemblance in the principal kinds of hypogene rocks, although +of very different ages and countries; but it has been shown that each of +these are, in fact, geological families of rocks, and not definite +mineral compounds. They are much more uniform in aspect than sedimentary +strata, because these last are often composed of fragments varying +greatly in form, size, and colour, and contain fossils of different +shapes and mineral composition, and acquire a variety of tints from the +mixture of various kinds of sediment. The materials of such strata, if +melted and made to crystallize, would be subject to chemical laws, +simple and uniform in their action, the same in every climate, and +wholly undisturbed by mechanical and organic causes. + +Nevertheless, it would be a great error to assume that the hypogene rocks, +considered as aggregates of simple minerals, are really more homogeneous in +their composition than the several members of the sedimentary series. In +the first place, different assemblages of hypogene rocks occur in different +countries; and, secondly, in any one district, the rocks which pass under +the same name are often extremely variable in their component ingredients, +or at least in the proportions in which each of these are present. Thus, +for example, gneiss and mica-schist, so abundant in the Grampians, are +wanting in Cumberland, Wales, and Cornwall; in parts of the Swiss and +Italian Alps, the gneiss and granite are talcose, and not micaceous, as in +Scotland; hornblende prevails in the granite of Scotland--schorl in that of +Cornwall--albite in the plutonic rocks of the Andes--common felspar in +those of Europe. In one part of Scotland, the mica-schist is full of +garnets; in another it is wholly devoid of them: while in South America, +according to Mr. Darwin, it is the gneiss, and not the mica-schist, which +is most commonly garnetiferous. And not only do the proportional quantities +of felspar, quartz, mica, hornblende, and other minerals, vary in hypogene +rocks bearing the same name; but what is still more important, the +ingredients, as we have seen, of the same simple mineral are not always +constant (p. 369., and table, p. 377.). + +_The Metamorphic strata, why less calcareous than the fossiliferous._--It +has been remarked, that the quantity of calcareous matter in metamorphic +strata, or, indeed, in the hypogene formations generally, is far less than +in fossiliferous deposits. Thus the crystalline schists of the Grampians in +Scotland, consisting of gneiss, mica-schist, hornblende-schist, and other +rocks, many thousands of yards in thickness, contain an exceedingly small +proportion of interstratified calcareous beds, although these have been the +objects of careful search for economical purposes. Yet limestone is not +wanting in the Grampians, and it is associated sometimes with gneiss, +sometimes with mica-schist, and in other places with other members of the +metamorphic series. But where limestone occurs abundantly, as at Carrara, +and in parts of the Alps, in connection with hypogene rocks, it usually +forms one of the superior members of the crystalline group. + +The scarcity, then, of carbonate of lime in the plutonic and metamorphic +rocks generally, seems to be the result of some general cause. So long as +the hypogene rocks were believed to have originated antecedently to the +creation of organic beings, it was easy to impute the absence of lime to +the non-existence of those mollusca and zoophytes by which shells and +corals are secreted; but when we ascribe the crystalline formations to +plutonic action, it is natural to inquire whether this action itself may +not tend to expel carbonic acid and lime from the materials which it +reduces to fusion or semi-fusion. Although we cannot descend into the +subterranean regions where volcanic heat is developed, we can observe in +regions of spent volcanos, such as Auvergne and Tuscany, hundreds of +springs, both cold and thermal, flowing out from granite and other rocks, +and having their waters plentifully charged with carbonate of lime. The +quantity of calcareous matter which these springs transfer, in the course +of ages, from the lower parts of the earth's crust to the superior or newly +formed parts of the same, must be considerable.[487-A] + +If the quantity of siliceous and aluminous ingredients brought up by +such springs were great, instead of being utterly insignificant, it +might be contended that the mineral matter thus expelled implies simply +the decomposition of ordinary subterranean rocks; but the prodigious +excess of carbonate of lime over every other element must, in the course +of time, cause the crust of the earth below to be almost entirely +deprived of its calcareous constituents, while we know that the same +action imparts to newer deposits, ever forming in seas and lakes, an +excess of carbonate of lime. Calcareous matter is poured into these +lakes, and the ocean, by a thousand springs and rivers; so that part of +almost every new calcareous rock chemically precipitated, and of many +reefs of shelly and coralline stone, must be derived from mineral matter +subtracted by plutonic agency, and driven up by gas and steam from fused +and heated rocks in the bowels of the earth. + +Not only carbonate of lime, but also free carbonic acid gas is given off +plentifully from the soil and crevices of rocks in regions of active and +spent volcanos, as near Naples, and in Auvergne. By this process, fossil +shells or corals may often lose their carbonic acid, and the residual lime +may enter into the composition of augite, hornblende, garnet, and other +hypogene minerals. That the removal of the calcareous matter of fossil +shells is of frequent occurrence, is proved by the fact of such organic +remains being often replaced by silex or other minerals, and sometimes by +the space once occupied by the fossil being left empty, or only marked by a +faint impression. We ought not indeed to marvel at the general absence of +organic remains from the crystalline strata, when we bear in mind how often +fossils are obliterated, wholly or in part, even in tertiary +formations--how often vast masses of sandstone and shale, of different +ages, and thousands of feet thick, are devoid of fossils--how certain +strata may first have been deprived of a portion of their fossils when they +became semi-crystalline, or assumed the _transition_ state of Werner--and +how the remaining organic remains have been effaced when they were rendered +metamorphic. Some rocks of the last-mentioned class, moreover, must have +been exposed again and again to renewed plutonic action. + + +FOOTNOTES: + +[483-A] See notices of Savi, Hoffmann, and others, referred to by Boué, +Bull. de la Soc. Géol. de France, tom. v. p. 317.; and tom. iii. p. xliv.; +also Pilla, cited by Murchison, Quart. Geol. Journ., vol. v. p. 266. + +[487-A] See Principles, _Index_, "Calcareous Springs." + + + + +CHAPTER 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. + + +The manner in which metallic substances are distributed through the earth's +crust, and more especially the phenomena of those nearly vertical and +tabular masses of ore called mineral veins, from which the larger part of +the precious metals used by man are obtained,--these are subjects of the +highest practical importance to the miner, and of no less theoretical +interest to the geologist. + +The views entertained respecting metalliferous veins have been modified, +or, rather, have undergone an almost complete revolution, since the middle +of the last century, when Werner, as director of the School of Mines, at +Freiberg in Saxony, first attempted to generalize the facts then known. He +taught that mineral veins had originally been open fissures which were +gradually filled up with crystalline and metallic matter, and that many of +them, after being once filled, had been again enlarged or re-opened. He +also pointed out that veins thus formed are not all referable to one era, +but are of various geological dates. + +Such opinions, although slightly hinted at by earlier writers, had never +before been generally received, and their announcement by one of high +authority and great experience constituted an era in the science. +Nevertheless, I have shown, when tracing, in another work, the history and +progress of geology, that Werner was far behind some of his predecessors in +his theory of the volcanic rocks, and less enlightened than his +contemporary, Dr. Hutton, in his speculations as to the origin of +granite.[489-A] According to him, the plutonic formations, as well as the +crystalline schists, were substances precipitated from a chaotic fluid in +some primeval or nascent condition of the planet; and the metals, +therefore, being closely connected with them, had partaken, according to +him, of a like mysterious origin. He also held that the trap rocks were +aqueous deposits, and that dikes of porphyry, greenstone, and basalt, were +fissures filled with their several contents from above. Hence he naturally +inferred that mineral veins had derived their component materials from an +incumbent ocean, rather than from a subterranean source; that these +materials had been first dissolved in the waters above, instead of having +risen up by sublimation from lakes and seas of igneous matter below. + +In proportion as the hypothesis of a primeval fluid, or "chaotic +menstruum," was abandoned, in reference to the plutonic formations, and +when all geologists had come to be of one mind as to the true relation of +the volcanic and trappean rocks, reasonable hopes began to be entertained +that the phenomena of mineral veins might be explained by known causes, or +by chemical, thermal, and electrical agency still at work in the interior +of the earth. The grounds of this conclusion will be better understood when +the geological facts brought to light by mining operations have been +described and explained. + +_On different kinds of mineral veins._--Every geologist is familiarly +acquainted with those veins of quartz which abound in hypogene strata, +forming lenticular masses of limited extent. They are sometimes observed, +also, in sandstones and shales. Veins of carbonate of lime are equally +common in fossiliferous rocks, especially in limestones. Such veins appear +to have once been chinks or small cavities, caused, like cracks in clay, by +the shrinking of the mass, which has consolidated from a fluid state, or +has simply contracted its dimensions in passing from a higher to a lower +temperature. Siliceous, calcareous, and occasionally metallic matters, have +sometimes found their way simultaneously into such empty spaces, by +infiltration from the surrounding rocks, or by segregation, as it is often +termed. Mixed with hot water and steam, metallic ores may have permeated a +pasty matrix until they reached those receptacles formed by shrinkage, and +thus gave rise to that irregular assemblage of veins, called by the Germans +a "stockwerk," in allusion to the different floors on which the mining +operations are in such cases carried on. + +The more ordinary or regular veins are usually worked in vertical shafts, +and have evidently been fissures produced by mechanical violence. They +traverse all kinds of rocks, both hypogene and fossiliferous, and extend +downwards to indefinite or unknown depths. We may assume that they +correspond with such rents as we see caused from time to time by the shock +of an earthquake. Metalliferous veins, referable to such agency, are +occasionally a few inches wide, but more commonly 3 or 4 feet. They hold +their course continuously in a certain prevailing direction for miles or +leagues, passing through rocks varying in mineral composition. + +[3 Illustrations: Fig. 513. Fig. 514. Fig. 515. Vertical sections of the +mine of Huel Peever, Redruth, Cornwall.] + +_That metalliferous veins were fissures._--As some intelligent miners, +after an attentive study of metalliferous veins, have been unable to +reconcile many of their characteristics with the hypothesis of fissures, I +shall begin by stating the evidence in its favour. The most striking fact +perhaps which can be adduced in its support is, the coincidence of a +considerable proportion of mineral veins with _faults_, or those +dislocations of rocks which are indisputably due to mechanical force, as +above explained (p. 62.). There are even proofs in almost every mining +district of a succession of faults, by which the opposite walls of rents, +now the receptacles of metallic substances, have suffered displacement. +Thus, for example, suppose _a a_, fig. 513., to be a tin lode in Cornwall, +the term _lode_ being applied to veins containing metallic ores. This lode, +running east and west, is a yard wide, and is shifted by a copper lode (_b +b_), of similar width. + +The first fissure (_a a_) has been filled with various materials, partly +of chemical origin, such as quartz, fluor-spar, peroxide of tin, +sulphuret of copper, arsenical pyrites, bismuth, and sulphuret of +nickel, and partly of mechanical origin, comprising clay and angular +fragments or detritus of the intersected rocks. The plates of quartz and +the ores are, in some places, parallel to the vertical sides or walls of +the vein, being divided from each other by alternating layers of clay, +or other earthy matter. Occasionally the metallic ores are disseminated +in detached masses among the veinstones. + +It is clear that, after the gradual introduction of the tin and other +substances, the second rent (_b b_) was produced by another fracture +accompanied by a displacement of the rocks along the plane of _b b_. This +new opening was then filled with minerals, some of them resembling those in +_a a_, as fluor-spar (or fluate of lime) and quartz; others different, the +copper being plentiful and the tin wanting or very scarce. + +We must next suppose the shock of a third earthquake to occur, breaking +asunder all the rocks along the line c _c_, fig. 514.; the fissure in this +instance, being only 6 inches wide, and simply filled with clay, derived, +probably, from the friction of the walls of the rent, or partly, perhaps, +washed in from above. This new movement has heaved the rock in such a +manner as to interrupt the continuity of the copper vein (_b b_), and, at +the same time, to shift or heave laterally in the same direction a portion +of the tin vein which had not previously been broken. + +Again, in fig. 515. we see evidence of a fourth fissure (_d d_), also +filled with clay, which has cut through the tin vein (_a a_), and has +lifted it slightly upwards towards the south. The various changes here +represented are not ideal, but are exhibited in a section obtained in +working an old Cornish mine, long since abandoned, in the parish of +Redruth, called Huel Peever, and described both by Mr. Williams and Mr. +Carne.[491-A] The principal movement here referred to, or that of _c c_, +fig. 515., extends through a space of no less than 84 feet; but in this, as +in the case of the other three, it will be seen that the outline of the +country above, or the geographical features of Cornwall, are not affected +by any of the dislocations, a powerful denuding force having clearly been +exerted subsequently to all the faults. (See above, p. 69.) It is commonly +said in Cornwall, that there are eight distinct systems of veins which can +in like manner be referred to as many successive movements or fractures; +and the German miners of the Hartz Mountains speak also of eight systems of +veins, referable to as many periods. + +Besides the proofs of mechanical action already explained, the opposite +walls of veins are frequently polished and striated, as if they had +undergone great friction, and this even in cases where there has been no +shift. We may attribute such rubbing to a vibratory motion known to +accompany earthquakes, and to produce trituration on the opposite walls of +rents. Similar movements have sometimes occurred in mineral veins which +had been wholly or partially filled up; for included pieces of rock, +detached from the sides, are found to be rounded, polished, and striated. + +That a great many veins communicated originally with the surface of the +country above, or with the bed of the sea, is proved by the occurrence +in them of well rounded pebbles, agreeing with those in superficial +alluviums, as in Auvergne and Saxony. In Bohemia, such pebbles have been +met with at the depth of 180 fathoms. In Cornwall, Mr. Carne mentions +true pebbles of quartz and slate in a tin lode of the Relistran Mine, at +the depth of 600 feet below the surface. They were cemented by oxide of +tin and bisulphuret of copper, and were traced over a space more than 12 +feet long and as many wide.[492-A] Marine fossil shells, also, have been +found at great depths, having probably been engulphed during submarine +earthquakes. Thus, a gryphæa is stated by M. Virlet to have been met +with in a lead-mine near Sémur, in France, and a madrepore in a compact +vein of cinnabar in Hungary.[492-B] + +When different sets or systems of veins occur in the same country, those +which are supposed to be of contemporaneous origin, and which are filled +with the same kind of metals, often maintain a general parallelism of +direction. Thus, for example, both the tin and copper veins in Cornwall run +nearly east and west, while the lead-veins run north and south; but there +is no general law of direction common to different mining districts. The +parallelism of the veins is another reason for regarding them as ordinary +fissures, for we observe that contemporaneous trap dikes, admitted by all +to be masses of melted matter which have filled rents, are often parallel. +Assuming, then, that veins are simply fissures in which chemical and +mechanical deposits have accumulated, we may next consider the proofs of +their having been filled gradually and often during successive +enlargements. I have already spoken of parallel layers of clay, quartz, and +ore. Werner himself observed, in a vein near Gersdorff, in Saxony, no less +than thirteen beds of different minerals, arranged with the utmost +regularity on each side of the central layer. This layer was formed of two +beds of calcareous spar, which had evidently lined the opposite walls of a +vertical cavity. The thirteen beds followed each other in corresponding +order, consisting of fluor-spar, heavy spar, galena, &c. In these cases, +the central mass has been last formed, and the two plates which coat the +outer walls of the rent on each side are the oldest of all. If they consist +of crystalline precipitates, they may be explained by supposing the fissure +to have remained unaltered in its dimensions, while a series of changes +occurred in the nature of the solutions which rose up from below; but such +a mode of deposition, in the case of many successive and parallel layers, +appears to be exceptional. + +If a veinstone consist of crystalline matter, the points of the crystals +are always turned inwards, or towards the centre of the vein; in other +words, they point in that direction where there was most space for the +development of the crystals. Thus each new layer receives the impression of +the crystals of the preceding layer, and imprints its crystals on the one +which follows, until at length the whole of the vein is filled: the two +layers which meet dovetail the points of their crystals the one into the +other. But in Cornwall, some lodes occur where the vertical plates, or +_combs_, as they are there called, exhibit crystals so dovetailed as to +prove that the same fissure has been often enlarged. Sir H. De la Beche +gives the following curious and instructive example (fig. 516.) from a +copper-mine in granite, near Redruth.[493-A] Each of the plates or combs +(_a_, _b_, _c_, _d_, _e_, _f_) are double, having the points of their +crystals turned inwards along the axis of the comb. The sides or walls (2, +3, 4, 5, and 6) are parted by a thin covering of ochreous clay, so that +each comb is readily separable from another by a moderate blow of the +hammer. The breadth of each represents the whole width of the fissure at +six successive periods, and the outer walls of the vein, where the first +narrow rent was formed, consisted of the granitic surfaces 1 and 7. + +[Illustration: Fig. 516. Copper lode, near Redruth, enlarged at +six successive periods.] + +A somewhat analogous interpretation is applicable to numbers of other +cases, where clay, sand, or angular detritus, alternate with ores and +veinstones. Thus, we may imagine the sides of a fissure to be encrusted +with siliceous matter, as Von Buch observed, in Lancerote, the walls of +a volcanic crater formed in 1731 to be traversed by an open rent in +which hot vapours had deposited hydrate of silica, the incrustation +nearly extending to the middle.[493-B] Such a vein may then be filled +with clay or sand, and afterwards re-opened, the new rent dividing the +argillaceous deposit, and allowing a quantity of rubbish to fall down. +Various metals and spars may then be precipitated from aqueous solutions +among the interstices of this heterogeneous mass. + +That such changes have repeatedly occurred, is demonstrated by +occasional cross-veins, implying the oblique fracture of previously +formed chemical and mechanical deposits. Thus, for example, M. Fournet, +in his description of some mines in Auvergne worked under his +superintendence, observes, that the granite of that country was first +penetrated by veins of granite, and then dislocated, so that open rents +crossed both the granite and the granitic veins. Into such openings, +quartz, accompanied by sulphurets of iron and arsenical pyrites, was +introduced. Another convulsion then burst open the rocks along the old +line of fracture, and the first set of deposits were cracked and often +shattered, so that the new rent was filled, not only with angular +fragments of the adjoining rocks, but with pieces of the older +veinstones. Polished and striated surfaces on the sides or in the +contents of the vein also attest the reality of these movements. A new +period of repose then ensued, during which various sulphurets were +introduced, together with hornstone quartz, by which angular fragments +of the older quartz before mentioned were cemented into a breccia. This +period was followed by other dilatations of the same veins, and other +sets of mineral deposits, until, at last, pebbles of the basaltic lavas +of Auvergne, derived from superficial alluviums, probably of Miocene or +older Pliocene date, were swept into the veins. I have not space to +enumerate all the changes minutely detailed by M. Fournet, but they are +valuable, both to the miner and geologist, as showing how the supposed +signs of violent catastrophes may be the monuments, not of one +paroxysmal shock, but of reiterated movements. + +Such repeated enlargement and re-opening of veins might have been +anticipated, if we adopt the theory of fissures, and reflect how few of +them have ever been sealed up entirely, and that a country with fissures +only partially filled must naturally offer much feebler resistance along +the old lines of fracture than any where else. It is quite otherwise in the +case of dikes, where each opening has been the receptacle of one continuous +and homogeneous mass of melted matter, the consolidation of which has taken +place under considerable pressure. Trappean dikes can rarely fail to +strengthen the rocks at the points where before they were weakest; and if +the upheaving force is again exerted in the same direction, the crust of +the earth will give way anywhere rather than at the precise points where +the first rents were produced. + +A large proportion of metalliferous veins have their opposite walls nearly +parallel, and sometimes over a wide extent of country. There is a fine +example of this in the celebrated vein of Andreasberg in the Hartz, which +has been worked for a depth of 500 yards perpendicularly, and 200 +horizontally, retaining almost every where a width of 3 feet. But many +lodes in Cornwall and elsewhere are extremely variable in size, being 1 or +2 inches in one part, and then 8 or 10 feet in another, at the distance of +a few fathoms, and then again narrowing as before. Such alternate swelling +and contraction is so often characteristic as to require explanation. The +walls of fissures in general, observes Sir H. De la Beche, are rarely +perfect planes throughout their entire course, nor could we well expect +them to be so, since they commonly pass through rocks of unequal hardness +and different mineral composition. If, therefore, the opposite sides of +such irregular fissures slide upon each other, that is to say, if there be +a fault, as in the case of so many mineral veins, the parallelism of the +opposite walls is at once entirely destroyed, as will be readily seen by +studying the annexed diagrams. + +[Illustration: Fig. 517. Schematic sketch.] + +[Illustration: Fig. 518. Schematic sketch.] + +[Illustration: Fig. 519. Schematic sketch.] + +Let _a b_, fig. 517., be a line of fracture traversing a rock, and let _a +b_, fig. 518., represent the same line. Now, if we cut a piece of paper +representing this line, and then move the lower portion of this cut paper +sideways from _a_ to _a'_, taking care that the two pieces of paper still +touch each other at the points 1, 2, 3, 4, 5, we obtain an irregular +aperture at _c_, and isolated cavities at _d d d_, and when we compare such +figures with nature we find that, with certain modifications, they +represent the interior of faults and mineral veins. If, instead of sliding +the cut paper to the right hand, we move the lower part towards the left, +about the same distance that it was previously slid to the right, we obtain +considerable variation in the cavities so produced, two long irregular open +spaces, _f f_, fig. 519., being then formed. This will serve to show to +what slight circumstances considerable variations in the character of the +openings between unevenly fractured surfaces may be due, such surfaces +being moved upon each other, so as to have numerous points of contact. + +[Illustration: Fig. 520. Schematic sketch.] + +Most lodes are perpendicular to the horizon, or nearly so; but some of +them have a considerable inclination or "hade," as it is termed, the +angles of dip varying from 15° to 45°. The course of a vein is +frequently very straight; but if tortuous, it is found to be choked up +with clay, stones, and pebbles, at points where it departs most widely +from verticality. Hence at places, such as _a_, fig. 520., the miner +complains that the ores are "nipped," or greatly reduced in quantity, +the space for their free deposition having been interfered with in +consequence of the pre-occupancy of the lode by earthy materials. When +lodes are many fathoms wide, they are usually filled for the most part +with earthy matter, and fragments of rock, through which the ores are +much disseminated. The metallic substances frequently coat or encircle +detached pieces of rock, which our miners call "horses" or "riders." +That we should find some mineral veins which split into branches is also +natural, for we observe the same in regard to open fissures. + +_Chemical deposits in veins._--If we now turn from the mechanical to the +chemical agencies which have been instrumental in the production of mineral +veins, it may be remarked that those parts of fissures which were not +choked up with the ruins of fractured rocks must always have been filled +with water; and almost every vein has probably been the channel by which +hot springs, so common in countries of volcanos and earthquakes, have made +their way to the surface. For we know that the rents in which ores abound +extend downwards to vast depths, where the temperature of the interior of +the earth is more elevated. We also know that mineral veins are most +metalliferous near the contact of plutonic and stratified formations, +especially where the former send veins into the latter, a circumstance +which indicates an original proximity of veins at their inferior extremity +to igneous and heated rocks. It is moreover acknowledged that even those +mineral and thermal springs which, in the present state of the globe, are +far from volcanos, are nevertheless observed to burst out along great lines +of upheaval and dislocation of rocks.[496-A] It is also ascertained that +all the substances with which hot springs are impregnated agree with those +discharged in a gaseous form from volcanos. Many of these bodies occur as +veinstones; such as silex, carbonate of lime, sulphur, fluor-spar, sulphate +of barytes, magnesia, oxide of iron, and others. I may add that, if veins +have been filled with gaseous emanations from masses of melted matter, +slowly cooling in the subterranean regions, the contraction of such masses +as they pass from a plastic to a solid state would, according to the +experiments of Deville on granite (a rock which may be taken as a +standard), produce a reduction in volume amounting to 10 per cent. The slow +crystallization, therefore, of such plutonic rocks supplies us with a force +not only capable of rending open the incumbent rocks by causing a failure +of support, but also of giving rise to faults whenever one portion of the +earth's crust subsides slowly while another contiguous to it happens to +rest on a different foundation, so as to remain unmoved. + +Although we are led to infer, from the foregoing reasoning, that there has +often been an intimate connection between metalliferous veins and hot +springs holding mineral matter in solution, yet we must not on that account +expect that the contents of hot springs and mineral veins would be +identical. On the contrary, M. E. de Beaumont has judiciously observed that +we ought to find in veins those substances which, being least soluble, are +not discharged by hot springs,--or that class of simple and compound bodies +which the thermal waters ascending from below would first precipitate on +the walls of a fissure, as soon as their temperature began slightly to +diminish. The higher they mount towards the surface, the more will they +cool, till they acquire the average temperature of springs, being in that +case chiefly charged with the most soluble substances, such as the alkalis, +soda and potash. These are not met with in veins, although they enter so +largely into the composition of granitic rocks.[496-B] + +To a certain extent, therefore, the arrangement and distribution of +metallic matter in veins may be referred to ordinary chemical action, or +to those variations in temperature, which waters holding the ores in +solution must undergo, as they rise upwards from great depths in the earth. +But there are other phenomena which do not admit of the same simple +explanation. Thus, for example, in Derbyshire, veins containing ores of +lead, zinc, and copper, but chiefly lead, traverse alternate beds of +limestone and greenstone. The ore is plentiful where the walls of the rent +consist of limestone, but is reduced to a mere string when they are formed +of greenstone, or "toadstone," as it is called provincially. Not that the +original fissure is narrower where the greenstone occurs, but because more +of the space is there filled with veinstones, and the waters at such points +have not parted so freely with their metallic contents. + +"Lodes in Cornwall," says Mr. Robert W. Fox, "are very much influenced +in their metallic riches by the nature of the rock which they traverse, +and they often change in this respect very suddenly, in passing from one +rock to another. Thus many lodes which yield abundance of ore in +granite, are unproductive in clay-slate, or killas, and _vice versâ_. +The same observation applies to killas and the granitic porphyry called +elvan. Sometimes, in the same continuous vein, the granite will contain +copper, and the killas tin, or _vice versâ_."[497-A] Mr. Fox, after +ascertaining the existence at present of electric currents in some of +the metalliferous veins in Cornwall, has speculated on the probability +of the same cause having acted originally on the sulphurets and muriates +of copper, tin, iron, and zinc, dissolved in the hot water of fissures, +so as to determine the peculiar mode of their distribution. After +instituting experiments on this subject, he even endeavoured to account +for the prevalence of an east and west direction in the principal +Cornish lodes by their position at right angles to the earth's +magnetism; but Mr. Henwood and other experienced miners have pointed out +objections to the theory; and it must be owned that the direction of +veins in different mining districts varies so entirely that it seems to +depend on lines of fracture, rather than on the laws of voltaic +electricity. Nevertheless, as different kinds of rock would be often in +different electrical conditions, we may readily believe that electricity +must often govern the arrangement of metallic precipitates in a rent. + +"I have observed," says Mr. R. Fox, "that when the chloride of tin in +solution is placed in the voltaic circuit, part of the tin is deposited in +a metallic state at the negative pole, and part at the positive one, in the +state of a peroxide, such as it occurs in our Cornish mines. This +experiment may serve to explain why tin is found contiguous to, and +intermixed with, copper ore, and likewise separated from it, in other parts +of the same lode."[497-B] + +_Relative age of the different metals._--After duly reflecting on the facts +above described, we cannot doubt that mineral veins, like eruptions of +granite or trap, are referable to many distinct periods of the earth's +history, although it may be more difficult to determine the precise age of +veins; because they have often remained open for ages, and because, as we +have seen, the same fissure, after having been once filled, has frequently +been re-opened or enlarged. But besides this diversity of age, it has been +supposed by some geologists that certain metals have been produced +exclusively in earlier, others in more modern times,--that tin, for +example, is of higher antiquity than copper, copper than lead or silver, +and all of them more ancient than gold. I shall first point out that the +facts once relied upon in support of some of these views are contradicted +by later experience, and then consider how far any chronological order of +arrangement can be recognized in the position of the precious and other +metals in the earth's crust. In the first place, it is not true that veins +in which tin abounds are the oldest lodes worked in Great Britain. The +government survey of Ireland has demonstrated, that in Wexford veins of +copper and lead (the latter as usual being argentiferous) are much older +than the tin of Cornwall. In each of the two countries a very similar +series of geological changes has occurred at two distinct epochs,--in +Wexford, before the Devonian strata were deposited; in Cornwall, after the +carboniferous epoch. To begin with the Irish mining district: We have +granite in Wexford, traversed by granite veins, which veins also intrude +themselves into the Silurian strata, the same Silurian rocks as well as the +veins having been denuded before the Devonian beds were superimposed. Next +we find, in the same county, that elvans, or straight dikes of porphyritic +granite, have cut through the granite and the veins before mentioned, but +have not penetrated the Devonian rocks. Subsequently to these elvans, veins +of copper and lead were produced, being of a date certainly posterior to +the Silurian, and anterior to the Devonian; for they do not enter the +latter, and, what is still more decisive, streaks or layers of derivative +copper have been found near Wexford in the Devonian, not far from points +where mines of copper are worked in the Silurian strata.[498-A] + +Although the precise age of such copper lodes cannot be defined, we may +safely affirm that they were either filled at the close of the Silurian or +commencement of the Devonian period. Besides copper, lead, and silver, +there is some gold in these ancient or primary metalliferous veins. A few +fragments also of tin found in Wicklow in the drift are supposed to have +been derived from veins of the same age.[498-B] + +Next, if we turn to Cornwall, we find there also the monuments of a very +analogous sequence of events. First the granite was formed; then, about +the same period, veins of fine-grained granite, often tortuous (see fig. +496., p. 445.), penetrating both the outer crust of granite and the +adjoining fossiliferous or primary rocks, including the coal-measures; +thirdly, elvans, holding their course straight through granite, granitic +veins, and fossiliferous slates; fourthly, veins of tin also containing +copper, the first of those eight systems of fissures of different ages +already alluded to, p. 491. Here, then, the tin lodes are newer than the +elvans. It has indeed been stated by some Cornish miners that the elvans +are in some few instances posterior to the oldest tin-bearing lodes, but +the observations of Sir H. De la Beche during the survey led him to an +opposite conclusion, and he has shown how the cases referred to in +corroboration can be otherwise interpreted.[499-A] We may, therefore, +assert that the most ancient Cornish lodes are younger than the +coal-measures of that part of England, and it follows that they are of a +much later date than the Irish copper and lead of Wexford and some +adjoining counties. How much later it is not so easy to declare, +although probably they are not newer than the beginning of the Permian +period, as no tin lodes have been discovered in any red sandstone of the +Poikilitic group, which overlies the coal in the south-west of England. + +There are lead veins in the Mendip hills which extend through the mountain +limestone into the Permian or Dolomitic conglomerate, and others in +Glamorganshire which enter the lias. Those worked near Frome, in +Somersetshire, have been traced into the Inferior Oolite. In Bohemia, the +rich veins of silver of Joachimsthal cut through basalt containing olivine, +which overlies tertiary lignite, in which are leaves of dicotyledonous +trees. This silver, therefore, is decidedly a tertiary formation. In regard +to the age of the gold of the Ural Mountains, in Russia, which, like that +of California, is obtained chiefly from auriferous alluvium, we can merely +affirm that it occurs in veins of quartz in the schistose and granitic +rocks of that chain. Sir R. Murchison observes, that no gold has yet been +found in the Permian conglomerates which lie at the base of the Ural +Mountains, although large quantities of iron and copper detritus are mixed +with the rolled pebbles of these same Permian strata. Hence it seems that +the Uralian quartz veins, containing gold and platinum, were not exposed to +aqueous denudation during the Permian era. But we cannot feel sure, from +any data yet before us, that such auriferous veins of quartz may not be as +old as the tin lodes of Cornwall, in which, as well as the more ancient +copper lodes of Ireland, some gold has been detected. We are also unable at +present to assign to the gold veins of Brazil, Peru, or California, their +respective geological dates. But, although enough is known to show that +Ovid's line about the "Age of Gold," "Aurea prima sata est ætas," would, by +no means, be an apt motto for a treatise on mining, it would be equally +rash in the present state of our inquiries to affirm, as some have done, +that gold was the last-formed of metals. + +It has been remarked by M. de Beaumont, that lead and some other metals are +found in dikes of basalt and greenstone, as well as in mineral veins +connected with trap rocks, whereas tin is met with in granite and in veins +associated with the granitic series. If this rule hold true generally, the +geological position of tin in localities accessible to the miners will +belong, for the most part, to rocks older than those bearing lead. The tin +veins will be of higher relative antiquity for the same reason that the +"underlying" igneous formations or granites which are visible to man are +older, on the whole, than the overlying or trappean formations. + +If different sets of fissures, originating simultaneously at different +levels in the earth's crust, and communicating, some of them, with +volcanic, others with heated plutonic masses, be filled with different +metals, it will follow that those formed farthest from the surface will +usually require the longest time before they can be exposed superficially. +In order to bring them into view, or within reach of the miner, a greater +amount of upheaval and denudation must take place in proportion as they +have lain deeper when first formed. A considerable series of geological +revolutions must intervene before any part of the fissure, which has been +for ages in the proximity of the plutonic rocks, so as to receive the gases +discharged from it when it was cooling, can emerge into the atmosphere. But +I need not enlarge on this subject, as the reader will remember what was +said in the 30th, 34th, and 37th chapters, on the chronology of the +volcanic and hypogene formations. + + * * * * * + +_Concluding Remarks._--The theory of the origin of the hypogene rocks, at a +variety of successive periods, as expounded in two of the chapters just +cited, and still more the doctrine that such rocks may be now in the daily +course of formation, has made and still makes its way, but slowly, into +favour. The disinclination to embrace it has arisen partly from an inherent +obscurity in the very nature of the evidence of plutonic action when +developed on a great scale, at particular periods. It has also sprung, in +some degree, from extrinsic considerations; many geologists having been +unwilling to believe the doctrine of the transmutation of fossiliferous +into crystalline rocks, because they were desirous of finding proofs of a +beginning, and of tracing back the history of our terraqueous system to +times anterior to the creation of organic beings. But if these expectations +have been disappointed, if we have found it impossible to assign a limit to +that time throughout which it has pleased an Omnipotent and Eternal Being +to manifest his creative power, we have at least succeeded beyond all hope +in carrying back our researches to times antecedent to the existence of +man. We can prove that man had a beginning, and that, all the species now +contemporary with man, and many others which preceded, had also a +beginning, and that, consequently, the present state of the organic world +has not gone on from all eternity, as some philosophers have maintained. + +It can be shown that the earth's surface has been remodelled again and +again; mountain chains have been raised or sunk; valleys formed, filled +up, and then re-excavated; sea and land have changed places; yet +throughout all these revolutions, and the consequent alterations of +local and general climate, animal and vegetable life has been sustained. +This has been accomplished without violation of the laws now governing +the organic creation, by which limits are assigned to the variability of +species. The succession of living beings appears to have been continued +not by the transmutation of species, but by the introduction into the +earth from time to time of new plants and animals, and each assemblage +of new species must have been admirably fitted for the new states of the +globe as they arose, or they would not have increased and multiplied and +endured for indefinite periods.[501-A] + +Astronomy had been unable to establish the plurality of habitable worlds +throughout space, however favourite a subject of conjecture and +speculation; but geology, although it cannot prove that other planets +are peopled with appropriate races of living beings, has demonstrated +the truth of conclusions scarcely less wonderful,--the existence on our +own planet of so many habitable surfaces, or worlds as they have been +called, each distinct in time, and peopled with its peculiar races of +aquatic and terrestrial beings. + +The proofs now accumulated of the close analogy between extinct and recent +species are such as to leave no doubt on the mind that the same harmony of +parts and beauty of contrivance which we admire in the living creation, has +equally characterized the organic world at remote periods. Thus as we +increase our knowledge of the inexhaustible variety displayed in living +nature, and admire the infinite wisdom and power which it displays, our +admiration is multiplied by the reflection, that it is only the last of a +great series of pre-existing creations, of which we cannot estimate the +number or limit in times past.[501-B] + + +FOOTNOTES: + +[489-A] Principles, &c. chap. iv. 8th ed. p. 49. + +[491-A] Geol. Trans. vol. iv. p. 139.; Trans. Roy. Geol. Society +Cornwall, vol. ii. p. 90. + +[492-A] Carne, Trans. of Geol. Soc. Cornwall, vol. iii. p. 238. + +[492-B] Fournet, Etudes sur les Dépots Metalliferes. + +[493-A] Geol. Rep. on Cornwall, p. 340. + +[493-B] Principles, ch. xxvii. 8th ed. p. 422. + +[496-A] See Dr. Daubeny's Volcanos. + +[496-B] Bulletin, iv. p. 1278. + +[497-A] R. W. Fox on Mineral Veins, p. 10. + +[497-B] Ibid. p. 38. + +[498-A] I am indebted to Sir H. De la Beche for this information. See also +maps and sections of Irish Survey. + +[498-B] Sir H. De la Beche, MS. notes on Irish Survey. + +[499-A] Report on Geology of Cornwall, p. 310. + +[501-A] See Principles of Geol., Book 3. + +[501-B] See the author's Anniv. Address to the Geol. Soc. 1837. Proceedings +of G. S. No. 49. p. 520. + + + + +INDEX. + + + A. + + Ægean Sea, mud of, 35. + animal life in depths of, 137. + + Agassiz, M., cited, 192. 276. 300. 335. 344. 345. + on parallel roads, 87. + on fossil fishes of molasse and faluns, 171. + on fossil fish of Lias, 275. + on fossil fish in Permian marl-slate, 304. + on fish from Sheppey, 202. + on foot-prints, 299. + on fishes of brown coal, 417. + on glaciers, 140. 143. + + Age of formation determined by fragments of older rock, 101. + of metamorphic rocks, 482. + test of, in plutonic rocks by relative position, 449. + of Spanish volcanos, 414. + of volcanic rocks, how tested, 397-400. + + Aix-la-Chapelle, hot spring at, 477. + + Alabaster defined, 13. + + Alabama, cretaceous shingle of, 225. + + Alberti on the Keuper, 287. + + Alexander, Capt., marine shells in crag, found by, 149. + + Alluvium, term explained, 79. + in Auvergne, 80. + of the Wealden, 252. + + Alps, nummulitic formation of, 205. + curved strata of, 58. + Swiss and Savoy, cleavage of, 470. + of Switzerland, 483. + + Alpine blocks on the Jura, 142. + erratics, 140. + + Altered rocks, 381. 456. + by subterranean gases, 476. + + Alternations of rocks, 14. + of marine and freshwater formations, 32. + + Alumine in rocks, 11. + + _Amblyrhynchus cristatus_, 279. + + America, North, lithodomi in beaches of, 78. + South, cretaceous strata, 225. + South, gradual rise of parts of, 46. + South, fossils of, 157. + + Amygdaloid, 372. + + Amphitherium, 268. + + Andelys, chalk cliffs at, 239. + + Andernach, strata near, 417. + + Andes, plutonic rocks of, 453. + rocks drifted from to Chiloe, 144. + + Anthracite in Rhode Island, 478. + + Anticlinal line, 48. 57. + + Antrim, rocks altered by dikes in, 382. + + Antwerp, strata like Suffolk crag near, 166. + + _Apateon pedestris_, a carboniferous reptile, 336. + + Apennines, limestone in, 482. + + Appalachian coal-field, 329. + + Appalachians, altered rocks in, 478. + + _Apteryx_ in New Zealand, 158. + + Aqueous rocks defined, 2. + rocks, mineral character of, 97. + deposits, superposition of, 96. + + Arbroath, section from, to the Grampians, 48. + + Archegosaurus, figure of, 337. + + Archiac, M., cited, 143. + on fossils in chalk, 221. + on shells in French Lower Eocene, 196. + + Ardèche, lava in, 385. + + Arenaceous rocks described, 11. + + Argillaceous rocks, 11. + schist, 465. + + Argile plastique, or Lower Eocene, 196. + + Argyleshire, trap-vein in cliff, 379. + + Arran, age of granite in, 459. + section of, 461. + dike of greenstone in, 379. + + Arthur's Seat, altered strata of, 383. + + Ashby-de-la-Zouch, fault in coal-field of, 69. + + Ascension, lamination of volcanic rocks in, 480. + + Asterophyllites, 314. + + Asti, formations at, 167. + + Atherfield, cretaceous strata of, 219. + + Augite, 369. + + Aurillac, freshwater strata of, 188. + + Austen, Mr., R. A. C., on phosphate of lime, 219. + + Australian cave-breccias, 155. + + Auvergne freshwater formations, 186. + succession of changes in, 180. + lacustrine strata, 181. + mineral veins of, 493. + indusial limestone, 184. + extinct volcanos of, 422. + alluvium in, 80. + + Aymestry limestone, 352. + + + B. + + Bagshot sands, 199. + + Bacillaria, fossil in tripoli, 25. + + Baiæ, Bay of, strata in, 403. + + Bakewell, Mr., on cleavages of Alps, 470. + + Balgray, near Glasgow, stumps of trees in coal, 317. + + Bahia Blanca, fossil remains at, 148. + + Baltic, brackish water strata on coast of, 114. + + Barcombe, chalk flints near, 253. + + Barton Cliff, 198. + + Barrande, M., on trilobites, 358. + + Basterot, M. de, on tertiaries of south of France, 105. + + Basalt, 371. + columnar in the Eifel, 387. + columnar, near Vicenza, 386. + columnar, structure of, 384. + + Basset, term explained, 56. + + Batrachian, eggs of, in Old Red, Scotland, Postscript, x. + + Bayfield, Capt., on fossil shells in Canada, 134. + on inland cliffs in Gulf of St. Lawrence, 78. + + Bean, Mr., shells similar to those in Norwich crag found in Yorkshire + by, 149. + + Bean, Mr., on fossil shells from oolite, 272. + + Beachy Head, chalk cliffs near, 246. + + Beaumont, M. E. de, on rocks of Hautes Alpes, 455. + on lamination of volcanic rocks, 480. + + Beaumont, M. E. de, on Swiss Alps, 484. + on quartz, 439. + on oolite formation in France, 221. + + Beck, Dr., on kelp, 217. + on graptolites, 357. + cited, 162. 186. + + Belemnite in Oxford clay, 262. + + Berger, Dr., on rocks altered by dikes, 382. + + Bergmann on trap, 366. + + Berlin, tertiary strata near, 177. + + Bermuda Islands, lagoons in, 216. + rocks of, 78. + + Bernese Alps, gneiss in, 484. + + Berthier, on augite and hornblende, 369. + + Beudant, M., on Hungary, 421. + + Beyrich, Prof., on tertiary strata near Berlin, 177. + + Biaritz, calcareous cliffs of, 72. + + Bilin, tripoli, composed of infusoria, 25. + + Binney, Mr., on stigmaria and sigillaria, 315. + + Birds, footprints of, 298. + fossil, scarcity of, Postscript, xix. + + Bischoff, Prof., experiments on heat, 476. + on steam at a high temperature, 477. + + Blainville, on number of genera of mollusca, 28. + + Boase, Dr., cited, 479. + + Boblaye, M., on inland cliffs, 73. + cited, 431. + + Bog-iron ore, 26. + + Borrowdale, black-lead of, 38. + + Bordeaux, tertiary deposits of, 171. + + Bosquet, M., on Maestricht beds, 210. + + Bothnia, Gulf of, land upheaved, 45. + + Boué, M., on arrangement of rocks, 95. + on fossil shells in Hungary, 421. + on Carrara marble, 482. + on Swiss Alps, 484. + + Bonelli, on strata in Italy, 106. + + Boulder formation in Canada, 133. + period, fauna of, 126. + formation, mineral ingredients of, 126. + formation in England, 130. + + Boulders, 123. + striated, 136. + + Boutigny, M., cited, 441. + + Bowen, Lieut. A., R.N., drawings of rocks in Gulf of St. Lawrence, 78. + + Bowerbank, Mr., on fossil flora of Sheppey, 200. + + Bowman, Mr., on coal-seams, 330. + + Bracklesham Bay, characteristic shells of, 199. + + Brash, term, explained, 81. + + Bravard, M., on Auvergne mammalia, 188. 425. + + Breccia on ancient coast lines, 73. + + Brickenden, Captain, on Elgin fossils, Postscript, ix. + + Brighton, elephant bed of, 256. + + Bristol, dolomitic conglomerate near, 305. + section of strata near, 102. + + Brocchi, on Subapennines, 105. 167. + + Brockedon, Mr., on black-lead, 38. + + Broderip, Mr., cited, 270. + + Brodie, Rev. P.B., on fossil insects, 281. + cited, 207. + + Bromley, oyster-bed near, 204. + + Brongniart, M. Adolphe, on Eocene flora, 200. + on flora of cretaceous period, 223. + on fossil plants in lias, 282. + on plants of Bunter sandstein, 288. + on fossil fir-cones, 313. + on Permian flora, 307. + on sigillaria, 314. + on asterophyllites, 314. + on stigmaria, 315. + age of acrogens, 316. + on endogens, 316. + + Brongniart, M. Alex., on Paris tertiaries, 104. + on Eocene formation, 175. + on shells of nummulitic formation, 205. + on coal mine near Lyons, 319. + + Brora, coal formation, 272. + + Brora, granite near, 458. + + Brown, Mr. Richard, on stigmariæ, 315. + on coal formation, 415. + on Cape Breton coal-field, 324. 334. + on carboniferous rain-prints, Postscript, xii. + + Buckland, Dr., on cave at Kirkdale, 154. + on coal plants, 317. + on coprolites in chalk, 216. + on fish of Lias, 276. + on footprints, 291. + on mountains of Caernarvonshire, 130. + on oyster bed near Bromley, 204. + on parallel roads, 87. + on term Poikilitic, 286. + on saurians of Lias, 278. + on sudden destruction of saurians, 280. + cited, 155. 231. 233. 267. 268. + + Buddle, Mr., on creeps in coal mines, 50. + on ancient river-channels of coal period, 334. + + Buist, Dr. G., on saltness of Red Sea, 296. + + Bunbury, Mr. C. J. F., on plants of coal-field, 285. + + Bunter sandstein, 288. + + Burmeister on trilobites, 358. + + Burnes, Sir A., cited, 295. + + + C. + + Caernarvonshire, ancient glaciers of, 130. + + Calamites, figures of, 313. + near Pictou, 319. + + Calcaire grossier, 193. + siliceux, 195. + + Calcareous rocks, 12. + rocks of Gulf of Spezia, 482. + cliffs of Biaritz, 72. + + Caldcleugh, Mr., cited, 399. + + Caldera of Palma, 392. + + Cambrian group, 361. + volcanic rocks, 435. + + Campagna di Roma, tuffs of, 408. + + Canada, shells in drift of, 134. + + Cantal, freshwater formation of, 188. + igneous rocks of, 429. + freshwater beds of, 429. + + Cape Breton, coal measures of, 324. + Wrath, granite veins in, 444. + + Caradoc sandstone, 356. + + Carbonaceous shale, 271. + + Carbonate of lime scarce in metamorphic rocks, 487. + + Carbonate of lime in rocks, how tested, 12. + + Carboniferous group, 308. + flora, 310. + period, plutonic rocks of, 456. + period, volcanic rocks of, 432. + reptiles, 335. + + Carne, Mr., on Cornish lodes, 491. 492. + + Carrara marble, 482. + + _Caryophyllia cæspitosa_, bed of, in Sicily, 151. + + Castrogiovanni, bent strata near, 58. + + Catalonia, volcanic region of, 408. + + Cautley, Captain, on Sewâlik hills, 173. + + Caves in Europe, 155. + at Kirkdale, 154. + in Sicily, 153. + in Australia, 156. + + Central France, Upper Eocene of, 178. + + Cetacea, fossil, rarity of, Postscript, xxi. + + Chalk, pinnacle of, near Sherringham, 129. + of Faxoe, 210. and Postscript, xv. + white, fossils of, 26. + white, section of, 211. + white, extent and origin of, 215. + white, animal origin of, 216. + pebbles in, 217. + difference of, in north and south of Europe, 221. + + Chalk cliffs, inland, on Seine, 238. + needles of, in Normandy, 241. + flints, bed of, near Barcombe, 253. + + Chambers, Mr., cited, 88. + + Chamisso, cited, 217. + + Chara, in freshwater strata, 31. + in flints of Cantal, 189. + in Eocene strata of France, 176. + in Purbeck beds, 232. + + Charlesworth, Mr. E., cited, on Crag, 162. + + Charpentier, M., on Alpine glaciers, 140. + on Swiss glaciers, 143. + + Cheirotherium, footprints of, 290. 337. + + Chemical and mechanical deposits, 33. + + Chili, earthquake in, 61. + gold mines in, 472. + + Chiloe, rocks drifted from Andes to, 144. + + Chlorite schist, 465. + + Christiania, dike near, 380. + trap rocks, passage of granite into, at, 441. + granite near, 457. + gneiss near, 446. + intrusion of granite into beds near, 446. + + Chronological groups, 101. + + Cinder-bed, Purbeck, 231. + + Claiborne, marine shells of, 206. + + Clausen, Mr., cited, 158. + + Clay, defined, 11. + + Clay-slate, 465. 468. + + Clay-ironstone, 326. + + Clays, plastic, 203. + + Cleavage of rocks, 468. + + Climate of drift period, 139. + of coal period, 335. + + Coal, zigzag flexures of, near Mons + group, 308. + measures, 308. 309. + how formed, 317. + pipes, danger of, 318. + mine, near Lyons, 319. + seam at Brownsville, Pennsylvania, view of, 332. + conversion of into lignite, 333. + formation at Brora, 272. + seams, continuity of, 334. + period, climate of, 335. + strata, footprints of reptiles in, 337. + + Coal-field at Burdiehouse, 325. + of Ashby-de-la-Zouch, 69. + United States, diagram of, 327. + of Yorkshire, fossils of, 325. + + Coalbrook Dale, beetles in coal of, 335. + fossil cones in, 313. + coal measures of, 324. + faults in, 62. + + Cockfield Fell, rocks altered by dikes, 383. + + Columbia, vinegar river of, 191. + + Colchester, Mr., on mammalian remains at Kyson, 203. + + Côme, ravine in lava of, 427. + + Cones in Val di Noto, 389. + and craters, absence of, in England, 6. + and craters, 367. + + Conifers, fossil trees, 316. + + Concretionary structure, 37. + + Conglomerate, or pudding-stone, 11. + dolomitic, 305. + vertical in Scotland, &c., 47. + + Connecticut, valley of the, 297. + beds, antiquity of, 300. + + Conrad, Mr., on cretaceous rocks, 224. + + Conybeare, Mr., cited, 64. 69. 244. 274. + on Plesiosaurus, 278. + on oolite and lias, 283. + on term Poikilitic, 286. + on crocodiles, 201. + + Cook, Capt., on _Fucus giganteus_, 217. + + Coprolites in chalk, 216. + + Coralline crag, fossils in, 164. + + Coral islands and reefs, 34. 46. + rag of Oolite, 260. + + Corals, figures of, in crag, 165. + of Devonian system, 346. + of Devonian strata in United States, 349. + in Wenlock formation, 355. + + Corinth, corrosion of rocks by gases near, 477. + + Cornbrash, 263. + + Cornwall, granite veins in, 445. 474. + mineral veins in, 490. 494. + tin of, newer than Irish copper, 499. + + Cotta, Dr. B., on granite in Saxony, 459. + + Crag coralline, fossils in, 164. + comparison of faluns and, 170. + of Suffolk, red and coralline, 105. 162. + fluvio-marine, Norwich, 148. + + Craigleith fossil trees, 40. + quarry, slanting tree in, 320. + + Crater of Island of St. Paul, 395. + + Craven fault, 64. + + Creeps in coal-mines described, 52. + + Credneria in Quadersandstein, Postscript, xvi. + + Cretaceous rocks of Pyrenees, 455. + group, 209. 219. and Postscript, xvi. + strata in South America and India, 225. + period, plutonic rocks of, 455. + volcanic rocks, 431. + rocks in United States, 224. + + Crocodiles near Cuba, 279. + + Croizet, M., on Auvergne fossil mammalia, 188. + + Cromer, contorted drift near, 129. + + "Crop out," term explained, 55. + + Crust of earth defined, 2. + + Crystalline limestone, 302. + rocks, erroneously termed primitive, 9. + schists defined, 7. + + Curved strata, 47. + strata, experiments to illustrate, 49. + + Cutch, Runn of, 295. + + Cuvier, M., on Eocene formation, 175. + on Amphitherium, 268. + cited, 192. + on tertiary strata near Paris, 104. + on fossils of Montmartre, 191. + + Cyclopian Islands, 401. + + Cypris in Lias, 281. + in Wealden, 228. + in marl of Auvergne, 183. + + Cystideæ in Silurian rocks, 358. + + + D. + + Dana, Mr., on coprolites of birds, 299. + on coral reef in Sandwich Islands, 216. + on volcanos of Sandwich Islands, 394. 406. 423. + + Dartmoor, granite of, 456. + + Darwin, Mr., cited, 217. + on boulders and glaciers in South America, 144. + on cleavage in South America, 471. + on coral islands of Pacific, 216. + on dike in St. Helena, 406. + on habits of ostrich, 299. and Postscript, xx. + on fossils in South America, 148. + on _Fucus giganteus_, 217. + on gradual rise of part of S. America, 46. + on lamination of volcanic rocks, 480. + on parallel roads, 87. + on plutonic rocks of Andes, 453. + on recent strata near Lima, 115. + on saurians in Galapagos Islands, 279. + on sinking of coral reefs, 46. + on Welsh glaciers, 131. + + Daubeny, Dr., on the Solfatara, 477. + + Daubeny, Dr., on volcanos in Auvergne, 428. + + Dax, inland cliff at, 72. + + Deane, Dr., on footprints, 298. + + Dean, forest of, coal in, 334. + + Dechen, Prof. von, on reptiles in Saarbrück coal-field, 336. + + De Koninck, cited, 176. 178. + + De la Beche, Sir H., cited, 231. 233. 281. + on Carrara marble, 482. + on clay beds, 283. + on clay-ironstone, 326. + on coal-measures near Swansea, 309. + on fossil trees, S. Wales, 318. + on granite of Dartmoor, 474. + on mineral veins, 493. 495. 498. + on term supracretaceous, 103. + on trap of New Red Sandstone period, 432. + + Deluge, 4. + + Denudation explained, 66. + of the Weald Valley, 242. + terraces of, in Sicily, 75. + + Derbyshire, lead veins of, 497. + + Deshayes, M., identification of shells, 176. + on fossil shells in Hungary, 421. + on Lower Eocene shells, 196. + on tertiary classification, 110. + + Desmarest, cited, 183. + on trappean rocks, 91. + + Desnoyers, M., on Faluns of Touraine, 106. + + Desor, M., on glacial fauna in N. America, 133. + + Devonian flora, 349. + strata in United States, 349. + system, term explained, 346. + + Diagonal, or cross stratification, 16. + + Dicotyledonous leaves in chalk, Postscript, xvi. + + Dike in St. Helena, 406. + + Dikes at Palagonia in Sicily, 407. + trappean, crystalline in centre, 380. + defined, 6. + in Scotland, 378. + of Somma, 404. + + Diluvium, popular explanation of term, 132. + + Dip, term explained, 53. + + Dolerite, or greenstone, 372. + + Dolomite defined, 13. + + Dolomitic conglomerate, 305. + + Doue, M. B. de, on volcanos of Velay, 428. + + Drift contorted, near Cromer, 129. + in Ireland, 131. + in Norfolk, 126. + meteorites in, 145. + northern, in Scotland, 125. + northern, in North Wales, 130. + of Scandinavia, North Germany, and Russia, 121. + period, climate of, 139. + period, subsidence in, 135. + shells in Canada, 134. + + Dudley limestone, 354. + shales of coal near, 474. + + Dufrénoy, M., on granite of Pyrenees, 475. + + Duff, Mr. P., on reptile of Old Red, Postscript, ix. + on hill of Gergovia, 430. + + Dunker, Dr., on Wealden of Hanover, 237. + + + E. + + Echinoderms of coralline crag, 166. + + Echinus, figure of, 23. + + Egerton, Mr., on fossils of Southern India, 225. + + Egerton, Sir P., on fish of marl slate, 304. + on fossil fish of Connecticut beds, 300. + on fossils of Isle of Wight, 198. + on saurians and fish in New Red Sandstone, 289. + on Ichthyosaurus, 276. + + Eggs, fossil, of snake, 120. + + Ehrenberg, Prof., on bog-iron ore, 26. + on infusoria, 24. + + Elephant bed, Brighton, 256. + + _Elephas primigenius_, jaw figured, 159. + + Elvans of Ireland and Cornwall, 498. + term explained, 457. + + Encrinites, figure of, 264. + + Endogens, 316. + + Eocene, foraminifera, 194. + formations, 174. + formations in England, 197. + granite, 451. + lower, in France, 176-191. + middle, in France, 191. + strata, in United States, 206. + upper, near Louvain, 177. + term defined, 111. + upper, of Central France, 178. + volcanic rocks, 429. + + Equisetaceæ, 313. + + Equisetum of Virginian oolite, 284. + + _Equisetum_ giganteum, 314. + + Erman on meteoric iron in Russia, 145. + + Erratics, Alpine, 140. + northern origin of, 123. + + Escher, M., on boulders of Jura, 143. + + Etna, deposits of, 401. + + Eurite, 440. + + Euritic porphyry described, 447. + + Exogens, 316. + + + F. + + Faluns of Touraine, 106. 168. + + Faluns, comparison of, and crag, 170. + + Falconer, Dr., on Sewâlik Hills, 173. + + Falkland Islands, 88. + + Farnham, phosphate of lime near, 219. + + Fault, term explained, 62. + + Faults, origin of, 64. + + Faxoe, chalk of, 210. and Postscript, xv. + + Felixstow, remains of cetacea found near, 166. + + Felspar, 369. + + Ferns in coal-measures, 310. + + Fife, altered rock in, 383. + + Fifeshire, trap dike in, 434. + Megalichthys found in Cannel coal in, 336. + + Fishes, fossil, of Upper Cretaceous, 214. + of Old Red Sandstone, 343. + of Wealden, 229. + fossil, of brown coal, 416. + + Fissures filled with metallic matter, 490. + _See_ mineral veins. + + Fitton, Dr., on division of lower cretaceous formation, 219. + cited, 227. 231. 233. 237. 244. 247. + + Fleming, Dr., on scales of fish in Old Red, 343. + on trap-rocks in coal-field of Forth, 432. + on trap dike in Fifeshire, 434. + + Flora, carboniferous, 310. + cretaceous, 223. + Devonian, 349. + of London Clay, 200. + permian, 305. 307. + + Flötz, term explained, 91. + + Flysch, explanation of term, 206. + + Footprints of birds, 297. and Postscript, xx. + of reptilians, 337. + fossil, 289. 290. 291. 297. + + Foraminifera in chalk, 26. + Eocene, 194. + + Forbes, Prof. E., on Caradoc sandstone, 359. + on Cystideæ, 358. + on shells in crag deposits, 162. + on cretaceous fossil shells, 224. + on fossils of the faluns, 169. + on fossils in drift in South Ireland, 131. + on deep-sea origin of Silurian strata, 360. + on echinoderms of coralline crag, 166. + on fauna of boulder period, 125. + on migrations of mollusca in glacial period, 166. + on fossils of Purbeck group, 231. 233. + on strata at Atherfield, 219. + on changes of Wealden testacea, 235. + on volcanic rocks of Oolite period, 432. + on depth of animal life in Ægean, 35. 137. + cited, 225. + + Forbes, Prof. James, on zones in volcanic rocks, 480. + on the Alps, 143. + + Forchhammer, on scratched limestone, 122. + + Forest, fossil, in Norfolk, 127. 130. + + Forfarshire, Old Red Sandstone in, 479. + + Formation, term defined, 3. + + Fossil, term defined, 4. + + Fossils of chalk and greensand, figures of, 212. + in chalk at Faxoe, 210. + of coralline crag, 164. + of Devonian system, 346. and Postscript, x. xi. + of Eocene strata in United States, 207. + in faluns of Touraine, 169. + freshwater and marine, 27. + of Isle of Wight, 198. + of Lias, 274. + of Ludlow formation, 352. + of mountain limestone, 340. + of London Clay, 200. + of Maestricht beds, 209. + of Lower Greensand, 220. + of New Red Sandstone, 287. and Postscript, xiii. + of Oolite, 259. 266. + of Red Crag, 164. + of Silurian rocks, 353. and Postscript, vii. + of Solenhofen, 260. + of Upper Greensand, 218. + of Wealden, 236. + test of the age of formations, 98. + + Fossil fish of Permian limestone, 303. + of Connecticut beds, 300. + of Richmond, U. S., strata, 285. + of Old Red Sandstone, 343. + scales of Permian, figured, 305. + footsteps, 289. 290. 291. + ferns in carbonaceous shale, 271. + forest in Nova Scotia, 321. + forest near Wolverhampton, 319. + forest in Isle of Portland, 233. + plants in Wealden, 230. + plants of Lias, 282. + plants of Bunter sandstein, 288. + trees erect, 317. + wood, petrifaction of, 39. + wood perforated by Teredina, 24. + remains in caves, 154. + shells from Etna, 401. + shells near Grignon, 193. + shells of Mayence strata, 178. + shells in Virginia, 172. + + Fossiliferous strata, tabular view of, 361. + + Fournet, M., on mineral veins of Auvergne, 493. + on disintegration of rocks, 476. + on quartz, 439. + + Fox, Mr. R. W., 472. + on Cornish lodes, 497. + + Fox, Rev. Mr., on extinct quadrupeds of Isle of Wight, 198. + + Freshwater beds of Isle of Wight, 197. + deposits in valley of Thames, 146. + land shells numerous in, 27. + + Freshwater formations of Auvergne, 186. + + Freshwater formation, how distinguished from marine, 27. 28. 30. + remains of fish in, 32. + associated with Norfolk drift, 127. + Chara in, 31. + Cypris in, 31. + + Freshwater shells in brown coal near Bonn, 417. + + _Fucus giganteus_, 217. + _vesiculosus_, growth of, in Jutland, 217. + _vesiculosus_ in Lym-Fiord, 33. + + Fundy, Bay of, impressions in red mud of, 297. + + + G. + + Gaillonella fossil in Tripoli, 25. + ferruginea in bog-iron ore, 26. + + Galapagos Islands, animals of, 279. + + Garnets in altered rock, 382. + + Gases, subterranean rocks altered by, 476. + + Gault, 218. + + Gavarnie, flexures of strata, 59. + + Geology defined, 1. + + Gergovia, hill of, 430. + + Giant's Causeway, columns at, 384. + + Gibbes, R. W., cited, 207. + + Glacial phenomena, northern, origin of, 132. + + Glaciers, Alpine, 140. + + Glaciers on Caernarvonshire mountains, 130. + + Glasgow, marine strata near, 148. + + Glen Roy, parallel roads of, 86. + + Glen Tilt, granite of, 442. + + Gneiss, altered by granite, 445. + in Bernese Alps, 484. + at Cape Wrath, 444. + near Christiania, 446. + described, 464. + + Gold, age of, in Ireland, 498. + age of, in Ural Mountains, 499. + + Goldfuss, Prof., on reptiles in coal-field, 336. + + Göppert, Prof., on beds of coal, 316. + on petrifaction, 40. + + Graham's Island, 389. 407. + + Grampians, old red conglomerates in, 47. + + Granite described, 7. 436. 438. 444. + passage of into trap, 441. + porphyritic, 439. + and limestone, junction of in Glen Tilt, 442. + syenitic, 440. + talcose, 440. + schorly, 440. + of Cornwall and Dartmoor, 474. + of Swiss Alps, 484. + rocks in connection with mineral veins, 500. + of Saxony, 459. + + Granites, oldest, 458. + varieties of, 444. + veins in Cornwall, 445. + veins in Cape Wrath, 444. + veins in Table Mountain, 443. + vein in White Mountains, 450. + of Arran, age of, 459. + near Christiania, 457. + dikes in Mount Battock, 443. + + Graphite, powder of, consolidated by pressure, 38. + + Graptolites, 357. + + Grateloup, M., on fossils in chalk, 223. + + Grauwacke, term explained, 350. + + Greenland, sinking of coast, 46. + + Greensand, upper, 218. + fossils of, 212. + + Greensburg, Pennsylvania, footprints of reptile in coal strata at, 337. + + Greenstone or Dolerite, 372. + dike of, in Arran, 379. + + Grès de Beauchamp, Paris Basin, 193. + + Grignon, fossil shells near, 193. + + Grit defined, 11. + + Guadaloupe, human skeleton of, 115. + + Guidoni on Carrara marble, 482. + + Gutbier, Col. von, on Permian flora, 305. 307. + + Gryphæa, fossil figure of, 22. + + Gypseous marls, 186. + series, 191. + + Gypsum defined, 13. + + + H. + + Hall, Sir Jas., experiments on fused minerals, 406. + on curved strata, 48. + Capt. B., cited, 378. 401. 443. + + Hamilton, Sir W., on eruption of Vesuvius, 405. + + Harris, Major, on salt lake in Ethiopia, 296. + + Hartz, Bunter sandstein of, 288. + + Hastings, Lady, fossils collected by, 198. + + Hastings sand, 229. + bed, shells of, 229. + + Hautes Alpes, rocks of, 455. + + Haüy cited, 369. + + Hawkshaw, Mr., on fossil trees in coal, 317. + + Hayes, T. L., on icebergs, 123. + + Hébert, M., cited on Upper Eocene beds, 176. + + Hebrides, dikes of trap in, 379. + + Heidelberg, varieties of granite near, 444. + + Henfrey, Mr. A., on food of Mastodon, 138. + + Henslow, Prof., on fossil cetacea in Suffolk, 166. + on fossil forests, 233. + on dike and altered rock near Plas Newydd, 381. + + Henry, Mr., cited, 476. + + Herschel, Sir J., on slaty cleavage, 472. + + Hertfordshire pudding-stone, 35. + + Hibbert, Dr., on volcanic rocks, 428. + on coal field at Burdiehouse, 325. + cited, 419. + + High Teesdale, garnets in altered rock at, 382. + + Hildburghausen, footprints of reptile at, 289. 290. + + Hippurite limestone, 221. + + Hitchcock, Prof., on footprints, 297. + + Hoffmann, Mr., on Lipari Islands, cited, 476. + on cave near Palermo, 74. + on Carrara marble, 482. + + Hooghly river, analysis of water, 41. + + Hopkins, Mr., on fractures in Weald, 251. + + Horizontality of strata, 15. + of roads of Lochaber, 88. + + Hornblende, 369. + schist, 464. 478. + + Horner, Mr., on geology of Eifel, 415. + on Megalichthys, 336. + + Hubbard, Prof., on granite vein in White Mountains, 450. + + Hugi, M., on Swiss Alps, 484. + + Humboldt, cited, 314. + on uniform character of rocks, 486. + + Hungary, trachyte of, 442. + volcanic rocks of, 421. + + Hunt, Mr., experiments on clay-ironstone, 326. + + Hutton, opinions of, 60. + + Huttonian theory, 92. + + Hypogene, term defined, 9. + rocks, mineral character of, 485. + or metamorphic limestone, 465. + + + I. + + Ibbetson, Capt., on chalk Isle of Wight, 215. + + Ice, rocks drifted by, 122. + + Icebergs, stranding of, 129. 137. + + Iceland, icebergs drifted to, 137. + + Ichthyolites of Old Red Sandstone, 349. + + _Ichthyosaurus communis_, figure of, 277. + + Igneous rocks, 6. + of Siebengebirge and Westerwald, 417. + rocks of Val di Noto, 389. + + _Iguanodon Mantelli_, 227. 229. + + India, cretaceous system in, 225. + freshwater deposits of, 173. + oolitic formation in, 285. + + Indusial limestone, Auvergne, 184. + + Infusoria in tripoli, 24. + + Inland sea-cliffs in South of England, 71. + + Insects in Lias, 281. + + Ireland, drift in, 131. + + Ischia, volcanic cones in, 403. + Post-Pliocene strata of, 113. + + Isle of Wight, freshwater beds of, 197. + + Isomorphism, theory of, 370. + + + J. + + Jackson, Dr. C. T., analysis of fossil bones, 138. + + James, Capt., on fossils in drift South Ireland, 131. + + Java, stream of sulphureous water, 191. + + Jobert, M., on hill of Gergovia, 430. + + Joints, 469. + + Jorullo, lava stream of, 450. + + Jura, alpine blocks on, 142. + limestone, 261. + structure of, 55. + + + K. + + Kangaroo, fossil and recent, jaws figured, 156. + + Kaup, Prof., on footprints of Cheirotherium, 290. + + Kaye, Mr., on fossils of Southern India, 225. + + Keeling Island, fragment of greenstone in, 217. + + Keilhau, Prof., cited, 457. 474. + on dike of greenstone, 380. + on gneiss near Christiania, 446. + on granite, 447. + + Kelloway rock, 34. + + Kentish chalk, sand-galls in, 82. + + Keuper, the, 287. + + Killas in granite of Cornwall, 474. + + Kimmeridge clay, 260. and Postscript, xxi. + + King, Dr., on footprints of reptile, 337. + + King, Mr., on Permian group and fossils, 301. 302. + + Kirkdale, cave at, 154. + + Kotzebue cited, 217. + + Kyson, in Suffolk, strata of, 202. + + + L. + + Labyrinthodon, 292. 288. 289. + + Lacustrine strata of Auvergne, 181. + + Lagoons at mouth of rivers, 33. + of Bermuda Islands, 216. + + Lake craters of Eifel, 419. + crater of Laach, 420. + + Lamarck on bivalve mollusca, 29. + + Land, rising and sinking, 45. + + Laterite, 376. + + Lava, 373. + current, Auvergne, 425. + relation to trap, 387. + stream of Jorullo, 450. + of Stromboli, 450. + + Lea, Mr., footprints of reptile discovered by, 340. + + Lead, veins of, in Permian rocks, 499. + + Lehman on classification of rocks, 90. + + Leibnitz, theory of, 94. + + Lepidodendra, 312. + + Lewes, coomb near, 250. + + Lias, 273. + period, Volcanic rocks, 431. + at Lyme Regis, 281. + plutonic rocks of, 455. + and oolite, origin of, 282. + fossil plants of, 282. + + Liebig, Prof., on conversion of coal into lignite, 333. + on preservation of fossil bones in caverns, 155. + + Lima, recent strata of, 115. + + Limagne d'Auvergne, freshwater formations of, 187. + + Lime, scarcity of, in metamorphic rocks, 487. + + Limestone, brecciated, 302. + crystalline, 302. + compact, 303. + fossiliferous, 303. + hippurite, 221. + indusial, Auvergne, 184. + of Jura, 261. + magnesian, 301. + mountain fossils of, 340. + primary or metamorphic, 465. + in Germany, of Devonian system, 348. + + Lindley, Dr., cited, 223. + on leaves in lignite, 416. + + Link, M., on footprints, 291. + + Lipari Islands, rocks altered by gases in, 476. + + Lisbon, marine tertiary strata near, 171. + + Lithodomi in beaches of N. America, 78. + in inland cliffs, 73. + + Llandeilo flags, 357. + + Loam defined, 13. + + Lochaber, parallel roads of, 86. + + Lodes. _See_ Mineral Veins, 490. + + Loess of valley of Rhine, 117. + fossil land shells of, figured, 120. + + Logan, Mr., on coal measures of South Wales, 310. + on fossil forest in Nova Scotia, 322. + on reptilian foot-prints in lowest Silurian in + Canada, Postscript, viii. + + London clay, 200. + + Lonsdale, Mr., cited, 152.; on corals, 173. + on corals of Normandy, 170. + on corals in Wenlock formation, 355. + on fossils in white chalk, 26. + on old red sandstone of S. Devon, 345. + on Stonesfield slate, 266. + + Louvain, Eocene strata near, 177. + + Lovén on shells of Norway, 114. + + Ludlow formation, 351. + + Lund, cited, 158. + + Lycett, Mr., on shells of oolite, 266. + + Lyme Regis, lias at, 281. + + Lym-Fiord invaded by the sea, 33. + kelp in, 217. + + Lyons, coal mine near, 319. + + + M. + + Macacus, in Eocene formation, 203. + + Maclaren, Mr., on erratic blocks in Pentlands, 125. + + Maclure, Dr., on volcanos in Catalonia, 409. + + MacCulloch, Dr., cited, 442. + on altered rock in Fife, 383. + on basaltic columns in Skye, 385. + on denudation, 67. + on granite of Aberdeenshire, 441. + on igneous rocks of Scotland, 390. + on Isle of Skye, 36. 456. + on hornblende schist, 478. + on overlying rocks, 8. + on parallel roads, 87. + on pebbles of granite, 460. + on trap vein in Argyleshire, 379. + + Madeira, view of dike in inland valley in, 378. + + Maestricht beds, 209. + + Magnesian limestone, concretionary structure of, 37. + defined, 13. + groups, 301. + + Maidstone, fossils in white chalk of, 214. + + Mammalia, extinct, above drift in United States, 138. + extinct, of basin of Mississippi, 116. + fossil teeth of, figured, 160. + + Mammat's "Geological Facts" cited, 69. + + Mammifer in trias near Stuttgart, Postscript, xiii. + + Mansfield in Thuringia, Permian formation at, 306. + + Mantell, Dr., cited, 217. 229. 231. 251. + on belemnite, 263. + on chalk flints, 253. + on Brighton elephant bed, 257. + on freshwater beds of Isle of Wight, 198. + on iguanodon, 227. + on Wealden group, 226. + on reptile in Old Red, Postscript, x. + + Marble defined, 12. + + Marl defined, 13. + in Lake Superior, 36. + red and green in England, 289. + + Marl-slate defined, 13. + + Martin, Mr., cited, 250. + on cross fractures in chalk, 245. + + Martins, Mr. C., on glaciers of Spitzbergen, 136. + + Map to illustrate denudation of Weald, 242. + + Map of Eocene beds of central France, 179. + + Massachusetts, plumbago in, 478. + + _Mastodon angustidens_, jaw, figure of, 159. + + _Mastodon giganteus_, in United States, 137. + + Mayence tertiary strata, 177. + + Mediterranean and Red Sea, distinct species in, 100. + deposits forming in, 99. + + Megalichthys in Cannel coal of Fifeshire, 336. + + Megatherium in South America, 158. + + Menai Straits, marine shells in drift, 130. + + Mendips, denudation in, 68. + + Metalliferous veins. _See_ Mineral Veins. + + Metals, supposed relative ages of, 497. + + Metamorphic rocks, 463. + defined, 8. + why less calcareous than fossiliferous, 487. + order of succession, 485. + glossary of, 466. + + Metamorphic strata, origin of, 467. + + Metamorphic structure, origin of, 477. + + Meteorites in drift, 145. + + Mexico, lamination of volcanic rocks in, 480. + + Meyer, M. H. von, cited, 147. + on fossil mammalia of Rhine, 178. + on reptile in coal, 336. 337. + on sandstone of Vosges, 288. + on Wealden of Hanover and Westphalia, 237. + + Mica schist, 465. + + Micaceous sandstone, origin of, 14. + + Microlestes antiquus, triassic mammifer, Postscr., xiv. + + Miller, Mr. H., on origin of rock salt, 295. + on old red sandstone, 343. + on fossil trees of coal near Edinburgh, 321. + + Minchinhampton, fossil shells at, 266. + + Mineral character of aqueous rocks, 97. + composition, test of age of volcanic rocks, 399. + springs, connected with mineral veins, 496. + veins and faults, 488. 490. + of different ages, 490. 498. 499. + veins, pebbles in, 492. + subsequently enlarged and re-opened, 492. + veins, various forms of, 489. + veins near granite, 496. + + Mineralization of organic remains, 38. + + Miocene formations, 168. + in United States, 171. + period, volcanic rocks of, 415. + term defined, 111. + + Mississippi, fluviatile strata and delta of, 115. 116. + + Mitchell, Sir T., on Australian caves, 156. + + Mitscherlich, Prof., on augite and hornblende, 369. + on isomorphism, 370. + on mineral composition of Somma, 404. + + Modon, lithodomi in cliff at, 73. + + Molasse of Switzerland, 171. + + Mons, flexures of coal at, 53. + + Mont Blanc, granite of, 453. + + Mont Dor, Auvergne, 422. + + Monte Calvo, section of, 18. + + Montlosier, M., on Auvergne volcanos, 427. + + Moraine, term explained, 123. + + Moraines of glaciers, 141. + + Morea, inland sea-cliffs of, 73. + trap of, 431. + + Morris, Mr., cited, 177. + on fossils at Brentford, 147. + + Morton, Dr., on cretaceous rocks, 224. + + Morven, basaltic columns in, 385. + + Mosasaurus in St. Peter's Mount, 210. + + Mountain limestone, fossils of, 340. + + Munster, Count, on fossils of Solenhofen, 260. + + Murchison, Sir R., cited, 248. 324. + on new red sandstone, 290. + on age of Alps, 206. + on age of gold in Russia, 499. + on erratic blocks of Alps, 144. + on granite, 456. 459. + on primary strata in Russia, 124. + on joints and cleavage, 469. 471. + on old red sandstone of S. Devon, 345. 348. + on pentamerus, 353. + on Permian flora, 305. + on Silurian strata of Shropshire, 434. + on Swiss Alps, 484. + on term Permian, 301. + on term Silurian, 350. + on tilestones, 351. + + Muschelkalk, 287. + + + N. + + Naples, post-pliocene formations near, 403. + recent strata near, 112. + + Navarino, lithodomi found in cliff at, 73. + + Necker, M. L. A., cited, 445. + on composition of cone of Somma, 404. + on granite in Arran, 460. + on granitic rocks, 447. + on Swiss Alps, 484. + terms granite underlying, 8. + + Nelson, Lieut., drawing of Bermuda, 79. + on Bermuda Island, 216. + + Neptunian theory, 91. + + Newcastle coal field, great faults in, 64. + + Newcastle, fossil tree near, 312. 318. + + New Jersey, _Mastodon giganteus_ in, 137. + + New red sandstone, distinction from old, 286. + its subdivisions, 287. + of United States, 297. + trap of, 432. + + New Zealand, absence of quadrupeds, 158. + + Niagara, recent shells in valley of, 138. + + Noeggerath, M., cited, 415. + + Nomenclature, changes of, 93. + + Norfolk, buried forest, 127. 130. 147. + drift, 126. + + Normandy chalk, cliffs, and needles, 241. + + Northwich, beds of salt at, 294. + + Norwich crag, fluvio-marine, 148. + sand-pipes near, 82. + + Nova Scotia, coal seams of Cape Breton, 315. + fossil forest of coal in, 321. + + Nummulites, figures of, 200. 205. + + Nummulitic formation, 205. + + Nyst, M., cited, 176. + + + O. + + Oeynhausen, M. von, on Cornish granite veins, 445. + + Olot, extinct volcanos near, 408. + + Old red sandstone, 342. + in Forfarshire, 478. + trap of, 434. + + Oolite, 257. + and lias, origin of, 282. + inferior, fossils of, 272. + in France, 259. + plutonic rocks of, 455. + term defined, 12. + volcanic rocks of, 431. + + Oolitic group in France, 283. + + Orbigny, M. d', cited, 222. + on fossils of nummulitic limestone, 206. + on subdivisions of cretaceous series, 209. + + Organic remains, criterion of age of formation, 98. + test of age of volcanic rocks, 399. + + Ormerod, Mr., on trias of Cheshire, 295. + + Overlying, term applied to volcanic rocks, 8. + + Owen, Prof., cited, 155. 166. 229. 267. 268. 270. 291. + on amphitherium, 269. + on birds in New Zealand, 158. + on caves in England, 154. + on footprints, 298. + on fossils in Australia, 156. + on fossil monkey, 202. + on fossil quadrupeds, 157. + on ichthyosaurus, 276. + on reptile in coal, 337. + on serpent of Bracklesham, 199. + on snake at Sheppey, 201. + on thecodont saurians, 306. + on zeuglodon, 207. 208. + on reptile in Silurian rocks, Postscript, viii. + + Oxford clay, 262. + + Oyster beds, 204. + + + P. + + Pacific, coral reefs of, 215. + + Palæontology, term explained, 103. + + Palagonia, dikes at, 407. + + _Paleotherium magnum_, figure of, 192. + tooth of, 193. + + Palermo, caves near, 74. + + Palma, Isle of, map and view of, 391. + + Parallel roads, 86. + + Pareto, M., on Carrara marble, 482. + + Paris basin, 93. + + Parkinson, Mr., on crag, 105. + + Parrot, Dr. F., on salt lakes of Asia, 295. + + Pebbles in chalk, 217. + + Pegmatite, 440. + + _Pentamerus Knightii_, 352. + + Pentland hills, Mr. Maclaren on, 125. + + Pepys, Mr., cited, 41. + + Permian flora, distinct from coal, 305. + formation in Thuringia, 306. + group, term explained, 301. + + Petrifaction of fossil wood, 39. + + Petrifaction, process of, 43. + + Philippi, Dr., on fossil shells near Naples, 113. + on marine shells in caves of Sicily, 154. + on tertiary shells of Sicily, 150. + + Phillips, Prof., cited, 274. 309. + on cleavage, 471. + on terminology, 103. + + Phillips, Mr. W., on kaolin of China, 11. + + Phosphate of lime, 219. + + Phryganea, figure of, 185. + indusiæ of, 186. + + Pictou, Nova Scotia, calamites near, 319. + + Pilla, M., on age of Carrara marble, 482. + + Planitz, tripoli of, 26. + + Plas Newydd, rock altered by dike near, 381. + + Plastic clays, 203. + + Playfair, cited, 45. 92. 383. + on faults, 62. + on Huttonian theory of stratification, 60. + + Plesiosaurus, figure of, 277. + + Plieninger, Professor, on triassic mammifer, Postscript, xiii. + + Pliocene, newer period, 121. + newer, strata, 146. + strata in Sicily, 150. + older, in United States, 171. + strata, 161. + period, volcanic rocks of, 407. 408. + term defined, 111. + + Plomb du Cantal, described, 429. + + Plumbago in Massachusetts, 478. + + Plutonic rocks, 7. 446. + age of, 439. + of carboniferous period, 456. + of oolite and lias, 455. + recent and pliocene, 450. + of Silurian period, 457. + age, how tested, 449. + + Plutonic and sedimentary rocks, diagram of, 452. + + Poggendorf, cited, 476. + + Poikilitic formation, 301. + term explained, 286. + + Pomel, M., on mammalia of Auvergne, 188. 425. + + Ponza Islands, structure of, 387. 480. + + Porphyritic granite, 439. + + Porphyry, 372. + + Portland, Isle of, fossil forest in, 233. + + Portland stone, 259. + + Post-pliocene formations, 111. + period, volcanic rocks, 401. + + Potsdam sandstone, reptilian, Postscript, vii. xviii. + + Pottsville, coal seams near, 329. + footprints of reptile near, 340. + + Pozzolana, 36. + + Pratt, Mr., on ammonites, 262. + on extinct quadrupeds of Isle of Wight, 198. + + Predazzo, altered rocks at, 456. + + Prestwich, Mr., cited, 69. + on English Eocene strata, 197. 198. 200. + on coal measures of Coalbrook Dale, 62. 324. + + Prevost, M. C., on Paris basin, 175. 176. 195. + + Progressive development, theory of, Postscript, xvi. + + Psaronites in Germany and France, 307. + + Pumice, 373. + + Purbeck beds, 231. + + Puy de Tartaret, 425. + + Puy de Pariou, 428. + + Puzzuoli, elevation and depression of land at, 403. + + Pyrenees, cretaceous rocks of, 455. + curvatures of strata, 58. + granite of, 475. + nummulitic formation of, 205. + + + Q. + + Quadrumana fossil, Postscript, xvii. + + Quarrington Hill, basaltic dike near, 398. + + Quartz, 438. + + Quartzite, or quartz rock, 465. + + + R. + + Radnorshire, stratified trap of, 425. + + Rain-prints, fossil in coal shale, Postscript, xii. + + Ramsay, Prof. A. C., on denudation, 68. + on granite in Arran, 460. + on section near Bristol, 102. + on Welsh glaciers, 131. + + Recent strata defined, 112. + near Naples, 112. + + Redfield, Mr., on glacial fauna in America, 133. + on fossil fish, 300. + + Red sandstone, origin of, 293. + + Red Sea and Mediterranean, distinct species in, 100. + + Red Sea, saltness of, 296. + + Reptiles, carboniferous, 335. 336. + of lias, 276. + fossil eggs of, 120. + + Reptile, in Lower Silurian, Postscript, vii. + in Old Red Sandstone of Morayshire, Postscript, ix. + + Rhine, valley, loess of, 117. + + Ripple-mark, formation of, 19. + + River channels, ancient, 334. + + River, excavation through lava by, 413. + terraces, 85. + + Rock, term defined, 2. + + Rocks, four classes of, contemporaneous, 9. + classification of, 90. + composed of fossil zoophytes and shells, 24. + trappean, 91. + + Roderberg, extinct volcano of, 420. + + Rogers, Prof. H. D., on coal field, United States, 328. + cited, 340. + on reptilian footprints in coal, Postscript, xi. + + Rogers, Prof. W. B., on oolitic coal field, United States, 284. 328. + + Rome, formations at, 168. + + Römer, F., on chalk in Texas, 225. + M. F. A., on flora of Hartz, 350. + + Rose, Prof. G., cited, 374. 434. + on hornblende, 369. + + Rosenlaui, limestone scratched by glacier of, 122. + + Ross, Captain, on greenstone in Keeling Island, 217. + + Ross-shire, denudation in, 67. + + Rothliegendes, lower, or Permian, 306. + + Rozet, M., cited, 191. + + Rubble, term explained, 81. + + Russia, erratic blocks in, 124. + fossil meteoric iron in, 145. + Permian rocks in, 306. + + + S. + + Saarbrück coal field, reptile found in, 336. + + St. Abb's Head, curved strata near, 49. + + St. Andrews, trap rocks in cliffs near, 432. 433. + + St. Helena, basalt in, 385. 406. + + St. Lawrence, gulf of, inland beaches and cliffs, 78. + + St. Mihiel, inland cliffs near, 77. + + St. Paul, island of, 394. + + St. Peter's Mount, Maestricht, fossils in, 210. + sand-pipes in, 83. + + Salisbury Crag, altered strata of, 383. + + Salt rock, origin of, 294. + precipitation of, 294. + at Northwich, 294. + lakes of Asia, 296. + + Salter, Mr., on fossil of Caradoc sandstone, 356. + + Sand-pipes near Maestricht, 83. + or sand-galls, term explained, 82. + near Norwich, 82. + + Sandstone, siliceous, 218. + with cracks in Wealden, 230. + + Sandwich Islands, coral reef in, 216. + volcanos of, 394. 406. 423. + + Saurians of lias, 278. + thecodont, 306. + + Saussure, M., on moraines, 141. + on vertical conglomerates, 47. + + Savi, M., on Carrara marble, 482. + + Saxony, granite in, 459. + + Schist, hornblende, and mica, 464. 465. + argillaceous, 465. + chlorite, 465. + + Schorl rock and schorly granite, 440. + + Scoresby on icebergs, 122. + + Scoriæ, 373. + + Scotland, carboniferous traps of, 432. + northern drift in, 125. + old red sandstone of, 343. + + Scrope, Mr., cited, 181. 263. 419. 423. 425. 427. 430. + on globular structure of traps, 387. + on Ponza Islands, 480. + on trachyte, basalt, and tuff, 374. 400. + + Sea cliffs, inland, 71. + + Section of Wealden, 243. + + Section of white chalk from England to France, 211. + + Section of volcanic rocks, Auvergne, 424. + + Sedgwick, Prof., cited, 309. 383. + on brecciated limestone, 302. + on concretionary magnesian limestone, 37. + on Devonian group, 348. + on garnets in altered rock, 382. + on granite, 456. 459. + on Permian sandstones, 305. + on joints and cleavage, 469. 471. + on mineral composition of granite, 444. + on old red of Devon and Cornwall, 345. + on structure of rocks, 468. + on trap rocks of Cumberland, 435. + + Segregation in mineral veins, 489. + + Semi-opal, infusoria in, 26. + + Serpulæ, on volcanic rocks, in Sicily, 151. + + Sewâlik Hills, freshwater deposits, 173. + + Shale, carbonaceous, 271. + defined, 11. + + Shales of coal near Dudley, 474. + + Sharpe, Mr. D., on mollusca in Silurian strata, 359. + on slaty cleavage, 471. + + Shells, fossil, in Purbeck, 231. + fossil, useful in classification, 109. + in Canada drift, 134. + recent, in valley of Niagara, 138. + species of, near Lisbon, 171. + + Sheppey, Isle of, fossil flora of, 200. + + Sherringham, mass of chalk in drift, 129. + + Shetland, granite of, 441. 444. + hornblende schist of, 478. + + Shrewsbury, coal deposit near, 324. + + Sicily, Fiume Salso in, 191. + inland cliffs in, 74. + newer pliocene strata of, 150. + terraces of denudation in, 75. + + Sidlaw Hills, trap of old red sandstone, 434. + + Siebengebirge, igneous rocks of, 417. + + Sienna, formations at, 167. + + Sigillaria, 314. 318. + + Siliceous limestone defined, 12. + rocks defined, 11. + + Silliman, Prof., cited, 450. + + Silurian, name explained, 350. + period, plutonic rocks of, 457. + rocks, table of, 351. + strata, mineral character of, 360. + strata of United States, 359. + strata, thickness of, 358. + strata, reptile in, Postscript, vii. + volcanic rocks, 434. + + Simpson, Mr., on ice islands, 129. + + Sivatherium described, 173. + + Skaptar Jokul, eruption of, 399. + + Skye, rocks of, 383. 456. + basaltic columns in, 385. + dikes in Isle of, 380. + sandstone in, 36. + + Slaty cleavage, 468. + + Slickensides, term defined, 61. + + Smith, Mr., of Jordan Hill, on Pleistocene, 134. + on shells near Lisbon, 171. + + Snags, fossil, 320. + + Snakes' eggs, fossil at Tonna near Gotha, 120. + + Solenhofen, lithographic stone of, 260. + + Solfatara, decomposition of rocks in the, 477. + + Somma, 404. + lava at, 380. + + Sopwith, Mr. T., models by, 57. + + Sortino, cave in valley of, 154. + + South Devon and Cornwall, old red of, 315. + + South Downs, view of, 245. + + Sowerby, Mr. G., cited, 162. + + Spatangus, figure of, 23. + + Spezia, gulf of, calcareous rocks in, 482. + + Spitzbergen, glaciers of, 136. + + Sponges, figures of, in chalk, 213. + + Spongilla of Lamarck, in tripoli, 25. + + Springs, mineral. See Mineral Springs, 490. + + Staffa, basaltic columns in, 385. + + Steno on classification of rocks, 90. + + Stigmaria, 310. 315. + in fossil forest, Nova Scotia, 322. + + Stirling Castle, rock of, altered by dike, 383. + + Stokes, Mr., on petrifaction, 43. + + Stonesfield slate, 266. + + Stonesfield, fossil mammalia, 268. and Postscript, xviii. + + Storton Hill, footprints at, 291. + + Strata, term defined, 2. + arrangement of, determined by fossils, 21. 22. + consolidation of, 34. + curved and vertical, 47. 58. + elevation of, above the sea, 44. + fossiliferous, tabular view of, 361. + horizontality of, 15. 45. + metamorphic origin of, 467. + mineral composition of, 10. + outcrop of, 56. + tertiary classification of, 134. + + Stratification, forms of, 13. 16. 47. + unconformable, 59. + + Strickland, Mr., on new red sandstone, 290. + + Strike, term explained, 53. + + Stromboli, lava of, 450. + + Studer, M., on Swiss Alps, 484. + on boulders of Jura, 143. + + Stutchbury, Mr., cited, 306. + + Subapennine strata, 105. 166. + + Subsidence in drift period, 135. + + Suffolk crag, 162. + + Sullivan, Capt., chart of Falkland Islands, 88. + + Superior, Lake, marl in, 36. + + Superposition of aqueous deposits, 96. + of volcanic rocks, test of age, 327. + + Supracretaceous, term explained, 103. + + Sussex marble, 228. + + Swansea, coal measures near, 309. + valley stems of _Sigillaria_, 317. + + Sydney coal field, Cape Breton, 324. + + Syenite, 440. + + Syenitic granite, 440. + greenstone, 372. + + Synclinal line, term defined, 48. + + + T. + + Table Mountain, strata horizontal, 45. + Mountain, granite veins in, 443. + + Talcose granite, 440. + + Tartaret, Puy de, cone of, 425. + + Teeth of fossil mammalia, figures of, 160. + + Teredina, fossil wood bored by, 24. + + Teredo navalis boring wood, 23. + + Terra del Fuego, 139. + _Fucus giganteus_ in, 217. + + Tertiary, term explained, 104. + strata, tabular view of, 362. + + Touraine, faluns of, 168. + + Trachyte, 372. + of Hungary, 442. + + Trachytic rocks, older than basalt, 400. + + Transition, term explained, 92. + + Trap, term explained, 366. + dike in Fifeshire, 434. + globular structure of, 387. + intrusion of, between strata, 384. + various ages of, 432. 434. + passage of granite into, 441. + in Radnorshire, 435. + rocks, relation to lava, 387. + rocks, lithological character of, 400. + in Lower Eifel, 420. + + Trappean rocks, 91. + + Trap-tuff, 374. + + Tertiary deposits, 171. 177. 178. + + Texas, chalk in, 225. + + Thames valley, freshwater deposits in, 146. + + Thecodont Saurians, 306. + Saurians, age of, Postscript, xv. + + Thirria, M., on oolitic group in France, 283. + + Thurmann, M., cited, 55. 252. 266. + + _Thuja occidentalis_ in stomach of mastodon, 138. + + Till, term explained, 121. + origin of, 123. + + Tilestone, 351. + + Tilgate Forest, remains in, 229. + + Tin, veins of, in Cornwall, 490. 498. + + Tiverton trap, porphyry near, 432. + + Travertin, how deposited, 34. + + Tree ferns in Permian formation, 307. + + Trias, or new red sandstone, 286. 289. and Postsc., xiii. + in Cheshire and Lancashire, 290. 295. + + Trilobite in Devonian strata, 348. + + Trilobites of Lower Silurian, 357. + + Trimmer, Mr., on sand-galls, 82. + on shells in drift near Menai Straits, 130. + + Tripoli composed of infusoria, 24. + + Tuff, volcanic, and trap, 6. 374. + + Tuffs on Wrekin and Caer Caradoc, 434. + + Tuomey, Mr., cited, 208. + + Turner, Dr., cited, 41. 42. + + Tuscany, volcanic rocks of, 408. + + Tynedale fault, 64. + + Tynemouth Cliff, limestone at, 302. + + + U. + + Uddevalla, shells of, compared with those near Naples, 108. + + Underlying, term applied to granite, 8. + + United States, coal field of, 326. + cretaceous formation in, 224. + Devonian strata in, 349. + Eocene strata in, 206. + older Pliocene and Miocene formations in, 171. + oolite and lias of, 284. + Silurian strata of, 359. + + Upsala, strata containing Baltic shells near, 124. + + + V. + + Val di Noto, composition of, 407. + igneous rocks of, 389. + inland cliffs in, 76. + + Valleys, origin of, 70. + transverse of Weald, 244. + + Valorsine granite, 445. + + Veins, mineral. See Mineral Veins, 488. + + Veinstones in parallel layers, 493. + + Velay, volcanos of, 428. + + Venetz, M., on Alpine glaciers, 140. + + Verneuil, M. de, on Devonian Flora, 350. + on horizontal strata in Russia, 124. + on the old red sandstone in Russia, 348. + on _Pentamerus Knightii_, 353. + on Permian flora, 305. + + Vesuvius, eruption of, 405. + + Vicenza, basaltic columns near, 386. + + Vidal, Capt., survey by, 393. + + Virginia, U. S., fossil shells in, 172. + + Virlet, M., on corrosion of rocks by gases, 477. + on geology of Morea, 431. + on inland cliffs, 73. + + Volcanic mountains, form of, 5. 390. + dikes, 378. + + Volcanic rocks, age of, 397. + described, 5. 385. + analysis of minerals in, 377. + Cambrian, 435. + composition and nomenclature, 368. + of Hungary, 421. + post-pliocene period, 401. + test of age of, 400. + Silurian, 434. + + Volcanic tuff, 374. + + Volcanos of Auvergne, 422. + extinct, 408. 420. 422. + newer, of Eifel, 418. + in Spain, age of, 414. + round Olot in Catalonia, 410. + + Von Buch, Baron, cited, 373. 456. 457. + on boulders of Jura, 143. + on Canary Islands, 392. + on Cystideæ, 358. + on land rising, 45. + + Von Dechen, M., on granite veins in Cornwall, 445. + Oeynhausen, M., cited, 415. + + + W. + + Waller quoted, 93. + + Warren, Dr. J. C., on skeleton of _Mastodon giganteus_, 138. + + Waterhouse, Mr., cited, 188. 269. + on triassic mammifer, Postscript, xiv. + + Watt, Mr. G., experiments on fused rocks, 406. 475. + + Weald clay, 227. + + Weald valley, denuded at what period, 254. + + Wealden, term explained, 225. 226. + the fracture and upheaval of, 251. + extent of formation, 236. + period, changes during, 235. + + Wealden, plants and animals of, 229. 236. + + Webster, Mr. T., cited, 105. 231. 233. + + Wellington Valley, caves in, 156. + + Wener Lake, horizontal Silurian strata of, 45. + + Wenlock formation, 354. + + Werner on classification of rocks, 90. + on mineral veins, 488. + on volcanic rocks, 369. + + Westerwald, igneous rocks of, 417. + + Westwood, Mr., on beetles in lias, 282. + + Whin-Sill, intrusion of trap between strata, 384. + + White chalk, 211. + + White mountains, granite vein in, 450. + + Wigham, Mr., on fossils near Norwich, 149. + + Wolverhampton, fossil forest near, 319. + + Wood, Mr. Searles, on fossils of crag, 162. + on fossils of Isle of Wight, 198. + on number of shells in crag, 149. + on cetacea of crag, 166. + cited, 170. 177. + + Woodward, Mr., on mammoth bones, Norfolk, 147. + + Wrekin, trap of, 70. + + Wyman, Dr., cited, 208. + + + Z. + + _Zamia_, at Lyme Regis, 282. + + _Zamia spiralis_, figure of, 233. + + Zechstein, 306. + + _Zeuglodon cetoides_, 207. and Postscript, xxi. + + + LONDON: + SPOTTISWOODES and SHAW, + New-street-Square. + + + + ALBEMARLE STREET, + _July 5, 1851_. + + + + + MR. MURRAY'S + + =List of Recent Works= + + * * * * * + + + HISTORY OF THE ROMAN STATE; + + FROM 1815-1850. BY LUGIA CARLO FARINI. + + TRANSLATED FROM THE ITALIAN + BY THE RIGHT HON. W. E. GLADSTONE, M.P. + + 2 Vols. 8vo. 24_s._ + + * * * * * + + + THE EXPOSITION OF 1851; + +OR, VIEWS OF THE INDUSTRY, THE SCIENCE, AND THE GOVERNMENT OF ENGLAND. + + BY CHARLES BABBAGE, ESQ., + Author of the "Economy of Manufactures and Machinery." + + _Second Edition_, with an Appendix. 8vo. 6_s._ 6_d._ + + * * * * * + + + THE DOVECOTE AND THE AVIARY; + +OR, THE NATURAL HISTORY OF PIGEONS AND OTHER DOMESTIC BIRDS, WITH HINTS + FOR THEIR MANAGEMENT. + + BY THE REV. EDMUND SAUL DIXON, M.A., + Author of "Ornamental and Domestic Poultry." + + With Numerous Woodcuts. Fcap. 8vo. 7_s_. 6_d_. + + * * * * * + + + MEMOIR OF THE LIFE OF THE LATE BISHOP STANLEY. + + PREFIXED TO A SELECTION FROM HIS ADDRESSES AND CHARGES. + + BY THE REV. 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Mr. Stanley brings out into +strong relief the more attractive parts of his father's character, and +suggests the best defence--namely, the consistent uprightness and perfect +sincerity of his motives--for the more questionable policy, on some +memorable occasions, of the bishop." + + _Morning Chronicle._ + + * * * * * + + + A PASTORAL LETTER ON THE STATE OF THE CHURCH, + + BY HENRY LORD BISHOP OF EXETER. + + _Eighth Edition._ 8vo. 4_s._ + + * * * * * + + + THE ACTS OF THE SYNOD OF EXETER. + + HOLDEN IN THE CHAPTER HOUSE OF THE CATHEDRAL CHURCH OF EXETER. + + ON JUNE 25, 26, and 27, 1851. + + 8vo. + :: _The Sermon may be had separately, price 1s._ + + * * * * * + + + THE EVANGELICAL AND TRACTARIAN MOVEMENTS. + + BY ARCHDEACON WILBERFORCE. + +(A Charge delivered to and published by Request of the Clergy.) + 8vo. 1_s._ + + * * * * * + + + A HISTORY OF ERASTIANISM. + + BY ARCHDEACON WILBERFORCE. + + 16mo. 3_s._ + + * * * * * + + + CATHOLIC SAFEGUARDS + + AGAINST THE ERRORS, CORRUPTIONS, AND NOVELTIES OF THE CHURCH OF ROME. + + BEING DISCOURSES AND TRACTS SELECTED FROM THE WORKS OF EMINENT DIVINES OF + THE CHURCH OF ENGLAND WHO LIVED DURING THE 17TH CENTURY; WITH PREFACE, + RECORDS, AND A CAREFULLY COMPILED INDEX. + + BY REV. JAMES BROGDEN, M.A. + + 3 Vols. 8vo. 36_s._ + + * * * * * + + + RECORDS OF THE SUPREMACY OF THE CROWN, + + AND THE CIVIL AND RELIGIOUS LIBERTIES OF THE PEOPLE OF ENGLAND. + + BY REV. JAMES BROGDEN. + + 8vo. 4_s._ + + * * * * * + + + HORÆ ÆGYPTIACÆ; + + OR, THE CHRONOLOGY OF ANCIENT EGYPT. + + DISCOVERED FROM ASTRONOMICAL AND HIEROGLYPHIC RECORDS UPON + ITS MONUMENTS INCLUDING MANY DATES FOUND IN COEVAL INSCRIPTIONS. + + BY REGINALD STUART POOLE, ESQ. + + With Plates. 8vo. 10_s._ 6_d._ + +"The substance of Mr. Poole's valuable work appeared originally in a +series of papers in this journal. Since their publication the author has +devoted further time and attention to the subject; and it may safely be +asserted that in their present amended and enlarged form, they are among +the most important contributions that have yet been made to the study of +Egyptian Chronology and history. We are indebted for the publication of +the present valuable work to the liberality of the Duke of +Northumberland, whose warm and generous support of literature and art +deserves our grateful acknowledgments." + + _Literary Gazette._ + + * * * * * + + + LAVENGRO; + THE SCHOLAR--THE GIPSY--AND THE PRIEST. + + BY GEORGE BORROW, ESQ. + + Author of "The Bible in Spain," "The Gipsies of Spain," &c. &c. + With a Portrait. 3 Vols. Post 8vo. 30_s._ + +"We trust our extracts have exhibited enough of one at least of the many +aspects of 'Lavengro' to convince the reader that neither is it a work to +be read cursorily, nor to be handled easily, by any of the silver-fork +school of critics. These volumes are indeed replete with life, with earnest +sympathy for all genuine workers, with profound insight into the wants and +wishes of the poor and uneducated, and a lofty disdain of the conventional +'shams' and pretensions which fetter the spirits or impede the energies of +mankind. Nor is a feeling for the beautiful less conspicuous in its pages. +A quiet market-town, environed by green meadows or bosomed in tufted trees; +an old mercantile and ecclesiastical city, with a history stretching from +the times of the Cæsars to the times of George III.; the treeless plain, +the broad river, the holt, the dingle, the blacksmith's forge, are all in +their turn sketched freely and vividly by Mr. Borrow's pencil. In his +portraitures of ruder life he is unsurpassed; a dog-fight, a prize-fight, +an ale-house kitchen, Greenwich Fair, a savage group of wandering tinkers, +are delineated in words as Wilkie or Hogarth might have depicted them in +colours. We are embarrassed by the riches spread before us. + +"We have not touched upon the gipsy scenes in 'Lavengro' because in any +work of Mr. Borrow's these will naturally be the first to draw the reader's +attention. Neither have we aimed at abridging or forestalling any portions +of a book which has a panoramic unity of its own, and of which scarcely a +page is without its proper interest. If we have succeeded in persuading our +readers to regard Mr. Borrow as partly an historian and partly as a poet, +as well as to look for more in his volumes than mere excitement or +amusement, our purpose is attained, and we may securely commend him to the +goodly company he will find therein. 'Lavengro,' however, is not concluded; +a fourth volume will explain and gather up much of what is now somewhat +obscure and fragmentary, and impart a more definite character to the +philological and physiological hints comprised in those now before us. +Enough, indeed, and more than enough, is written to prove that the author +possesses, in no ordinary measure, 'the vision and the faculty divine' for +discerning and discriminating what is noble in man and what is beautiful in +nature. We trust Mr. Borrow will speedily bring forth the remaining acts of +his 'dream of adventure,' and with good heart and hope pursue his way +rejoicing, regardless of the misconceptions or misrepresentations of +critics who judge through a mist of conventionalities, and who themselves, +whether travelled or untravelled, have not, like Lavengro, grappled with +the deeper thoughts and veracities of human life."--_Tait's Magazine._ + + * * * * * + + + ENGLAND IN THE NINETEENTH CENTURY: + + POLITICAL, SOCIAL, AND INDUSTRIAL. + + BY WILLIAM JOHNSTON, ESQ. + + 2 Vols. Post 8vo. 18_s._ + +"This book is a somewhat undigested mass of valuable matter, interspersed +occasionally with reflections of much interest and observations of +considerable originality. The author is unquestionably a man of talent; he +writes with vigour and smartness; he has taken pains in the collection of +most of his materials; and his statistics are arranged with great care and +managed with unusual skill. In this point he is much superior to his +prototype and apparent master, Mr. Alison." + +"Mr. Johnston's work is readable and well-written, abounding with +information of many kinds."--_Edinburgh Review._ + + * * * * * + + + THE SAXON IN IRELAND: + + BEING NOTES OF THE RAMBLES OF AN ENGLISHMAN IN THE WEST OF IRELAND + IN SEARCH OF A SETTLEMENT. + + With Map. Post 8vo. 9_s._ 6_d._ + +"A valuable testimony to the capabilities of Ireland."--_Observer._ + +"Let the intending emigrant devote a few hours to the perusal of this +volume. The work possesses deeper interest than even could be claimed for +it from its fascinating descriptions."--_Illustrated News._ + + * * * * * + + + SLEEP AND DREAMS: + + TWO LECTURES DELIVERED AT THE BRISTOL LITERARY AND PHILOSOPHICAL + INSTITUTION. + + BY JOHN ADDINGTON SYMONDS, M.D., + + Consulting Physician to the Bristol General Hospital. + + 8vo. 2_s._ 6_d._ + + * * * * * + + + THE PALACES OF + NINEVEH AND PERSEPOLIS RESTORED. + + AN ESSAY ON ANCIENT ASSYRIAN AND PERSIAN ARCHITECTURE. + + BY JAMES FERGUSSON ESQ., + + With Woodcuts. 8vo. 16_s._ + +"This book contains many things of general interest relating to one of the +most wonderful discoveries that has occurred in the history of the world. +Mr. Fergusson writes very dispassionately. What he has said deserves +serious consideration."--_Gentleman's Magazine._ + + * * * * * + + + SHALL WE KEEP THE CRYSTAL PALACE + + AND HAVE RIDING AND WALKING IN ALL WEATHERS, AMONG FLOWERS, + SCULPTURE, AND FOUNTAINS? + + BY DENARIUS. + + 8vo. 6_d._ + + * * * * * + + + MEMOIRS OF ROBERT PLUMER WARD. + + WITH HIS CORRESPONDENCE, DIARIES, AND LITERARY REMAINS. + + BY THE HON. EDMUND PHIPPS. + + With Portrait. 2 Vols. 8vo. 28_s._ + +"The most valuable portions of Mr. Ward's diary are its illustrations of +the character of the Duke of Wellington. The great soldier, then in the +flush of his military triumph, was also in the prime of his power and +activity; and Mr. Ward gives us an insight into his business habits, his +method of arguing public questions, his ready resource and never-tiring +energy, which possesses occasionally a striking interest."--_Examiner._ + + * * * * * + + + THE MILITARY EVENTS IN ITALY, 1848-9. + + TRANSLATED FROM THE GERMAN. + BY THE RIGHT HON. THE EARL OF ELLESMERE. + + With a Map. Post 8vo. 9_s._ + +"Military history is, as the Earl of Ellesmere declares, a rare article in +English literature; and, therefore, he thought that the most authentic +extant narrative of the operations implied in the title page of the present +book, written by an impartial Swiss, would not be an unwelcome addition to +the British library. His lordship has judged rightly; the work of which he +has presented a version is a worthy labour, and the events to which it +relates are of the last importance. It is written with judgment, and has +been translated with care."--_Morning Chronicle._ + + * * * * * + + + A TRANSPORT VOYAGE TO THE MAURITIUS, + + BY WAY OF THE CAPE OF GOOD HOPE AND ST. HELENA. + + BY THE AUTHOR OF "PADDIANA." + + Post 8vo. 9_s._ 6_d._ + +"This book reminds us of one of those pleasant fellows, whom one sometimes +meets with in company, who has an anecdote or a story ready à propos of +everything, whose fund of amusing tales is inexhaustible, and who rattling +on from one thing to another, will keep a whole table in a roar, or a whole +drawing-room in high glee. Even such is our author. He gossips on and on, +telling now of one adventure, and then of another; his volume is a perfect +chaos of racy reminiscences graphically told."--_John Bull._ + + * * * * * + + + ADMIRALTY MANUAL OF SCIENTIFIC ENQUIRY + + FOR THE USE OF OFFICERS AND TRAVELLERS IN GENERAL. + + BY PROFESSORS WHEWELL, AIRY, OWEN, SIR W. HOOKER, CAPT. BEECHEY, + J. R. HAMILTON, ESQ., SIR JOHN HERSCHEL, &c. + + EDITED BY SIR JOHN F. HERSCHEL, BART. + Second Edition. Post 8vo. 10_s._ 6_d._ + +:: _Published by Authority of the Lords Commissioners of the Admiralty._ + + * * * * * + + + LIVES OF THE CHIEF JUSTICES OF ENGLAND. + + FROM THE NORMAN CONQUEST TO THE DEATH OF LORD MANSFIELD. + + BY LORD CHIEF JUSTICE CAMPBELL. + + 2 Vols. 8vo. 30_s._ + +"There is, indeed, in Lord Campbell's works much instruction; his subjects +have been so happily selected, that it was scarcely possible that there +should not be. An eminent lawyer and statesman could not write the lives of +great statesmen and lawyers without interweaving curious information, and +suggesting valuable principles of judgment and useful practical maxims: but +it is not for these that his works will be read. Their principal merit is +their easy animated flow of interesting narrative. No one possesses better +than Lord Campbell the art of telling a story: of passing over what is +commonplace; of merely suggesting what may be inferred; of explaining what +is obscure; and of placing in strong light the details of what is +interesting."--_Edinburgh Review._ + + * * * * * + + + THE FORTY-FIVE. + + BEING A NARRATIVE OF THE REBELLION IN SCOTLAND OF 1745; + + BY LORD MAHON. + + Post 8vo. 3_s._ + +"This is a very comprehensive and lively sketch of the famous 'Rebellion' +so vividly remembered, even after the lapse of a century, by the people of +Scotland. The incidents of that unfortunate invasion from first to last, +from the landing of Charles (July 25th) in Borrodale, with the 'seven men +of Moidart,' to the fatal battle of Culloden (16th April, 1746), are +minutely and faithfully recorded; but we have no doubt the reader will be +most and mainly interested in the personal history and adventures of the +Pretender himself. The character of the Prince is admirably drawn, and +generously vindicated from the calumnies heaped upon him by his adversaries +after his fall. It will perhaps surprise some to learn, that he was so +illiterate as scarcely to be master of the most common elements of +education. 'His letters,' says Lord Mahon, 'which I have seen among the +Stuart papers, are written in a large, rude, rambling hand, like a +schoolboy's. In spelling they are still more deficient.' We recommend Lord +Mahon's narrative as a very agreeable sketch of a stirring and eventful +period."--_Edinburgh Advertiser._ + + * * * * * + + + A HISTORY OF GREECE. + + FROM THE EARLIEST PERIOD TO THE END OF THE PELOPONNESIAN WAR. + + BY GEORGE GROTE, ESQ. + + Vols. I.-VIII. With Maps. 8vo. 16_s._ each. + _The Work may be obtained in Portions, as it was published_:-- + + VOLS. I.-II. + + LEGENDARY GREECE. + GRECIAN HISTORY TO THE REIGN OF PEISISTRATUS AT ATHENS. + + VOLS. III.-IV. + + HISTORY OF EARLY ATHENS, AND THE LEGISLATION OF SOLON. + GRECIAN COLONIES. + VIEW OF THE CONTEMPORARY NATIONS SURROUNDING GREECE. + GRECIAN HISTORY DOWN TO THE FIRST PERSIAN INVASION, AND THE BATTLE OF + MARATHON. + + VOLS. V.-VI. + + PERSIAN WAR AND INVASION OF GREECE BY XERXES. + PERIOD BETWEEN THE PERSIAN AND THE PELOPONNESIAN WARS. + PELOPONNESIAN WAR DOWN TO THE EXPEDITION OF THE ATHENIANS AGAINST + SYRACUSE. + + VOLS. VII.-VIII. + + THE PEACE OF NIKIAS DOWN TO THE BATTLE OF KNIDUS [B.C. 421 TO 394.] + SOCRATES AND THE SOPHISTS. + + * * * * * + + + KUGLER'S HANDBOOK ILLUSTRATED. + + THE SCHOOLS OF PAINTING IN ITALY. + FROM THE EARLIEST TIMES. + + TRANSLATED FROM THE GERMAN BY A LADY, AND EDITED WITH NOTES + + BY SIR CHARLES LOCK EASTLAKE, + President of the Royal Academy. + + _A New Edition._ 2 Vols. Post 8vo. 24_s._ + +"We cannot leave this subject (_Christian Art, its present state and its +prospects_), without reverting to Sir C. Eastlake's edition of Kugler's +Handbook of Painting, not for the sake of reviewing it,--for it is a work +now of established reputation,--but for the purpose of recommending it as +being upon the whole by far the best manual we are acquainted with, for +every one who, without the opportunity of foreign and particularly Italian +travel, desires to make a real study of art. Its method, its chronological +arrangement, and its generally judicious criticism, make it most +instructive to a learner. We may add that the present edition is enlarged +just where the former one needed enlargement, and the Handbook is now far +more satisfactory as to the early religious schools than it was before. The +edition is beautifully got up, and so profusely and judiciously illustrated +by one hundred woodcuts drawn by Scharf, that it would be next to +impossible to speak too highly in its praise, even were its matter less +valuable and important than it is."--_The Ecclesiastic._ + + * * * * * + + + CHRISTIANITY IN CEYLON: + + ITS INTRODUCTION AND PROGRESS UNDER THE PORTUGUESE, DUTCH, + BRITISH, AND AMERICAN MISSIONS. + + BY SIR JAMES EMERSON TENNENT, K.C.S., LL.D. + + With Illustrations. 8vo. 14_s._ + +"To those who take either a religious or a philosophical interest in the +subject, Sir Emerson Tennent's volume may be safely recommended, as a +clear, succinct, sensible, and flowing account. The work also possesses a +living animation arising from the author's knowledge of the country and the +people."--_Spectator._ + + * * * * * + + + THE LEXINGTON PAPERS. + + THE COURTS OF LONDON AND VIENNA + IN THE 17TH CENTURY. + + EXTRACTED FROM THE PRIVATE AND OFFICIAL CORRESPONDENCE OF + LORD LEXINGTON, WHILE BRITISH MINISTER AT VIENNA, 1694-98. + + EDITED BY THE HON. H. MANNERS SUTTON. + + 8vo. 14_s._ + + * * * * * + + + THE LAW AND PRACTICE OF NAVAL COURTS-MARTIAL. + + BY WILLIAM HICKMAN, R.N., + Late Secretary to Commodore Sir Charles Hotham, K.C.B. + + 8vo. 10_s._ 6_d._ + + * * * * * + + + A MANUAL OF ELEMENTARY GEOLOGY; + + OR, THE ANCIENT CHANGES OF THE EARTH AND ITS INHABITANTS, + AS ILLUSTRATED BY ITS GEOLOGICAL MONUMENTS. + + BY SIR CHARLES LYELL, F.R.S., P.G.S. + + Third Edition, thoroughly revised, + and illustrated with 520 Woodcuts. + 8vo. 12_s._ + +"The production of one of our most eminent geologists in an age of many. +Though styled a 'third edition,' it is in reality a new book. This could +not be otherwise if the task were well done; for the science of which Sir +Charles Lyell treats is assuming new aspects every year. It is continually +advancing and ever growing. As it advances, its steps become firmer and +surer; as it grows, its framework becomes more compact, and its +organization more perfect. They who take up the hammer to follow it must +toil with unflagging tread to keep pace with its onward progress. If they +lag behind, they can scarcely hope to overtake. None among its votaries has +marked each movement more minutely, or weighed its value and purpose more +judiciously, than the distinguished author of this Manual. He has indeed +done his task well, and both the beginner and the experienced investigator +will find his book an invaluable guide and companion."--_Literary Gazette._ + + * * * * * + + + COMMENTARIES ON + THE WAR IN RUSSIA AND GERMANY OF 1813-14. + + BY COLONEL THE HON. GEORGE CATHCART, + Deputy-Lieutenant of the Tower of London. + + With Plans. 8vo. 14_s._ + +"As a Treatise on the Science of War, these Commentaries ought to find +their way into the hands of every soldier. In them is to be found an +accurate record of events of which no military man should be +ignorant."--_Morning Chronicle._ + + * * * * * + + + MODERN DOMESTIC COOKERY. + + FOUNDED UPON PRINCIPLES OF ECONOMY AND PRACTICAL KNOWLEDGE. + AND ADAPTED FOR THE USE OF PRIVATE FAMILIES. + + With 100 Woodcuts. Post 8vo. 6_s._ + +"The advanced state of cookery having rendered Mrs. Rundell's work +obsolete, the publisher has caused it to be remodelled and improved to +such an extent as to give it a claim to the title of an original +production. The receipts of the late Miss Emma Roberts have been revised +and added to the work; and it has had the advantage of being subjected +besides to the careful inspection of a 'professional gentleman'--Economy +combined with excellence--is the aim, end, and object which it cannot be +doubted will be obtained if its prescriptions are attended to. It is +fuller than the former _Domestic Cookery_, of which it is an improved +and amended edition--it is more simple and comprehensible in its +language; it contains several diagrams not to be found in its +predecessor; and it possesses various minor qualities, which increase +its value in a tenfold degree, and make it, to say the least, equal to +any other book of the kind in the English language."--_Observer._ + + + + + ALBEMARLE STREET, + _July 5, 1851_. + + + + + MR. MURRAY'S + + =List of Works in the Press.= + + * * * * * + + + Selections from the Despatches of the Duke of + Wellington. + + BY THE LATE COL. GURWOOD, C.B., K.C.T.S. + + A New Edition. One Volume. 8vo. + + * * * * * + + + History of England, from the Peace of Utrecht. + + VOLS. 5 & 6--THE FIRST YEARS OF THE AMERICAN WAR: 1763--1780. + + BY LORD MAHON, M.P. + + 2 Vols. 8vo. + + * * * * * + + + Lives of the Friends and Contemporaries of Lord + Chancellor Clarendon. + + ILLUSTRATIVE OF PORTRAITS IN HIS GALLERY; WITH AN ACCOUNT OF THE + ORIGIN OF THE COLLECTION; AND A DESCRIPTIVE CATALOGUE + OF THE PICTURES. + + BY LADY THERESA LEWIS. + + With Portraits. 2 Vols. 8vo. + + * * * * * + + + The Treasures of Art in Great Britain. + + BEING AN ACCOUNT OF THE CHIEF COLLECTIONS OF PAINTINGS, SCULPTURE, + MSS. MINIATURES, &c., &c., + + OBTAINED FROM PERSONAL INSPECTION DURING VISITS IN 1836 AND 1850. + + (BEING A REVISED AND GREATLY ENLARGED VERSION OF + "ART AND ARTISTS IN ENGLAND.") + + BY DR. WAAGEN, + Director of the Royal Gallery of Pictures at Berlin. + + 2 Vols. 8vo. + + * * * * * + + + The Grenville Papers; + + BEING + THE PRIVATE CORRESPONDENCE OF RICHARD GRENVILLE, EARL TEMPLE, K.G., + AND HIS BROTHER, THE RIGHT HONOURABLE GEORGE GRENVILLE, + THEIR FRIENDS AND CONTEMPORARIES, + + FORMERLY PRESERVED AT STOWE--NOW FOR THE FIRST TIME MADE PUBLIC. + +_Among the contents of this highly important accession to the History +of Great Britain in the middle of the Eighteenth Century, will be +found Letters from_ + + H. M. KING GEORGE THE THIRD. + + H. R. H. WILLIAM DUKE OF CUMBERLAND. + + DUKES OF:-- + NEWCASTLE. + DEVONSHIRE. + GRAFTON. + BEDFORD. + + MARQUESS:-- + GRANBY. + + EARLS:-- + BUTE. + TEMPLE. + SANDWICH. + EGREMONT. + HALIFAX. + HARDWICKE. + CHATHAM. + MANSFIELD. + NORTHINGTON. + SUFFOLK. + HILLSBOROUGH. + HERTFORD. + + LORDS:-- + LYTTLETON. + CAMDEN. + HOLLAND. + CLIVE. + GEORGE SACKVILLE. + ---- + MARSHAL CONWAY. + HORACE WALPOLE (EARL OF ORFORD). + EDMUND BURKE. + GEORGE GRENVILLE. + JOHN WILKES. + WILLIAM GERARD HAMILTON. + AUGUSTUS HERVEY. + MR. JENKINSON (first EARL OF LIVERPOOL). + MR. WHATELY. + MR. WEDDERBURN (EARL OF ROSLYN). + MR. CHARLES YORKE. + MR. HANS STANLEY. + MR. CHARLES TOWNSEND. + MR. CALCRAFT. + MR. RIGBY. + MR. KNOX. + MR. CHARLES LLOYD. + + + AND THE + + _AUTHOR OF THE LETTERS OF JUNIUS_. + + INCLUDING ALSO, + + Mr. Grenville's Diary of Political Events; + + PARTICULARLY DURING THE PERIOD OF HIS ADMINISTRATION AS FIRST LORD + OF THE TREASURY, FROM 1763 TO 1765. + + EDITED BY WILLIAM JAMES SMITH, ESQ. + + 8vo. + + * * * * * + + + Personal Narrative of an Englishman Domesticated + in Abyssinia. + + BY MANSFIELD PARKYNS, ESQ. + + With Illustrations. 8vo. + + * * * * * + + + Lives of the Three Devereux, Earls of Essex, + + FROM 1540 TO 1646. + + 1. THE EARL MARSHALL OF IRELAND.--2. THE FAVOURITE.--3. THE GENERAL OF + THE PARLIAMENT. + + FOUNDED UPON LETTERS AND DOCUMENTS CHIEFLY UNPUBLISHED. + + BY THE HON. CAPTAIN DEVEREUX, R.N. + + 2 Vols. 8vo. + + * * * * * + + + The Present State of the Republic of the Rio de la + Plata (Buenos Ayres). + + ITS GEOGRAPHY, RESOURCES, STATISTICS, COMMERCE, DEBT, ETC., DESCRIBED. + + WITH THE HISTORY OF THE CONQUEST OF THE COUNTRY BY THE SPANIARDS. + + BY SIR WOODBINE PARISH, F.R.S., K.C.H, F.G.S., + Formerly Her Majesty's Consul General and Chargé d' Affaires + at Buenos Ayres. + + With New Map and Illustrations. 8vo. + + * * * * * + + + Contrasts of Foreign and English Society; + + OR, RECORDS AND RECOLLECTIONS OF A RESIDENCE IN VARIOUS PARTS + OF THE CONTINENT AND ENGLAND. + + BY MRS. AUSTIN. + + 2 Vols. Post 8vo. + + * * * * * + + + The Hand; + + ITS MECHANISM AND ENDOWMENTS, AS EVINCING DESIGN. + + BY THE LATE SIR CHARLES BELL. + + _A New Edition._ Woodcuts. Post 8vo. + + * * * * * + + + Naval and Military Technological Dictionary. + + ENGLISH AND FRENCH.--FRENCH AND ENGLISH. + + FOR THE USE OF SOLDIERS, SAILORS, AND ENGINEERS. + + BY COLONEL BURN, Assistant Inspector of Artillery. + + Small 8vo. + + * * * * * + + + The Life and Reminiscences of Thomas Stothard, R.A. + + BY MRS. BRAY. + + With numerous Illustrations from his Chief Works, drawn on Wood by + G. SCHARF, Jun., and printed in a novel and beautiful style. + + With a Portrait. Small 4to. + + * * * * * + + + Life and Works of Alexander Pope. + + EDITED WITH NOTES. + + BY THE RIGHT HON. JOHN WILSON CROKER. + + Portraits. 4 vols. 8vo. + + * * * * * + + + Dictionary of Greek and Roman Geography. + + BY WILLIAM SMITH, LL.D. + + With an Historical Atlas. 8vo. + + * * * * * + + + A Church Dictionary. + + BY WALTER FARQUHAR HOOK, D.D., Vicar of Leeds. + + _Sixth Edition_, revised and enlarged. One Volume. 8vo. + +"In this edition, besides the addition of many new articles, all those +relating to important Doctrinal and Liturgical Subjects have been enlarged. +The authorities on which statements have been made, are given, with copious +extracts from the works of our Standard Divines. Special reference has been +made to the Romish Controversy. Attention has also been paid to the +subjects of Ecclesiastical and Civil Law, and to the Statute Law of England +in Church Matters."--_Extract from the Preface._ + + * * * * * + + + History of Ancient Pottery; + + EGYPTIAN, ASIATIC, GREEK, ROMAN, ETRUSCAN, AND CELTIC. + + BY SAMUEL BIRCH, F.S.A. + Assistant Keeper of the Antiquities in the British Museum. + + With Illustrations. 8vo. + + UNIFORM WITH "MARRYAT'S MODERN POTTERY AND PORCELAIN." + + * * * * * + + + A Sketch of Madeira in 1850. + + BY EDWARD VERNON HARCOURT. + + A HANDBOOK FOR THE USE OF TRAVELLERS OR INVALIDS VISITING THE ISLAND. + + With a Map and Woodcuts. Post 8vo. + + * * * * * + + + The History of Herodotus. + + A NEW ENGLISH VERSION. TRANSLATED FROM THE TEXT OF GAISFORD, AND EDITED + + BY REV. GEORGE RAWLINSON, M.A., Exeter College, Oxford. + ASSISTED BY + COLONEL RAWLINSON, C.B., AND SIR J. G. WILKINSON, F.R.S., + + WITH COPIOUS NOTES AND APPENDICES, ILLUSTRATING THE HISTORY AND + GEOGRAPHY OF HERODOTUS, FROM THE MOST RECENT SOURCES + OF INFORMATION, + + EMBODYING THE CHIEF RESULTS, HISTORICAL AND ETHNOGRAPHICAL, WHICH HAVE + BEEN ARRIVED AT IN THE PROGRESS OF CUNEIFORM AND + HIEROGLYPHICAL DISCOVERY. + + 4 Vols. 8vo. + +The translation itself has been undertaken from a conviction of the +entire inadequacy of any existing version to the wants of the time. The +gross unfaithfulness of Beloe, and the extreme unpleasantness of his +style, render his translation completely insufficient in an age which +dislikes affectation and requires accuracy; while the only other +complete English versions which exist are at once too close to the +original to be perused with any pleasure by the general reader, and also +defective in respect of scholarship. + + * * * * * + + + A Treatise on Naval Gunnery, + + FOR THE USE OF OFFICERS AND FOR THE TRAINING OF SEAMEN GUNNERS. + + WITH DESCRIPTIONS OF THE GUNS INTRODUCED SINCE THE LATE WAR. + + BY LIEUT.-GEN. SIR HOWARD DOUGLAS, BART., G.C.B. + + _Third Edition_, revised. Plates. 8vo. + + * * * * * + + + Considerations on Steam Warfare and Naval Shell-Firing; + + BY LIEUT.-GEN. SIR HOWARD DOUGLAS, BART. + + 8vo. + + * * * * * + + + Letters and Journals of General Sir Hudson Lowe, + + REVEALING THE TRUE HISTORY OF NAPOLEON AT ST. 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Post 8vo. + + * * * * * + + + BRADBURY AND EVANS, PRINTERS, WHITEFRIARS. + + + + +Transcriber's Notes: + +Passages in italics are indicated by _underscore_. +Passages in fracture style are indicated by =fracture=. +All Greek has been transliterated and placed between +Greek+. + +Illustrations have been moved from the middle of a paragraph to the +closest paragraph break. + +In the footnote 336-A (page 336) lib. or liv. might be printed wrong. + +On page 185 a footnote anchor was added to fig. 160 185-A. + +On page 215 an potential anchor for footnote 215-A was guessed and +added. + +On page 245 an anchor for footnote 245-A was added. + +On page 280 a footnote anchor 280-B was added. + +Other than the corrections listed below, printer's inconsistencies in +spelling, punctuation, hyphenation, and ligature usage have been retained. + +The use of capital letters in names, scientific classifications, locations, +and time periods/eras is not consistent in this book, they have been kept +as printed and only changed when an obvious error occurred. + +The system of abbreviations and punctuation in citations and figure +captions can vary, the text has been kept as printed and only changed when +an obvious error occurred. + +The punctuation in the index was inconsistent, all commas in listings for +page numbers have been changed into full stops, they are not specially +mentioned/marked in the list of changes. The alphabetic order in the index +is sometimes inconsistent but has been kept as printed. + +Palæomæryx (page 178) is known in the literature by Paleomeryx +(http://www.paleodatabase.org) as well as Palaeomeryx. + +Palæoniscus is known in the literature as Palaeoniscus. + +Inoceramus Cuvieri is today known as Inoceramus cuvieri (ref: Cretaceous +Fossils of North America). + +Different spelling of Ashby de la Zouch (text) and Ashby-de-la-Zouch +(index) was retained. + +Older or unusual forms of spelling of some German and French towns and +locations have been retained (e.g. Bertrich-Baden--Bad Bertrich, +Roderberg--Rodderberg, Gemunder Maar--Gemünder Maar, Boulade--Boulaide, +Pont Gibaud--Pontgibaud, Saarbrück--Saarbrücken). + +The following words have been retained in both versions: + + Agas. and Agass. + brachiopod, brachiopods and brachiopoda (as well as with capital + letters or lower case) + Bunter Sandstein and Bunter-Sandstein (as well as various combinations + with Bunter, bunter, sandstein, Sandstein) + Cheirotherium and Chirotherium as cheirotherian and chirotherian + Didelphis and Didelphys + dike/s and dyke/s + foot-print/s and footprint/s + foot-marks and footmarks + gault and Gault + G/grauwacke and G/grauwacké and their English translations (greywacke) + greensand and Greensand as well as their variations + Holoptichius (e.g. Lyell) and Holoptychius (general usage) + Ichthyolites and Icthyolites + iron-stones and ironstones + jaw-bone and jawbone + Keuper and keuper + Lias and lias + Liége and Liege + Muschelkalk and muschelkalk + non-fossiliferous and nonfossiliferous + Old Red Sandstone and old red sandstone with all variations + P/palæo** and P/paleo** with all variations from paleontological to + paleozoic + Pozzolana and Pozzuolana (recent form) + primæval and primeval + quâquâversal and qua-quaversal + Rhinoceros tichorhinus and Rhinoceros tichorinus + scoria and scoriæ + Sénonien and Senonien + tilestone/s and T/tile-stone/s + +The following misprints have been corrected: + + changed "to recognise rocks" into "to recognize rocks" page vi + changed "a fresh-water or" into "a freshwater or" page viii + changed "belong to gasterodous" into "belong to gasteropodous" page x + changed "Ova in a carbonised state." into "Ova in a carbonized state." + page xi (fig. 523a) + changed "Würtembergisch. Naturwissen Jahreshefte" into "Würtembergisch. + Naturwissen. Jahreshefte" footnote xiii-A + changed "by Herman von Meyer" into "by Herman von Meyer." page xiv + (fig. 530) + changed "near Stuttgart, Wurtemberg." into "near Stuttgart, + Würtemberg." page xiv + changed "is characterised by" into "is characterized by" page xvi + changed "genus Sauricthys, Hybodus," into "genus Saurichthys, Hybodus," + page xv + changed "Sauricthys Mougeotii, is" into "Saurichthys Mougeotii, is" + page xv + changed "in the Quader Sand-stein and" into "in the Quadersandstein + and" page xvi + changed "of organisation in fossils" into "of organization in fossils" + page xix + changed "or to Plerodactyles" into "or to Pterodactyles" page xix + changed "class Aves have hither to" into "class Aves have hitherto" + page xix + changed "bored by teredina" into "bored by Teredina" page xxiii + changed "near St. Andrew's" into "near St. Andrews" page xxix + changed "Sub-marine lava" and into "Submarine lava and" page xxix + changed "Granite of Dartmore altering" into "Granite of Dartmoor + altering" page xxx + changed "Concluding remarks 489" into "Concluding remarks 488" page xxxi + changed "occasionally characterised" by into "occasionally + characterized" by page 3 + changed "are all characterised" into "are all characterized" page 5 + changed "Loire, and Ardêche," into "Loire, and Ardèche," page 5 + changed "Giants' Causeway, called" into "Giant's Causeway, called" page 6 + changed "cooled and crystallised," into "cooled and crystallized," page 7 + changed "by Dr. Mac Culloch" into "by Dr. MacCulloch" page 8 + changed "afterwards super-imposed, and" into "afterwards superimposed, + and" page 9 + changed "causes, while super-imposed" into "causes, while superimposed" + page 9 + changed "(Green-sand formation.)" into "(Greensand formation.)" page 16 + changed "annexed fig. (7.)," into "annexed fig. 7.," page 18 + changed "(Green-sand formation?)" into "(Greensand formation?)" page 18 + changed "bored by teredina" into "bored by Teredina" page 21 + changed "great bed of tripoli, Bilin." into "great bed of Tripoli, + Bilin." page 25 (figs. 19/20) + changed figure number figure "34" into figure "33" page 29 + changed "information from icthyolites" into "information from + ichthyolites" page 32 + changed "confined to vein-stones." into "confined to veinstones." page 34 + changed "the drying and skrinking" into "the drying and shrinking" + page 63 + changed "conglomerate, N. 2. clay," into" conglomerate, No. 2. clay," + page 67 + changed "described by Dr. Macculloch," into "described by Dr. + MacCulloch," page 67 + changed "of Ross-shire. (Macculloch.)" into "of Ross-shire. + (MacCulloch.)" page 67 (fig. 90) + changed "Dax, near Bourdeaux" into "Dax, near Bordeaux" page 72 + changed "indicate the intermittance" into "indicate the intermittence" + page 74 + changed figure number "96" to figure "93" page 75 + changed "Modica, precipitious" into "Modica, precipitous" page 77 + changed "them by Dr. Macculloch," into "them by Dr. MacCulloch," page 86 + changed "Dr. Macculloch and" into "Dr. MacCulloch and" page 87 + changed "have ever re-appeared" into "have ever reappeared" page 98 + changed "fossilisation of certain" into "fossilization of certain" + page 106 + changed "hills called Bruder Holz" into "hills called Bruderholz" + page 120 + changed "near Stuttgardt, in" into "near Stuttgart, in" page 120 + changed "stones have travelled" into "stones have travelled." page 121 + changed "already characterised by" into "already characterized by" + page 124 + changed "neighbourhood of Upsal," into "neighbourhood of Upsala," + page 124 + changed "Isles of sub-aerial glaciers." into "Isles of subaerial + glaciers." page 130 + added "BOULDER FORMATION--continued." to chapter heading page 131 + changed "its materials rearranged" into "its materials re-arranged" + page 136 + changed "chapters 7 and 8.," into "chapters 7. and 8.," page 139 + changed "to coexist in" into "to co-exist in" page 147 + changed "class of warm-blodded" into "class of warm-blooded" page 148 + changed "speces of deer" into "species of deer" page 154 + changed "skeletons of Magatherium," into "skeletons of Megatherium," + page 157 + changed "student to recognise the" into "student to recognize the" + page 159 + changed "b. nat. size of a and b." into "c. nat. size of a and b." + page 161 (fig. 141) + changed "overan are a" into "over an area" page 162 + changed "concretionary rearrangement of" into "concretionary + re-arrangement of" page 164 + changed "Faseicularia aurantium" into "Fascicularia aurantium" page 165 + (fig. 148) + changed "v. exterior." into "a. exterior." page 165 (fig. 148.) + changed "climates, such a" into "climates, such as" page 165 + changed "clayslate, and various" into "clay-slate, and various" page 169 + changed "from the Appenines" into "from the Apennines" page 168 + changed "17 per cent," into "17 per cent.," page 172 + changed "beds (Sables inferieurs" into "beds (Sables inférieurs" page 175 + changed "inferieurs et argiles" into "inférieurs et argiles" page 175 + changed "Upper Marine or Fontainbleau" into "Upper Marine or + Fontainebleau" page 177 + changed "M. de Koninck of Liége" into "M. De Koninck of Liége" page 178 + changed "or Caddice-fly" into "or Caddis-fly" page 185 + changed "lake of the Lemagne" into "lake of the Limagne" page 187 + changed "Bagshot and Brocklesham division" into "Bagshot and + Bracklesham division" page 190 + changed "Nome of them" into "None of them" page 192 + changed "genera Emys and Trionix." into "genera Emys and Trionyx." + page 192 + changed "Sables Moyens. divide" into "Sables Moyens, divide" page 193 + changed "of the English Eocenestrata," into "of the English Eocene + strata," page 197 + changed "Headen Hill, on" into "Headon Hill, on" page 197 + changed "Egerton has recognised" into "Egerton has recognized" page 198 + changed "brown and blueish gray" into "brown and blueish grey" page 200 + changed "beds Nos. 1, 2. are" into "beds Nos. 1, 2., are" page 208 + changed "places for mill-stones." into "places for millstones." page 208 + changed "of D'Orbigny before" into "of d'Orbigny before" page 208 + changed "sea-cliffs at Stevensklint" into "sea-cliffs at Stevens Klint" + page 210 + changed "and Ostrea, vesicularis." into "and Ostrea vesicularis." + page 215 + changed "bivalves (figs. 203. 205," into "bivalves (figs. 203, 205," + page 216 + changed "the Dammura of" into "the Dammara of" page 216 + changed "afterwards recognised by" into "afterwards recognized by" + page 216 + changed "of the Radack achipelago," into "of the Radack archipelago," + page 217 + changed "observations of Ferdinand Roemer;" into "observations of + Ferdinand Römer;" page 224 + changed "the marl-stones are" into "the marlstones are" page 224 + changed "Wealden (see Nos. 5" into "Wealden (see Nos. 5." page 225 + changed "purely fresh-water origin." into "purely freshwater origin." + page 227 + changed "Auvergne (see above, p. 183.)" into "Auvergne (see above, p. + 183.)." page 228 + changed "genera Trioynx and Emys," into "genera Trionyx and Emys," + page 229 + changed "See Flinder's Voyage." into "See Flinders' Voyage." + footnote 233-A + changed "Author's Annivers. Address," into "Author's Anniv. Address," + footnote 237-C + changed "those from the Gualt" into "those from the Gault" page 242 + changed "eological Map of" into "Geological Map of" page 242 (fig. 252) + changed "(fig. 254.), where" into "(fig. 253.), where" page 244 + changed "in the north" into "in the North" page 245 + changed "In the wood-cut" into "In the woodcut" page 246 + changed "South Downs at Beachy head." into "South Downs at Beachy Head." + page 246 + changed "fail to recognise in" into "fail to recognize in" page 246 + changed "voll. ii. p. 98." into "vol. ii. p. 98." footnote 248-A + changed "of clay aud limestone," into "of clay and limestone," page 258 + changed "Coral rag," into "Coral rag." page 261 (fig. 273) + changed "in their orginal" into "in their original" page 264 + changed "says Mr Lycett," into "says Mr. Lycett," page 266 + changed "such as Pleiosaur," into "such as Plesiosaur," page 267 + changed "obtained by Dr Buckland" into "obtained by Dr. Buckland" + page 268 + changed "that the Thuia," into "that the Thuja," page 270 + changed "Buckland's Bridgw. Treat." into "Buckland's Bridgew. Treat." + page 271 (fig. 294) + changed "lower shales are wel" into "lower shales are well" page 271 + changed "the Oolitic system generally" into "the Oolitic system + generally." page 272 + changed "1/3 nat size." into "1/3 nat. size." page 273 (fig. 301) + changed "(G. arcuata, Lam)" into "(G. arcuata, Lam.)" page 274 (fig. 304) + changed "their own predacious race" into "their own predaceous race" + page 278 + changed "both of Icthyosaur and Plesiosaur" into "both of Ichthyosaur + and Plesiosaur" page 278 + changed "for swimming (see fig. 313.)" into "for swimming (see fig. + 313.)." page 279 + changed "Sir H. de la Beche," into "Sir H. De la Beche," page 281 + changed "of the Haute Saône," into "of the Haute-Saône," page 283 + changed "in Germany-Keupar" into "in Germany-Keuper" page 286 + changed "Buckland, Bridg. Treat.," into "Buckland, Bridgew. Treat.," + footnote 286-A + changed calcaire coquillier into "calcaire coquillier." page 287 + changed "Württemberg, and is" into "Würtemberg, and is" page 287 + changed "genera Sauricthys and Gyrolepis" into "genera Saurichthys and + Gyrolepis" page 287 + changed "near Strazburg, on" into "near Strasburg, on" page 288 + changed "the "gres bigarré," or" into "the "grès bigarré," or" page 288 + changed "vol. v. p. 347" into "vol. v. p. 347." footnote 290-B + changed "in the gray, and" into "in the grey, and" page 294 + changed "with ornithicnites on" into "with ornithichnites on" page 300 + changed "and botroidal character." into "and botryoidal character." + page 302 + changed "the icthyolites which" into "the ichthyolites which" page 304 + changed "Pygopteris mandibularis" into "Pygopterus mandibularis" page 305 + (fig. 346) + changed "Gutbier are Asterophillites" into "Gutbier are Asterophyllites" + page 307 + changed "Lepidodendra, Calamites, Asterophillites," into "Lepidodendra, + Calamites, Asterophyllites," page 308 + changed "same bands of" into "some bands of" page 309 + changed "sometimes called fire-stone," into "sometimes called firestone," + page 309 + changed "Geol. Soc Proceedings," into "Geol. Soc. Proceedings," + footnote 317-B + changed "f. 4. feet oal" into "f. 4. feet coal." page 321 (fig. 372) + changed "at an angle of 8°," into "at an angle of 8°." page 324 + changed "genus called Michroconchus" into "genus called Microconchus" + page 324 + changed "of Sigillaria, Lepidodrendon," into "of Sigillaria, + Lepidodendron," page 324 + changed "frequently recognised. Thus," into "frequently recognized. + Thus," page 324 + changed "be recognised at still" into "be recognized at still" page 324 + changed "Clay iron-stone.--Bands and nodules of clay iron-stone" into + "Clay-iron-stone.--Bands and nodules of clay-iron-stone" + page 326 + changed Dome-shaped out-crop of into Dome-shaped outcrop of page 327 + changed "ornithichnites (see p. 297.)." into "ornithichnites (see p. + 327.)." page 328 + changed "The out-crop of" into "The outcrop of" page 328 + changed "olifiant gas. The" into "olefiant gas. The" page 333 + changed "American Journ. of Sci," into "American Journ. of Sci.," + footnote 334-A + changed "a neucleus of granite," into "a nucleus of granite," page 343 + changed "Scale of Holoptychus nobilissimus," into "Scale of + Holoptychius nobilissimus," page 344 (fig. 395) + changed "peculiar lamelli-branchiate" into "peculiar lamellibranchiate" + page 347 + changed "south from St. Petersburgh." into "south from St. Petersburg." + page 348 + changed "of the Astræa." into "of the Astrea." page 349 + changed "lowest or mud-stone beds," into "lowest or mudstone beds," + page 352 + changed "showing siphuncle. Ludlow" into "showing siphuncle. Ludlow." + page 354 (fig. 417) + changed "the Welch mountains afford." into "the Welsh mountains afford." + page 359 + changed "Kleyn Spawen beds," into "Kleyn Spauwen beds," page 362 + changed "belong to neighboring" into "belong to neighbouring" page 362 + changed "with gypsum--Wirtemberg," into "with gypsum--Würtemberg," + page 364 + changed "Crinoidians abundant" into "Crinoideans abundant" page 365 + changed "like chelonians, Ptericthys," into "like chelonians, + Pterichthys," page 365 + changed "were recognised as" into "were recognized as" page 366 + changed "Their igneons origin" into "Their igneous origin" page 366 + changed "recognised by a peculiar" into "recognized by a peculiar" + page 370 + changed "One half I scoriaceous," into "One half is scoriaceous," + page 373 + changed "others are Andesitic," into "others are andesitic," page 373 + changed "tom. 8. p. 22. 1835." into "tom. 8. p. 22. 1835.)" page 375 + changed "A green porphyritic rocks" into "A green porphyritic rock" + page 376 + changed "Saussurite, a mineral" into "saussurite, a mineral" page 376 + changed "oxyde of iron." into "oxide of iron." page 376 + changed "of talc. Burat's" into "of talc. (Burat's" page 376 + changed "Sub-marine lava and" into "Submarine lava and" page 378 + changes "much as 20 per cent of" into "much as 20 per cent. of" page 382 + changed "of Hutt. Theory, s. 253." into "of Hutt. Theory, p. 253." + footnote 383-B + changed "Giants' Causeway, in Ireland." into "Giant's Causeway, in + Ireland." page 384 + changed "bottom of a shallow sea" into "bottom of a shallow sea." + page 388 + changed "to larva and" into "to lava and" page 388 + changed "PORM, STRUCTURE, AND" into "FORM, STRUCTURE, AND" page 390 + changed "Baranco de las Angustias." into "Barranco de las Angustias." + page 391 + changed "lie uncomformably to" into "lie unconformably to" page 398 + changed "trap-dikes of Etna," into "trap dikes of Etna," page 401 + changed "the accompanying wood-cut" into "the accompanying woodcut" + page 404 + changed "in once instance" into "in one instance" page 404 + changed "Punto del Nasone on Somma" into "Punta del Nasone on Somma" + page 405 (fig. 467) + changed "we recognise the ordinary" into "we recognize the ordinary" + page 418 + changed "near St. Andrew's" into "near St. Andrews" page 422 + changed "crystals of mesotyge" into "crystals of mesotype" page 431 + changed "H. de la Beche during" into "H. De la Beche during" page 432 + changed "Geol. Trans, 2d" into "Geol. Trans., 2d" footnote 435-E + changed "silex, thay have" into "silex, they have" page 439 + changed "except when mineralogicaly" into "except when mineralogically" + page 440 + changed "Bontigny's experiments have" into "Boutigny's experiments have" + page 441 + changed "mineral camposition-Test" into "mineral composition-Test" + page 449 + changed "Granite of Dartmore altering" into "Granite of Dartmoor + altering" page 449 + changed "are many vareties" into "are many varieties" page 450 + changed "the gritz quartzose" into "the grits quartzose" page 456 + changed "ay at smome" into "may at some" page 462 + changed "and their synonymes." into "and their synonymies." page 465 + changed "These aeriform fluids," into "These aëriform fluids," page 476 + changed "fumeroles have been" into "fumaroles have been" page 476 + changed "its being nonfossiliferous," into "its being non-fossiliferous," + page 479 + changed "have become matamorphic" into "have become metamorphic" page 484 + changed "MM. Studer, and Hugi," into "MM. Studer and Hugi," page 484 + changed "hornblende-schist, chlorine-schist," into "hornblende-schist, + chlorite-schist," page 485 + changed "enlarged or reopened." into "enlarged or re-opened." page 488 + changed "vein of Andreasburg" into "vein of Andreasberg" page 494 + changed "greenstone, or "toad-stone,"" into "greenstone, or "toadstone,"" + page 497 + changed "can be recognised in" into "can be recognized in" page 498 + changed "H. de la Beche during" into "H. De la Beche during" page 499 + changed "Lithodomi in beaches" into "lithodomi in beaches," page 502 + changed "Barrarde, M., on trilobites, 358." into "Barrande, M., on + trilobites, 358." page 502 + changed "Argile plastiqne, or" into "Argile plastique, or" page 502 + changed "or inland" into "on inland" page 502 + changed "on cornish lodes," into "on Cornish lodes," page 503 + changed "on Sewalik hills," into "on Sewâlik hills," page 503 + changed "Caryophillia cespitosa, bed" into "Caryophyllia cæspitosa, + bed" page 503 + changed "Cystidiæ in Silurian rocks, 358." into "Cystideæ in Silurian + rocks, 358." page 504 + changed "Decken, Prof. von, on reptiles in Saarbrück coalfield, 336." + into "Dechen, Prof. von, on reptiles in Saarbrück coal-field, + 336." page 505 + changed "France, 176-196." into "France, 176-191." page 505 + changed "Doué, M. B. de, on" into "Doue, M. B. de, on" page 505 + changed "Desroyers, M., on" into "Desnoyers, M., on" page 505 + changed "on Icthyosaurus, 276." into "on Ichthyosaurus, 276." page 505 + changed "hill of Gergovla," into "hill of Gergovia," page 505 + changed "on Cystidiæ, 358." into "on Cystideæ, 358." page 505 + changed "Glenroy, parallel" into "Glen Roy, parallel" page 506 + changed "sienitic, 440." into "syenitic, 440." page 506 + changed "vesiculosus in Lym-fiord, 33." into "vesiculosus in Lym-Fiord, + 33." page 506 + changed "Hamilton. Sir W.," into "Hamilton, Sir W.," page 507 + changed "Hooghley river, analysis" into "Hooghly river, analysis" + page 507 + changed "Icthyolites of Old" into "Ichthyolites of Old" page 507 + changed "Icthyosaurus communis, figure" into "Ichthyosaurus communis, + figure" page 507 + changed "period, Volcanic rocks," into "period. Volcanic rocks," page 507 + changed "Kentish chalk, sandgalls" into "Kentish chalk, sand-galls" + page 507 + changed "Limestone, fosslliferous," into "Limestone, fossiliferous," + page 508 + changed "Lochabar, parallel roads" into "Lochaber, parallel roads" + page 508 + changed "in cannel coal" into "in Cannel coal" page 508 + changed "enlarged and reopened, 492." into "enlarged and re-opened, + 492." page 508 + changed "teeth of. figured," into "teeth of, figured," page 508 + changed "Mammifer in trlas" into "Mammifer in trias" page 508 + changed "on Stonefield slate, 266." into "on Stonesfield slate, 266." + page 508 + changed "Mososaurus in St. Peter's" into "Mosasaurus in St. Peter's" + page 509 + changed "Oeynhansen, M. von, on" into "Oeynhausen, M. von, on" page 509 + changed "Saarbruck coal field," into "Saarbrück coal field," page 510 + changed "sandpipes near," into "sand-pipes near," page 509 + changed "St. Andrew's, trap" into "St. Andrews, trap" page 510 + changed "Plutonic rocks, 7-446." into "Plutonic rocks, 7. 446." page 510 + changed "Rose, Frof. G.," into "Rose, Prof. G.," page 510 + changed "of Colebrook Dale," into "of Coalbrook Dale," page 510 + changed "sandpipes in, 83." into "sand-pipes in, 83." page 510 + changed "Sandpipes near Maestricht" into "Sand-pipes near Maestricht" + page 510 + changed "or sandgalls, term" into "or sand-galls, term" page 510 + changed "Seacliffs, inland, 71." into "Sea cliffs, inland, 71." page 510 + changed "Sedgewick, Prof., cited," into "Sedgwick, Prof., cited," + page 510 + changed "Sedgewick, Prof., on" into "Sedgwick, Prof., on" page 511 + changed "Sewalik Hills, freshwater" into "Sewâlik Hills, freshwater" + page 511 + changed "Skapter Jokul, eruption" into "Skaptar Jokul, eruption" page 511 + changed "Sub-Apennine strata, 105. 166." into "Subapennine strata, 105, + 166." page 511 + changed "on sand galls, 82." into "on sand-galls, 82." page 512 + added Header "W." in index page 512 + changed "Wenlok formation, 354." into "Wenlock formation, 354." page 512 + changed "Whin-Sil, intrusion of" into "Whin-Sill, intrusion of" page 512 + changed "on Cystidæ, 358." into "on Cystideæ, 358." page 512 + changed "in 'Lavengro.' because" into "in 'Lavengro' because" + Advertisements + changed "Vols. I-VIII. With" into "Vols. I.-VIII. With" Advertiesements + changed "early religous schools" into "early religious schools" + Advertisements + changed "its organisation more" into "its organization more" + Advertisements + changed "with unfagging tread" into "with unflagging tread" + Advertisements + changed "of time and money" into "of time and money." Advertisements + changed "A CONDENSED HAND-BOOK OF ALL ENGLAND" into "A CONDENSED + HANDBOOK OF ALL ENGLAND" Advertisements + changed "Leicester, Bucks Nottinghamshire." into "Leicester, Bucks, + Nottinghamshire." 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