diff options
Diffstat (limited to 'old/3772-h/files/ch20.html')
| -rw-r--r-- | old/3772-h/files/ch20.html | 538 |
1 files changed, 538 insertions, 0 deletions
diff --git a/old/3772-h/files/ch20.html b/old/3772-h/files/ch20.html new file mode 100644 index 0000000..4ca2487 --- /dev/null +++ b/old/3772-h/files/ch20.html @@ -0,0 +1,538 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> +<!-- saved from url=(0036)http://../Lyell/The Student's Elements of Geology --> +<html> +<head> +<meta name="generator" content="HTML Tidy, see www.w3.org"> +<title>The Student's Elements of Geology: Title</title> +<meta content="text/html; charset=iso-8859-1" http-equiv= +"Content-Type"> +<meta content="MSHTML 5.00.2919.6307" name="GENERATOR"> +<link rel="stylesheet" href="geology.css" type="text/css"> +</head> +<body> +<p><b>The Student’s Elements of Geology</b></p> + +<hr> +<p class="page"><a name="page 353">[ 353 ]</a></p> + +<p> </p> + +<center><b>Chapter XX</b><br> +<br> +JURASSIC GROUP—<i>continued</i>—LIAS.</center> + +<p class="intro">Mineral Character of Lias. — Numerous +successive Zones in the Lias, marked by distinct Fossils, without +Unconformity in the Stratification, or Change in the Mineral +Character of the Deposits. — Gryphite Limestone. — +Shells of the Lias. — Fish of the Lias. — 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. — Fossil Plants. +— The origin of the Oolite and Lias, and of alternating +Calcareous and Argillaceous Formations.</p> + +<p><b>Lias.</b>—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. 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, having a +surface which becomes 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 +ribbon-like appearance.</p> + +<p>The Lias has been divided in England into three groups, the +Upper, Middle, and Lower. The Upper Lias consists first of sands, +which were formerly regarded as the base of the Oolite, but which, +according to Dr. Wright, are by their fossils more properly +referable to the Lias; secondly, of clay shale and thin beds of +limestone. The Middle Lias, or marl-stone series, has been divided +into three zones; and the Lower Lias, according to the labours of +Quenstedt, Oppel, Strickland, Wright, and others, into seven zones, +each marked by its own group of fossils. This Lower Lias averages +from 600 to 900 feet in thickness.</p> + +<p>From Devon and Dorsetshire to Yorkshire all these divisions, +observes Professor Ramsay, are constant; and from top to bottom we +can not assert that anywhere there is actual unconformity between +any two subdivisions, whether of the larger or smaller kind.</p> + +<p>In the whole of the English Lias there are at present known +about 937 species of mollusca, and of these 267 are Cephalopods, of +which class more than two-thirds are Ammonites,</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 354">[ 354 ]</a></p> + +<p>the Nautilus and Belemnite also abounding. The whole series has +been divided by zones characterised by particular Ammonites; for +while other families of shells pass from one division to another in +numbers varying from about 20 to 50 per cent, these cephalopods are +almost always limited to single zones, as Quenstedt and Oppel have +shown for Germany, and Dr. Wright and others for England.</p> + +<p>As no actual unconformity is known from the top of the Upper to +the bottom of the Lower Lias, and as there is a marked uniformity +in the mineral character of almost all the strata, it is somewhat +difficult to account even for such partial breaks as have been +alluded to in the succession of species, if we reject the +hypothesis that the old species were in each case destroyed at the +close of the deposition of the rocks containing them, and replaced +by the creation of new forms when the succeeding formation began. I +agree with Professor Ramsay in not accepting this hypothesis. No +doubt some of the old species occasionally died out, and left no +representatives in Europe or elsewhere; others were locally +exterminated in the struggle for life by species which invaded +their ancient domain, or by varieties better fitted for a new state +of things. Pauses also of vast duration may have occurred in the +deposition of strata, allowing time for the modification of organic +life throughout the globe, slowly brought about by variation +accompanied by extinction of the original forms.</p> + +<center><img src="../images3/fig361.jpg" width="412" height="251" alt= +"Fig. 361: Plagiostoma (Lima) giganteum. Fig. 362: Gryphæa incurva."> +</center> + +<p><b>Fossils of the Lias.</b>—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 <i> +Gryphæa</i> (Fig. 362). A large heavy shell called</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 355">[ 355 ]</a></p> + +<center><img src="../images3/fig363.jpg" width="433" height="495" alt= +"Fig. 363: Avicula inæquivalvis. Fig. 364: Avicula cygnipes. Fig. 365: Hippopodium ponderosum. Fig. 366: Spiriferina (Spirifera). Fig. 367: Leptæna Moorei."> +</center> + +<p><i>Hippopodium</i> (Fig. 365), allied to <i>Cypricardia,</i> is +also characteristic of the upper part of the Lower Lias. In this +formation occur also the Aviculas, Figs. 363 and 364. The Lias +formation is also remarkable for being the newest of the secondary +rocks in which brachiopoda of the genera <i>Spirifer</i> and <i> +Leptæna</i> (Figs. 366, 367) occur, although the former is +slightly modified in structure so as to constitute the subgenus +Spiriferina, Davidson, and the Leptæna has dwindled to a +shell smaller in size than a pea. No less than eight or nine +species of Spiriferina are enumerated by Mr. Davidson as belonging +to the Lias. Palliobranchiate mollusca predominate greatly in</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 356">[ 356 ]</a></p> + +<p>strata older than the Trias; but, so far as we yet know, they +did not survive the Liassic epoch.</p> + +<center><img src="../images3/fig368.jpg" width="412" height="492" alt= +"Fig. 368: Ammonites Bucklandi. Fig. 369: Ammonites planorbis. Fig. 370: Nautilus truncatus. Fig. 371: Ammonites bifrons."> +</center> + +<p>Allusion has already been made, p. 354, to numerous zones in the +Lias having each their peculiar Ammonites. Two of these occur near +the base of the Lower Lias, having a united thickness, varying from +40 to 80 feet. The upper of these is characterised by <i>Ammonites +Bucklandi,</i> and the lower by <i>Ammonites planorbis</i> (see +Figs. 368, 369).* Sometimes, however, there is a third intermediate +zone, that of <i>Ammonites angulatus,</i> which is the equivalent +of the zone called the infra-lias on the Continent, the species of +which are for the</p> + +<p class="fnote">* Quart. Journ., vol. xvi, p. 376.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 357">[ 357 ]</a></p> + +<img src="../images3/fig372.jpg" width="238" height="194" alt= +"Fig. 372: Ammonites margaritatus." align="right"> + +<p>most part common to the superior group marked by <i>Ammonites +Bucklandi.</i></p> + +<p>Among the Crinoids or Stone-lilies of the Lias, the +Pentacrinites are conspicuous. (See Fig. 373.) Of <i> +Palæocoma (Ophioderma) Egertoni</i> (Fig. 374), referable to +the <i>Ophiuridæ</i> of Muller, perfect specimens have been +met with in the Middle Lias beds of Dorset and Yorkshire.</p> + +<br> +<center><img src="../images3/fig373.jpg" width="407" height="350" alt= +"Fig. 373: Extracrinus (Pentacrinus) Briareus. Fig. 374: Palæocoma (Ophioderma) tenuibrachiata."> +</center> + +<p>The <i>Extracrinus Briareus</i> (removed by Major Austin from +Pentacrinus on account of generic differences) occurs in tangled +masses, forming thin beds of considerable extent, in the Lower Lias +of Dorset, Gloucestershire, and Yorkshire. The remains are often +highly charged with pyrites. This Crinoid, with its innumerable +tentacular arms, appears to have been frequently attached to the +driftwood of the liassic sea, in the same manner as Barnacles float +about on wood at the present day. There is another species of <i> +Extracrinus</i> and several of</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 358">[ 358 ]</a></p> + +<p><i>Pentacrinus</i> in the Lias; and the latter genus is found in +nearly all the formations from the Lias to the London Clay +inclusive. It is represented in the present seas by the delicate +and rare <i>Pentacrinus caput-medusæ</i> of the Antilles, +which, with Comatula, is one of the few surviving members of the +ancient family of the Crinoids, represented by so many extinct +genera in the older formations.</p> + +<center><img src="../images3/fig375.jpg" width="434" height="366" alt= +"Fig. 375: Scales of Lepidotus gigas. Fig. 376: a. Scales of Æchmodus Leachii, b. Æchmodus (restored outline), c. Scales of Dapedius monilifer."> +</center> + +<p><b>Fishes of the Lias.</b>—The fossil fish, of which there +are no less than 117 species known as British, resemble generically +those of the Oolite, but differ, according to M. Agassiz, from +those of the Cretaceous period. Among them is a species of <i> +Lepidotus</i> (<i>L. gigas,</i> Agassiz), Fig. 375, which is found +in the Lias of England, France, and Germany.* This genus was before +mentioned (<a href="ch18.html#page 316">p. 316</a>) as occurring in +the Wealden, and is supposed to have frequented both rivers and +sea-coasts. Another genus of Ganoids (or fish with hard, shining, +and enamelled scales), called <i>Æchmodus</i> (Fig. 376), is +almost exclusively Liassic. The teeth of a species of <i> +Acrodus,</i> also, are very abundant in the Lias (Fig. 377).</p> + +<p class="fnote">* Agassiz, Poissons Fossiles, vol. ii, tab. 28, +29.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 359">[ 359 ]</a></p> + +<center><img src="../images3/fig377.jpg" width="353" height="310" alt= +"Fig. 377: Acrodus nobilis. Fig. 378: Hybodus reticulatus, a. Part of fin, commonly called Ichthyodorylite, b. Tooth."> +</center> + +<p>But the remains of fish which have excited more attention than +any others are those large bony spines called ichthyodorulites (a, +Figure 378), 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.</p> + +<img src="../images3/fig379.jpg" width="248" height="175" alt= +"Fig. 379: Chimæra monstrosa.*" align="right"> + +<p>These last appear to have been bony spines which formed the +anterior part of the dorsal fin, like that of the living genera <i> +Cestracion</i> and <i>Chimæra</i> (see <i>a,</i> Figure 379). +In both of these genera, the posterior concave face is armed with +small spines, as in that of the fossil <i>Hybodus</i> (Fig. 378), a +placoid fish 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 <i>Chimæra</i> (Fig. 379), to raise and depress the fin, +their action</p> + +<p class="fnote">* Agassiz, Poissons Fossiles, vol. iii, tab. C, +Fig. 1.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 360">[ 360 ]</a></p> + +<p>resembling that of a movable mast, raising and lowering backward +the sail of a barge.”*</p> + +<p><b>Reptiles of the Lias.</b>—It is not, however, the +fossil fish which form the most striking feature in the organic +remains of the Lias; but the <i>Enaliosaurian</i> reptiles, which +are extraordinary for their number, size, and structure. Among the +most singular of these are several species of <i>Ichthyosaurus</i> +and <i>Plesiosaurus</i> (Figs. 380, 381). The genus <i> +Ichthyosaurus,</i> or fish-lizard, is not confined to this +formation, but has been found in strata as high as the White Chalk +of England, and as low as the Trias of Germany, a formation which +immediately succeeds the Lias in the descending order. 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 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.</p> + +<p>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 <i> +Plesiosaurus.</i>† (See Figs. 380, 381.) 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 <i> +Ichthyosaurus,</i> 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.‡ Some of the reptiles +above mentioned were of formidable dimensions. One specimen of <i> +Ichthyosaurus platydon,</i> from the Lias at Lyme, now in the +British Museum, must have belonged to an animal more than 24 feet +in length; and there are species of <i>Plesiosaurus</i> which +measure from 18 to 20 feet in length. The form of the <i> +Ichthyosaurus</i> may have fitted it to cut through the waves like +the porpoise; as it was furnished besides its paddles with a +tail-fin so constructed as to be a powerful organ of motion; but it +is supposed that the <i>Plesiosaurus,</i> at least the long-necked +species (Fig. 381), was better suited to fish in shallow creeks and +bays defended from heavy breakers.</p> + +<p>It is now very generally agreed that these extinct saurians must +have inhabited the sea; and it was urged that as there are now +chelonians, like the tortoise, living in fresh water,</p> + +<p class="fnote">* Bridgewater Treatise, p. 290.<br> +† Geol. Soc. Transactions, Second Series, vol. i, p. 49.<br> +‡ Conybeare and De la Beche, Geol. Trans., First Series, +vol. v, p. 559; and Buckland, Bridgewater Treatise, p. 203.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 361">[ 361 ]</a></p> + +<center><img src="../images3/fig380.jpg" width="593" height="371" alt= +"Fig. 380: Skeleton of Ichthyosaurus communis, restored by Conybeare and Cuvier. Fig. 381: Skeleton of Plesiosaurus dolichodeirus, restored by Rev. W. D. Conybeare."> +</center> + +<p>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</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 362">[ 362 ]</a></p> + +<p>Pinos (Isle of Pines), south of Cuba, and in the open sea round +the coast. In 1835 a curious lizard (<i>Amblyrhynchus +cristatus</i>) was discovered by Mr. Darwin in the Galapagos +Islands.* It was found to be exclusively marine, swimming easily by +means of its flattened tail, and subsisting chiefly on seaweed. One +of them was sunk from the ship by a heavy weight, and on being +drawn up after an hour was quite unharmed.</p> + +<p>The families of Dinosauria, crocodiles, and Pterosauria or +winged reptiles, are also represented in the Lias.</p> + +<p><b>Sudden Destruction of Saurians.</b>—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.</p> + +<p>“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.”† 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.”‡ It is further 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.</p> + +<p>Numerous specimens of the Calamary or pen-and-ink fish, +(<i>Geoteuthis bollensis</i>) 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</p> + +<p class="fnote">* See Darwin, Naturalist’s Voyage, p. 385. +Murray.<br> +† Bridgewater Treatise, p. 115.<br> +‡ Geological Researches, p. 334.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 363">[ 363 ]</a></p> + +<p>sediment; for, if long exposed after death, the membrane +containing the ink would have decayed.*</p> + +<p>As we know that river-fish are sometimes stifled, even in their +own element, by muddy water during floods, it can not be doubted +that the periodical discharge of large bodies of turbid fresh water +in 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 +indescribable multitudes of dead fishes have been seen floating on +the sea after a discharge of noxious vapours during similar +convulsions. 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.</p> + +<img src="../images3/fig382.jpg" width="181" height="124" alt= +"Fig. 382: Wing of a neuropterous insect." align="right"> + +<p><b>Fresh-water Deposits.—Insect-beds.</b>—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 have an +estuarine character, and must have been formed within the influence +of rivers. At the base of the Upper and Lower Lias respectively, +insect-beds appear to be almost everywhere present throughout the +Midland and South-western districts of England. These beds are +crowded with the remains of insects, small fish, and crustaceans, +with occasional marine shells. One band in Gloucestershire, rarely +exceeding a foot in thickness, has been named the “insect +limestone.” It passes upward, says the Reverend P. B. +Brodie,† into a shale containing <i>Cypris</i> and <i> +Estheria,</i> and is full of the wing-cases of several genera of +Coleoptera, with some nearly entire beetles, of which the eyes are +preserved. The nervures of the wings of neuropterous insects +(Figure 382) are beautifully perfect in this bed. Ferns, with +Cycads and leaves of monocotyledonous plants, and some apparently +brackish and fresh-water shells, accompany the insects in several +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. +After studying 300 specimens of these insects from the Lias, Mr. +Westwood declares that they comprise both</p> + +<p class="fnote">* Buckland, Bridgewater Treatise, p. 307.<br> +† A History of Fossil Insects, etc., 1846. London.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 364">[ 364 ]</a></p> + +<p>wood-eating and herb-devouring beetles, of the Linnean genera +<i>Elater, Carabus,</i> etc., besides grasshoppers +(<i>Gryllus</i>), and detached wings of dragon-flies and may-flies, +or insects referable to the Linnean genera <i>Libellula, Ephemera, +Hemerobius,</i> and <i>Panorpa,</i> 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.</p> + +<p><b>Fossil Plants.</b>—Among the vegetable remains of the +Lias, several species of <i>Zamia</i> have been found at Lyme +Regis, and the remains of coniferous plants at Whitby. M. Ad. +Brongniart enumerates forty-seven liassic acrogens, most of them +ferns; and fifty gymnosperms, of which thirty-nine are cycads, and +eleven conifers. Among the cycads the predominance of <i> +Zamites,</i> and among the ferns the numerous genera with leaves +having reticulated veins (as in <a href="../images2/fig349.jpg">Fig. +349</a>), are mentioned as botanical characteristics of this era.* +The absence as yet from the Lias and Oolite of all signs of +dicotyledonous angiosperms is worthy of notice. The leaves of such +plants are frequent in tertiary strata, and occur in the +Cretaceous, though less plentifully (see <a href= +"ch17.html#page 303">p. 303</a>). The angiosperms seem, therefore, +to have been at the least comparatively rare in these older +secondary periods, when more space was occupied by the Cycads and +Conifers.</p> + +<p><b>Origin of the Oolite and Lias.</b>—The entire group of +Oolite and Lias 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 now considered +(see <a href="#page 353">p. 353</a>) as belonging to the same +formation, though formerly referred to the Inferior Oolite, and +these sands again by the shelly and coralline limestone called the +Great or Bath Oolite. 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 (see <a href="../images2/fig298.jpg">Fig. 298</a>).† +The clay beds, however, as Sir H. de la Beche remarks, can be +followed over larger areas than the sand or sandstones.‡ It +should also be remembered that while the Oolite 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.</p> + +<p class="fnote">* Tableau des Vég. Foss., 1849, p. 105.<br> +† Conybeare and Philips’s Outlines, etc., p. 166.<br> +‡ Geological Researches, p. 337.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 365">[ 365 ]</a></p> + +<p>Nevertheless, some of the clays and intervening limestones do +retain, in reality, a pretty uniform character for distances of +from 400 to 600 miles from east to west and north to south.</p> + +<p>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.</p> + +<p>The increased thickness of the limestones in those regions, as +in the Alps and Jura, where the clays are comparatively thin, +arises from the calcareous matter having been derived from species +of corals and other organic beings which live in clear water, far +from land, to the growth of which the influx of mud would be +unfavourable. Portions therefore of these clays and limestones have +probably been formed contemporaneously to a greater extent than we +can generally prove, for the distinctness of the species of organic +beings would be caused by the difference of conditions between the +more littoral and the more pelagic areas and the different depths +and nature of the sea-bottom. Independently of those ascending and +descending movements which have given rise to the superposition of +the limestones and clays, and by which the position of land and sea +are made in the course of ages to vary, the geologist has the +difficult task of allowing for the contemporaneous thinning out in +one direction and thickening in another, of the successive organic +and inorganic deposits of the same era.</p> + +<br> +<hr> +<small><a href="contents.html">Contents</a> / <a href="ch19.html"> +Chapter XIX</a> / <a href="ch21.html">Chapter XXI</a></small> +</body> +</html> + |