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diff --git a/old/3772-h/files/ch19.html b/old/3772-h/files/ch19.html new file mode 100644 index 0000000..36673d8 --- /dev/null +++ b/old/3772-h/files/ch19.html @@ -0,0 +1,1616 @@ +<!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 321">[ 321 ]</a></p> + +<p> </p> + +<center><b>Chapter XIX</b><br> +<br> +JURASSIC GROUP.—PURBECK BEDS AND OOLITE.</center> + +<p class="intro">The Purbeck Beds a Member of the Jurassic Group. +— Subdivisions of that Group. — Physical Geography of +the Oolite in England and France. — Upper Oolite. — +Purbeck Beds. — New Genera of fossil Mammalia in the Middle +Purbeck of Dorsetshire. — Dirt-bed or ancient Soil. — +Fossils of the Purbeck Beds. — Portland Stone and Fossils. +— Kimmeridge Clay. — Lithographic Stone of Solenhofen. +— Archæopteryx. — Middle Oolite. — Coral +Rag. — Nerinæa Limestone. — Oxford Clay, +Ammonites and Belemnites. — Kelloway Rock. — Lower, or +Bath, Oolite. — Great Plants of the Oolite. — Oolite +and Bradford Clay. — Stonesfield Slate. — Fossil +Mammalia. — Fuller’s Earth. — Inferior Oolite and +Fossils. — Northamptonshire Slates. — Yorkshire Oolitic +Coal-field. — Brora Coal. — Palæontological +Relations of the several Subdivisions of the Oolitic group.</p> + +<p><b>Classification of the Oolite.</b>—Immediately below the +Hastings Sands we find in Dorsetshire another remarkable +fresh-water formation, called <i>the Purbeck,</i> because it was +first studied in the sea-cliffs of the peninsula of Purbeck in that +county. These beds are for the most part of fresh-water origin, but +the organic remains of some few intercalated beds are marine, and +show that the Purbeck series has a closer affinity to the Oolitic +group, of which it may be considered as the newest or uppermost +member.</p> + +<p>In England generally, and in the greater part of Europe, both +the Wealden and Purbeck beds are wanting, and the marine cretaceous +group is followed immediately, in the descending order, by another +series called the Jurassic. In this term, the formations commonly +designated as “the Oolite and Lias” are included, both +being found in the Jura Mountains. The Oolite was so named because +in the countries where it was first examined the limestones +belonging to it had an Oolitic structure (see <a href= +"ch2.html#page 37">p. 37</a>). These rocks occupy in England a zone +nearly thirty miles in average breadth, which 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.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 322">[ 322 ]</a></p> + +<center>OOLITE</center> + +<center> +<table border="1" cellspacing="0" cellpadding="4" width="60%" +summary="Upper, Middle and Lower Oolite systems."> +<tr> +<td align="left" valign="middle">Upper</td> +<td align="left"><i>a.</i> Purbeck beds.<br> +<i>b.</i> Portland stone and sand.<br> +<i>c.</i> Kimmeridge clay.</td> +</tr> + +<tr> +<td align="left" valign="middle">Middle</td> +<td align="left"><i>d.</i> Coral rag.<br> +<i>e.</i> Oxford clay, and Kelloway rock.</td> +</tr> + +<tr> +<td align="left" valign="middle">Lower</td> +<td align="left"><i>f.</i> Cornbrash and Forest marble.<br> +<i>g.</i> Great Oolite and Stonesfield slate.<br> +<i>h.</i> Fuller’s earth.<br> +<i>i.</i> Inferior Oolite.</td> +</tr> +</table> +</center> + +<p>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 accepted English +type, notwithstanding the thinness or occasional absence of the +clays, is more perfect than in Yorkshire or Normandy.</p> + +<p><b>Physical Geography.</b>—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 bands of country where +the argillaceous strata crop out; and between these valleys the +limestones are observed, forming 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.</p> + +<center><img src="../images2/fig298.jpg" width="368" height="106" alt= +"Fig. 298: Configuration of surface."></center> + +<p>Fig. 298 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.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 323">[ 323 ]</a></p> + +<p>It will be remarked, that the lines of steep slope, 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.</p> + +<p>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. It is evident, +therefore, that the denuding causes (see <a href= +"ch6.html#page 105">p. 105</a>) have acted similarly over an area +several hundred miles in diameter, removing the softer clays more +extensively than the limestones, and causing these last to form +steep slopes or escarpments wherever the harder calcareous rock was +based upon a more yielding and destructible formation.</p> + +<br> + + +<center><small>UPPER OOLITE.</small></center> + +<p><b>Purbeck Beds.</b>—These strata, which we class as the +uppermost member of the Oolite, are of limited geographical extent +in Europe, as already stated, but they acquire importance when we +consider the succession of three distinct sets of fossil remains +which they contain. Such repeated changes in organic life must have +reference to the history of a vast lapse of ages. The Purbeck beds +are finely exposed to view in Durdlestone Bay, near Swanage, +Dorsetshire, and at Lulworth Cove and the neighbouring bays between +Weymouth and Swanage. At Meup’s Bay, in particular, Professor +E. Forbes examined minutely, in 1850, the organic remains of this +group, displayed in a continuous sea-cliff section, and it appears +from his researches 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.</p> + +<p><i>Upper Purbeck.</i>—The highest of the three divisions +is purely fresh-water, the strata, about fifty feet in thickness, +containing shells of the genera <i>Paludina, Physa, Limnæa, +Planorbis, Valvata, Cyclas,</i> and <i>Unio,</i> with <i> +Cyprides</i> and fish. All the species seem peculiar, and among +these the <i>Cyprides</i></p> + +<p> </p> + +<hr> +<p class="page"><a name="page 324">[ 324 ]</a></p> + +<p>are very abundant and characteristic (see Fig. 299, <i>a, b, +c.</i>)</p> + +<p>The stone called “Purbeck Marble,” formerly much +used in ornamental architecture in the old English cathedrals of +the southern counties, is exclusively procured from this +division.</p> + +<center><img src="../images2/fig299.jpg" width="410" height="171" alt= +"Fig. 299: Cyprides from the Upper Purbecks."></center> + +<p><i>Middle Purbeck.</i>—Next in succession is the Middle +Purbeck, about thirty feet thick, the uppermost part of which +consists of fresh-water limestone, with cyprides, turtles, and +fish, of different species from those in the preceding strata. +Below the limestone are brackish-water beds full of <i>Cyrena,</i> +and traversed by bands abounding in <i>Corbula</i> and <i> +Melania.</i> These are based on a purely marine deposit, with <i> +Pecten, Modiola, Avicula,</i> and <i>Thracia.</i> Below this, +again, come limestones and shales, partly of brackish and partly of +fresh-water origin, in which many fish, especially species of <i> +Lepidotus</i> and <i>Microdon radiatus,</i> are found, and a +crocodilian reptile named <i>Macrorhynchus.</i> Among the mollusks, +a remarkable ribbed <i>Melania,</i> of the section <i>Chilina,</i> +occurs.</p> + +<center><img src="../images2/fig300.jpg" width="358" height="190" alt= +"Fig. 300: Ostrea distorta. Fig. 301: Hemicidaris Purbeckensis."> +</center> + +<p>Immediately below is a great and conspicuous stratum, twelve +feet thick, formed of a vast accumulation of shells of <i>Ostrea +distorta</i> (Fig. 300), long familiar to geologists under the +local name of “Cinder-bed.” In the uppermost part +of</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 325">[ 325 ]</a></p> + +<center><img src="../images2/fig302.jpg" width="411" height="171" alt= +"Fig. 302: Cyprides from the Middle Purbecks."></center> + +<p>this bed Professor Forbes discovered the first echinoderm (Fig. +301) as yet known in the Purbeck series, a species of <i> +Hemicidaris,</i> a genus characteristic of the Oolitic period, and +scarcely, if at all, distinguishable from a previously known +Oolitic fossil. It was accompanied by a species of <i>Perna.</i> +Below the Cinder-bed fresh-water strata are again seen, filled in +many places with species of <i>Cypris</i> (Fig. 302, <i>a, b, +c</i>), and with <i>Valvata, Paludina, Planorbis, Limnæa, +Physa</i> (Fig. 303), and <i>Cyclas,</i> all different from any +occurring higher in the series. It will be seen that <i>Cypris +fasciculata</i> (Fig. 302, <i>b</i>) has tubercles at the end only +of each valve, a character by which it can be immediately +recognised. In fact, these minute crustaceans, almost as frequent +in some of the shales as plates of mica in a micaceous sandstone, +enable geologists at once to identify the Middle Purbeck in places +far from the Dorsetshire cliffs, as, for example, in the Vale of +Wardour in Wiltshire. Thick beds of chert occur in the Middle +Purbeck filled with mollusca and cyprides of the genera already +enumerated, in a beautiful state of preservation, often converted +into chalcedony. Among these Professor Forbes met with gyrogonites +(the spore-vessels of <i>Chara</i>), plants never until 1851 +discovered in rocks older than the Eocene. About twenty feet below +the “Cinder-bed” is a stratum two or three inches +thick, in which fossil mammalia presently to be mentioned occur, +and beneath this a thin band of greenish shales, with marine shells +and impressions of leaves like those of a large <i>Zostera,</i> +forming the base of the Middle Purbeck.</p> + +<img src="../images2/fig303.jpg" width="130" height="125" alt= +"Fig. 303: Physa Bristovii" align="right"> + +<p><i>Fossil Mammalia of the Middle Purbeck.</i>—In 1852,* +after alluding to the discovery of numerous insects and +air-breathing mollusca in the Purbeck strata, I remarked that, +although no mammalia had then been found, “it was too soon to +infer</p> + +<p class="fnote">* Elements of Geology, 4th edition.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 326">[ 326 ]</a></p> + +<p>their non-existence on mere negative evidence.” Only two +years after this remark was in print, Mr. W. R. Brodie found in the +Middle Purbeck, about twenty feet below the +“Cinder-bed” above alluded to, in Durdlestone Bay, +portions of several small jaws with teeth, which Professor Owen +recognised as belonging to a small mammifer of the insectivorous +class, more closely allied in its dentition to the <i> +Amphitherium</i> (or <i>Thylacotherium</i>) than to any existing +type.</p> + +<p>Four years later (in 1856) the remains of several other species +of warm-blooded quadrupeds were exhumed by Mr. S. H. Beckles, +<small>F.R.S.</small>, from the same thin bed of marl near the base +of the Middle Purbeck. In this marly stratum many reptiles, several +insects, and some fresh-water shells of the genera <i>Paludina, +Planorbis,</i> and <i>Cyclas,</i> were found.</p> + +<p>Mr. Beckles had determined thoroughly to explore the thin layer +of calcareous mud from which in the suburbs of Swanage the bones of +the Spalacotherium had already been obtained, and in three weeks he +brought to light from an area forty feet long and ten wide, and +from a layer the average thickness of which was only five inches, +portions of the skeletons of six new species of mammalia, as +interpreted by Dr. Falconer, who first examined them. Before these +interesting inquiries were brought to a close, the joint labours of +Professor Owen and Dr. Falconer had made it clear that twelve or +more species of mammalia characterised this portion of the Middle +Purbeck, most of them insectivorous or predaceous, varying in size +from that of a mole to that of the common polecat, <i>Mustela +putorius.</i> While the majority had the character of insectivorous +marsupials, Dr. Falconer selected one as differing widely from the +rest, and pointed out that in certain characters it was allied to +the living Kangaroo-rat, or <i>Hypsiprymnus,</i> ten species of +which now inhabit the prairies and scrub-jungle of Australia, +feeding on plants, and gnawing scratched-up roots. A striking +peculiarity of their dentition, one in which they differ from all +other quadrupeds, consists in their having a single large +pre-molar, the enamel of which is furrowed with vertical grooves, +usually seven in number.</p> + +<p>The largest pre-molar (see <a href="../images2/fig304.jpg">Fig. +305</a>) in the fossil genus exhibits in like manner seven parallel +grooves, producing by their termination a similar serrated edge in +the crown; but their direction is diagonal—a distinction, says Dr. +Falconer, which is “trivial, not typical.” As these +oblique furrows form so marked a character of the majority of the +teeth, Dr. Falconer gave to the fossil the generic name of <i> +Plagiaulax.</i> The shape and relative size of the incisor, <i> +a,</i> <a href="../images2/fig306.jpg">Fig. 306,</a> exhibit</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 327">[ 327 ]</a></p> + +<center><img src="../images2/fig304.jpg" width="374" height="162" alt= +"Fig. 304: Pre-molar of the recent Australian Hypsiprymnus Gaimardi, showing 7 grooves at right angles to the length of the jaw. Fig. 305: Third and largest pre-molar (lower jaw) of Plagiaulax Becklesii, showing 7 diagonal grooves."> +</center> + +<p>a no less striking similarity to Hypsiprymnus. Nevertheless, the +more sudden upward curve of this incisor, as well as other +characters of the jaw, indicate a great deviation in the form of +Plagiaulax from that of the living kangaroo-rats.</p> + +<img src="../images2/fig306.jpg" width="280" height="247" alt= +"Fig. 306: Plagiaulax Becklessi. Right ramus of lower jaw." align= +"right"> + +<p>There are two fossil specimens of lower jaws of this genus +evidently referable to two distinct species extremely unequal in +size and otherwise distinguishable. The <i>Plagiaulax Becklesii</i> +(Fig. 306) was about as big as the English squirrel or the flying +phalanger of Australia (<i>Petaurus Australis,</i> Waterhouse). The +smaller fossil, having only half the linear dimensions of the +other, was probably only one-twelfth of its bulk. It is of peculiar +geological interest, because, as shown by Dr. Falconer, its two +back molars bear a decided resemblance to those of the Triassic <i> +Microlestes</i> (<a href="../images3/fig389.jpg">Fig. 389</a>), the +most ancient of known mammalia, of which an account will be given +in Chapter XXI.</p> + +<p>Up to 1857 all the mammalian remains discovered in secondary +rocks had consisted solely of single branches of the lower jaw, but +in that year Mr. Beckles obtained the upper portion of a skull, and +on the same slab the lower jaw of another quadruped with eight +molars, a large canine, and a broad and thick incisor. It has been +named Triconodon from its bicuspid teeth, and is supposed to have +been a small insectivorous marsupial, about the size of a hedgehog. +Other jaws have since been found indicating a larger species of the +same genus.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 328">[ 328 ]</a></p> + +<p>Professor Owen has proposed the name of <i>Galestes</i> for the +largest of the mammalia discovered in 1858 in Purbeck, equalling +the polecat (<i>Mustela putorius</i>) in size. It is supposed to +have been predaceous and marsupial.</p> + +<p>Between forty and fifty pieces or sides of lower jaws with teeth +have been found in oolitic strata in Purbeck; only five upper +maxillaries, together with one portion of a separate cranium, occur +at Stonesfield, and it is remarkable that with these there were no +examples in Purbeck of an entire skeleton, nor of any considerable +number of bones in juxtaposition. In several portions of the matrix +there were detached bones, often much decomposed, and fragments of +others apparently mammalian; but if all of them were restored, they +would scarcely suffice to complete the five skeletons to which the +five upper maxillaries above alluded to belonged. As the average +number of pieces in each mammalian skeleton is about 250, there +must be many thousands of missing bones; and when we endeavour to +account for their absence, we are almost tempted to indulge in +speculations like those once suggested to me by Dr. Buckland, when +he tried to solve the enigma in reference to Stonesfield; +“The corpses,” he said, “of drowned animals, when +they float in a river, distended by gases during putrefaction, have +often their lower jaw hanging loose, and sometimes it has dropped +off. The rest of the body may then be drifted elsewhere, and +sometimes may be swallowed entire by a predaceous reptile or fish, +such as an ichthyosaur or a shark.”</p> + +<p>As all the above-mentioned Purbeck marsupials, belonging to +eight or nine genera and to about fourteen species, insectivorous, +predaceous, and herbivorous, have been obtained from an area less +than 500 square yards in extent, and from a single stratum no more +than a few inches thick, we may safely conclude that the whole +lived together in the same region, and in all likelihood they +constituted a mere fraction of the mammalia which inhabited the +lands drained by one river and its tributaries. They afford the +first positive proof as yet obtained of the co-existence of a +varied fauna of the highest class of vertebrata with that ample +development of reptile life which marks all the periods from the +Trias to the Lower Cretaceous inclusive, and with a gymnospermous +flora, or that state of the vegetable kingdom when cycads and +conifers predominated over all kinds of plants, except the ferns, +so far, at least, as our present imperfect knowledge of fossil +botany entitles us to speak.</p> + +<p>The following table will enable the reader to see at a glance +how conspicuous a part, numerically considered, the mammalian +species of the Middle Purbeck now play when compared</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 329">[ 329 ]</a></p> + +<p>with those of other formations more ancient than the Paris +gypsum, and, at the same time, it will help him to appreciate the +enormous hiatus in the history of fossil mammalia which at present +occurs between the Eocene and Purbeck periods, and between the +latter and the Stonesfield Oolite, and between this again and the +Trias.</p> + +<center><i>Number and Distribution of all the known Species of +Fossil Mammalia from Strata older than the Paris Gypsum, or than +the Bembridge Series of the Isle of Wight.</i></center> + +<table border="1" cellspacing="0" cellpadding="4" summary= +"Number and Distribution of all the known Species of Fossil Mammalia from Strata older than the Paris Gypsum, or than the Bembridge Series of the Isle of Wight."> +<tr> +<td align="left" valign="middle" rowspan="7"> +T<small>ERTIARY</small></td> +<td align="left">Headon Series and beds between the Paris Gypsum +and the Grès de Beauchamp</td> +<td align="right" valign="top">14</td> +<td align="left">10 English<br> + 4 French</td> +</tr> + +<tr> +<td align="left">Barton Clay and Sables de Beauchamp</td> +<td align="right">0</td> +<td> </td> +</tr> + +<tr> +<td align="left" valign="top">Bagshot Beds, Calcaire Grossier, and +Upper Soissonnais of Cuisse-Lamotte</td> +<td align="right" valign="top">20</td> +<td align="left" valign="top">16 French<br> + 1 English<br> + 3 U. States*</td> +</tr> + +<tr> +<td align="left">London Clay, including the Kyson Sand</td> +<td align="right">7</td> +<td>English</td> +</tr> + +<tr> +<td align="left" valign="top">Plastic Clay and Lignite</td> +<td align="right" valign="top">9</td> +<td>7 French<br> +2 English</td> +</tr> + +<tr> +<td align="left">Sables de Bracheux</td> +<td align="right">1</td> +<td>French</td> +</tr> + +<tr> +<td align="left">Thanet Sands and Lower Landenian of Belgium</td> +<td align="right">0</td> +<td> </td> +</tr> + +<tr> +<td valign="middle" align="left" rowspan="20"> +S<small>ECONDARY</small></td> +<td align="left">Maestricht Chalk</td> +<td align="right">0</td> +<td> </td> +</tr> + +<tr> +<td align="left">White Chalk</td> +<td align="right">0</td> +<td> </td> +</tr> + +<tr> +<td align="left">Chalk Marl</td> +<td align="right">0</td> +<td> </td> +</tr> + +<tr> +<td align="left">Chloritic Series (Upper Greensand)</td> +<td align="right">0</td> +<td> </td> +</tr> + +<tr> +<td align="left">Gault</td> +<td align="right">0</td> +<td> </td> +</tr> + +<tr> +<td align="left">Neocomian (Lower Greensand)</td> +<td align="right">0</td> +<td> </td> +</tr> + +<tr> +<td align="left">Wealden</td> +<td align="right">0</td> +<td> </td> +</tr> + +<tr> +<td align="left">Upper Purbeck Oolite</td> +<td align="right">0</td> +<td> </td> +</tr> + +<tr> +<td align="left">Middle Purbeck Oolite</td> +<td align="right">14</td> +<td align="left">Swanage</td> +</tr> + +<tr> +<td align="left">Lower Purbeck Oolite</td> +<td align="right">0</td> +<td> </td> +</tr> + +<tr> +<td align="left">Portland Oolite</td> +<td align="right">0</td> +<td> </td> +</tr> + +<tr> +<td align="left">Kimmeridge Clay</td> +<td align="right">0</td> +<td> </td> +</tr> + +<tr> +<td align="left">Coral Rag</td> +<td align="right">0</td> +<td> </td> +</tr> + +<tr> +<td align="left">Oxford Clay</td> +<td align="right">0</td> +<td> </td> +</tr> + +<tr> +<td align="left">Great Oolite</td> +<td align="right">4</td> +<td align="left">Stonesfield</td> +</tr> + +<tr> +<td align="left">Inferior Oolite</td> +<td align="right">0</td> +<td> </td> +</tr> + +<tr> +<td align="left">Lias</td> +<td align="right">0</td> +<td> </td> +</tr> + +<tr> +<td align="left" valign="top">Upper Trias</td> +<td align="right" valign="top">4</td> +<td align="left">Wurtemberg<br> +Somersetshire<br> +N. Carolina</td> +</tr> + +<tr> +<td align="left">Middle Trias</td> +<td align="right">0</td> +<td> </td> +</tr> + +<tr> +<td align="left">Lower Trias</td> +<td align="right">0</td> +<td> </td> +</tr> + +<tr> +<td align="left" valign="middle" rowspan="6"> +P<small>RIMARY</small></td> +<td align="left">Permian</td> +<td align="right">0</td> +<td> </td> +</tr> + +<tr> +<td align="left">Carboniferous</td> +<td align="right">0</td> +<td> </td> +</tr> + +<tr> +<td align="left">Devonian</td> +<td align="right">0</td> +<td> </td> +</tr> + +<tr> +<td align="left">Silurian</td> +<td align="right">0</td> +<td> </td> +</tr> + +<tr> +<td align="left">Cambrian</td> +<td align="right">0</td> +<td> </td> +</tr> + +<tr> +<td align="left">Laurentian</td> +<td align="right">0</td> +<td> </td> +</tr> +</table> + +<p class="fnote">* I allude to several Zeuglodons found in Alabama, +and referred by some zoologists to three species.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 330">[ 330 ]</a></p> + +<p>The Sables de Bracheux, enumerated in the Tertiary division of +the table, supposed by Mr. Prestwich to be somewhat newer than the +Thanet Sands, and by M. Hébert to be of about that age, have +yielded at La Fere the <i>Arctocyon (Palæocyon) +primævus,</i> the oldest known tertiary mammal.</p> + +<p>It is worthy of notice, that in the Hastings Sands there are +certain layers of clay and sandstone in which numerous footprints +of quadrupeds have been found by Mr. Beckles, and traced by him in +the same set of rocks through Sussex and the Isle of Wight. They +appear to belong to three or four species of reptiles, and no one +of them to any warm-blooded quadruped. They ought, therefore, to +serve as a warning to us, when we fail in like manner to detect +mammalian footprints in older rocks (such as the New Red +Sandstone), to refrain from inferring that quadrupeds, other than +reptilian, did not exist or pre-exist.</p> + +<p>But the most instructive lesson read to us by the Purbeck strata +consists in this: They are all, with the exception of a few +intercalated brackish and marine layers, of fresh-water origin; +they are 160 feet in thickness, have been well searched by skillful +collectors, and by the late Edward Forbes in particular, who +studied them for months consecutively. They have been numbered, and +the contents of each stratum recorded separately, by the officers +of the Geological Survey of Great Britain. They have been divided +into three distinct groups by Forbes, each characterised by the +same genera of pulmoniferous mollusca and cyprides, these genera +being represented in each group by different species; they have +yielded insects of many orders, and the fruits of several plants; +and lastly, they contain “dirt-beds,” or old +terrestrial surfaces and vegetable soils at different levels, in +some of which erect trunks and stumps of cycads and conifers, with +their roots still attached to them, are preserved. Yet when the +geologist inquires if any land-animals of a higher grade than +reptiles lived during any one of these three periods, the rocks are +all silent, save one thin layer a few inches in thickness; and this +single page of the earth’s history has suddenly revealed to +us in a few weeks the memorials of so many species of fossil +mammalia, that they already outnumber those of many a subdivision +of the tertiary series, and far surpass those of all the other +secondary rocks put together!</p> + +<p><i>Lower Purbeck.</i>—Beneath the thin marine band +mentioned at <a href="#page 324">p. 324</a> as the base of the +Middle Purbeck, some purely fresh-water marls occur, containing +species of <i>Cypris</i> (Fig. 307 <i>a, c</i>), <i>Valvata,</i> +and <i>Limnæa,</i> different from those of the</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 331">[ 331 ]</a></p> + +<img src="../images2/fig307.jpg" width="185" height="151" alt= +"Fig. 307: Cyprides from the Lower Purbeck." align="right"> + +<p>Middle Purbeck. This is the beginning of the inferior division, +which is about 80 feet thick. Below the marls are seen, at +Meup’s Bay, more than thirty feet of brackish-water strata, +abounding in a species of <i>Serpula,</i> allied to, if not +identical with, <i>Serpula coacervites,</i> found in beds of the +same age in Hanover. There are also shells of the genus <i> +Rissoa</i> (of the subgenus <i>Hydrobia</i>), and a little <i> +Cardium</i> of the subgenus <i>Protocardium,</i> in these marine +beds, together with <i>Cypris.</i> 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 <i>Cycadeæ,</i> which I +shall presently describe, underlies these marls, and rests upon the +lowest fresh-water limestone, a rock about eight feet thick, +containing <i>Cyclas, Valvata,</i> and <i>Limnæa,</i> of the +same species as those of the uppermost part of the Lower Purbeck, +or above the dirt-bed. The fresh-water limestone in its turn rests +upon the top beds of the Portland stone, which, although it +contains purely marine remains, often consists of a rock +undistinguishable in mineral character from the Lowest Purbeck +limestone.</p> + +<img src="../images2/fig308.jpg" width="166" height="235" alt= +"Fig. 308: Mantellia nidiformis." align="left"> + +<p><i>Dirt-bed or ancient Surface-soil.</i>—The most +remarkable of all the varied succession 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 and sub-angular fragments +of stone, from 3 to 9 inches in diameter, in such numbers that it +almost deserves the name of gravel. I also saw in 1866, in +Portland, a smaller dirt-bed six feet below the principal one, six +inches thick, consisting of brown earth with upright <i>Cycads</i> +of the same species, <i>Mantellia nidiformis,</i> as those found in +the upper bed, but no <i>Coniferæ.</i> The weight of the +incumbent strata squeezing down the compressible dirt-bed has +caused the <i>Cycads</i> to assume that form which has</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 332">[ 332 ]</a></p> + +<p>led the quarrymen to call them “petrified birds’ +nests,” which suggested to Brongniart the specific name of +<i>nidiformis.</i> I am indebted to Mr. Carruthers for Figure 308 +of one of these Purbeck specimens, in which the original +cylindrical figure has been less distorted than usual by +pressure.</p> + +<p>Many silicified trunks of coniferous trees, and the remains of +plants allied to <i>Zamia</i> and <i>Cycas,</i> are buried in this +dirt-bed, and must have become fossil on the spots where they grew. +The stumps of the trees stand erect for a height of from one to +three feet, and even in one instance to six feet, with their roots +attached to the soil at about the same distances from one another +as the trees in a modern forest. The carbonaceous matter is most +abundant immediately around the stumps, and round the remains of +fossil <i>Cycadeæ.</i></p> + +<p>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 three or four 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 root is about one foot; but I +measured one myself, in 1866, which was 3½ feet in diameter, +said by the quarrymen to be unusually large. Root-shaped cavities +were observed by Professor Henslow to descend from the bottom of +the dirt-bed into the subjacent fresh-water stone, which, though +now solid, must have been in a soft and penetrable state when the +trees grew. The thin layers of calcareous slate (Fig. 309) 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.</p> + +<center><img src="../images2/fig309.jpg" width="391" height="144" alt= +"Fig. 309: Section in Isle of Portland, Dorset."></center> + +<p>The dirt-bed is by no means confined to the island of Portland, +where it has been most carefully studied, but is seen</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 333">[ 333 ]</a></p> + +<p>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. 310).</p> + +<center><img src="../images2/fig310.jpg" width="384" height="189" alt= +"Fig. 310: Section of cliff east of Lulworth Cove."></center> + +<p>From the facts above described we may infer, first, that those +beds of the Upper 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 permit the growth of the <i>Zamia</i> and +<i>Cycas.</i> Secondly. 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. +Thirdly. 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 fresh-water 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 taken place.</p> + +<p>The forest of the dirt-bed, as before hinted, was not everywhere +the first vegetation which grew in this region. Besides the lower +bed containing upright <i>Cycadeæ,</i> before mentioned, +another has sometimes been found above it, which implies +oscillations in the level of the same ground, and its alternate +occupation by land and water more than once.</p> + +<p><i>Subdivisions of the Purbeck.</i>—It will be observed +that the division of the Purbecks into upper, middle, and lower, +was made by Professor Forbes strictly on the principle of the</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 334">[ 334 ]</a></p> + +<p>entire distinctness of the species of organic remains which they +include. The lines of demarkation 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 organised beings. +“The causes which led to a complete change of life three +times during the deposition of the fresh-water 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.”</p> + +<p>Each dirt-bed may, no doubt, be the memorial of many thousand +years or centuries, because we find that two or three 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 imagine +the fossil soils of the Lower Purbeck to represent as many ages, we +need not be surprised to find that they do not constitute lines of +separation between strata characterised 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 Purbeck dirt-beds were probably formed in +succession and annihilated, besides those few which now remain.</p> + +<p>The plants of the Purbeck beds, so far as our knowledge extends +at present, consist chiefly of Ferns, Coniferæ, and +Cycadeæ (<a href="../images2/fig308.jpg">Fig. 308</a>), without +any angiosperms; the whole more allied to the Oolitic than to the +Cretaceous vegetation. The same affinity is indicated by the +vertebrate and invertebrate animals. Mr. Brodie has found the +remains of beetles and several insects of the homopterous and +trichopterous orders, some of which now live on plants, while +others are of such forms as hover over the surface of our present +rivers.</p> + +<p><b>Portland Oolite and Sand</b> (<i>b,</i> Table <a href= +"#page 321">p. 321</a>).—The Portland Oolite has already been +mentioned as forming in Dorsetshire the foundation on which the +fresh-water limestone of the Lower Purbeck reposes (see <a href= +"#page 331">p. 331</a>). It supplies the well-known building-stone +of which St. Paul’s and so many of the principal edifices of +London are constructed. About fifty species of mollusca occur in +this formation, among which are some ammonites of large size. The +cast of a spiral univalve</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 335">[ 335 ]</a></p> + +<img src="../images2/fig311.jpg" width="159" height="277" alt= +"Fig. 311: Cerithium Portlandicum." align="right"> + +<p>called by the quarrymen the “Portland screw” +(<i>a,</i> Figure 311), is common; the shell of the same (<i>b</i>) +being rarely met with. Also <i>Trigonia gibbosa</i> (Fig. 313) and +<i>Cardium dissimile</i> (<a href="../images2/fig314.jpg">Fig. +314</a>). This upper member rests on a dense bed of sand, called +the Portland Sand, containing similar marine fossils, below which +is the Kimmeridge Clay. In England these Upper Oolite formations +are almost wholly confined to the southern counties. But some +fragments of them occur beneath the Neocomian or Speeton Clay on +the coast of Yorkshire, containing many more fossils common to the +Portlandian of the Continent than does the same formation in +Dorsetshire. Corals are rare in this formation, although one +species is found plentifully at Tisbury, Wiltshire, in the Portland +Sand, converted into flint and chert, the original calcareous +matter being replaced by silex (Fig. 312).</p> + +<center><img src="../images2/fig312.jpg" width="351" height="235" alt= +"Fig. 312: Isastræa oblonga. Fig. 313: Trigonia gibbosa."> +</center> + +<p><b>Kimmeridge Clay.</b>—The <i>Kimmeridge Clay</i> +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, with +skeletons of fish and saurians, the bitumen</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 336">[ 336 ]</a></p> + +<p>may perhaps be of animal origin. Some of the saurians +(Pliosaurus) in Dorsetshire are among the most gigantic of their +kind.</p> + +<center><img src="../images2/fig314.jpg" width="375" height="346" alt= +"Fig. 314: Cardium dissimile. Fig. 315: Ostrea expansa. Fig. 316: Cardium striatulum. Fig. 317: Ostrea deltoidea. Fig. 318: Gryphæa (Exogyra) virgula."> +</center> + +<p>Among the fossils, amounting to nearly 100 species, may be +mentioned <i>Cardium striatulum</i> (Fig. 316) and <i>Ostrea +deltoidea</i> (Fig. 317), the latter found in the Kimmeridge Clay +throughout England and the north of France, and also in Scotland, +near Brora. The <i>Gryphæa virgula</i> (Fig. 318), also met +with in the Kimmeridge 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.</p> + +<img src="../images2/fig319.jpg" width="98" height="120" alt= +"Fig. 319: Trigonellites latus." align="left"> + +<p>The <i>Trigonellites latus</i> (<i>Aptychus</i> of some authors) +(Fig. 319) is also widely dispersed through this clay. The real +nature of the shell, of which there are many species in oolitic +rocks, is still a matter of conjecture. Some are of opinion that +the two plates have been the gizzard of a cephalopod; others, that +it may have formed a bivalve operculum of the same.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 337">[ 337 ]</a></p> + +<img src="../images2/fig320.jpg" width="152" height="284" alt= +"Fig. 320: Skeleton of Pterodactylus crassirostris." align="right"> + + +<p><b>Solenhofen Stone.</b>—The celebrated lithographic stone +of Solenhofen in Bavaria, appears to be of intermediate age between +the Kimmeridge clay and the Coral Rag, presently to be described. +It 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 <i>species</i> of flying reptiles or +pterodactyls (see Fig. 320), 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 +pterodactyls, and other contemporaneous air-breathers, +resorted.</p> + +<p>In the same slate of Solenhofen a fine example was met with in +1862 of the skeleton of a bird almost entire, and retaining even +its feathers so perfect that the vanes as well as the shaft are +preserved. The head was at first supposed to be wanting, but Mr. +Evans detected on the slab what seems to be the impression of the +cranium and beak, much resembling in size and shape that of the jay +or woodcock. This valuable specimen is now in the British Museum, +and has been called by Professor Owen <i>Archæopteryx +macrura.</i> Although anatomists agree that it is a true bird, yet +they also find that in the length of the bones of the tail, and +some other minor points of its anatomy, it approaches more nearly +to reptiles than any known living bird. In the living +representatives of the class Aves, the tail-feathers are attached +to a coccygian bone, consisting of several vertebræ united +together, whereas in the Archæopteryx the tail is composed of +twenty vertebræ, each of which supports a pair of +quill-feathers. The first five only of the vertebræ, as seen +in A, have transverse processes, the fifteen remaining ones become +gradually longer and more tapering. The feathers diverge outward +from them at an angle of 45°.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 338">[ 338 ]</a></p> + +<center><img src="../images2/fig321.jpg" width="422" height="440" alt= +"Fig. 321: Tail and feather of Archæopteryx, from Solenhofen, and tail of living bird for comparison."> +</center> + +<p>Professor Huxley in his late memoirs on the order of reptiles +called Dinosaurians, which are largely represented in all the +formations, from the Neocomian to the Trias inclusive, has shown +that they present in their structure many remarkable affinities to +birds. But a reptile about two feet long, called Compsognathus, +lately found in the Stonesfield slate, makes a much greater +approximation to the class Aves than any Dinosaur, and therefore +forms a closer link between the classes Aves and Reptilia than does +the Archæopteryx.</p> + +<p>It appears doubtful whether any species of British fossil, +whether of the vertebrate or invertebrate class, is common to the +Oolite and Chalk. But there is no similar break or discordance as +we proceed downward, and pass from one to another of the several +leading members of the Jurassic group, the Upper, Middle, and Lower +Oolite, and the Lias, there being often a considerable proportion +of the mollusca, sometimes as much as a fourth, common to such +divisions as the Upper and Middle Oolite.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 339">[ 339 ]</a></p> + +<center><small>MIDDLE OOLITE.</small></center> + +<p><b>Coral Rag.</b>—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, most of +them retaining the position in which they grew at the bottom of the +sea. In their forms they more frequently resemble the reef-building +polyparia of the Pacific than do the corals of any other member of +the Oolite. They belong chiefly to the genera <i>Thecosmilia</i> +(Fig. 322), <i>Protoseris,</i> and <i>Thamnastræa,</i> and +sometimes form masses of coral fifteen feet thick.</p> + +<center><img src="../images2/fig322.jpg" width="369" height="192" alt= +"Fig. 322: Thecosmilia annularis. Fig. 323: Thamnastræa."> +</center> + +<img src="../images2/fig324.jpg" width="166" height="365" alt= +"Fig. 324: Ostrea gregaria. Fig. 325: Nerinæa Goodhallii." +align="right"> + +<p>In Fig. 323 of a <i>Thamnastræa</i> 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, until +those on the left side are nearly filled up. The last-mentioned +stars are supposed to represent a perfected condition, and the +others an immature state. These coralline strata extend through the +calcareous hills of the north-west of Berkshire, and north of +Wilts, and again recur in Yorkshire, near Scarborough. The <i> +Ostrea gregarea</i> (Fig. 324) is very characteristic of the +formation in England and on the Continent.</p> + +<p>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;</p> + +<br> +<br> +<p> </p> + +<hr> +<p class="page"><a name="page 340">[ 340 ]</a></p> + +<p><i>Nerinæa</i> being an extinct genus of univalve shells +(Fig. 325) much resembling the <i>Cerithium</i> in external form. +The section shows the curious and continuous ridges on the +columnella and whorls.</p> + +<p><b>Oxford Clay.</b>—The coralline limestone, or +“Coral Rag,” above described, and the accompanying +sandy beds, called “calcareous grits,” of the Middle +Oolite, rest on a thick bed of clay, called the “Oxford +Clay,” sometimes not less than 600 feet thick. In this there +are no corals, but great abundance of cephalopoda, of the genera +Ammonite and Belemnite (Figs. 326 and 327). In some of the finely +laminated clays ammonites are very perfect, although somewhat +compressed, and are frequently found with the lateral lobe extended +on each side of the opening of the mouth into a horn-like +projection (Figure 327). These were discovered in the cuttings of +the Great Western Railway, near Chippenham, in 1841, and have been +described by Mr. Pratt (<i>An. Nat. Hist.,</i> Nov., 1841).</p> + +<center><img src="../images2/fig326.jpg" width="418" height="322" alt= +"Fig. 326: Belemnites hastatus. Fig. 327: Ammonites Jason."> +</center> + +<p>Similar elongated processes have been also observed to extend +from the shells of some Belemnites discovered by Dr. Mantell in the +same clay (see Figure 328), who, by the aid of this and other +specimens, has been able to throw much light on the structure of +singular extinct forms of cuttle-fish.*</p> + +<p class="fnote">* See Phil. Trans. 1850, p. 363; also Huxley, +Memoirs of Geol. Survey, 1864; Phillips, Palæont. Soc.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 341">[ 341 ]</a></p> + +<img src="../images2/fig328.jpg" width="137" height="573" alt= +"Fig. 328: Belemnites Puzosianus." align="right"> + +<p><b>Kelloway Rock.</b>—The arenaceous limestone which +passes under this name is generally grouped as a member of the +Oxford clay, in which it forms, in the south-west of England, +lenticular masses, 8 or 10 feet thick, containing at Kelloway, in +Wiltshire, numerous casts of ammonites and other shells. But in +Yorkshire this calcareo-arenaceous formation thickens to about 30 +feet, and constitutes the lower part of the Middle Oolite, +extending inland from Scarborough in a southerly direction. The +number of mollusca which it contains is, according to Mr. +Etheridge, 143, of which only 34, or 23½ per cent, are +common to the Oxford clay proper. Of the 52 Cephalopoda, 15 (namely +13 species of ammonite, the <i>Ancyloceras Calloviense</i> and one +Belemnite) are common to the Oxford Clay, giving a proportion of +nearly 30 per cent.</p> + +<center><small>LOWER OOLITE.</small></center> + +<p><b>Cornbrash and Forest Marble.</b>—The upper division of +this series, which is more extensive than the preceding or Middle +Oolite, is called in England the Cornbrash, as being a brashy, +easily broken rock, good for corn land. It consists of clays and +calcareous sandstones, which pass downward 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 crustaceans, which +walked over the soft wet sands, are still visible. In the same +stone the claws of crabs, fragments</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 342">[ 342 ]</a></p> + +<p>of echini, and other signs of a neighbouring beach, are +observed.*</p> + +<p><b>Great (or Bath) Oolite.</b>—Although the name of Coral +Rag has been appropriated, as we have seen, to a member of the +Middle Oolite before described, some portions of the Lower Oolite +are equally entitled in many places to be called coralline +limestones. Thus the Great Oolite near Bath contains various +corals, among which the <i>Eunomia radiata</i> (Fig. 329) is very +conspicuous, single individuals forming masses several feet in +diameter; and having probably required, like the large existing +brain-coral (<i>Meandrina</i>) of the tropics, many centuries +before their growth was completed.</p> + +<center><img src="../images2/fig329.jpg" width="337" height="221" alt= +"Fig. 329: Eunomia radiata."></center> + +<p>Different species of crinoids, or stone-lilies, are also common +in the same rocks with corals; and, like them, must have enjoyed a +firm bottom, where their base of attachment remained undisturbed +for years (<i>c,</i> Fig. 330). Such fossils, therefore, are almost +confined to the limestones; but an exception occurs at Bradford, +near Bath, where they are enveloped in clay sometimes 60 feet +thick. 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 encrinite, were +scattered at random through the argillaceous deposit in which some +now lie prostrate. These appearances are represented in the section +<i>b,</i> Fig. 330, where the darker strata represent the Bradford +clay, which is however a formation</p> + +<p class="fnote">* P. Scrope, Proc. Geol. Soc., March, 1831.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 343">[ 343 ]</a></p> + +<center><img src="../images2/fig330.jpg" width="356" height="288" alt= +"Fig. 330: Apiocrinites rotundus, or Pear Eucrinite."></center> + +<p>of such local development that in many places it can not easily +be separated from the clays of the overlying +“forest-marble” and underlying “fuller’s +earth.” 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 <i>serpulæ.</i> Now +these <i>serpulæ</i> 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 <i>serpulæ</i> were full grown, they had become +incrusted over with a bryozoan, called <i>Diastopora diluviana</i> +(see <i>b,</i> Fig. 331);</p> + +<center><img src="../images2/fig331.jpg" width="362" height="241" alt= +"Fig. 331: a. Aingle plate of body of Apiocrinus, overgrown with serpulæ and bryozoa; b. Portion of same magnified, showing the bryozoan Diastopora diluviana covering one of the serpulæ."> +</center> + +<p> </p> + +<hr> +<p class="page"><a name="page 344">[ 344 ]</a></p> + +<p>and many generations of these molluscoids had succeeded each +other in the pure water before they became fossil.</p> + +<p>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.</p> + +<p>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 <i>stations</i> +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 <i>stations,</i> or how far it +has been caused by time or the law of variation above alluded to. +But we recognise 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, although there +is scarcely any lithological resemblance, 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.</p> + +<p>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.” In such +shallow-water</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 345">[ 345 ]</a></p> + +<p>beds shells of the genera <i>Patella, Nerita, Rimula, +Cylindrites</i> are common (see Figs. 334 to 337); while +cephalopods are rare, and instead of ammonites and belemnites, +numerous genera of carnivorous trachelipods appear. Out of 224 +species of univalves obtained from the Minchinhampton beds, Mr. +Lycett found no less than 50 to be carnivorous. They belong +principally to the genera <i>Buccinum, Pleurotoma, Rostellaria, +Murex, Purpuroidea</i> (Fig. 333), and Fusus, and exhibit a +proportion of zoophagous species not very different from that which +obtains in seas of the Recent period. These zoological 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, and a great number of other +representatives of the same class of chambered shells, had become +extinct.</p> + +<center><img src="../images2/fig332.jpg" width="397" height="341" alt= +"Fig. 332: Terebratula digona. Fig. 333: Purpuroidea nodulata. Fig. 334: Cylindrites acutus. Fig. 335: Patella rugosa. Fig. 336: Nerita costulata. Fig. 337: Rimula (Emarginula) clathrata."> +</center> + +<p><b>Stonesfield Slate: Mammalia.</b>—The slate of +Stonesfield has been shown by Mr. Lonsdale to lie at the base of +the Great Oolite.* It is a slightly oolitic shelly limestone, +forming large lenticular masses imbedded in sand only six feet +thick,</p> + +<p class="fnote">* Proceedings Geol. Soc., vol. i, p. 414.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 346">[ 346 ]</a></p> + +<p>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 elytra or wing-covers of beetles, are +perfectly preserved (see Fig. 338), some of them approaching nearly +to the genus Buprestis. The remains, also, of many genera of +reptiles, such as <i>Plesiosaur, Crocodile,</i> and <i> +Pterodactyl,</i> have been discovered in the same limestone.</p> + +<img src="../images2/fig338.jpg" width="84" height="190" alt= +"Fig. 338: Elytron of Buprestis?" align="left"> + +<p>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, had been discovered until the +<i>Spalacotherium</i> of the Purbeck beds came to light in 1854. +Yet we have seen that terrestrial plants were not wanting in the +Upper Cretaceous formation (see <a href="ch17.html#page 302">p. +302</a>), and that in the Wealden there was evidence of fresh-water +sediment on a large scale, containing various plants, and even +ancient vegetable soils. We had also in the same Wealden many +land-reptiles and winged insects, which render 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.</p> + +<p>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 ten specimens of lower +jaws of mammiferous quadrupeds, belonging to four different species +and to three distinct genera, for which the names of <i> +Amphitherium, Phascolotherium,</i> and <i>Stereognathus</i> have +been adopted.</p> + +<img src="../images2/fig339.jpg" width="218" height="122" alt= +"Fig. 339: Tupaia Tana. Right ramus of lower jaw." align="right"> + +<p>It is now generally admitted that these or really the remains of +mammalia (although it was at first suggested that they might be +reptiles), and the only question open to controversy is limited to +this point, whether the fossil mammalia found in the Lower +Oolite</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 347">[ 347 ]</a></p> + +<center><img src="../images2/fig340.jpg" width="408" height="231" alt= +"Fig. 340: Part of lower jaw of Tupaia Tana. Fig. 341: Side view of same. Fig. 342: Part of lower jaw of Didelphys Azaræ. Fig. 343: Side view of same. Fig. 344: Amphitherium Prevostii."> +</center> + +<p>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 (<i>c,</i> +Figs. 342 and 343) of the lower jaw, as a character of the genus +<i>Didelphys</i>; and Professor Owen has since confirmed the +doctrine of its generality in the entire marsupial series. In all +these pouched quadrupeds this process is turned inward, as at <i>c, +d,</i> Fig. 342, in the Brazilian opossum, whereas in the placental +series, as at <i>c,</i> Figs. 340 and 341, there is an almost +entire absence of such inflection. The <i>Tupaia Tana</i> of +Sumatra has been selected by Mr. Waterhouse for this illustration, +because the jaws of that small insectivorous quadruped bear a great +resemblance to those of the Stonesfield <i>Amphitherium.</i> By +clearing away the matrix from the specimen of <i>Amphitherium +Prevostii</i> here represented (Fig. 344), Professor Owen +ascertained that the angular process (<i>c</i>) bent inward 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 +made him doubt whether</p> + +<center><img src="../images2/fig344.jpg" width="374" height="179" alt= +"Fig. 344: Amphitherium Prevostii."></center> + +<p> </p> + +<hr> +<p class="page"><a name="page 348">[ 348 ]</a></p> + +<p>the <i>Amphitherium</i> might not be an insectivorous placental, +although it offered some points of approximation in its osteology +to the marsupials, especially to the <i>Myrmecobius,</i> a small +insectivorous quadruped of Australia, which has nine molars on each +side of the lower jaw, besides a canine and three incisors.* +Another species of <i>Amphitherium</i> has been found at +Stonesfield (Fig. 345), which differs from the former (Fig. 344) +principally in being larger.</p> + +<center><img src="../images2/fig345.jpg" width="375" height="126" alt= +"Fig. 345: Amphitherium Broderipii. Fig. 346: Phascolotherium Bucklandii."> +</center> + +<p>The second mammiferous genus discovered in the same slates was +named originally by Mr. Broderip <i>Didelphys Bucklandi</i> (see +Fig. 346), and has since been called <i>Phascolotherium</i> 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 pre-molar and molar teeth is +complete.†</p> + +<p>In 1854 the remains of another mammifer, small in size, but +larger than any of those previously known, was brought to light. +The generic name of <i>Stereognathus</i> was given to it, and, as +is usually the case in these old rocks (see <a href="#page 328">p. +328</a>), it consisted of part of a lower jaw, in which were +implanted three double-fanged teeth, differing in structure from +those of all other known recent or extinct mammals.</p> + +<p><b>Plants of the Oolite.</b>—The Araucarian pines, which +are now abundant in Australia and its islands, together with +marsupial quadrupeds, are found in like manner to have accompanied +the marsupials in Europe during the Oolitic period (see <a href= +"../images2/fig347.jpg">Fig. 348</a>). In the same rock endogens of +the most perfect structure are met with, as, for example, fruits +allied to the Pandanus, such as the <i>Kaidacarpum ooliticum</i> of +Carruthers in the Great Oolite, and the <i>Podocarya</i> of +Buckland (see <a href="../images2/fig347.jpg">Fig. 347</a>) in the +Inferior Oolite.</p> + +<p><b>Fuller’s Earth.</b>—Between the Great and +Inferior Oolite near Bath, an argillaceous deposit, called +“the fuller’s earth,”</p> + +<p class="fnote">* A figure of this recent <i>Myrmecobius</i> will +be found in my Principles of Geology, chap. ix.<br> +† Owen’s British Fossil Mammals, p. 62.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 349">[ 349 ]</a></p> + +<center><img src="../images2/fig347.jpg" width="410" height="272" alt= +"Fig. 347: Portion of a fossil fruit of Podocarya Bucklandii. Fig. 348: Cone of fossil Araucaria sphærocarpa."> +</center> + +<p>occurs; but it is wanting in the north of England. It abounds in +the small oyster represented in Fig. 349. The number of mollusca +known in this deposit is about seventy; namely, fifty +Lamellibranchiate Bivalves, ten Brachiopods, three Gasteropods, and +seven or eight Cephalopods.</p> + +<img src="../images2/fig349.jpg" width="97" height="114" alt= +"Fig. 349: Ostrea acuminata." align="right"> + +<p><b>Inferior Oolite.</b>—This formation consists of a +calcareous freestone, usually of small thickness, but attaining in +some places, as in the typical area of Cheltenham and the Western +Cotswolds, a thickness of 250 feet. It sometimes rests upon yellow +sands, formerly classed as the sands of the Inferior Oolite, but +now regarded as a member of the Upper Lias. These sands repose upon +the Upper Lias clays in the south and west of England. The +Collyweston slate, formerly classed with the Great Oolite, and +supposed to represent in Northamptonshire the Stonesfield slate, is +now found to belong to the Inferior Oolite, both by community of +species and position in the series. The Collyweston beds, on the +whole, assume a much more marine character than the Stonesfield +slate. Nevertheless, one of the fossil plants <i>Aroides +Stutterdi,</i> Carruthers, remarkable, like the Pandanaceous +species before mentioned (Fig. 347) as a representative of the +monocotyledonous class, is common to the Stonesfield beds in +Oxfordshire.</p> + +<p>The Inferior Oolite of Yorkshire consists largely of shales and +sandstones, which assume much the aspect of a true</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 350">[ 350 ]</a></p> + +<img src="../images2/fig350.jpg" width="258" height="186" alt= +"Fig. 350: Hemitelites Brownii." align="left"> + +<p>coal-field, thin seams of coal having actually been worked in +them for more than a century. A rich harvest of fossil ferns has +been obtained from them, as at Gristhorpe, near Scarborough (Fig. +350). They contain also Cycadeæ, of which family a +magnificent specimen has been described by Mr. Williamson under the +name Zamia gigas, and a fossil called <i>Equisetum Columnare</i> +(see <a href="../images3/fig397.jpg">Fig. 397</a>), which maintains an +upright position in sandstone strata over a wide area. Shells of +<i>Estheria</i> and <i>Unio,</i> collected by Mr. Bean from these +Yorkshire coal-bearing beds, point to the estuary or fluviatile +origin of the deposit.</p> + +<p>At Brora, in Sutherlandshire, a coal formation, probably coeval +with the above, or at least 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 three and a half feet +thick, and there are several feet more of pyritous coal resting +upon it.</p> + +<center><img src="../images2/fig351.jpg" width="396" height="167" alt= +"Fig. 351: Terebratula fimbria. Fig. 352: Rhynchonella spinosa. Fig. 353: Pholadomya fidicula."> +</center> + +<p>Among the characteristic shells of the Inferior Oolite, I may +instance <i>Terebratula fimbria</i> (Fig. 351), <i>Rhynchonella +spinosa</i> (Fig. 352), and <i>Pholadomya fidicula</i> (Fig. 353). +The extinct genus <i>Pleurotomaria</i> is also a form very common +in this division as well as in the Oolitic system generally. It +resembles the <i>Trochus</i> in form, but is marked by a deep cleft +(<i>a,</i> Figs. 354, 355) on one side of the mouth. The</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 351">[ 351 ]</a></p> + +<p><i>Collyrites (Dysaster) ringens</i> (Fig. 356) is an Echinoderm +common to the Inferior Oolite of England and France, as are the two +Ammonites (Figs. 357, 358).</p> + +<center><img src="../images2/fig354.jpg" width="428" height="604" alt= +"Fig. 354: Pleurotomaria granulata. Fig. 355: Pleurotomaria ornata. Fig. 356: Collyrites (Dysaster) ringens. Fig. 357: Ammonites Humphresianus. Fig. 358: Ammonites Braikenridgii. Fig. 359: Ostrea Marshii."> +</center> + +<p><b>Palæontological Relations of the Oolitic +Strata.</b>—Observations have already been made on the +distinctness of the organic remains of the Oolitic and Cretaceous +strata, and</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 352">[ 352 ]</a></p> + +<p>the proportion of species common to the different members of the +Oolite. Between the Lower Oolite and the Lias there is a somewhat +greater break, for out of 256 mollusca of the Upper Lias, +thirty-seven species only pass up into the Inferior Oolite.</p> + +<img src="../images2/fig360.jpg" width="144" height="163" alt= +"Fig. 360: Ammonites macrocephalus." align="left"> + +<p>In illustration of shells having a great vertical range, it may +be stated that in England some few species pass up from the Lower +to the Upper Oolite, as, for example, <i>Rhynchonella obsoleta, +Lithodomus inclusus, Pholadomya ovalis,</i> and <i>Trigonia +costata.</i></p> + +<p>Of all the Jurassic Ammonites of Great Britain, <i>A. +macrocephalus</i> (Fig. 360), which is common to the Great Oolite +and Oxford Clay, has the widest range.</p> + +<p>We have every reason to conclude that the gaps which occur, both +between the larger and smaller sections of the English Oolites, +imply intervals of time, elsewhere represented by fossiliferous +strata, although no deposit may have taken place in the British +area. This conclusion is warranted by the partial extent of many of +the minor and some of the larger divisions even in England.</p> + +<br> +<hr> +<small><a href="contents.html">Contents</a> / <a href="ch18.html"> +Chapter XVIII</a> / <a href="ch20.html">Chapter XX</a></small> +</body> +</html> + |