The Student's Elements of Geology Part 42

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.

LIAS.

The English provincial name of Lias has been very generally adopted for a formation of argillaceous limestone, marl, and clay, which forms the base of the Oolite, and is cla.s.sed 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, a.s.sume a striped and ribbon-like appearance.

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.

From Devon and Dorsetshire to Yorkshire all these divisions, observes Professor Ramsay, are constant; and from top to bottom we can not a.s.sert that anywhere there is actual unconformity between any two subdivisions, whether of the larger or smaller kind.

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 cla.s.s more than two-thirds are Ammonites, the Nautilus and Belemnite also abounding. The whole series has been divided by zones characterised by particular Ammonites; for while other families of sh.e.l.ls pa.s.s 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.

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.

FOSSILS OF THE LIAS.

(FIGURE 361. Plagiostoma (Lima) giganteum, Sowerby. Inferior Oolite and Lias.)

(FIGURE 362. Gryphaea incurva, Sowerby. (G. arcuata, Lam.) Lias.)

(FIGURE 363. Avicula inaequivalvis, Sowerby. Lower Lias.)

(FIGURE 364. Avicula cygnipes, Phil. Lower Lias, Gloucestershire and Yorkshire.

a. Lower valve.

b. Upper valve.)

(FIGURE 365. Hippopodium ponderosum, Sowerby. 1/4 diameter. Lias, Cheltenham)

(FIGURE 366. Spiriferina (Spirifera) Walcotti, Sowerby. Lower Lias.)

(FIGURE 367. Leptaena Moorei, Davidson. Upper Lias, Ilminster.)

The name of Gryphite limestone has sometimes been applied to the Lias, in consequence of the great number of sh.e.l.ls which it contains of a species of oyster, or Gryphaea (Figure 362). A large heavy sh.e.l.l called Hippopodium (Figure 365), allied to Cypricardia, is also characteristic of the upper part of the Lower Lias. In this formation occur also the Aviculas, Figures 363 and 364. The Lias formation is also remarkable for being the newest of the secondary rocks in which brachiopoda of the genera Spirifer and Leptaena (Figures 366, 367) occur, although the former is slightly modified in structure so as to const.i.tute the subgenus Spiriferina, Davidson, and the Leptaena has dwindled to a sh.e.l.l 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 strata older than the Trias; but, so far as we yet know, they did not survive the Lia.s.sic epoch.

(FIGURE 368. Ammonites Bucklandi, Sowerby. Ammonites bisulcatus, Brug. One- eighth diameter of original.

a. Side view.

b. Front view, showing mouth and bisulcated keel. Characteristic of the lower part of the Lias of England and the Continent.)

(FIGURE 369. Ammonites planorbis, Sowerby. One-half diameter of original. From the base of the Lower Lias of England and the Continent.)

(FIGURE 370. Nautilus truncatus, Sowerby. Lias.)

(FIGURE 371. Ammonites bifrons, Brug. Ammonites Walcotti, Sowerby. Upper Lias shales.)

(FIGURE 372. Ammonites margaritatus, Montf. Syn. Ammonites Stokesi, Sowerby.

Middle Lias.)

Allusion has already been made 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 Ammonites Bucklandi, and the lower by Ammonites planorbis (see Figures 368, 369). (Quarterly Journal volume 16 page 376.) Sometimes, however, there is a third intermediate zone, that of Ammonites angulatus, which is the equivalent of the zone called the infra-lias on the Continent, the species of which are for the most part common to the superior group marked by Ammonites Bucklandi.

(FIGURE 373. Extracrinus (Pentacrinus) Briareus. Miller. 1/2 natural size.

(Body, arms, and part of stem.) Lower Lias, Lyme Regis.)

(FIGURE 374. Palaeocoma (Ophioderma) tenuibrachiata. E. Forbes. Middle Lias, Seatown, Dorset.)

Among the Crinoids or Stone-lilies of the Lias, the Pentacrinites are conspicuous. (See Figure 373.) Of Palaeocoma (Ophioderma) Egertoni (Figure 374), referable to the Ophiuridae of Muller, perfect specimens have been met with in the Middle Lias beds of Dorset and Yorkshire.

The Extracrinus Briareus (removed by Major Austin from Pentacrinus on account of generic differences) occurs in tangled ma.s.ses, 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 lia.s.sic sea, in the same manner as Barnacles float about on wood at the present day. There is another species of Extracrinus and several of Pentacrinus 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 Pentacrinus caput-medusae 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.

FISHES OF THE LIAS.

(FIGURE 375. Scales of Lepidotus gigas. Aga.s.s.

a. Two of the scales detached.)

(FIGURE 376. Aechmodus Leachii and Dapedius monilifer.

a. Aechmodus. Restored outline.

b. Scales of Aechmodus Leachii.

c. Scales of Dapedius monilifer.)

(FIGURE 377. Acrodus n.o.bilis, Aga.s.siz (tooth); commonly called "fossil leech."

Lias, Lyme Regis, and Germany.)

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. Aga.s.siz, from those of the Cretaceous period. Among them is a species of Lepidotus (L.

gigas, Aga.s.siz), Figure 375, which is found in the Lias of England, France, and Germany. (Aga.s.siz Poissons Fossiles volume 2 tab. 28, 29.) This genus was before mentioned (Chapter 18) 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 Aechmodus (Figure 376), is almost exclusively Lia.s.sic. The teeth of a species of Acrodus, also, are very abundant in the Lias (Figure 377).

(FIGURE 378. Hybodus reticulatus, Aga.s.siz. Lias, Lyme Regis.

a. Part of fin, commonly called Ichthyodorulite.

b. Tooth.)

(FIGURE 379. Chimaera monstrosa. (Aga.s.siz Poissons Fossiles volume 3 tab. C Figure 1.) a. Spine forming anterior part of the dorsal fin.)

SBut 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. Aga.s.siz has shown to be neither the one nor the other. The spines, in the genera last mentioned, articulate with the backbone, whereas there are no signs of any such articulation in the ichthyodorulites. These last appear to have been bony spines which formed the anterior part of the dorsal fin, like that of the living genera Cestracion and Chimaera (see a, Figure 379). In both of these genera, the posterior concave face is armed with small spines, as in that of the fossil Hybodus (Figure 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 Chimaera (Figure 379), to raise and depress the fin, their action resembling that of a movable mast, raising and lowering backward the sail of a barge." (Bridgewater Treatise page 290.)

REPTILES OF THE LIAS.

(FIGURE 380. Skeleton of Ichthyosaurus communis, restored by Conybeare and Cuvier.

a. Costal vertebrae.)

(FIGURE 381. Skeleton of Plesiosaurus dolichodeirus, restored by Reverend W.D.

Conybeare.

a. Cervical vertebra.)

It is not, however, the fossil fish which form the most striking feature in the organic remains of the Lias; but the Enaliosaurian reptiles, which are extraordinary for their number, size, and structure. Among the most singular of these are several species of Ichthyosaurus and Plesiosaurus (Figures 380, 381).

The genus Ichthyosaurus, 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 vertebrae, 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.