A Manual of Elementary Geology Part 41

_Inferior Oolite._--Between the Great and Inferior Oolite, near Bath, an argillaceous deposit called "the fuller's earth," occurs, but is wanting in the north of England. The Inferior Oolite is a calcareous freestone, usually of small thickness, which sometimes rests upon, or is replaced by, yellow sands, called the sands of the Inferior Oolite. These last, in their turn, repose upon the lias in the south and west of England.

Among the characteristic sh.e.l.ls of the Inferior Oolite, I may instance _Terebratula spinosa_ (fig. 297.), and _Pholadomya fidicula_ (fig. 298.).

The extinct genus _Pleurotomaria_ is also a form very common in this division as well as in the Oolitic system generally. It resembles the _Trochus_ in form, but is marked by a singular cleft (_a_, fig. 299.) on the right side of the mouth.

[Ill.u.s.tration: Fig. 297. _Terebratula spinosa._ Inferior Oolite.]

[Ill.u.s.tration: Fig. 298.

_a._ _Pholadomya fidicula_, 1/3 nat. size. Inf. Ool.

_b._ Heart-shaped anterior termination of the same.]

[Ill.u.s.tration: Fig. 299. _Pleurotomaria ornata._ Ferruginous Oolite, Normandy. Inferior Oolite, England.]

As ill.u.s.trations of sh.e.l.ls having a great vertical range, I may allude to _Trigonia clavellata_, found in the Upper and Inferior Oolite, and _T.

costata_, common to the Upper, Middle, and Lower Oolite; also _Ostrea Marshii_ (fig. 300.), common to the Cornbrash of Wilts and the Inferior Oolite of Yorkshire; and _Ammonites striatulus_ (fig. 301.) common to the Inferior Oolite and Lias.

[Ill.u.s.tration: Fig. 300. _Ostrea Marshii._ 1/2 nat. size. Middle and Lower Oolite.]

[Ill.u.s.tration: Fig. 301. _Ammonites striatulus_, Sow. 1/3 nat. size.

Inferior Oolite and Lias.]

Such facts by no means invalidate the general rule, that certain fossils are good chronological tests of geological periods; but they serve to caution us against attaching too much importance to single species, some of which may have a wider, others a more confined vertical range. We have before seen that, in the successive tertiary formations, there are species common to older and newer groups, yet these groups are distinguishable from one another by a comparison of the whole a.s.semblage of fossil sh.e.l.ls proper to each.

FOOTNOTES:

[259-A] See Chapters VI. and XIX.

[261-A] Fitton, Geol. Trans., Second Series, vol. iv. pl. 23. fig. 12.

[262-A] S. P. Pratt, Annals of Nat. Hist., November, 1841.

[263-A] See Phil. Trans. 1850, p. 393.

[263-B] P. Scrope, Geol. Proceed., March, 1831.

[265-A] For a fuller account of these Encrinites, see Buckland's Bridgewater Treatise, vol. i. p. 429.

[266-A] Lycett, Quart. Geol. Journ. vol. iv. p. 183.

[266-B] Proceedings Geol. Soc. vol. i. p. 414.

[267-A] See Buckland's Bridgewater Treatise; and Brodie's Fossil Insects, where it is suggested that these elytra may belong to _Priomus_.

[267-B] Vol. i. p. 115.

[269-A] I have given a figure in the Principles of Geology, chap. ix., of another Stonesfield specimen of _Amphitherium Prevostii_, in which the sockets and roots of the teeth are finely exposed.

[269-B] A figure of this recent _Myrmecobius_ will be found in the Principles, chap. ix.

[270-A] Owen's British Fossil Mammals, p. 62.

[271-A] Ibbetson and Morris, Report of Brit. a.s.s., 1847, p. 131.

CHAPTER XXI.

OOLITE AND LIAS--_continued_.

Mineral character of Lias--Name of Gryphite limestone--Fossil sh.e.l.ls and fish--Ichthyodorulites--Reptiles of the Lias--Ichthyosaur and Plesiosaur--Marine Reptile of the Galapagos Islands--Sudden destruction and burial of fossil animals in Lias--Fluvio-marine beds in Gloucestershire and insect limestone--Origin of the Oolite and Lias, and of alternating calcareous and argillaceous formations--Oolitic coal-field of Virginia, in the United States.

_LIAS._--The English provincial name of Lias has been very generally adopted for a formation of argillaceous limestone, marl, and clay, which forms the base of the Oolite, and is cla.s.sed by many geologists as part of that group. They pa.s.s, indeed, into each other in some places, as near Bath, a sandy marl called the marlstone of the Lias being interposed, and partaking of the mineral characters of the upper lias and inferior oolite.

These last-mentioned divisions have also some fossils in common, such as the _Avicula inaequivalvis_ (fig. 302.). Nevertheless the Lias may be traced throughout a great part of Europe as a separate and independent group, of considerable thickness, varying from 500 to 1000 feet, containing many peculiar fossils, and having a very uniform lithological aspect. Although usually conformable to the oolite, it is sometimes, as in the Jura, unconformable. In the environs of Lons-le-Saulnier, for instance, in the department of Jura, the strata of lias are inclined at an angle of about 45, while the inc.u.mbent oolitic marls are horizontal.

[Ill.u.s.tration: Fig. 302. _Avicula inaequivalvis_, Sow.]

The peculiar aspect which is most characteristic of the Lias in England, France, and Germany, is an alternation of thin beds of blue or grey limestone with a surface becoming light-brown when weathered, these beds being separated by dark-coloured narrow argillaceous partings, so that the quarries of this rock, at a distance, a.s.sume a striped and riband-like appearance.[274-A]

Although the prevailing colour of the limestone of this formation is blue, yet some beds of the lower lias are of a yellowish white colour, and have been called white lias. In some parts of France, near the Vosges mountains, and in Luxembourg, M. E. de Beaumont has shown that the lias containing _Gryphaea arcuata_, _Plagiostoma giganteum_ (see fig. 303.), and other characteristic fossils, becomes arenaceous; and around the Hartz, in Westphalia and Bavaria, the inferior parts of the lias are sandy, and sometimes afford a building stone.

[Ill.u.s.tration: Fig. 303. _Plagiostoma giganteum._ Lias.]

[Ill.u.s.tration: Fig. 304. _Gryphaea incurva_, Sow. (_G. arcuata_, Lam.)]

[Ill.u.s.tration: Fig. 305. _Nautilus truncatus._ Lias.]

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_ (fig. 304., see also fig. 30. p. 29.). Many cephalopoda, also, such as _Ammonite_, _Belemnite_, and _Nautilus_ (fig.

305.), prove the marine origin of the formation.

[Ill.u.s.tration: Fig. 306. Scales of _Lepidotus gigas_, Agas.

_a._ two of the scales detached.]

The fossil fish resemble generically those of the oolite, belonging all, according to M. Aga.s.siz, to extinct genera, and differing remarkably from the ichthyolites of the Cretaceous period. Among them is a species of _Lepidotus_ (_L. gigas_, Agas.) (fig. 306.), which is found in the lias of England, France, and Germany.[275-A] This genus was before mentioned (p.

229.) as occurring in the Wealden, and is supposed to have frequented both rivers and coasts. The teeth of a species of _Acrodus_, also, are very abundant in the lias (fig. 307.).

[Ill.u.s.tration: Fig. 307. _Acrodus n.o.bilis_, Agas. (tooth); commonly called fossil leach. Lias, Lyme Regis, and Germany.]

[Ill.u.s.tration: Fig. 308. _Hybodus reticulatus_, Agas. Lias, Lyme Regis.

_a._ Part of fin, commonly called Ichthyodorulite.

_b._ Tooth.]

But the remains of fish which have excited more attention than any others, are those large bony spines called _ichthyodorulites_ (_a_, fig. 308.), which were once supposed by some naturalists to be jaws, and by others weapons, resembling those of the living _Balistes_ and _Silurus_; but which M. 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_, fig. 309.). In both of these genera, the posterior concave face is armed with small spines like that of the fossil _Hybodus_ (fig. 308.), one of the shark family found fossil at Lyme Regis. Such spines are simply imbedded in the flesh, and attached to strong muscles. "They serve," says Dr. Buckland, "as in the _Chimaera_ (fig. 309.), to raise and depress the fin, their action resembling that of a moveable mast, raising and lowering backwards the sail of a barge."[276-A]