Part 41 (1/2)

_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 Mars.h.i.+_ (fig. 300.), common to the Cornbrash of Wilts and the Inferior Oolite of Yorks.h.i.+re; and _Ammonites striatulus_ (fig. 301.) common to the Inferior Oolite and Lias.

[Ill.u.s.tration: Fig. 300. _Ostrea Mars.h.i.+._ 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.