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CHAPTER XVII.

OOLITE AND LIAS.

Subdivisions of the Oolitic group- Fossil shells Corals in the calcareous divisions only — Buried forest of Encrinites in Bradford clay — Changes in organic life during accumulation of Oolites - Characteristic fossils — Signs of neighbouring land and shoals - Supposed cetacea in Oolite. Oolite of Yorkshire and Scotland.

OOLITE. Below the freshwater group last described, or, where this is wanting, immediately. beneath the Cretaceous formation, a great series of marine strata, commonly called "the Oolite,” occurs in many parts of Europe. This group has been so named, because, in England and other places where it was first examined, the limestones belonging to it had an oolitic structure (see p. 29.). These rocks occupy in England a zone which is nearly thirty miles in average breadth, and extends across the island, from Yorkshire on the north-east, to Dorsetshire on the south-west.* Their mineral characters are not uniform throughout this region; but the following are the names

* For details respecting this formation in England, see Conybeare and Phillips's Geology, chap. iii.

of the principal subdivisions observed in the central and south-eastern parts of England:

e. Cornbrash and Forest Marble.

Lower f. Great Oolite and base of Fullers' earth.
g. Inferior Oolite.

The Lias then succeeds to the Inferior Oolite.

The upper oolitic system of the above Table has usually the Kimmeridge clay for its base, and 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-west of France, and the Jura mountains, which separate that country from Switzerland, and in which, though distant above 400 geographical miles, the analogy to the English type above mentioned is more perfect than in Yorkshire or Normandy.

To enter upon a systematic description of this complicated series of strata would require many

chapters; the following facts, therefore, are selected from a multitude of others, with a view of illustrating the origin of the oolitic rocks, and of showing the state of organic life and geographical condition of part of the globe when they were formed.

In almost all the minor divisions enumerated in the above Table, Ammonites and Belemnites are found (see Figs. 213. 215.), but of species different from those of the cretaceous period. The ammonites are of various sizes, from the size of a small carriage-wheel to less than an inch diameter.

It is not uncommon to find belemnites in different members of the series, with full grown serpulæ attached to them. As these shells, like the bone of the cuttle-fish, so often thrown on our shores, were internal, it is clear, that after the death of the cephalopod the belemnite remained for some time unburied at the bottom of the sea, so that the serpulæ grew upon it.

These cephalopoda, swimming about in the open sea, left their shells to be imbedded indifferently in whatever sediment was then in the course of deposition, whether calcareous or argillaceous. But the corals are almost entirely confined to the limestones, and are wanting in the dense formations of interposed clay, as also in the lias, these zoophytes requiring, not only carbonate of lime for their support, and clear water, but a bottom remaining

for years unchanged, either by the shifting of sand or the accession of fresh sediment.

In the Upper Oolite of England, corals are rare, although one species is found plentifully at Tisbury, in Wiltshire, in the Portland sand, converted into flint and chert, the original calcareous matter being replaced by silex. (Fig. 199.) 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, for the most part retaining the position in which they grew at the bottom of the sea. They belong chiefly to the genera Caryophyllia (Fig. 200.), Agaricia, and

Astrea, and sometimes form masses of coral fifteen feet thick. These coralline strata extend through the calcareous hills of the N. W. of Berkshire, and north of Wilts, and again recur in Yorkshire,

near Scarborough. Although the name of coral rag has been thus appropriated, there are portions of the lower oolite, as for example the Great and Inferior Oolite (ƒ. g. Table, p. 369.), which are equally entitled in many places to be called coralline limestones. Thus the Great Oolite near Bath contains various corals, among which the Eunomia radiata (Fig. 201.) is very conspicuous, Fig. 201.

Eunomia radiata, Lamouroux.

a. section transverse to the tubes.

b. vertical section, showing the radiation of the tubes.

c. portion of interior of tubes magnified, showing striated surface.

single individuals forming masses several feet in diameter; and having probably required, like the large existing brain-coral (Meandrina) of the tropics, many centuries before their growth was completed.

Different species of Crinoideans, or stone-lilies, are also common in the same rocks with corals; and, like them, must have enjoyed a firm bottom, where their root, or base of attachment, remained undisturbed for years (c Fig. 202.) Such fossils,