A Manual of Elementary Geology / or, The Ancient Changes of the Earth and its Inhabitants as Illustrated by Geological Monuments

by Sir Charles Lyell

Fig. 270.

Trigonia gibbosa. ¹/₂ nat. size. a. the hinge.

Portland Oolite, Tisbury.

The Kimmeridge clay 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, the bitumen may perhaps be of animal origin.

The celebrated lithographic stone of Solenhofen, in Bavaria, belongs to one of the upper divisions of the oolite, and 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 species of flying lizards, or pterodactyls, 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 flying lizards, and other contemporaneous reptiles, resorted.

Middle Oolite.

Coral Rag.—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. 271.]), Agaricia, and Astrea, and sometimes form masses of coral 15 feet thick. In the annexed figure of an Astrea, 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, till those on the left side are nearly filled up. The last-named stars are supposed to be Polyparia of advanced age. 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.