Inoceramus concentricus (from the Gault).
3. Oolites (or Roe-stones) are so named because the characteristic limestones of this formation resemble very much the roe of a fish. The small round grains, of which the typical examples are built up, when cut or broken through will be seen to be formed of numerous layers of carbonate of lime, disposed like the coats of an onion, around some central nucleus, generally a grain of sand, a fragment of coral, or the shell of one of the Foraminifera. They are divided into Upper, Middle, and Lower Oolites, and these again are subdivided as follows—
Upper Oolite.
a. Purbeck Beds, a series of fresh-water, with a few estuarine, or marine beds, which in point of fact connect the deposits we are next coming to with the Wealden just passed. They contain numerous fresh-water shells—Paludina, Physa, Limnæa, etc., with the microscopic valves of the little fresh-water crustacean Cypris, whose descendants are abundant in the rivers and lakes of to-day. An oyster occurs in the "Cinder Bed" and Plant remains in the "Dirt Beds." But the Purbecks are best known for the numerous remains of small mammals (Plagiaulax) allied to the kangaroo rat, at present living in Australia.
b. The Portland Stone and Sand, which come next in order, are largely quarried in the island whence they take their name. The quarrymen point out fossils in the stone, which they call "Horses'-heads" and "Portland screws." The former is the cast of a Trigonia shell; the latter, that of a tall spired univalve (Cerithium).
In Wiltshire, a coral (Isastrea oblonga) is found in the sandy beds, the original calcareous matter of which has been replaced by silex.
c. Kimmeridge Clay. This, by the pressure of the rocks subsequently deposited on it, has in greater part been hardened, and possesses a tendency to split in thin layers, and hence is termed by geologists a shale. It is seen at various points between Kimmeridge on the Dorsetshire coast and the Vale of Pickering in Yorkshire, and forms broad valleys. The principal fossils in it are Ammonites, a triangular-shaped oyster (Ostrea deltoidea), and one resembling a comma (Exogyra virgula).
Middle Oolites.
a. The Coral Rag, or Coralline Oolite, comprises a most variable set of beds, but principally a series of limestone, with fossil corals still in the position in which they grew, and resembling in form the reef-building corals of the Pacific. They rest on
b. Oxford Clay, a dark blue or slate-coloured clay without any corals, but containing a great many Ammonites and Belemnites. The Kelloway Rock, a sandy limestone at the base of the Oxford Clay, is well developed in Yorkshire, and furnishes amongst other fossils a large belemnite and an oyster (Gryphæa dilatata).