Oolite (Gr., an, and?., a stone), limestone composed of rounded particles, like the roe or eggs of a fish. Each of the grains has usually a small fragment of sand as a nucleus, around which concentric layers of calcareous matter have accumulated. Ferruginous oolites are also found, as in the Clinton group in the United States, the investing material being in this case red oxide of iron. The name oolite is also applied to a group of strata of the Jurassic period, largely developed in England, in which limestone of this character occurs. The oolitic epoch is embraced between the liassic below and the Wealden above, these three epochs forming the Jurassic period, the middle period of the reptilian age. The oolitic formation contains limestones, part of which are oolitic in texture, and others arenaceous and clayey, all being of marine origin except the Purbeck beds which rest upon the upper or Portland oolite. The following subdivisions are recognized: 1. Lower or Bath oolite, containing, among other groups, fuller's earth and the great oolitic limestone, beneath which are the Stonesfield slates, near Oxford, noted for the remains of saurians, insects, and the earliest British mammals.
There is also in the lower oolite in Sutherlandshire a bed of good coal 3½ ft. thick, which has been worked for a long time. 2. Middle oolite, containing calcareous grits with Oxford clay between.
3. Upper oolite, containing Kimmeridge clay, which furnishes material for Portland cement.
4. The Purbeck beds, consisting of lower, middle, and upper Purbeck, which contain freshwater marls, the "Portland dirt bed " being in the lower, and containing the second deposit of British mammals.