The Trias is succeeded by a great and complex system of formations, usually known as the Jurassic, from its admirable development and exposure in the range of the Jura; but which the English geologists often name the "Oolitic," from the occurrence in it of beds of Oolite or roe-stone. This rock, of which the beautiful cream-coloured limestone of Bath is an illustration, consists of an infinity of little spheres, like seeds or the roe of a fish. Under the microscope these are seen to present concentric layers, each with a radiating fibrous: structure, and often to have a minute grain of sand or fragment of shell in the centre. They are, in short, miniature concretions, produced by the aggregation of the calcareous matter around centres, by a process of molecular attraction to which fine sediments, and especially those containing much lime, are very prone. This style of limestone is very abundant in the Jurassic system, but it is not confined to it. I have seen very perfect Oolites in the Silurian and the Carboniferous. The Jurassic series, as developed in England, may be divided into three triplets or cycles of beds, in the following way: {Purbeck Beds. Upper Jurassic {Portland Limestone. {Portland Sand. {Kimmeridge Clay, etc. Middle Jurassic {Coral Rag, Limestone. {Lower Calcareous Grit, Oxford Clay, etc. {Cornbrash and Forest Marble. Lower Jurassic[AD] {Great and inferior Oolite, Limestone. {Lias Clays and Limestones. [AD] This last group is very complex, and might perhaps admit of sub division, locally at least, into subordinate cycles.