{ Corallian Middle Oolites { Oxfordian { Callovian Lower Oolites { Bathonian { Bajocian { Toarcian Lias { Liassian { Sinemurian. Many of these series have been still farther subdivided into smaller stages, and the whole differentiated into a number of zones characterised by different forms of Ammonites. Dr E. von Mojsisovics gives thirty-two Ammonite zones, of which fourteen occur in the Lias, eight in the Lower Oolites, six in the Middle Oolites, and four in the Upper Oolites. _Characters of the strata._ The whole of the Jurassic rocks and also those of Lower Cretaceous age may be regarded as having been deposited during the first shallow water phase of the third marine period, but this shallow water phase is represented by strata which are varied owing to numerous marine changes resulting in the production of land at times, and estuarine conditions, shallow water, marine conditions, and somewhat deeper sea conditions respectively at other times, and accordingly the strata of the British Isles vary greatly when traced laterally. That the uplifts of the Permo-Triassic periods produced some effect on the nature and distribution of the Jurassic rocks is certain, but it is not quite clear how far the ridges produced by these uplifts were submerged and denuded during the deposition of the main portion of the Jurassic strata. Viewed broadly, the Jurassic rocks of Britain may be regarded as consisting of three great clay deposits, the Lias, Oxford and Kimmeridge Clays, alternating with the deposits of variable lithological characters, which compose the Bajocian, Bathonian, Corallian, Portlandian and Purbeckian subdivisions. This essentially argillaceous character of a large part of the deposits of Jurassic age is often overlooked, as, owing to their sameness and the comparative paucity of organisms constituting the faunas in the clays, their description in text-books can be given at much shorter length than that of the more variable and highly fossiliferous deposits which separate the clays. The following figure (Fig. 23) roughly represents the nature of the different divisions of the rocks of this system when traced across England from south-west to north-east. [Illustration: Fig. 23. Vertical scale: 1 in. = about 1000 feet.] It will be seen that the greatest variations in lithological character occur in the Bathonian and Bajocian beds, and it will be of interest to give some account of the principal variations and to attempt to account for them. In so doing it will be convenient to consider the four major divisions of the Jurassic rocks separately, and to enter into particulars concerning the local classification applied to the rocks of these divisions.