It is one of the many striking facts of geology that, as we pass from one great group of rocks to another, we find a considerable change both in lithologioal and zoological characters. Sometimes the transition is abrupt and sudden, at others, slow and gradual. In the former case, the marine fauna of the older deposit had perished, and a new race sprung up and succeeded to its place; while, in the latter, there was only a gradual dying out of some species unable to exist under the changed oceanic conditions, others more hardy still continuing to flourish in the later formation. But this change of genera and species is not confined necessarily to the breaks which seem to have taken place between one great rock-system and another; it is often to be noted in a greater or less degree, as we have seen in our review of the Oolites in this county, in the minor divisions and sub-divisions of each formation. The student must also bear in mind that the strata which form the crust of the globe were, for the most part, deposited by the sea; a very small proportion only being the products of fresh water, and still fewer being purely terrestrial. Therefore, in our researches into the history of the past, we have to deal more usually with the varied inhabitants of the ancient seas, and to mark the changes which took place from time to time as we pass through either entire periods, or the parts and details into which they may be divided.