To the upper Jurassic beds, which contained much more limestone and also occasional beds of sandstone, the name of Oolite was given. The Oolitic strata have a special interest for English geologists, for it was in them that William Smith, the west of England surveyor, first made out (about the year 1790) the _order of succession_ of the strata, and by this was led to his great discovery that "strata could be identified by their organic remains," that is by their fossils. He noticed that some of the limestone beds of the strata we are about to describe consisted of small rounded grains, which made them resemble the roe of a fish--indeed, they were called "roestone" by the workmen. Hence Smith--when seeking a name for this set of strata--bethought himself of the term "Oolite," which means "egg-stone" (Gr. _oon_, an egg, and _lithos_, a stone). Where the grains are very large the limestone is called "pea-grit" or _pisolite_ (Lat. _pisum_, a pea). Some beds which contain numerous and irregularly shaped fragments of shells, corals, etc., are called _rag-stones_. [Illustration: Plesiosaurs Different species ranged from ten to forty feet in length.] The Jurassic strata of Great Britain were sediments laid down in warm seas surrounding an archipelago of which Dartmoor, Wales, and Cumberland formed some of the islands. The whole of Western Europe was sinking and had sunk; and the waters of the open ocean were admitted into and mingled with the salt mineral waters of the great Triassic lakes. The change was at first very like what would happen at the present day if the coast of Palestine were depressed, so that the waters of the Mediterranean flowed into the Dead Sea. The few fish of the salt lakes were killed; and as the land continued to sink, the sea at last flowed all over Central and West England, bringing with it an abundance of marine life. But the reptiles were far from being finished with; and the progress of the small mammals was extremely slow.