While the higher members of the Cretaceous system, as seen in the chalk cliffs and downs, represent the upraised calcareous accumulations on the floor of a fairly deep and clear sea, the lower members testify to shallower water within reach of river-borne sand and mud. 'During the Chalk period,' as Huxley wrote, 'not one of the present great physical features of the globe was in existence. Our great mountain ranges, Pyrenees, Alps, Himalayas, Andes, have all been upheaved since the chalk was deposited, and the Cretaceous sea flowed over the sites of Sinai and Ararat'(29). The Wealden strata, at the base of the Cretaceous system, as seen on the Sussex coast, in parts of the Isle of Wight, in the Weald district of Kent and neighbouring counties, point to the existence of a lake over a portion of the south of England and of the English Channel. The remains of a rich Wealden flora have been collected from these Wealden sediments, notably from the plant-beds of Ecclesbourne near Hastings, in which, so far as we know, flowering plants played no part or at most occupied a very subordinate position. A few fossil leaves have been described from rocks assigned to a Wealden age,--and from the older Stonesfield Slate, of Jurassic age, a single leaf is recorded,--which seem to be those of Dicotyledons; but it is certain that even in the early days of the Cretaceous period the present dominant group in the plant kingdom was in its infancy and in many regions probably unrepresented. When we glance at the geological table and consider that in all the floras from the Wealden down to the Devonian period, flowering plants played no part, we are able to appreciate the fact of their rapid development, referred to in a previous chapter, when once this highest type had become established. The rocks comprised in the Jurassic system extend from East Yorkshire to the coast of Dorsetshire; they consist of a succession of limestones, clays, sandstones, and a few thin beds of impure coal. Sediments of this age also occur, though to a much less extent, on the north-east coast of Scotland and in a few places in the Inner Hebrides. Many of the Jurassic strata contain only marine shells, and corals are occasionally abundant, though in the lower members of the system in the cliffs near Lyme Regis and at Whitby fossil plants are fairly common. It is, however, from the middle Jurassic beds, in the cliffs between Whitby and Scarborough, and in some inland quarries in East Yorkshire, that we have obtained the richest Jurassic flora. Rivers from a northern land laden with sediment and carrying drift-wood, leaves and other plant fragments, deposited their burden in an estuary which occupied the eastern edge of Yorkshire. Sedimentary rocks laid down towards the close of the Jurassic period in the island of Portland in the south and on the Sutherland coast in the north have furnished valuable records of plant-life.