These rocks occupy a large space in England, as the names above given will serve to show; and they are also largely distributed over the continent of Europe and Asia which had evidently three great and long-continued dips under water, indicated by the three great limestones. In America the case was different. The Jurassic has not been distinctly recognised in any part of the eastern coast of that continent, which then perhaps extended farther into the Atlantic then it does at present; so that no marine beds were formed on its eastern border. But in the west, along the base of the Rocky Mountains and also in the Arctic area, there were Jurassic seas of large extent, swarming with characteristic animals. At the close of the Jurassic period our continents seem to have been even more extensive then at present. In England and the neighbouring parts of the continent of Europe, according to Lyell, the fresh-water and estuarine beds known as the Wealden have been traced 320 miles from west to east, and 200 miles from north-west to south-east, and their thickness in one part of this area is estimated at no less then 2,000 feet. Such a deposit is comparable in extent with the deltas of such great rivers as the Niger or even the Mississippi, and implies the existence of a continent much more extensive and more uniform in drainage then Europe as it at present exists. Lyell even speculates on the possible existence of an Atlantic continent west of Europe. America also at this time had, as already stated, attained to even more then its present extension eastwards. Thus this later Jurassic period was the culmination of the Mesozoic, the period of its most perfect continental development, corresponding in this to the Carboniferous in the Palæozoic.