THE MESOZOIO AGES. 191 



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 than it does at present ; so that no 

 marine beds were formed on its eastern border. But 

 in the west, along the base of the Eocky Mountains 

 and also in the Arctic area, there were Jurassic seas 

 of large extent, sw.arming with characteristic animals. 

 At the close of the Jurassic period our continents 

 seem to have been even more extensive than at pre- 

 sent. In England and the neighbouring parts of 

 the continent of Europe, according to Lyell, the 

 freshwater 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 than 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 exist- 

 ence of a continent much more extensive and more 

 uniform in drainage than Europe as it at present 

 exists. Lyell even speculates on the possible exist- 

 ence of an Atlantic continent west of Europe. 

 America also at this time had, as already stated, 

 attained to even more than its present extension 



