LATER GEOLOGICAL HISTORY OF NORTH AMERICA 17 



Continental outlines have changed greatly during the vast lapse 

 of geologic time, but the student of modern floras need not concern 

 himself with the earlier and more profound changes due to the 

 past movements of the strand. Those changes in geography, 

 topography and climate which have gone hand in hand with the 

 development of the present flora of North America may be sketched 

 in a few words and need not be carried back farther than the Mid- 

 Cretaceous. 



The Mid-Cretaceous shows in the character and distribution of 

 its rocks that the continental outline of North America had assumed 

 much its modern form except for the extension of the Gulf of Mexico 

 over Texas and into Colorado and Kansas as is indicated upon the 

 accompanying sketch map (fig. 1). The waters of the Pacific had 

 submerged a narrow strip along the western coast and the Coast 

 Range was not yet elevated. In marked contrast with modern 

 conditions the present Rocky Mountain region was then one of 

 low relief upon which were being deposited scattered fluviatile, 

 lacustrine, swamp and terrestrial sediments. 



As time passed both the Atlantic and more of the Pacific con- 

 tinental borders became submerged and the waters of the Gulf of 

 Mexico passed northward over the present sites of the Great Plains 

 and Rocky Mountains to become mingled with the waters of the 

 Arctic Ocean which had advanced up the valley of the Mackenzie- 

 River, thus forming a vast but extremely shallow Mediterranean 

 Sea and widely separating northeastern North America from the 

 Pacific coast strip which was connected directly with Siberia across 

 what is now Bering Sea. This stage in the history of North America 

 is shown in the sketch map, figure 2. ^ 



' The whole continent at that time was low and heavily wooded, ' 

 i and enjoyed an abundant rainfall throughout and a climate that 

 was much more uniform than it is at the present time. There seems 

 to have been a total absence of frost, and palms and figs were able 

 to flourish far to the northward of the present international bound- 

 ary. Naturally this flora was very different from that of today as 

 it contained many elements which are now extinct as well as others 

 which are no longer American, and a curious mixture of types 



