SH 



HISTORICAL GEOLOGY 



and seas which cut them into a number of large and small islands. 

 The submergence of central and northern Russia is of especial in- 

 terest, since in this basin was developed a peculiar fauna which 

 spread into the great mediterranean sea of the western interior of 

 North America. An arm of the sea covering the site of the Hima- 

 layas separated India from northern Asia. This Upper Jurassic sub- 

 mergence in Europe and Asia was one of the greatest in all the re- 

 corded geological history of these continents. 



Lower Cretaceous (Comanchean) 



Atlantic and Gulf Coasts. — No marine deposits of the Lower 

 Cretaceous (Fig. 485) have been found on the Atlantic coast, but 



a belt of continental 

 sediments, stretch- 

 ing from Marthas 

 Vineyard island to 

 Georgia (the Poto- 

 mac group), occurs, 

 which seldom attains 

 a thickness of more 

 than 600 feet. The 

 lesson which these 

 deposits teach of the 

 physical condition 

 at, and immediately 

 preceding, the time 

 of their formation is 

 interesting. They 

 show that at the 

 close of the Jurassic 

 the eastern portion 

 of the continent had 

 been reduced to a 

 comparatively level 

 plain over wide 

 areas, the surface of 

 Fig. 485. — Map showing the probable outline of which was covered 

 North America during a portion of the Lower Creta- 11 • l l 



ceous. Continental deposits are shown in solid black. deeply With weath- 

 (Modified after Schuchert.) ered rock, resulting 



