Handbook of Paleontology 433 



the emergence of the continent continued from the Trias- 

 sic ; and there were no deposits except perhaps fresh- 

 water deposits in Maryland along the Potomac river and 

 marine deposits in Mexico where the present Gulf of 

 Mexico had extended westward. In the western interior 

 no early Jurassic deposits are known. Later in the period 

 a shallow arm of the sea extended from Alaska as far 

 south as Utah and as far east as South Dakota but was 

 soon drained through elevation of the land. Another area 

 of deposition was in the great western geosyncline along 

 the present Pacific coast. At the close of the period the 

 enormous accumulation of sediments in this trough 

 (Paleozoic, Triassic and Jurassic) were raised into the 

 Sierra Nevada mountains, and about at the same time 

 the growth of the Coast and Cascade ranges and the 

 Klamath mountains was started. 



The Jurassic period received its name from the occur- 

 rence of rocks of this age in the Jura mountains. 



Cretaceous Period 



The Cretaceous period or "Age of Chalk" received its 

 name from the conspicuous chalk (Latin, creta) deposits 

 of this time. 



No marine deposits of the Lower Cretaceous or Co- 

 manchean occur along the Atlantic coast, but there are 

 deposits of gravel and clays laid down as deltas and flood 

 plains or in marshes or shallow lakes which extend from 

 Martha's Vineyard, Mass., to Georgia. There was an 

 expansion of the Gulf of Mexico westward and north- 

 westward, covering large areas of Mexico, Texas and 

 New Mexico, in which marine deposits were laid down. 

 In the western interior only nonmarine beds (sometimes 

 with coal) occur, and on the Pacific coast deposition took 



