CHAPTER XXIII 

 THE CRETACEOUS PERIOD 1 



Renewed inundation of the continent. Throughout 

 the first three periods of the Mesozoic era the seas had over- 

 spread only small portions of the United States. The areas 

 submerged were (1) the Pacific coast in the Triassic, with the 

 addition of the northwestern mountain region in the Jurassic, 

 and (2) the southwest part of the Great Plains in the 

 Comanchean. 



The Cretaceous period, on the other hand, was charac- 

 terized by a widespread inundation of this and most other 

 continents. Fof North America, at least, it was the last 

 time that any large part of the continent was covered by 

 the sea. 



The coastal plain submerged. It will be recalled that 

 along the Atlantic coast the Comanchean strata are sands 

 and clays, which were not deposited in the sea, but more 

 probably along rivers and marshes. A series of marine beds 

 of Cretaceous age lies upon them, but the two are separated 

 by a slight unconformity. The unconformity represents an 

 interval during which the plain was land, and was subject to 

 erosion. 



The Cretaceous beds are, like the Comanchean, almost 

 unconsolidated, but they differ in some respects from the 

 latter. Besides clay and ordinary sand, one of the constit- 

 uents of the Cretaceous, especially in New Jersey, is green- 

 sand,' which seems to be a chemical precipitate formed in 

 relatively shallow water. 



1 As used here, the name Cretaceous refers to the " Upper Cretaceous " of 

 many writers. 



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