CHAPTER XXV 

 THE COMANCHEAN (LOWER CRETACEOUS) PERIOD 1 



Introductory 



The history of the Cretaceous period, as formerly defined, was 

 complex. At its beginning, the larger part of the North American 

 continent was above the sea. During its progress, the sequence 

 of events in our continent was somewhat as follows: (1) A somewhat 

 wide-spread warping of the continental surface, resulting in ex- 

 tensive submergence in Mexico and Texas, and a lesser submergence 

 along the Pacific coast. At about the same time the Atlantic and 

 Gulf coasts and some parts of the western interior were brought 

 into such an attitude as to become sites of deposition, though not 

 submerged. A prolonged period of sedimentation followed. (2) 

 The period of sedimentation was followed by other geographic 

 changes which inaugurated a prolonged interval of erosion which 

 affected the recent deposits as well as older formations. (3) Later, 

 the sea encroached upon the Atlantic and Gulf borders, extending 

 somewhat beyond the non-marine formations of the earlier stage. 

 It again covered Texas, and presently extended northward over 

 the Great Plains, probably to the Arctic Ocean. On the Pacific 

 coast, too, the sea gained on the land. Few greater transgressions 

 of the land by the ocean are recorded in the long history of the 

 North American continent. A long period of deposition was initi- 

 ated by the submergence. It was succeeded by (4) a wide-spread 

 withdrawal of the waters from the continent, leaving the land area 

 nearly or quite as large as now. 



The formations of the Cretaceous period, as outlined under (1) 

 and (3) above, have been divided, commonly, into two main series, 

 a Lower and an Upper. To the former were referred the deposits 



1 For a full review of the American Cretaceous, up to 1891, see White 

 (C. A.), Bull. 82, U. S. Geol. Surv. 



727 



