

CHAPTER XXIV 

 THE COMANCHEAN (LOWER CRETACEOUS) PERIOD 



Definition. 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: 

 (i) A somewhat widespread warping of the continental surface, 

 resulting in extensive 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 

 si us of deposition, though not submerged. Prolonged sedimenta- 

 tion followed. (2) Geographic changes which inaugurated a long 

 period of erosion that affected the recent deposits as well as older 

 formations. (3) Encroachment of the sea submerging the Coastal 

 Plain of the Atlantic and the Gulf of Mexico, and presently the 

 drrut Plains, probably to the Arctic Ocean. On the Pacific coast, 

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

 by sea are recorded in the long history of the North American con- 

 tinent. "A long period of deposition was initiated by the sub- 

 mergence. It was succeeded by (4) a widespread withdrawal of 

 the waters from the continent, leaving the land area nearly or quite 

 as large as now. 



The -formations of the Cretaceous period have been divided, 

 commonly, into two main series, a Lower and an Upper. To the 

 former were referred the deposits of the earlier and lesser submer- 

 gence, and to the latter; those of the later and more extensive sub- 

 mergence. The distinctness of the Lower and Upper Cretaceous is, 

 however, so great that it is more in keeping with the spirit of modern 

 classification to regard them as separate systems, and the corre- 

 sponding divisions of time as periods. What was formerly called 

 the Lower Cretaceous series is here called the Comanchean system. 

 The propriety of this classification is the more striking, since it is 

 applicable to other continents as well as to our own. 



