THE COMANCHEAN PERIOD 735 



The Close of the Comanchean (Lower Cretaceous) period in North 



America 



Considerable changes in the geography of North America brought 

 the Comanchean period to a close. Along the Atlantic and Gulf 

 borders considerable tracts were converted from areas of depo- 

 sition into areas of erosion. The sea was withdrawn from Texas, 

 and the Comanchean system somewhat deformed and faulted; in 

 Mexico the deformation of the system was notable. Along some 

 parts of the western coast, there was folding of the Lower Creta- 

 ceous beds, 1 accompanied by volcanic activity, as in the southern 

 Coast Range of California, while in other places the sea spread itself 

 over areas which had been land. Still other areas in the west 

 appear to have emerged at this time, and never to have been sub- 

 merged since. 2 



On the whole, the deformative movements at the close of the 

 Comanchean period were more extensive than those which occurred 

 in the midst of any one of the Paleozoic periods as now denned, 

 if the Mississippian and Pennsylvanian are regarded as separate 

 periods. The force of this point in its bearing on the distinctness 

 of the Comanchean period from the Cretaceous, is increased by the 

 fact that the latter was inaugurated by a notable submergence, 

 affecting great areas. During this submergence, Cretaceous beds 

 of marine origin were deposited on the eroded surface of the Coman- 

 chean system, somewhat generally, and the younger system over- 

 lapped the older in most regions where both are present. The 

 reverse was the case, however, in British Columbia. On strati- 

 graphic grounds, therefore, the distinctness of the two systems is 

 clear. The case is hardly less clear on the paleontological side. In 

 Texas, for example, no species of marine life is known to have lived 

 over the time-interval recorded by the unconformity between the 

 two systems. 



1 Fairbanks, Jour. Geol., Vol. Ill, pp. 415-430, and San Luis folio, U. S. 

 Geol. Surv. 



2 Ransome, Bisbee, Ariz, folio, U. S. Geol. Surv. 



