. THE COMANCHEAN PERIOD 415 



These are known locally as the Potomac series. Similar 

 strata are found in the eastern Gulf states, and there, too, 

 they are not of marine origin. In Texas and Mexico the sea 

 spread far inland during the early part of the Comanchean 

 period. That the submergence came on gradually and 

 disappeared equally slowly may be readily inferred from the 

 character of the rocks of that age. Thus the lowest beds of 

 Comanchean age in Texas do not contain marine fossils. They 

 are mixed sands and clays, with traces of marsh vegetation, 

 facts which indicate that they were laid down upon a low- 

 lying land, perhaps in the lagoons and flats along meandering 

 rivers. Upon these earlier strata shales and limestones are 

 found, and the marine shells which they contain prove that 

 they were deposited in the sea, which was then advancing 

 northward over the land. Above the limestones, however, 

 are more sandy beds, which indicate that the shore line was 

 on its retreat southward, and the sea was becoming shallower, 

 until finally Texas and neighboring regions were left dry again. 



Reduction of the Pacific mountains. On the Pacific slope 

 a vast thickness of gravel, sand, and mud accumulated during 

 the Comanchean period. No other evidence is needed to 

 show that the mountains which had been upraised at the close 

 of the Jurassic were being rapidly eroded and the products 

 of their decay carried into the adjacent sea. Hundreds and 

 doubtless thousands of feet of rock were carried away by these 

 slow but incessant processes, resulting in the uncovering of 

 even the deep-seated batholiths of granite, which had been 

 intruded at the time of the folding. 



Emergence of the continent. Above the Comanchean 

 strata there is almost everywhere a distinct unconformity, 

 which tells of long-continued erosion after the sediments were 

 deposited. In the Atlantic and Gulf coastal formations the 

 unconformity is merely an irregular surface dividing the 

 strata above from those below. In that region there was no 

 notable deformation between the two epochs of deposition. 

 On the Pacific coast, however, the unconformity is more promi- 



