476 THE CRETACEOUS PERIOD 



thence southward to Georgia, where it turns . northwestward, fol- 

 lowing the Mississippi embayment into Tennessee, and from there 

 turning southwestward through Arkansas. In the northern part 

 of this region, from Nantucket to the Delaware River, only the 

 uppermost part of the Potomac series has been found. The 

 Potomac is nowhere marine, and everywhere rests unconformably 

 upon the underlying Triassic and older rocks. The accumulation 

 of sediments in these depressions went on for a very long time, 

 apparently throughout the whole of the Lower Cretaceous, and by 

 some geologists it is believed that the process began in the Juras- 

 sic, to which period they refer the lowest members of the Potomac 

 series. As the thickness of sediment is not great (not exceeding 

 600 feet), the process of deposition must have been very slow. 



While along the Atlantic border the land was more extended 

 than at present, in the southern part of the continent a different 

 order of events was brought about. In southern Mexico occurred 

 a depression which submerged the land almost from ocean to 

 ocean, through probably leaving some sort of land barrier between 

 the Atlantic and the Pacific. The transgression of the sea ex- 

 tended northward, covering most of Texas and Oklahoma, and 

 sending a bay into southern Kansas. At the base of the Lower 

 Cretaceous strata in Texas is found a deposit of fresh-water sands, 

 the Trinity stage, which is the recognized equivalent of the basal 

 Potomac. The advancing sea soon obliterated this body of fresh 

 water, and the continued depression soon established a clear and 

 quite deep sea, in which were formed the great masses of the 

 Comanche limestones, that are the surface rocks of nearly all 

 Mexico and cover a large part of Texas. The Ouachita Moun- 

 tains of Arkansas stood out as a promontory in the Lower Creta- 

 ceous sea and the ancient shore line has been traced around their 

 foot. Over much of Texas the Comanche limestones are soft, 

 and beds of chalk occur among them ; while in Mexico, where they 

 have been folded into mountain ranges, they have become much 

 harder and more compact. The thickness of the limestones in- 

 creases southward; from 1000 feet in northern central Texas, it 

 rises to 5000 feet on the Rio Grande, and on the Mexican plateau 



