AMERICAN 479 



converted them into rocks of great density, and from their ancient 

 appearance they were long supposed to be Carboniferous. 



The Upper Cretaceous rocks have a far wider distribution over 

 North America than have those of the lower division, which is due 

 to an enormous transgression of the sea over the land, one of the 

 greatest in all recorded geological history. Over the region of 

 the Great Plains the Upper Cretaceous was inaugurated by the 

 formation of a non-marine stage, the Dakota. These strata cover 

 much of Texas, lying unconformably upon the Comanche series, 

 and extend northward into Canada. It is very difficult to com- 

 prehend under what conditions these vast sheets of conglomerate 

 and sandstone could have been laid down, and there is much 

 reason to believe that not all the beds referred to the Dakota really 

 belong to it. On the western side of the Colorado uplift, the 

 Dakota is less distinctly a sandstone formation, and is charac- 

 terized by beds of shale and even coal seams of workable thick- 

 ness. In most parts of the Rocky Mountain region the Dakota 

 rests in apparent conformity upon the lowest fresh-water Creta- 

 ceous, and even upon the Jurassic. In the Uinta and Wasatch 

 ranges there is no apparent break in sedimentation from the 

 Palaeozoic to the end of the Cretaceous, though the whole Lower 

 Cretaceous is there wanting. From this we may infer that during 

 the long Lower Cretaceous time all these regions had been low- 

 lying lands, nearly or quite at base-level, and therefore not subject 

 to profound denudation. 



It was at the end of the Dakota age that the great subsidence 

 took place which affected nearly all parts of the continent, and 

 brought the sea in over vast areas where for ages had been dry 

 land. South of New England the Atlantic coastal plain was sub- 

 merged, and in New Jersey, at least, the waters covered even the 

 Triassic belt, bringing the sea up to the foot of the crystalline 

 highlands. The lowlands of Maryland, Virginia, and the Caro- 

 linas, and all of Florida were under the ocean, and the Gulf of 

 Mexico was extended northward in a great bay (the Mississippi 

 embayment), covering western Tennessee and Kentucky and ex- 

 tending into southern Illinois. Northeastern Mexico and Texas 



