Geographic and Geologic Relations 411 



American region. And to understand at least approximate- 

 ly the changing conditions of land and sea there, we can- 

 not do better than quote the excellent description by Cham- 

 berlin {8: 106). 



"The history of the Cretaceous period, as that term 

 has commonly been used, is rather complex. The general 

 sequence of events in North America is somewhat as fol- 

 lows: (i) Early in the period there was a somewhat wide- 

 spread warping of the continental surface, resulting in 

 sedimentation at many points within the continental borders. 

 Submergence was extensive in Mexico and Texas, and the 

 sea extended thence as far north as the Ouachita Moun- 

 tains, and temporarily beyond, while on the Pacific coast 

 a narrow border of the present land was beneath the sea. 

 Along the Atlantic and Gulf coasts, and in some parts of 

 the western interior, considerable tracts were brought so 

 low, or into such an attitude, as to become the sites of de- 

 position, though not submerged beneath the sea. A pro- 

 longed period of sedimentation followed these geographic 

 changes. (2) This period of sedimentation was followed 

 by an interval when most of the areas which had recently 

 been the sites of deposition, whether marine or non-marine, 

 were exposed to subaerial degradation. (3) After this 

 interval had been sufficiently long to allow of very con- 

 siderable erosion of the Early Cretaceous beds, the sea 

 encroached upon the Atlantic and Gulf borders, covering, 

 and in general spreading beyond, the non-marine formations 

 of the earlier stage. It again covered Texas, and presently 

 extended northward over the Great Plains to the Arctic 

 Ocean, forming a great Mediterranean sea several hundred 

 miles wide from the mouth of the Mackenzie River on the 

 north, to the mouth of the Rio Grande on the south, divid- 

 ing the continent into two unequal parts, a larger eastern, 

 and a smaller western. On the Pacific coast also, the sea 

 extended its area somewhat at the expense of the land. 

 There have been few greater incursions of the sea over 

 the land, and therefore few equally great geographic 

 changes, during the long history of the North American 

 continent. A long period of deposition was initiated by the 

 submergence, and this was succeeded in turn by (4) a wide- 



