288 GENERAL GEOLOGY. 



the boundary just described: and it is consequently inferred 

 that the space between that point the most easterly ones in 

 southwestern Iowa, was originally occupied by strata which 

 were then continuous with them. Again, as we pass westward 

 from the Upper Des Moines river, the strata of older date 

 than the Cretaceous, disappear beneath the surface along or 

 near the suggested eastern boundary of the latter, and are 

 seen no more in that direction, and the next rocks we find in 

 place are along the western boundary of the State in the 

 valleys of the Missouri and Big Sioux rivers, and are of 

 Cretaceous age. 



These remarks apply particularly to the northern half of 

 the area, the boundaries of which have just been given. In 

 its southern portion the aggregate thickness of the Cretaceous 

 strata is less, and they have been so frequently cut through 

 to the underlying coal-measure strata by the streams during 

 the process of the erosion of their valleys, that the borders of 

 the latter formations may be traced in a very satisfactory 

 manner, through a veil, as it were, of the much-eroded sand- 

 stone of Cretaceous age. 



The Cretaceous rocks of the Upper Missouri river region 

 have a greater aggregate thickness than rocks of that age are 

 known to have in any other part of North America, and have 

 been explored and ably reported upon by Messrs. Meek and 

 Hay den. These gentlemen divide them into "Earlier" and 

 "Later Cretaceous." The former is thought to be somewhat 

 later than the earliest Cretaceous strata of Europe, and 

 probably the earliest Cretaceous rocks yet found in America 

 are also later. They farther subdivide their Earlier Creta- 

 ceous into the "Dakota group," "Benton group," and 

 " Niobrara group." 



The Cretaceous strata of Iowa have so slight a development 

 in comparison with those farther up the Missouri river, that 

 it is difficult to determine their stratigraphical equivalents 

 with those, without actual comparison, which it has thus far 

 been impossible to make. There is no doubt however that all 

 the Iowa Cretaceous strata belong to the "Earlier Cretaceous" 



