hatden.] THE CRETACEOUS GROUP. 45 



would be mere repetition to refer to them in this report. The Jurassic 

 group in Colorado has little or no influence economically or in giving 

 form to the peculiar scenery. 



THE CRETACEOUS. 



The Dakota group is composed of massive beds of sandstones inter- 

 sected with layers of clay, and forms some of the most conspicuous 

 ridges or " hogbacks" along the eastern base of the Front or Colorado 

 range. Its importance, however, varies in different localities as much 

 as its texture ; sometimes it is scarcely seen and then again it forms one 

 or more of the most important ridges. Its aggregate thickness is 

 never great, varying from 200 to 400 feet, and may be represented by 

 a very narrow belt on the map. West of the 100th meridian it has 

 yielded very few organic remains, although it has a very extended geo- 

 graphical range. It is hardly ever wanting along the margins of the 

 mountain ranges east of the Wasatch Mountains, in Utah. From its 

 structure in the far West, I regard it as a sort of transitional group be- 

 tween the well-defined Cretaceous group and the Jurassic below. 



Numbers 2, 3 and 4, or the Fort Benton, Niobrara, and Fort Pierre 

 divisions, may be regarded as one group, under the name of the Colora- 

 do group, as adopted on Clarence King's beautiful geological map of the 

 Green River basin. To one who has never studied these divisions in the 

 Northwest, along the Upper Missouri River, there would seem to be no 

 occasion for their separation. Having studied these divisions with 

 much care in their typical localities, along the Missouri River and in 

 Eastern Kansas and Nebraska, I found very little difficulty in tracing 

 them across the country westward and southward, so far as my explor- 

 ations have extended. It is very doubtful, however, if any geologist 

 would have ever separated the Cretaceous beds between the Dakota 

 and Fox Hills groups into divisions, had they been first studied 

 in the interior of the continent. The Fox Hills group has a very impor- 

 tant influence on the physical history of a most important geological 

 period. It was at the close of this period that one of the most impor- 

 tant biological changes occurred in geological history. So far as we 

 know at the present time, no animal-remains, and very few, if 

 any, vegetable forms, passed above it. A few species of plants 

 probably began their existence in the Upper Cretaceous in the 

 Fox Hills group and continued on up into ihe Lignitic group, 

 where they reached their highest point of development. The gradual 

 approach of shallow seas is finely shown in the character of the sedi- 

 ments in the upper portion of the Fox Hills group. Not only the shal- 

 low seas but the gradual change of salt to brackish and then to purely 

 fresh waters was amply sufficient to destroy all traces of marine life, 

 which occur soabuudantly in the Fox Hills group. Fig. 2, Plate IV, pre- 

 sents a fine illustration of the remarkable concretionary masses which 

 characterize in many localities the upper portion of the Fox Hills 

 group as it passes into the brackish -water strata of the Lignitic or 

 Laramie group above. This cut, though intended to illustrate a portion 

 of the Dakota group in Eastern Kansas, serves perfectly to explain to 

 the eye the immense rusty-brown concretions which abound in the mud- 

 beds just beneath the lower sandstones of the Lignitic group at Canon 

 City and at Colorado Springs, and at other localities in Eastern Colo- 

 rado. These concretions are peculiar rounded, regularly stratified 

 masses, often merely resting upon the pedestals of the softer and 

 more regularly bedded sandstones below. So far as Colorado is con- 



