186 GEOLOGY OF THE BLACK HILLS. 



river. On the east side of the river the Cretaceous strata extend southeast- 

 ward in a broad flat mesa. Crossing it for six or seven miles one reaches the 

 abrupt edge of the prairie and looks iinexpectedly down upon the wonder- 

 ful and wiord bad land basin of the White River, which falls away from the 

 plain as though it had been sunk from a higher level by some catastrophe 

 of nature. It is hard to realize that one has been gradually ascending in 

 the geological scale while he crossed the even prairie, so that he now looks 

 down upon a formation higher than any to be seen along the river; but such 

 is nevertheless the case. The gentle dip of two or three, or perhaps four, 

 degrrees has carried down the Cretaceous until it is no lonjjer visible even 

 in the deepest water-ways, but is replaced in the topography by the middle 

 portions of the White River Tertiary, 



That the Cretaceous series once extended continuously across the area 

 which is now elevated to form the Black Hills there can be no reasonable 

 doubt. The strata have been elevated and tilted and their wasting edges 

 outcrop around the Hills at various angles, but still they conform perfectly 

 with the underlying rocks, sharing their curves and inclinations. Tiiis 

 relation could not subsist if they had been deposited after the uplifting ; 

 and being deposited before it, there could have been nothing to prevent their 

 continuity across its area. 



SECTION IX. 



THE TERTIAEY. 



There are no Tertiary strata within the area of the Hills, and all men- 

 tion of the formation might have been omitted at this place but for the 

 fact that the Tertiary deposits of the vicinity have furnished evidence 

 bearing on the question of the age of the Hills. A brief account will 

 therefore be given of the Tertiary of the Plains for the purpose of showing 

 the general relations of the particular beds that have interest in this con- 

 nection. 



