CHAPTER III. 



GEOLOGY. 



SECTION I. 

 INTRODUCTION. 



It has been remarked by Dr. Hayden that — 



The Black Hills of Dakota will form oue of most interesting studies on this con- 

 tinent. There is so much regularity in the upheaval that all obscurity is removed and 

 all the formations known in the West arc revealed in zones or belts around the gi-anite 

 nucleus in their fullest development. A careful detailed topographical and geological 

 survey of this range would be a most valuable contribution to science. In all the 

 western country I have never seen the cretaceous, Jurassic, triassic, or red-beds, the 

 carboniferous and Potsdam rocks, so well exposed for studj' as around the Black Hills.* 



His statement, founded upon a rapid reconnaissance of the foothills and 

 extremities of the region during the exploration of Lieutenant Warren in 

 1857, has been fully corroborated by the more thorough and complete survey 

 which our party was enabled to make during the past summer. Elevated 

 as they are like an island above the surrounding sea of the Plains, and 

 separated by more than one hundred miles from the nearest spur or sub- 

 range of the Rocky Mountains, the Black Hills are a complete study in 

 themselves. Exhibiting in the strata exposed and in the general character 

 of the elevation most of the principal features of the geology of the Rocky 

 Mountains, they are a geological epitome of the neighboring portions of that 

 great range. The geologist therefore finds in this region a monogra])hic 

 study of universal interest ; and by the regularity of the uplift, by the 

 absence of great faults in the strata, and by the splendid exposures of the 



* United States Geological Survey of Wyoming and Contiguous Territory, 1870. p. 98. 

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