1888.1 489 [Crosby. 



necessarily be given special prominence in this paper, it should be 

 understood that these are insignificant compared with those which 

 fully sustain his report. 



For a general view of the geological structure of the Black Hills 

 we may refer to Newton's introductory chapter : — "Around a nu- 

 cleal area of metamorphic slates and schists, containing masses of 

 granite, the various members of the sedimentary series of rocks, 

 the Potsdam, Carboniferous, Trias or Red Beds, Jura, Cretaceous 

 and Tertiary, lie in rudely concentric belts or zones of varying 

 width, dipping on all sides away from the elevatory axis or region 

 of the Hills. From the Hills outward the inclination of the beds 

 gradually diminishes until all evidence of the elevation is lost in 

 the usual rolling configuration of the plains. At numerous points, 

 also, within the area of the Hills are centres of volcanic eruption." 



That the Black Hills, as here indicated, offer an exceptionally 

 fine field to the geologist is the concurrent testimony of all who 

 have visited the region. The late Dr. Hay den said that, "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 study as around the Black Hills." 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 geo- 

 logical epitome of the neighboring portions of that great range. 

 The geologist, therefore, finds in this region a monographic 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 sedimentary rocks, he is given a piece of mountain geology 

 of great beauty, simplicity, and ease of elucidation (Newton). 



The Archaean or metamorphic rocks of the Black Hills occupy 

 the axial or nucleal area of the uplift, and are exposed almost con- 

 tinuously over a tract about sixty miles long north and south, and 

 twenty-five miles in its greatest width east and west. Made up 

 as it is of many alternations of slate, quartzite, schist and granite, 

 i. e., of rocks varying greatly and abruptly in hardness and dura- 

 bility, this area has developed, under the discriminating erosion 

 of the atmospheric agents and running water, and in the apparently 



