EROSION. 297 



of the auriferous quartz veins, and probably the intrusion of the granite of 

 the Harney Peak range, a most enormous denudation of the metamorphic 

 rocks occurred, and the greater portion of the resulting material was swept 

 away and lost. Only water-worn bowlders of quartz and the harder rocks 

 remain, forming the conglomerate at the base of the Potsdam. 



Most, if not all, of the area of metamorphic rocks was at one time cov- 

 ered by this conglomerate, and it is probable that the advancing Silurian 

 ocean, at least in part, produced this erosion of the slates and schists. The 

 conglomerate is often found to contain huge bowlders of quartz and quartzite 

 derived from the ledges in the slates in the immediate vicinity, but nowhere 

 were we able to find any pebbles of granite in the Potsdam, though frag- 

 ments of soft clay, slate, quartzite, and quartz from its lower layers, and 

 scales of mica are easily seen with a magnifying glass in the compact layers 

 of this sandstone. Granite occurs abundantly in the Tertiary and more 

 recent conglomerate, and I can only account for this absence of granite in 

 the conglomerate of the Potsdam by supposing that the feldspar yielded 

 more readily to decomposition by the atmospheric agencies of the Silurian 

 period than it has from the same forces since the elevation of the Black Hills. 



The Potsdam commences with the coarse conglomerate, a deposit formed 

 by a shallow and advancing sea, which seems to have gradually deepened, 

 as the sediments forming the sandstone regularly become finer in grain as 

 we ascend. 



Since the Cretaceous, the Black Hills have been above the ocean-level, 

 probably attaining their present altitude by a slight additional elevation at 

 the close of the Tertiary, strata of that age, on the Cheyenne, having a dip 

 of 3° to 5° from the direction of this range. From the Archaean to the 

 Cretaceous, these sedimentary rocks are an evidence of denudation; of a 

 wearing down of some continent, furnishing the enormous mass of material 

 which now composes the extensive strata of slate, sandstone, limestone, and 

 clay-shales. 



The second great erosion succeeded immediately the elevation of the 

 Black Hills, and occurred during the early part of the Tertiary period. 

 The resulting material forms the conglomerate between the top of No. 5 

 Cretaceous and the base of the Miocene or White River Tertiary. Only 



