AMERICAN GEOLOGY DECADE OF 1870-1879. 569 



val of erosion and metamorphism, a newer Archean, consisting mainly 

 of sandstones, was deposited, accompanied by abundant intrusions of 

 granitic rock. Then followed the period in which these rocks were 

 raised to a nearly vertical position, metamorphosed, and deeply eroded, 

 after which the various Potsdam, Carboniferous, Red Bed, Jura, and 

 I Jretaceous deposits were conformably laid down. Then followed the 

 uplift and subsequently the unconformable depositions of the White 

 River Tertiaries upon their flanks. The date of the uplift is therefore 

 set as the interval of time between the Cretaceous formation and the 

 beginning of the Miocene Tertiary. (See under Hayden, p. 591.) 



Newton showed that the drainage from the Black Hills was conse- 

 quent — that is, it conformed to the dip of the strata. This he con- 

 ceived to be caused by the streams having acquired their direction dur- 

 ing the uplift of the hills, or while they were being laid bare by the 

 drainage of a lake. In either case the drainage is consequent upon the 

 uplift. With the rivers — the BeJle Fourche and the South Fork of 

 the Cheyenne — the case is different, however. These cut across the 



Fig. 92.— Ideal cross-section of the Black Hills. (After Henry Newton.) 

 The vertical scale is about six times the horizontal: the dotted lines indicate the portion of the 

 uplift removed by erosion; 1, Archean slates and schists; 2, Granite; 3, Potsdam, resting unconform- 

 ably on 1 and 2; 4, Carboniferous; 5, Red Beds, with included limestone; (i, Jura; 7, Cretaceous; 8, 

 White River Tertiary, resting unconformably on 7. 



uplifted strata in a way to show that the drainage is superimposed. If 

 they existed prior to the deposition of the Tertiary sediments they 

 were completely blotted out. 



J. H. Caswell described the various metamorphic and eruptive rocks 

 collected by the survey, his paper forming the second on the subject 

 of micropetrography to appear from the Government press, Zirkel's 

 fortieth parallel report (1870) being the first. The report was accom- 

 panied by two plates with eight colored figures illustrating micro- 

 structure and composition, and is of particular interest as giving the 

 first authoritative account of the occurrence of the rock phonolite in 

 the United States. Caswell, although one of the first in a new and 

 fascinating field, seems to have rested on his laurels, content with the 

 one effort, his name in geologic literature appearing here for the first 

 and only time. 



W. P. Jenny, to whom fell the economic work, reported gold occur- 

 ring (1) in veins of quartz traversing Archean schists and slates; (2) 

 in slate mineralized by waters depositing silica and iron pyrites; (3) in 



