1888.] 515 [Crosby. 



the ferruginous slates and schists of the newer Archaean series is 

 derived from the associated porphyry dikes is an entirely indepen- 

 dent question. Prof. Carpenter has answered it in the negative ; 

 and he is certainly justified, in the main, by the fact that similar 

 ferruginous strata are distinctly auriferous outside the volcanic 

 area. Still, after studying the Ruby Basin, it seems impossible to 

 resist the conclusion that the porphyry is the actual source of 

 some of the gold in the Homestake district ; although its chief 

 favorable influence has probably been to render the gold originally 

 present more available. 



Age of the Volcanic Bocks, 



My observations sustain Newton's general conclusion that the 

 volcanic rocks of the northern Hills are probably all of about the 

 same age, and date from the period between the Middle Cretaceous 

 and the Miocene Tertiary. They have upturned strata belonging 

 to the Fort Benton Cretaceous group, and are enclosed as water- 

 worn fragments in the Miocene conglomerate. 



If the history of the Black Hills uplift is correctly outlined in 

 the preceding pages, the summit of the uplift, which is plainly ec- 

 centric, being marked by the Harney Range in the southern 

 Hills, must have been above the sea and exposed to erosion at 

 least as early as the beginning of Jurassic time. It is even prob- 

 able that erosion had cut entirely through the Paleozoic series, ex- 

 posing the Archaean at some points, before the beginning of the 

 volcanic activity ; and it may be that we have here an explanation 

 of the important fact that the igneous intrusions are limited to 

 the northern Hills, which were probably an area of deposition till 

 near the close of Mesozoic time. 



Absence of Glacial Phenomena. 

 Geologists appear to be substantially agreed that the region of 

 the Black Hills was not covered by the great ice sheet of the glacial 

 epoch, and that they have not, in recent geological times, been the 

 seat of any important local glaciers. These propositions are 

 clearly established by the complete absence of true glacial drift, 

 morainal deposits, large erratic bowlders and glacial striae, and 

 also by the wonderfully broken and jagged character of many of 

 the topographic features of the Hills. The outcrops of the granite 

 veins in the Harney Range, and of the vertical beds of Archaean 



