1888.] 513 [Crosby. 



obvious to every geologist familiar with the facts as they are now 

 exposed in the mining camps of the northern Hills. It is also- 

 plain that the thinness of many of the sheets and dikes implies a 

 degree of fluidity in the porphyry which is entirely inconsistent 

 with Newton's theory of viscid, pustular eruptions. On the other 

 hand, all the facts, so far as I have observed, fully support the 

 view that the great conical masses of Custer, Terry and other 

 peaks and buttes are typical laccolites, of which the intrusive 

 sheets so well exposed in the Ruby Basin and elsewhere are, in 

 many cases at least, merely extensions and branches, as figured by 

 Gilbert in his classic monograph on the laccolites of the Henry 

 Mountains. 



The laccolites seem to have accumulated in some cases between 

 the Archaean surface and the Potsdam, more commonly, like most. 

 of the intrusive sheets, in the Potsdam itself, and sometimes, as 

 in the case of Black Butte, mainly between the Potsdam and Car- 

 boniferous. Erosion has removed the covering strata in every 

 case, exposing the crown of the laccolite, but they still reach high 

 up on its slopes ; and on Terry Peak I observed isolated patches 

 of the Potsdam quartzite well up toward the summit. My atten- 

 tion was first directed to the intrusive sheets of the Ruby Basin by 

 Professor Carpenter, and this district is certainly an admirable 

 field for the study of the volcanic rocks. Ascending from the 

 lower part of the Basin, we cross, first, the vertical Archaean 

 slates and quartzites traversed by dikes of porplryry ; and then the 

 Potsdam beds with numerous intrusive sheets and d^kes, sloping 

 gentty up toward the massif of Terry Peak, over which they un- 

 doubtedly once arched. 



Contact Ore Deposits. 

 As in so many other districts in the Rocky Mountain region, 

 the sedimentary rocks, and especially the Potsdam quartzites, are 

 more or less highly metamorphosed and mineralized along the 

 contacts with the porphyry sheets and dikes. These mineralized 

 contact strata are the ore bodies of the gold and silver mines of 

 the Ruby Basin and some other mining districts of the Black Hills. 

 Being mainly horizontal, they are worked much after the manner 

 of coal mines. So far as observed, the lower contacts are the 

 most highly mineralized ; that is, the richest mines are those in 

 which the porphyry forms the roof rather than the floor. This fact 



PROCEEDINGS B. S. N. H. VOL. XXIII 33 JUNE, 1888. 



