BLACK HILLS GEOLOGY. 301 



with the ore on both sides. A large phonolite dike some hun- 

 dred feet or more in width occurs at the west of this mine and 

 separates the ore body from that of the Little Bonanza. 



Otlicr Mines. — Of the remaining mines in the Ruby Basin, 

 the most important is that of the Golden Reward Company. It 

 is the largest producer of this class of ore in the hills. The 

 writer was unable to obtain access to the Golden Reward prop- 

 erties, but presumably the ore shoots show no difference from 

 those just described except perhaps in size. Mention has been 

 made by Prof Smith ^ of several horizons at w^hich the ore occurs 

 in this region, but he has stated that these may be due to faults. 

 This has undoubtedly been the case, for the Algonkian itself 

 has been faulted to a very considerable extent in Nevada Gulch, 

 and many other faults can be readily distinguished. 



2. Portland or Green Mountain District. 



In this district all of the mines with one exception, the 

 Decorah, are situated from two to three hundred feet above the 

 base of the Cambrian. On Green Mountain and almost imme- 

 diately beneath the phonolite cap are the Trojan and Empire 

 State, the upper workings of the Decorah and other mines. 

 Along the Burlington and Missouri River Railroad are the 

 Clinton, Mark Twain, and Gunnison mines. In the bottom of 

 Dead wood Gulch, and separated by a thickness of two or three 

 hundred feet of strata from the upper working of the same mine 

 on Green Mountain, is the Decorah. 



The Decorah. — In the Decorah mine the conditions are the 

 same as those in the Union and Big Bonanza, the ore lying 

 directly on the quartzite. The mine is however, in the early 

 stages of its development, and no very extensive ore shoots 

 have yet been mined out. One peculiar feature of the mine, and 

 one which is met at no other place to the writer's knowledge, is 

 that the basal quartzite is thinner than usual, and seems to oc- 



1 Trans. Amer. Inst. Mining Engineers, Vol. XXVII, p. 416. July, 1897. 



