74 



GEOLOGY OF THE BLACK HILLS. 



500 feet in height and several miles in extent. Whatever their size, they 

 have always a lenticular horizontal section, that is, they are thickest near 

 the middle and taper off toward each end; but whether this same form 

 would appear in a vertical section we have no means of ascertaining. 

 Their form is illustrated in the annexed cut. 



Frequently, standing on one of these lenses of granite and 

 looking along the strike, the geologist can trace a series of 

 others trending in the same direction and on the same line. 

 They appear like masses of granite that have filled seams or 

 separations in the schists, pinching in and swelling out like 

 fissure veins. 



In the canon of French Creek a cliff was observed wherein 

 a stratum of granite is inclosed between two of quartzite, the 

 granite mass itself inclosing some gneiss and having distinctly 

 marked in its short exposure the swelling or lenticular form. 



The peculiar granite of the Black Hills resembles but 

 few of the described granites of the Rocky Mountains proper. 

 The rock of Rawhide Butte, mentioned in Chapter II, is very 

 similar in its highly feldspathic and coarse character and in 

 the abundance of tourmaline and rose quartz. 



Laramie Peak, too, presents many of the features of 

 the granites of the Hills. Dr. Havden* finds the granite 



Fig. 8.— Ground ° . 



Plan and Section bodies thinner and less abundant at a distance from the main 

 peak and the interstratified schistose rocks more prominent, 

 while the peak itself is composed almost entirely of granite. This he 

 describes as an "aggregation of large crystals of reddish feldspar, with 

 quartz and mica; the feldspar so predominating that it gives the character 

 to the rock. The mica usually occurs in small masses and in limited 

 quantities." 



Dr. Hayden states that the granite of the Wind River Mountains is 

 very uniform and gray,f and Prof. Theo. R. Comstock remarks a gradual 

 change from the schists to gneiss, gneissoid granite, and compact fine- 



* Geological Survey of the Territories, 1870, p. 18. 

 tld., p. 37. 



