BLACK HILLS GEOLOGY. 289 



next mineral to the hornblende and plagioclase in abund- 

 ance is a colorless mineral in grains and definite crystals, and 

 scattered in great profusion through the rock. It has a high 

 index of refraction, and was at first taken for pyroxene. The 

 interference colors are, however, extremely low, by which means 

 it may be readily distinguished from that mineral. It is also 

 usually confined to the plagioclase, of which it seems to be an 

 alteration product. The extinction is parallel. In the amphi- 

 bolite from Squaw Creek this mineral is in very much larger 

 crystals, and in far greater abundance, having developed at the 

 expense of the feldspar. The latter mineral decreases in amount 

 as it becomes a more prominent constituent of the rock. The 

 mineral is probably zoisite. 



Accessory ilmenite, calcite, apatite and quartz also occur. The 

 ilmenite is in large masses and is invariably surrounded by a 

 heavy border of leucoxene. The quartz and calcite are secondary. 



The other varieties of amphibolite differ from the above largely 

 in degree of alteration and the absence of a recognizable diabase 

 texture. That which forms the large, irregular mass in Squaw 

 Creek is a fine-grained, dense rock, with a light green color, 

 and is locally known as " diorite." The amphibole here, however, 

 is in much greater abundance. It is arranged in wide, fibrous 

 masses, made up of a series of parallel rods, which are often 

 curved and show a wavy extinction. 



An analysis of this rock gave 



SiU, 



49.19 



A1P3, 



15-13 



Fe,03, 



10.71 



CaO, 



9-55 



MgO, 



8.05 



K,0, 



Not det. 



Na,0, 



Not det. 



B. General Discussion of Petrography. 



The hills have twice been the seat of prolonged igneous 

 activity. The first period was previous to the metamorphism of 



