70 GEOLOGY OF THE BLACK HILLS. 



The quartz of the granite is commonly glassy and clear, but its varia- 

 tion in texture and color is great. It is usually crystalline, but no crystals 

 are found. It is frequently found opaque and white, and sometimes 

 lightly tinted with pink, while many large masses are of a delicate rose 

 color. From the decomposition of associated minerals it is often stained 

 by oxide of iron to a dull red or brown color. It composes approximately 

 about 20 per cent of the granite, but is distributed with great irregularity. 

 In many cases the quartz penetrates the feldspar mass in irregular seams or 

 fragments, which on certain fracture planes .produce the figures so suggest- 

 ive of oriental inscriptions, and which have given to the variety its name 

 of graphic granite. Sometimes it occurs in large compact bodies, sometimes 

 in the form of huge veins, and sometimes it forms a more equable mixture 

 with the feldspar and mica, making a normal and tolerably uniform but 

 coarse granite. 



The mica ranges from silvery white to dark brown in color. It is 

 always highly crystallized, and well defined hexagonal crystals two inches 

 in diameter are very common; mica plates six or more inches across have 

 been found. The mineral forms only about 5 per cent of the granite, and 

 though sometimes distributed generally through the mass it is more often 

 found in bunches or segregations. It was observed that it rarely accom- 

 panies the feldspar alone, but is almost always associated with quartz. 



Besides the three minerals essential to the formation of granite the only 

 ones found in abundance are rose quartz and tourmaline. The latter is 

 quite common and sometimes composes 3 or 4 per cent of the granite. 

 It is usually but not exclusively associated with the quartz. It is black 

 in color, and is generally highly crystallized, though sometimes massive. 

 At Harney Peak and elsewhere massive pieces of tourmaline were found 

 weighing several pounds and covered with small well defined crystals. 

 More generally it occurs in trigonal pyramids, well defined, and sometimes 

 with beautifully terminated ends. These crystals are often found from three 

 to eight inches in diameter and nearly a foot in length; but perfect speci- 

 mens could not be obtained from the weathered surfaces accessible to us. 

 Though very abundant in the granite it is strictly confined to that rock and 

 was never seen in either the schists or the slates. 



