BLACK HILLS GEOLOGY. 279 



grained, but is so thickly crowded with small phenocrysts of 

 feldspar that it will not be at first noticed. Quartz phenocrysts 

 also occur in abundance, and sometimes attain a size of one- 

 half of an inch in diameter, although most of them are smaller. 

 Besides these phenocrysts of quartz and feldspar, which range 

 about one-eighth inch in diameter, extremely large crystals of 

 sanidine of an older generation occur in such an abundance as 

 to give the rock almost the appearance of a granite-porphyry. 

 The latter crystals range from three-eighths of an inch to one 

 and one-half inches in length. They are invariably idiomorphic, 

 are always undecomposed and never show corroded boundaries. 

 Scattered through the groundmass are minute flakes of biotite. 



Microscopic CJiaracters. — Examined microscopically, the rock 

 shows an exceedingly fine-grained, granular groundmass, made 

 up of quartz and feldspar, in which are embedded decomposed 

 plates of biotite, quartz phenocrysts and alkali feldspar of two 

 generations. The biotite is much decomposed. The quartz is 

 in automorphic crystals, and much corroded, and contains num- 

 bers of inclusions. The most abundant of these are little bub- 

 bles of gas, and irregular patches of a mineral, with a very high 

 index of refraction, but whose nature was not determined. The 

 smaller feldspars are present in great numbers, and are almost 

 invariably decomposed. Many of them are an acid plagioclase, 

 probably albite. 



The larger phenocrysts are sanidine, very fresh and most fre- 

 quently in Carlsbad twins. They contain great numbers of in- 

 clusions, which can be seen in roughly zonal arrangement, 

 even in the hand spec men. Most important of these are quartz 

 and feldspar. The inclusions of quartz are often so large as to 

 be in the nature of true phenocrysts, when considered in relation 

 to the body of the rock. The feldspar inclusions are both or- 

 thoclase and albite. 



Whitetail Gulch Type. 



Still another type of quartz-porphyry occurs on the Black 

 Hills and Fort Pierre Railroad, in Whitetail Gulch, a little north 

 of the mouth of Fantail and Nevada Gulches. 



