BLACK HILLS GEOLOGY. 285 



as a core, although apparent crystals of sanidine were observed 

 with fragments of striated feldspar attached to them in different 

 orientation. It is probable that many of the sanidines de- 

 scribed by Caswell were merely plagioclase, cut parallel to the 

 twinning lamellae. Measurement of extinction angles on sec- 

 tions cut perpendicular to albite lamellae give a maximum angle 

 of 1 8 degrees, which places the feldspar among the oligoclase 

 andesines. 



The groundmass is composed of great numbers of automor- 

 phic sanidines of considerable size. They are prevailingly 

 square in section and are accompanied by other interstitial 

 masses, not so perfectly developed. Quartz has only attained 

 a slight development, although quite a few grains of this min- 

 eral seem to be present, giving higher colors than the gray feld- 

 spar. None of them are large enough to give an axial cross. 

 The silica determination published by Caswell, 67.36 per cent., 

 places the rock at the upper limits of andesites series and makes 

 the presence of considerable quartz probable. 



Quite a prominent accessory of the rock is titanite, which 

 forms crystals of considerable size. 



Deadwood Gulch Type. 



From the Crow Peak rock we may easily pass to another 

 rock which differs but little from it in general appearance, but is 

 very much more acidic. This rock occurs in a very large dike 

 on the loop of the Fremont, Elkhorn and Missouri Valley Rail- 

 road. Macroscopically, it is a very light, whitish gray, porphy- 

 ritic rock, with numerous irregularly bounded, and often very 

 large phenocrysts of quartz. 



The feldspars are prevailingly small, rarely attaining the 

 diameter of a fourth inch. Most of them are from four to five 

 millimeters in breath. The groundmass has a dense light gray 

 appearance, and is sharply contrasted with all the rocks yet de- 

 scribed in containing few if any ferruginous minerals. 



Under the microscope the rock is seen to be made up of a 

 very fine-grained granular groundmass, which is probably feld- 



