BLACK HILLS GEOLOGY. 287 



ognize the following minerals : plagioclase, quartz, orthoclase, 

 biotite and hornblende. These are arranged in granular tex- 

 ture, being largely without crystal boundaries. There is, how- 

 ever, a slight tendency toward an automorphic development in 

 the feldspar, such as would be anticipated in a rock of this 

 character. That we have here a rock of so nearly granitoid 

 texture is undoubtedly due to the very large size of the intruded 

 mass. 



The plagioclase slightly outranks the orthoclase in abundance. 

 Maximum extinction angles measured on albite lamellae did not 

 give very satisfactory results, but the mineral seems to be of 

 quite an acid type. The other minerals present no unusual 

 features. 



7. Lamprophyre Family. 



Augite-Vogesite. 



This rock was found cutting the diorite-porphyry of the 

 needles uplift in Bear Gulch on the west of Spearfish Creek. 



Megascopic Characters. — It is a dense, black rock, carrying 

 no noticeable phenocrysts, and of extremely fine grain. 



Microscopic Characters. — The microscope reveals the presence 

 of the following minerals : augite, hornblende, magnetite and 

 feldspar. The augite constitutes the main body of the rock, and 

 is always in automorphic crystals. These are small and usually 

 free from inclusions, but exhibit the characteristic cross-section, 

 and cleavage of the mineral. 



Hornblende is present in long needles, but compared to the 

 augite, is sparingly developed. 



Between the crystals of augite and hornblende, occurs a small 

 amount of glassy groundmass, in which from time to time, 

 singly twinned feldspar may be observed. The feldspar is prob- 

 ably orthoclase. From these facts, and from the almost perfect 

 panidiomorphic texture of the rock, it has been classed with 

 the vogesites. It exactly parallels the camptonites which are so 

 often associated with elseolite syenite magmas, except that the 

 component mineral is augite, instead of the usual brown basal- 

 tic hornblende. 



