284 IRVING. 



blende. No biotite is present, and the rock is of a very much 

 less basic type than the others described. The microscope 

 shows a fine-grained ground mass of irregular masses of pla- 

 gioclase, amongst which a few rectangular crystals of or- 

 thoclase occasionally appear, and through which grains of 

 magnetite are thickly scattered. The phenocrysts are plagio- 

 clase and sanidine in almost equal development, and horn- 

 blende, which is always in automorphic crystals. The plagio- 

 clase and hornblende show no unusual features and the chief 

 point of interest about the rock is that its affinities to the ande- 

 sites and trachytes are about equally divided. 



5. Dacite Family. 



Of this family two types are described : ist. Crow Peak 

 Type. 2d. Deadwood Gulch Type. 



Crow Peak Type. 



The first rock determined by the writer under this name, was 

 taken from the summit of Crow Peak. It was described by 

 CaswelP as rhyolite as follows : 



"The rock (141 and 142) from Crow Peak is a rhyolite, con- 

 taining plagioclase, and is much more crystalline than the pre- 

 ceding, having microscopical sanidine crystals plainly embedded 

 in a groundmass. They are white and very transparent. There 

 are also some black crystals of hornblende and empty cavities, 

 which were formerly filled with the mineral. In the section, 

 the microscope shows the rock to consist of large, clear crystals 

 of sanidine and plagioclase in a crystalline groundmass, also 

 containing broken biotite crystals and some quartz in grains and 

 crystals." 



In the section examined by the writer all of these minerals 

 were observed, but a few additional important features need to 

 be emphasized. The phenocrysts, with the exception of one or 

 two crystals, are all plagioclase, and the few sanidines present 

 are smaller than the plagioclases. Plagioclase was not observed 



1 Report of the Geol. and Resources of the Black Hills. U. S. G. G. Survey, 

 p. 506. 



