BLACK HILLS GEOLOGY, 



291 



as to belong more properly in the trachyte series. On the 

 other hand the invariable occurrence of plagioclase in all of the 

 quartz- porphyries and rhyolites, indicates a transition toward 

 the dacites. These relations will be brought out by the accom- 

 panying diagram. 



Syenite 



Trachyte 



Family 



U 



V' 



Diorite 

 Andesite 

 Family 



Increasing SiOj 



-"> 



Granite 



Rhyolite 



Family 



Dia^}-a?n ilbistrating affinities of rock types. 



Quartz Diorite 

 Dae te 

 Family 



1 . Diorite porphyry from the Needles to the west of Spear- 

 fish Creek. Contains less plagioclase and more orthoclase than 

 typical diorite porphyries, but more nearly approximates them 

 than the trachytes. Described on pages 283 and 284. 



2. Typical mica-diorite porphyry from the Rua Mine. Con- 

 tains much plagioclase, but little orthoclase, and is very basic. 

 Analysis on page 283. 



This rock has been described by Prof Smith as mica-ande- 

 site. Described on page 282. 



3. Rock from Crow Peak. Contains a very large propor- 

 tion of orthoclase, but almost all of the phenocrysts are of 

 plagioclase. Enough free silica is present to bring the rock 

 nearer to the rhyolite-dacite line than to that of the trachyte- 

 andesite series, and the plagioclase is in too great abundance 



