BLACK HILLS GEOLOGY. 247 



blende and biotite. The series varies in the amount of SiOj 

 present from 65 ^ to 76 ^ or 78 ^. Five type* are described. 



Andesite family. 



Under the andesite family have been placed the diorite por- 

 phyries of which there are quite extensive developments through- 

 out the district. They are rocks of a moderately dense tex- 

 ture and of quite basic character but markedly porphyritic and 

 of undoubted intrusive nature. 



They exhibit a fine-grained groundmass of plagioclase feld- 

 spar, accessory quartz and much chlorite, phenocrysts of plagio- 

 clase, orthoclase, hornblende and in most cases biotite. The 

 hornblende is very generally altered to chlorite. Orthoclase in 

 instances becomes so abundant as to cause the rock to resem- 

 ble the syenite -porphyries Analysis showed about 55 ^ SiOg. 



Dacite family. 



The dacites exhibit a fine-grained groundmass of quartz and 

 orthoclase in which are embedded phenocrysts of plagioclase, 

 orthoclase and quartz. Auxiliary titanite, magnetite and biotite 

 are generally present. No analysis of the dacites were made, 

 but one type showed an extremely large amount of quartz. 



Diorite family. 



Toiialite. — Only one exposure of this rock was found and that 

 an extremely large dike in Deadwood Gulch. It is a gray rock 

 composed of hornblende, plagioclase, quartz, biotite and acces- 

 sory orthoclase and is of granitoid texture, showing a slight 

 tendency to automorphism in the component minerals. 



Basalt family. 



Augite- Vogesite. — This rock occurs in small dikes west of 

 Spearfish canon and is composed of a fine automorphic aggre- 

 gate of augite and feldspar with accessory hornblende and mag- 

 netite. It is the final and basic representative of the soda-rich 

 magma that constitutes the principal Black Hill's eruptive series. 



Pre- Cambrian eniptives. 



Amphibolites. — These constitute a series of basic rocks in- 

 truded in the Algonkian series and metamorphosed with them. 



