BLACK HILLS GEOLOGY. 245 



besides those of Professor F. C. Smith have attracted the at- 

 tention of petrographers to this region and it has been sup- 

 posed that investigation would show the occurrence of rock 

 types of unusual interest. Such has proved to be the case. 



The series of eruptive rocks is quite a varied one. The dif- 

 ferent types collected by the writer were all gathered from a 

 rather restricted area, but even so show many different varieties. 



The following classification has been followed in the descrip- 

 tion of the eruptive rocks : 



Of post-Cretaceous age. 



Grorudite family. 



Quartz-aegirite -porphyry. 

 Pho7iolite family. 



Tinguaite. 



Phonolite. 



Trachytoid phonolite. 

 Rhyolite family. 



Quartz-porphyry. 

 And e site family. 



Mica-diorite -porphyry. 

 Dacite family. 



Dacite. 



Diorite family. 



Tonalite. 

 L amprophyre family. 



Augite-Vogesite. 



Of pre- Cambrian age. 

 Amphibolites. 



Post-Cretaceous Eruptives. 

 Grorudite fa mily . 



The grorudite family comprises a series of rocks of highly alka- 

 line character whose constituents are : orthoclase, quartz, segirine- 

 augite and segirine. In some types albite, microcline and biotite 

 appear as accessories. In the types resembling most closely 

 the rock described by Broegger as grorudite, quartz is confined 



