BLACK HILLS GEOLOGY. 



293 



Whether there is any gradation between the quartz-aegirite- 

 porphyries and the quartz-porphyry -andesite series has not been 

 definitely determined. Certain rocks gathered from the com- 

 plex mass on Foley Peak seem to indicate a transition between 

 these rock types, as also does the larger percentage of soda in 

 the quartz-porphyry from the Ulster Mine. 



The following diagram will illustrate the affinities of the 

 aegirine rocks. 



Phonolite 



or tinguaite 



Family 



A 



Increasinsr SiO^ — 



> 



Sdlvsbergites ? 



Quartz-EEgirite- 



porphyry or groru- 



dite Family 



Rock from the Ulster Mine. 



Trachyte 

 Family 



Quartz-porphyry 



or Rhyolite 



Family 



1. Rock from Foley Peak. This position as a transitional type is tentative. 

 Chemical analysis is necessary to confirm it. 



2. Second specimen from Foley Peak. Position likewise tentative. 



Concerning the succession of the different types, a few definite 

 data are available. The mica-andesite or mica-diorite-porphyry 

 on the divide beyond Twin Peaks is cut by a dike of tinguaite. A 

 dike of the same rock intersects the quartz-porphyry of the Port- 

 land type, on the western slope of the hill beyond the Rua 

 Mine. Both contacts are clearly visible. The same dike cuts 

 the porphyry of the War Eag e Hill type, in the Gulch below 

 the Rua Mine. The War Eagle Hill quartz-porphyry is again 

 cut in the bed of Squaw creek by a third dike of tinguaite, 

 which strikes east and west. In Bear Gulch the diorite-por- 



