BLACK HILLS GEOLOGY. 277 





Chemical Composition. 





/. 





//. 



SiO., 



69.78 



67.77 



AlA 



16.07 



17-57 



FeP3 



2.47 





FeO 



Not det. 



1-59 



CaO 



1. 17 



0.51 



MgO 



trace 



0.49 



K.p 



Not det. 



4-56 



Na,0 



Not det. 



6.20 



H,0 



0.65 



0-73 



Loss 



O.IO 



1.47 



100.89 



I. Quartz-porphyry known as "Bird's Eye porphyry" from 

 sheet along the Burhngton and Missouri Railroad on Crown 

 Hill. Analysis by J. D. Irving. 



II. Quartz-porphyry from large dike in the Ulster Mine, 

 near Preston, South Dakota. This rock forms the wall of the 

 ore deposits and is cut by a phonolite dike. The ore is asso- 

 ciated with the phonolite, and is stained with purple fluorite. 

 Analysis by Professor Flintermann, of Deadwood, South Da- 

 kota. 



In both of these rocks lime is very low, considering the very 

 considerable amount of plagioclase that is present. The per- 

 centage of soda is, however, so high that we would infer that 

 the plagioclase is quite probably an albite, which is also shown 

 by measurements on albite lamellae. 



War Eagle Hill Type. 



Further down the valley of Squaw Creek, and forming the 

 main mass of War Eagle Hill, occurs an exceedingly dense, 

 dark quartz porphyry, which apparently intersects the lighter 

 variety. 



Megascopic Appearance. — This rock has a grayish color, and 

 shows frequent phenocrysts of orthoclase. These are embedded 



