BLACK HILLS GEOLOGY. 281 



amphibolite is included and around it the flow lines make a 

 series of fantastic curves and eddies, following the contour of 

 the included fragment. The separate layers are of all thick- 

 nesses, but are so sharply marked off from one another that 

 they may be readily followed. Many of them thin out into 

 mere hairlike tongues, which may still, however, be distinguished 

 from the adjacent layers by their sharply contrasted color. 

 Under the microscope these different layers show simply a differ- 

 ence in the texture of the groundmass, and in the abundance of 

 phenocrysts, the latter occurring at much rarer intervals in the 

 darker and more fine-grained streaks. A photograph showing 

 these flow. lines may be seen in Plate XVI. 



4. Andesite Family. 

 Mica-Diorite-Poryhyry. 



This rock occurs in quite large development throughout the 

 region of Squaw Creek, and in the Ruby Basin. In the head 

 of that creek, in the upper shaft of the Rua Mine is a sheet, 

 and the same rock forms the large Redpath laccolite. It oc- 

 curs also in irregular masses in Squaw Creek and its tributaries, 

 and forms a sheet of considerable size at the mouth of that 

 creek. There is further a large development in the vicinity of 

 Carbonate Camp. 



The analyzed specimen is from the Rua Mine, and when 

 seen in hand specimens, it presents a very much darker and 

 more basic appearance than any of the rocks so far described. 



Megascopic Appearance. — It has a dense bluish black ground- 

 mass, in which are thickly disseminated small scales of biotite. 

 and when in the fresh condition large prisms of hornblende, 

 giving to the rock a very basic aspect. In the rock from the 

 flat northwest of Twin Peaks the hornblendes are present in 

 great development, but in the other occurrences are generally 

 decomposed and show only chloritic pseudomorphs. The feld- 

 spar phenocrysts are not in very great abundance and often 

 show an almond-shaped cross-section, rarely attaining a greater 

 length than one-fourth inch. When the rock is much altered the 



Annals N. Y. Acad. Sci., XII, December 5, 1899—18. 



