354 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



The groundroass is a mixture of delicate plagioclase staves, minute prisms of 

 hypersthene and augite, with magnetite anil a scanty glass base between them, the 

 latter devitrifled by brownish globulites. 



HTPERSTHENE-ANDESITE. 



On the northeast shoulder of the mountain a very dark compact rock occurs, which 

 seems to be an almost typical angite-andesite. Macroscopically there are numerous 

 small glassy feldspars visible and a few green grains and ore specks, but the black, 

 generally vitreous groundmass is much more prominent [144]. The microscopical ex- 

 amination shows a very close resemblance to the well-known Hungarian " augite- 

 andesites " of similar macroscopical habit. The rock contains no hornblende and 

 no biotite, while the pyroxene consists in part of hypersthene and in part of common 

 augite. Hypersthene is the more characteristic bisilicate in this rock, and the name is 

 therefore given as above. Its determination rests upon careful optical and chemical 

 investigations. 



Comparative study in connection with the above rock has shown that a large 

 number of so-called augite-andesites, both in this country and in Europe, are more cor- 

 rectly to be considered as hypersthene-andesites. Detailed investigations in regard 

 to the Buffalo Peak rock and a comparative microscopical examination of allied occur- 

 rences are given in Bulletin No. 1 of the series published by the United States Geo- 

 logical Survey, ''On Hypersthene- Andesite,'' &c. 



On Plate XXI are heliotype reproductions of photographs showing the com- 

 position and structure of the chief andesite types of the Buffalo Peaks. The result 

 is so unsatisfactory that the figures convey but an indistinct impression. In Fig. 2, 

 however, the small prisms of hypersthene are distinguishable from augite, which occurs 

 chiefly in peculiar aggregates, with magnetite, feldspar, and sometimes with biotite, 

 as shown in the lower left-hand portion of the figure. 



TUFACEOUS ANDESITES. 



The tufaceous rocks of the Buffalo Peaks are chiefly if not entirely of ande- 

 sitic character, although they exhibit a very wide range in composition and texture. 

 Some of them are loose or friable ash-beds,.others contain a large amount of dark 

 pearlitic glass with the ashy material, and still other beds are so compact as to resem- 

 ble massive rocks. In composition they vary greatly, especially in regard to the more 

 basic silicates, for hornblende, biotite, hypersthene, and augite are respectively the 

 characteristic minerals in different beds, while they frequently occur t< gether. 



The pebbles included in these tufas represent as many types of massive ande- 

 sites as are indicated by the various beds of tufa. Granite is also frequently found, 

 especially in some layers, and sometimes in large bowlders. 



RESUME. 



In the following lines are brought together, in concise form, the results and par- 

 ticular features of the preceding description which are deemed of special importance 

 or interest: 



