582 DESCRIPTIVE GEOLOGY 



fine breccia, of which the included fragments are so small and similar to the 

 paste that to the unaided eye it appears like one homogeneous rock. In 

 certain of the larger feldspars, a metamorphism has taken place, resulting in 

 a kaolin-like substance, penetrated in all directions by fine seams of chalce- 

 dony. Further evidence of alteration is observable in the cloudings of 

 rusty-brown which traverse the purple groundmass. This results no doubt 

 from the decomposition of the iron in the hornblendes, which conveys a 

 stain to the plagioclases, and renders their edges and cracked portions a yel- 

 lowish-brown. The mass possesses a general vertical structure, and never 

 overflowed in tabular flows. This is common to all the dacites, whereas 

 the andesite frequently overflows after the manner of the basalt, as at 

 Washoe and Berkshire Canon, Virginia Range. 



Cropping through the dacite along the centre of the ridge is an obscure 

 mass of dark-brown andesite, possessing, in a remarkable degree, the same 

 earthy, half-decomposed modification, as described in the neighboring dacite. 

 This is probably the crest of a ridge of earlier origin than the dacite, and 

 represents a limited area uncovered by erosion. The rock has somewhat 

 of a resinous lustre, and its feldspars are decomposed, and show a zonal 

 structure. Under the microscope, their twin striation is visible, and the 

 dark, yellow-brown crystals are seen to be augite, which brings this rock 

 among the augite-andesites. The groundmass is much discolored, changing 

 from the prevailing chocolate color to a rusty, iron-red and pale, yellow- 

 brown, and is seen under the microscope to have a gray, rather than the 

 usual brown, glass, which is probably caused by its proportionately greater 

 richness in augite. It contains no olivine nor quartz. Its chemical con- 

 stituents, as determined by Mr. R W. Woodward, are: 



Silica 61.64 



Alumina 17.44 



Ferric oxide 0.82 



Ferrous oxide 3.99 



Manganous oxide , ,. trace 



Lime ■. 5.86 



Magnesia 3.05 



