70 GEOLOGY OF THE COMSTOCK LODE. 



scopically distinguishable from other tufas, such as that of aiigite-andesite, 

 but inclosed masses commonly indicate its character. The exposure repre- 

 sented in Plate VII. is made up of coarse porphyries and tufa, and the engrav- 

 ing shows a species of bedding in the rock, no doubt due to variations in 

 eruption. Gray, tolerabl}' firm varieties, about as coarse as ordinary gran- 

 ular diorite, occur at the Sugar Loaf, F. 3, and near the Utah. At the latter 

 point columnar structure is very finely developed. Mount Abbie, C. 2, is 

 intermediate between the firm gray and the soft, highl}- porphyritic modifica- 

 tions, and the black glass}' occurrences require no further description. None 

 of these bear much resemblance to the prevalent varieties of earlier horn- 

 blende-andesite, but there is a considerable area to the northeast of Mount 

 Emma, and just outside of the map, where the resemblance is almost perfect. 

 This area seems to be strictly continuous with the more typical one, how- 

 ever, and transitions occur. Lithologically the presence of more or less 

 mica seems characteristic. 



The weathering of this rock is commonly confined to the separation of 

 ferric o.xide, not merely on the surface, but often for considerable distance 

 into the mass, where the latter is of an open texture. In the neighborhood 

 of the Sierra Nevada mine, however, chloritic degeneration of the bisili- 

 cates is perceptible. 



Distinctive characteristics. — No csscutial propcrty distiuguislies the younger 

 from the older hornblende-andesite, but in the Washoe Distkict it forms a 

 variety of andesite readily distinguishable in most cases by its loose struct- 

 ure, and the presence of mica. The glassy modification is more likely to be 

 confounded with augite-andesite, but the luster is not resinous, as in the 

 augitic rocks. ' The distinction is hardest to draw in the wild canons east 

 of Mount Kate, a region wholly unlikely ever to have any importance. 



Basalt plavs a very small part in the geology of the district, but the 

 rock is a thoroughly cliaracteristie representative of the species. It is dark 

 and compact, with many visible crystals of dark amber-colored olivine. 



Microscopical character. — Tlic basalt is a tlioroughly crystalline mixture of 



