THE COMSTOCK LODE. 33 



further evident, from the position of the andesite cappings, that nearly all the 

 surface erosion, which has determined the configuration of the whole district, 

 was subsequent to the andesite period. It occupied then an interval of 

 geological calm between the intense solfatarism and the turbulent conditions 

 of trachyte eruptions and erosion forces which followed. Nowhere else in 

 Nevada does this rock come into such close connection with a silver district. Its 

 usual j)osition in the volcanic group is normally continued here. Throughout 

 all of our observations on the volcanic rocks of the Great Basin it is invariably 

 subsequent to the propylites and anterior to the trachytes. With the single 

 exception of its lithological constitution, this rock follows closely the geological 

 habits of basalt, and until a comparatively recent period has been classed in 

 that family. 



Teachyte. — About on the level of Virginia City, and not far to the east 

 of it, there outcrops a line of trachytes. The most northerly of these is on 

 Graveyard spur, at what is known as the Quarry. Capping an abrupt hill is a 

 comparatively thin sheet of sanidin- trachyte. Throughout this outcrop is a 

 well-marked tendency to jointing planes which have a north and south direc- 

 tion, and a dip of about 70° to the south of east. These planes are again 

 jointed in rude hexagonal columns. The greater mass of the trachyte is 

 formed of these inclined prisms, varying in size from a foot to four feet 

 across. Unlike most volcanic columns, they are frequently curved, and in cer- 

 tain cases broken and pushed to one side. The rock weathers of a reddish- 

 gray, with an exceedingly rough surface, upon which protrude prominent 

 crystals of sanidin and hornblende. The base of the rock is a fine, gray 

 crystalline paste, chiefly formed of sanidin and black hornblende, throughout 

 the mass of which crystals of both these minerals are dispersed, associated 

 with spangles of black mica. Oligoclase is not unfrequently present. 



The second outcrop occupies a bench just above the Geiger grade, 

 near the Sierra Nevada works. It is quite unlike the former in litho- 

 logical characteristics, being of a heavy, dull grayish-brown base, contain- 

 ing but few sanidin crystals, occasional, but rare, augites, but neither horn- 

 blende nor mica. The mass is an unusually smooth, compact material, of a 

 very dark color and basic appearance. The outcrop is reddened and crumb- 



