30 GEOLOGY OF THE COMSTOCK LODE. 



of the open seams, the intervening rock being attacked from both sides 

 until the meeting of Several depositions of silica composed quartz bodies, 

 which in many cases had a thickness of several hundred feet. This quartz 

 was not argentiferous, and no ore was formed. A second important result 

 of this appearance of silicious waters is the almost entire removal of the 

 immense andesite cap, which was decomposed in the same manner as the 

 deeper-lying rocks. 



8. The trachyte epoch. — New crevices opened in the eastern part of the 

 district, and vast floods of trachyte poured out. Instead of resisting move- 

 ment like the andesite, it loaded down the hanging wall of the Lode so 

 heavily that it slid upon the foot wall. This action resulted in an entirely 

 new system of openings. Near the surface the new crevices abandoned the 

 old line of quartz deposition, and broke through the hanging wall in a more 

 or less nearly vertical direction. 



9. The argentiferous epoch. — Into these new crevices poured a second 

 stream of water containing minerals in solution, but differing from the first 

 in holding not only silica, but also silver and gold. 



"The facts here brought forward," says Mr. Church, "show that no 

 vein and nothing like a real vein exists in ground that has for years been 

 supposed to contain the boldest example of true fissure vein formation in 

 the world; that the largest bodies of ore can be formed from deep sources 

 of mineral supply without the agency of a fracture even of the smallest 

 dimensions; and that it is quite unnecessary to seek for great dynamical 

 convulsions to account for the formation of thick masses of ore within the 

 solid rock, a sufficient cause being found in the quiet action of the same 

 forces which have everywhere molded the crust of the earth." 



The Justice ore body Mr. Church regards as a deposit wholly distinct 

 from the Comstock, though attributable to the same general causes, and as 

 foi'med in a similar way. 



Physics. — Of the finely-divided quartz known as sugar quartz, he says ■} 

 "The grains are remarkable in never being crystalline, the microscope not 

 revealing one crystal in millions of particles." And again :^ "The lesson to 

 be derived from the sugar quartz is not that it has been crushed, but that it has 



'L. c, p. 85. 'L. c, p. 151. 



