PEBVIOUS INVESTIGATIONS. 29 



History of the lode. — Ml'. Cliurcli cUvides tlio liistoiy of the Comstock Lode 

 into nine epochs :^ 



1. The diorite epoch. — The horizontiil deposition of diorite, which is one 

 of the fine-grained, thin-running' hwas," in stratified hiyers, by a series of 

 eruptions. 



2. The subordinate pressure.— The system of diorite strata was acted 

 upon by a pressure which produced broad folds wiih east and west axes, 

 an uplift in Virginia, and a trough in Gold Hill. This important force 

 continued to aff'ect the rocks through the greater part of their history, 

 and is the dynamic cause of the Lode. 



3. The propylite epoch.— The horizontal deposition of the propylite, also 

 in stratified layers from successive fissures. The members of the new rock 

 are essentially parallel to the older layers. 



4. The principid elevation. — After the propylite, came a movement by 

 which the two sei-ies of eruptive depositions were raised into a mountain 

 system. This elevation took place about a north and south axis, and its 

 folds are therefore at right angles to those of the former movement. 



5. The andesite epoch. — A third period of eruption follows, the seat of 

 which is the upturned strata of the diorite and propylite. These are not 

 fractured except near the eroded surface, but the layers are separated, and 

 the andesite rises through the crevices, establishing an extensive system of 

 bedded dikes. The whole mass of erupted andesite is assumed to have 

 been some thousands of feet thick, and to have played an important part in 

 the history of the Lode by its weight and rigidity. 



6. The opening of the strata — The crests of folds already produced were 

 lifted forcibly against the rigid andesite cap, while the intervening troughs 

 were bent downward, relieving them from its weight. Under this action 

 the uplifted portions of the strata were squeezed sidevvise into the relieved 

 troughs, opening slightly the partings between the layers. 



7. The silicious epoch. — Thi'ough the small partings of the strata thus 

 opened, rose currents of water holding silica in solution. The strata sub- 

 jected to their action were dissolved or carried off mechanically, and quartz 

 with "base" metals was deposited in their place. This action went on in each 



'L. c, p. 128. «L. c, p. 153. 



