274 GEOLOGY OF THE COMSTOCK LODE. 



It appears to me necessary, therefore, to suppose that quartz and ore have 

 been deposited in openings. The space occupied by the bonanza can of 

 course never have been an uninterrupted cavern, but it would seem to have 

 been a space loosely filled by fragments of country rock, vi^hich are now 

 represented by the included horse matter. Though the country -rock is so 

 greatly fractured, a space of this kind is by no means impossible. If a 

 large opening were to be made anywhei-e in the diabase, fragments would 

 immediately fall from the sides and roof. The latter would assume the 

 shape of a dome, and though a complete arch of blocks would not form, a 

 portion of the weight of the overlying country would be distributed later- 

 ally, and the diminished pressure would most likely be insufficient to crush 

 the displaced fragments. The lenticular mass of diorite below the bonanza 

 does not appear to be in jjlace. It was probably partly separated from the 

 west wall at the diabase eruption, and since that time it seems to me to 

 have moved downwards. Owing to the irregularity in the walls consequent 

 upon its presence, and to the difference between its resistance and that of 

 the diabase to lamination by faulting, it left a rent in the hanging wall, 

 which has afforded an opportunity for the deposition of quartz in the man- 

 ner just explained. 



Cross-section through the Tunnel. Tlie UeXt SeCtioU SOUth of the C. (& C. is that 



through the Sutro Tunnel and the Savage shaft. It fails to cross any ore 

 but, as may be seen from the longitudinal vertical projection, it nevertheless 

 passes through nearly the lowest point of a fan-like group of bonanzas, 

 the "Virginia group," as it is often called, extending from the CJiollar to the 

 Gould d Curry. On this plane the secondary fissure leaves the west wall at 

 a lower point than in any other portion of the Lode, and all of the bonan- 

 zas were found in the secondary fissure. Throughout this portion of the 

 Lode the east and west fissures display the same general characteristics as 

 at and near the Andes. The west quartz was hard, according to Mr. King, 

 while the eastern quartz, as I have myself been able to observe, is crushed. 

 The great horse bod}^ is split by quartz-masses, which are not continuous, 

 however, thinning out in the strike and being replaced by others. Clay 

 seams are very heavy and intersect as well as follow the horses. The ore 

 was not "base," and much of it was extraordinarily rich. The bonanzas 



