220 GEOLOGY OF THE COMSTOCK LODE. 



from ten per cent, downwards; but in the graphitic slates forming the west 

 wall in the Gold Hill mines, bunches are met with in which it is the pre- 

 dominant constituent. These are, however, usually only a cubic foot or 

 two in size, and appear to occur only close to the vein. As a rule, the slates 

 are not much more pyritiferous than the diabase. 



Relations between ores and rocks. — There is au ovidcut rclatiou between the in- 

 closing rocks and the character of the ore. The rocks occurring at and 

 near the Justice, with its refractory ores and calcite gangue, are metamor- 

 phic diorite, mica-diorite, quartz-porphyry, and hornblende-andesite. The 

 Cedar Hill gold-quartz veins are in diorite. The ores of the more impor- 

 tant mines lie on the contact between diabase and diorite. 



There seem to be but two probable ways in which these differences 

 can have come about.^ The ore deposits might have taken place at differ- 

 ent times, and therefore under different conditions, or the contents of the 

 fissures may have been extracted from their walls at the same time, and the 

 differences be due to the composition of the surrounding rock. If the 

 Cedar Hill veins were deposited at a different time from the main mass of 

 the CoMSTOCK ore, it must have been at an earlier date, for the vast quan- 

 tities of solutions which reached the Comstock could not have failed to 

 penetrate the fissured diorite. Not only stringers from the Comstock, how- 

 ever, but even the "west vein," are of the same character as the Cedar Hill 

 quartz. When this west quartz was deposited the fissure below was cer- 

 tainly open, and had it been deposited before the argentiferous ore, it is 

 scarcely possible to suppose that it would not also have filled the vein at 

 lower points. If they were to be assigned to different periods, one would 

 also expect to find either gold veins in the east country, or silver veins in the 

 west. In short, there is much to show that these two classes of deposits 

 were contemporaneous; and I know of no evidence tending to show that 

 they are not ascribable to a single period. The Justice ore body is not 

 closely enough connected with the more important portion of the Cum- 



' It is also conceivable that the ores should have been precipitated from solution by the rock 

 forming the walls and the horses, and that the observed differences are due to the character of the 

 precipitant. All the evidence of ore deposits in general, and of the Comstock in particular, however, 

 appear to me to point to changes of temperature and pressure, evaporation and the action of liquid 

 reagents, as the causes of precipitation. In describing the Lode I shall be obliged to recur to this 

 subject. 



