394 GEOLOGY OF THE COMSTOCK LODE. 



ore, though nearly half its value is usually in gold. The proportion of the 

 two metals varies greatly in different portions of the Lode and even in the 

 same ore body. It is probable that the Comstock contains but little quartz 

 which is wholly barren; while, as is usual in silver veins, it is only in cer- 

 tain spots that the tenor reaches a point at which extraction is profitable. 

 These concentrations, or "bonanzas," usually occur in masses of quartz of 

 lower grade, and large bodies of quartz usuallj^ contain "bonanzas" when 

 they are associated with the diabase, though to this rule there are exceptions. 

 The Justice bonanza is the only one of any moment which is not associated 

 with that rock. The quartz is in great part in a highly crushed condition, 

 resembling nothing so much as ordinary commercial salt. When the fine 

 dust from such masses is examined under the microscope in polarized light, 

 it is immediately seen to consist of fragments of quartz crystals; the larger 

 particles can be shown to have the same origin by direct examination. 

 Very solid quartz bodies are also met with in certain positions. 



Crushing action. — The prcscnce of faults on the Comstock is abundantly 

 proved, as has already been shown. The secondary fissures form one evi- 

 dence of such a movement, and as a large portion of the ore occupies the 

 openings between the great horse and the east country, it is plain that the 

 deposition of ore was preceded by faulting. The only movement which can 

 have crushed the quartz must also have been in the nature of a fault, and 

 some of the bonanzas show a parallelism in the lines of dynamical action 

 to the dip of the Lodk. It is not probable that the solid masses of quartz 

 were formed at a later date than those now found in a crushed condition, 

 for it appears to be only when the quartz has been deposited in sheets par- 

 allel to the west wall that it has escaped comminution. Certain stringers of 

 rich ore in the bonanzas have seemed to possess great solidity, and may 

 possibly have been formed after the final cessation of movement; other- 

 wise the entire period of quartz deposition seems to have been embraced by 

 that of the fiuilting movement. It is much more probable that the total 

 fault was accomplished by a great number of small slips in the same sense 

 than tliat one large throw preceded and another followed the filling of the 

 vein. 



Clays. — The clays of the Comstock are not largely comjiosed of kaolin, 



