200 _ REVIEWS 



Superficial alteration of the rocks is apparent to depths of 300 and 

 400 feet with the usual effects of hydration, oxidation, and leaching. 



The value of the ores shipped varies from $20 to ^400 per 

 ton; an average is perhaps between ^50 and ^85. During the 

 first year's operation, the camp was essentially a shipper of high-grade 

 ores. Recently, however, there has been a change with the establish- 

 ment of a number of cyanide and chlorinating plants, which has 

 reduced treatment charges to I7.50 per ton. It is to be feared, how- 

 ever, that an understanding between the managers of these plants will 

 prevent a further reduction in these charges, if it does not cause an 

 increase over the above rate. It is of great importance to the future 

 of the camp that low freight and treatment charges be reached and 

 maintained as otherwise great quantities of low-grade ores will have to 

 be thrown aside. Any such movement, therefore, to maintain high 

 charges is to be opposed and may seriously damage the camp. 



Chapter III deals with the mode of occurrence of the ores. This 

 subject is one of special value and interest here. It is well summarized 

 iin the statement of the first paragraph, which is to the effect that the 

 «0res generally occur in fissures in the country rock, which usually 

 represent slight faulting and that the veins have been formed mostly 

 by replacement along these fissures and not by the filling of open gaps. 

 Indeed, there is often no cavity recognizable and little, if any evidence 

 of one ever having existed, the solutions apparently having filled the 

 imperceptible space between the fiat joint or fault planes of the rock, 

 ithe gold minerals having been deposited on the surface and the rock 

 litself having been merely silicified. Thus, in these deposits, the 

 scrapings from such surfaces often run very high in gold, while the 

 Tock itself is valueless. This is a very peculiar form of deposit and is 

 almost exclusively confined to this district. 



The deposits occur in a region of Tertiary volcanic breccias or 

 tuffs. The breccias are cut by numerous bodies of intrusive eruptive 

 rocks, such as phonolite and andesite, and they also surround eruptive 

 masses which are older than the breccia. The latter are themselves 

 surrounded by what Cross terms the granite-gneiss complex of the 

 Colorado range. The dikes intersect all of these rocks, passing from 

 granite into breccia, and they also intersect each other. They are 

 evidence of several epochs of fissuring. The veins also intersect all 

 these rocks in their courses. The vein fissures seem to be later than 

 the dikes, though some were apparently formed at the same time. 



