FEATURES OF THE FISSURES. 4-11 



the fissures. But they sometimes run in other courses. Between the 



Middle fork of the American and the Yuba there is a scries of fissures 

 trending directly across the strike of the rocks. One of these, on Jami- 

 son creek at the Plumas Eureka mine, has yielded a large product of 

 gold. At the same mine there are a number of so-called "Hat veins" 

 near and connected with the fissure, which are cleavage crevices enlarged 

 and filled with quartz and pyrite containing gold. 



Gold. — The occurrence ofgold is not only most important economically, 

 but is also a very important geological characteristic of the Sierra. The 

 gold is associated with quartz, various sulphides (pyrite, chalcopyrite, 

 galena, etc), and other minerals, hut the essential accompanying minerals 

 are quartz and pyrite. Gold mining in solid rock is called " quartz min- 

 ing," and the treatment ofgold ore consists principally in separating the 

 valuable metal from quartz and " sulphurets." It occurs with quartz and 

 pyrite both in fissure- and outside of fissures where the quartz is a pro- 

 duct of alteration of slates arid other rocks, and its occurrence seems to 

 he connected not only with the precipitation of quartz and pyrite in 

 fissures, but also with the presence of pyrite and the quartzitic alteration* 

 in the rocks of the range generally. The richest deposits of gold in the 

 solid rock, however, and all or nearly all that have been found rich 

 enough to he profitably worked on a large scale in the Sierra are in 

 fissures. The gold is very unequally distributed through the quartz of 

 the fissure; frequently only a pail of the thickness of the lode can be 

 worked, and profitable mining, where it exists at all, is always limited to 

 certain areas of the lode called ■•chutes " or "chimneys," and it would 

 in nearly all cases effect a large saving of cost to find and use every avail- 

 able means of determining as early as possible the trend of the axes and 

 outlines of these areas. 



Gold-bearing fissures occur in both the pre-Mesozoic and Mesozoic 

 rocks, [n the granites gold quartz lodes have been found more or less 

 productive, as at Granite basin, between the North and Middle forks of 

 the Feather, also between the Sanislaus and Tuolumne south of Sonora 

 and elsewhere, but E believe nonesuch have been found far from contact 

 with other rocks, and the great area of granite exposures, which includes 

 much the larger part of the Sierra, has been barren ground for miners. 

 In the pre-Mesozoic sedimentary rocks rich deposit- have been found at 

 and near Sonora. between the Sanislaus and Tuolumne rivers. These 

 rocks are traversed by dikes of eruptive matter, which to the naked eve 

 appears like the Mesozoic diabase, and the dikes were probably formed 

 and filled iii Mesozoic times. The gold occur- mostly in and near th< 

 dikes, and therefore it probably should be classed with the gold deposits 

 of (he Lower Mesozoic subgroup. The pre-Mesozoic rocks of the district 



