THE COMSTOCK LODE. 61 



it has been necessary to fill large chambers to prevent serious accidents from 

 caving in, as was the case in parts of the Savage and Gould and Curry mines, 

 rooms were opened in the thick bodies of clay, and an endless supply obtained 

 by its expansion. In the region of the Potosi strike, a chamber about twelve 

 feet high and fifteen feet across was opened in clay, which, on exposure to 

 the air, immediately began to swell and to flake off in conchoidal scales from 

 a foot to two feet in size. Two men wheeled these as fast as they fell from 

 the roof of the excavation, and dumped them into the open stopes. For weeks 

 the falling clay gave them ample material, the size of the cavity scarcely 

 changing meanwhile. After filling the stopes the chamber was abandoned; 

 and when next visited, at the end of a few weeks, was found to have filled 

 itself solid. 



The Ophir Group. — Atlas-Plate 5 comprises all of the materials that 

 can be assembled of the region lying between the Gould and Curry and the 

 northernmost workings of the Ophir. This, in reality, indicates the northern 

 limit of the productive lode. Continuing northward upon the slopes of Cedar 

 Hill, there are indeed several claims in which the quartz is still developed, 

 carrying small quantities of silver and gold, but so far nothing that can be 

 called a bonanza has been found. The Sierra Nevada, in its uppermost levels, 

 contains fragmentary masses of blocky quartz impregnated with native gold, 

 closely resembling the California auriferous quartz. Directly west of it, and 

 high upon the slope of Cedar Hill, gold recurs in the Sacramento mine, which 

 perhaps is the Sierra Nevada sheet, although more probably it lies behind that 

 body. Still further to the north of this, the Utah and Allen are unquestion- 

 ably parts of the same lode, but in an economical sense have proved almost 

 valueless; and their explorations are too limited to throw any additional light 

 upon the mode of occurrence of vein-materials. 



From the Gould and Curry to the north line of the Central the quartz- 

 bodies continue; but, instead of being charged with the characteristic silver 

 ore, contain bonanzas of base metals ; galena, blende, and iron pyrites predom- 

 inating. The 600 feet lying directly north of the South Ophir mine was at 

 one time the richest and most productive portion of the Comstock. This 

 highly complicated region is well shown on Fig. 4, Atlas-Plate 12, which is a 

 horizontal section upon the level of the Union Tunnel, 2 70 feet below the 



