260 GEOLOGY OF THE COMSTOCK LODE. 



Reasons for some fluctuations. — A part of tliB fluctuations of tlic observatioiLs in 

 the Wa.shoe Distkict can be reasonably accounted for. In the Foniiaii 

 shaft it will be observed that the water temperatures are somewhat lower 

 than the rock temperatures above a depth of 1,160 feet. The upper portion 

 of this shaft passes through decomposed, and in part disintegrated, augite- 

 andesite. Near its under surface, however, this rock is somew^hat fresher, 

 and is there unusually fine-grained and rhyolitic in structure. It therefore 

 offers some resistance to the rise of waters from below, and almost none to 

 the descent of the slig-ht atmospheric precipitation. The point at which the 

 water grows hotter than the rock is exactly that at which the shaft passes 

 from augite-andesite into the underlying hornblende-andesite. At 1,700 

 feet the shaft became so hot that it was necessary to shower cold watei* from 

 the surface. The subsequent water temperatures were excluded from the 

 calculation, and it is most likely that the rock temperatures were somewhat 

 affected. This offers a probable explanation for the abnormally low tem- 

 peratures of the rock immediately below this point. Mr. Forman informs 

 me that, from the 20U0-foot level on, the practice of showering water into 

 the shaft was abandoned. 



As may be seen from the section through the Yellow Jacket (Atlas-sheet 

 VII.), this shaft passes through diabase and mica-diorite alternately, and such 

 changes are likely to exaggerate the ordinary disturbing influences. That 

 portion of the Combination shaft in which observations were taken is wholly 

 in diabase, but there is evidence of disturbed conditions. The water in the 

 face of the Sutro Tunnel opposite the Combination shaft was about 5° 

 cooler than the rock in the shaft, while a reverse relation would have been 

 expected. The shaft observations also fluctuate somewhat violently near 

 this level, while for the interval from 1,900 to 2,100 feet the increment is 

 sensibly the same as in the other shafts. It seems probable, therefore, that 

 the high value of a and the low value of h resulting from the reduction of 

 the observations is somewhat misleading, and that local variations of struc- 

 ture only cause them to diflfer essentially from those obtained for the Yellow 

 Jacket and the Forman shafts. 



Conditions in the Sutro Tunnel. — It will probably at once occur to the reader 

 that the depth of the Satru Tunnel Ijelo^v tlie surface is far from uniform. 



^'4Si ''.^ 



