264 GEOLOGY OF THE COMSTOCK LODE. 



But it is cleai- that as time goes on the radius of curvature of the conduction 

 curve will increase, and that no illimitable time has elapsed since the Lode 

 first assumed a temperature of above 110° F. on the 1,900-foot level. 



Results independent of very accurate thermometers. It is Well knOWU that tllC tliermOm- 



eter is not an instrument which gives positively uniform results, and that 

 thermometric experiments aiming at a high degree of accuracy imply con- 

 stant rerating of even the best instruments, which probably change more 

 or less permanently at each fluctuation of temperature. The observations 

 discussed in the foregoing pages were only in part taken with first-class 

 thermometers, and some of them are very probably affected with errors of 

 1° or even 2° F. from this cause. This fact, however, does not at all impair 

 the general validity of the resi;lts obtained. Suppose the graduation of the 

 thermometers employed wholly arbitrary, and that the graduation of the 

 instruments used at each shaft bore no relation to those used at any other, 

 but that the calibration of each was good and permanent changes in volume 

 were absent; the results obtained would still show that the increment of 

 temperature from the surface downward was affected by no percejDtible law 

 other than that of direct proportionality to depth, and that in the tunnel 

 the rise of temperature as the Lode was approached was best expressed by 

 a geometric ratio. Or suppose that imperfection in calibration and the per- 

 manent effects of expansion induced any error for which a precedent can 

 be found, say, even 3° F., between the highest and lowest readings in either 

 of the shafts or the adit ; the differences themselves are so large (from 30° 

 to 60° F.) that the same general conclusions as to the great distance of the 

 source of heat and the method of its communication to the walls of the Lode 

 would follow. In short, the indications are so positive that no probable 

 errors in the theraiometers, however gross, could account for them or ob- 

 scure them. 



Conclusions. — The couutry rock, then, is heated from the Lode or the sys- 

 tem of fissures closely associated with it, and the focus of this heat is at a 

 vertical distance which can hardly be less than two miles from the surface, 

 and is more probably four — in short, at a volcanic distance. Only fluid sub- 



