262 GEOLOGY OF THE COMSTOCK LODE. 



acter. The Forman and the Yellow Jacket shafts show that the characteristic 

 increment is very close to 1 '^ F. for every 33 feet of vertical descent, and, 

 since there is no evidence of curvature, this rate may be expected to con- 

 tinue for a long distance below the present workings. As the source of 

 heat is approached the vertical increment must increase, and the true 

 expression for the relation of depth and temperature is probably very sim- 

 ilar to that found for the horizontal increment in the Sutro Tunnel. It 

 appears hardly possible that were the source of heat within two miles of 

 the surface no trace of curvature would be perceptible in the diagrams for 

 a depth of 2,000 feet. The probabilities seem to be that the focus is several 

 miles from the surface. 



Equations referred to the datum level. — Tlic cquatious for tlic shafts are referred to 

 the surface at the points where they are sunk, and equal values oft? do not, 

 therefore, answer to the same level. The Forman shaft is 356 feet and the 

 Yellow Jacket 343 feet below the datum level employed in surveys of the 

 mines. Referred to that level, the equations become: 



Forman Shaft: ^=38 -f 0.033 d 

 Yellow Jacket: irr42-f 0.033 d 



where d is the depth below the datum level. 



Correlation with the tunnel equation. — Thc difference bctwcen thc valucs of a in 

 these two equations is 4°. Now, the Yelloiv Jacket shaft is about 2,600 

 feet from the croppings of the Lode, and the Forman shaft is 950 feet 

 farther, and the curve obtained from the Sutro Tunnel shows that such a 

 difference should exist. Indeed, if in the equation 



^=8()-f 34 £»■'""'''* 



— 2,600 and —1)50 are successively substituted for x the difference in the 

 values of t obtained will be 4°.^ This shows very clearly that the law of 

 decrease of the temperature to the east of the Lode holds good for other 

 sections than that taken on the line of the Sutro Tunnel; and this inference 

 is strengthened by the similarity of the temperatures in the north and south 



'To give a to fractions of a degree would manifestly be absurd. If the fractions resulting from 

 cotupiitation were to be retained the dilference in the value of a calculated from the exponential equa- 

 tion would be the same as that derived from the ob.servations at the shafts within half a degree. 



