THE WASHOE ROCKS. 107 



that crystallization could progress after the cones were 

 complete though still hot. It is difficult to imagine any 

 influence other than pressure tending to modify the char- 

 acter of the rock in a horizontal direction excepting the 

 rate of cooling, which would depend upon the distance 

 from the nearest surface. The dip of the lode is 45^, an 

 angle greater than that of any volcanic cone/ hence the 

 rock at the lode on the 3000 foot level must have been 

 further from the surface of the supposed cone than that at 

 the croppings, besides being under enormously greater pres- 

 sure. Since no difference tending to confirm the views of 

 Messrs. Hague and Iddings is perceptible on the dip of the 

 lode, it seems improbable that any could be detected along 

 a horizontal line equally far removed from the surface. 



Bait of variation of crystallization. — It is very evident 

 from Messrs. Hague and Iddings' paper, that the rate of in- 

 crease of crystallization is more rapid near the inner end of 

 the tunnel than near the outer end. The difference in this 

 respect between the ordinary fine-grained diabases and the 

 diorites, supposed by them to be the same rock, is very 

 great; while they do not claim to have found anything like 

 so great a difference between dift'erent portions of those 

 tunnel rocks which I regard as pyroxene andesites. Now, 

 one cannot consider the laws of cooling and the curves and 

 functions representing them for a moment without perceiving, 

 that the difference of rate of cooling decreases very rapidly 

 near the surface of a cooling body, and almost disappears at 

 considerable distances from the radiating surface. Hence, it 

 would seem that if the difference in crystallization is de- 

 pendent on the rate of cooling, and if Messrs. Hague and 

 Iddings have correctly interpi'eted the structure of the dis- 

 trict, the rate of increase in the Sutro Tunnel should have 

 been greatest at the eastern edge of the pyroxene andesite 

 and nearly or quite imperceptible near the lode.'' 



Note ^. — American Journal of Science, 1885, vol. 30, p. 283. 



Note ^. — It is well known that iron-blast furnace slags, which are glassy if 



