106 CALIFORNIA ACADEMY OF SCIENCES. 



the adit as well as the new ones prepared for Messrs. 

 Hague and Iddings. In the greater part of these, 

 the gronndmass, as well as the porphyritic crystals, are 

 highly modified, and a very large proportion of the grains 

 so carefully measured by Mr. Iddings are neither more nor 

 less than secondary quartz. In my opinion, if his micro- 

 scopic analysis of the gronndmass of these rocks proves any- 

 thing, it is simply that solfataric action increased in inten- 

 sity as the distance from the lode decreased, an interesting 

 result but not a new one. 



Physical Conditions. — If the diabase and augite andesite 

 formed a single eruption, the original surface may have been 

 level. If so, there could have been no difference in pres- 

 sure or rate of cooling on any horizontal line. Those who 

 do not accept my theory of faulting on the Comstock will 

 probably regard the east country as a single continuous 

 mass. In that case, it is hard to see how there can have 

 been any notable increase of pressure or retardation of 

 cooling along the Sutro Tunnel. If the truth of my theory^ 

 of the faulting is granted, the tunnel strikes the east wall of 

 the Comstock at a point which was originally about 1000 

 feet lower than the eastern edge of the augite andesite. 

 But I have already shown that an increase of depth of 3000 

 feet makes no perceptible difference in the character of the 

 rock. The influence of a single thousand feet cannot pos- 

 sibly be traceable therefore. 



The supposed eruption may also have formed a volcanic 

 cone above the Comstock instead of a level surface. In 

 this case, too, horizontal planes would be level or equipo- 

 tential surfaces, or planes of equal pressure," and there 

 could be no tendency induced by pressure to more thorough 

 cry^stallization on horizontal lines, even if it were supposed 



Note ^ — This can readily be seen by considering extreme cases. Suppose 

 a hollow cone tilled with fluid. Then of course horizontal surfaces are sur- 

 faces of equal pressure. Suppose a perfectly rigid cone; the same result fol- 

 lows. From these extremes any intermediate case of a viscous cone follows. 



