104 CALIFORNIA ACADEMY OF SCIENCES. 



calculations, the faulting on the Comstock amounts, at this 

 locality, to about two thousand feet vertical dislocation. If 

 this is correct, and if the two walls are portions of the same 

 eruption, the fine-grained diabase of the 3,000-foot level 

 cooled under a pressure of at least one thousand feet greater 

 than the coarse granitoid rock which forms the west wall at 

 the croppings. It is also to be observed, that since these 

 rocks are separated only by the width of the fissure, and 

 must have been in contact before the fissure formed, it is 

 impossible to suppose those portions of the rocks which 

 were originally on one level subject to different physical 

 conditions in cooling, if they originally formed parts of one 

 eruption. It is of course open to all to doubt the correct- 

 ness of my theory of the faulting on the Comstock. If I 

 am wrong, the fault may have been greater, but I think 

 few geologists who have studied the district would be 

 willing to admit a fault of above three thousand feet. If the 

 vertical displacement is supposed three thousand feet, the 

 fine-grained diabase of the 3,000-foot level must have cooled 

 under a pressure not less than that of the granitoid diorite 

 west of the croppings, if the two ro*cks formed portions of 

 the same eruption. On the other hand, this would involve 

 as a consequence the assumption of an immense erosion 

 since the fault took place, an hypothesis at variance with 

 many observed relations. One of these is on Messrs. 

 Hague and Iddings' hypothesis, the survival of glassy 

 portions of the great eruption of porphyritic pyroxene 

 rock. There being no limit to suppositions, however, any 

 amount of faulting may be supposed. It then appears 

 that if the texture of these rocks is a function of the 

 depths at which they cooled, the coarseness and granula- 

 tion increasing with the depth, though slowly, the amount 

 of faulting which will account for the character actually 

 observed must exceed six thousand feet by a distance 

 which is indefinite but certainly enormous. This no one 

 will maintain for a moment. 



