102 CALIFORNIA ACADEMY OF SCIENCES. 



rounded by porjjhyritic rock. Neither one nor the other 

 form inchisions. They resemble the dark spots so constantly 

 found in granite and show in innumerable instances, a tran- 

 sition from one structure to the other. In some cases this 

 transition is rapid though unmistakable, in others it is 

 very gradual, so that it would be impossible to say within 

 some inches where the mass should be called granular, and 

 where porphyritic. In a great proportion of cases, the por- 

 phyritic portions contain hornblende recognizable with the 

 naked eye. Under the microscope, hornblende is seen to be 

 abundant, and augite almost entirely wanting. Now, I know 

 of no reason to suppose that the change from a porphyry 

 Avith a granular ground mass to a thoroughly granular struc- 

 ture is regularly accompanied by a change of the bisilicate 

 from hornblende to augite; indeed, there is ample direct 

 evidence that this is not necessarily the case. The infer- 

 ence then is strong that where these patches occur, and I 

 know of no part of the mountain which is free from them, 

 the mass is essentiall}^ and originally hornblendic. 



Crystallization of diorite does not vary loitli depth. — But even 

 if it could be shown that the granular west wall of the Com- 

 stock were of the same mineralogical composition as the 

 east wall, as I believe impossible, it would be a necessary 

 inference from the whole nature of the occurrence that the 

 two rocks Avhich I call granular diorite and porphyritic dia- 

 base are diiFerent eruptions which have cooled under wholly 

 different conditions. The diorite is now exposed on the 

 3,000 foot level of the Chollar mine. It is at this point ab- 

 solutely identical in mineralogical and physical character 

 with the rock on the surface. This statement is not founded 

 on general impressions. I gathered every variety of the 

 diorite which was to be found on the 3,000 level, and 

 took the specimens with me to the flume above the crop- 

 pings. I found no difficalty in matching each of them per- 

 fectly as to structure and coarseness. The only trace of 

 difference was in the color, which was of course a bluish 



