118 CALIFORNIA ACADEMY OF SCIENCES. 



similarity Avliicli appears to exist between this material and 

 a quartzose felsitic mass distant over two miles, they do not 

 hesitate to identify the two, structural evidence again to the 

 contrary notwithstanding. While a large part of the granular 

 diorite is, beyond question, hornblendic, and a still greater 

 portion is of such a character that it is now impossible to say 

 with certainty whether the green amphibole is original, 

 uralitic, or results from an alteration of brown hornblende, 

 they regard the whole mass as altered pyroxene rock. Yet 

 when any specimen of this rock is found to contain mica, 

 they pronounce it later hornblende andesite, no matter how 

 it may be involved in the mass of their supposed pyroxene 

 andesite. But mica is not a more significant mineral than 

 hornblende or augite, as I feel confident from many observ- 

 ations. It would seem to me as reasonable to call the later 

 hornblende andesite, diabase, because it contains some 

 augite as to call the micaceous spots in the granular mass 

 of Mount Davidson later hornblende andesite because they 

 carry mica. Black dike again they identify with the very 

 dissimilar basalt of the district. The structure and physical 

 character of this rock are exactly similar to the commonest 

 variety of diabase elsewhere. They state, indeed, that they 

 have seen basalts of the same structure, but these are cer- 

 tainly rare, for though I have had occasion to make micro- 

 scopic examinations of many basalts, I have never seen one 

 which at all resembled black dike. The excellent represent- 

 ation of this rock in my memoir will enable lithologists to 

 judge for themselves on this point. Its bearing on Messrs. 

 Hague and Iddings' theory is manifest, for, if it is a diabase, 

 the surrounding masses must bo pre-Tertiary; but if it is 

 basalt, it is in so far possible that the enclosing rocks may 

 be Tertiary or later. 



It is surely unnecessary to go into further detail on the 

 subject of the Washoe rocks. I find that several of them 

 at least, extend into the area of the Gold belt of California, 



