114 CALIFORNIA ACADEMY OF SCIENCES. 



as a whole, however, is much tlie most uniform in the dis- 

 trict, and no such local exception to the representative min- 

 eral composition can properly affect its classification. 



Fluid inclusions in quartz porphyry. — According to Messrs. 

 Hague and Iddings, the microscopic characteristics of most 

 of this rock are exactly the same as those of rhyolites from 

 the Great Basin. They will not, however, deny that they 

 are also exactly similar to those of well known pre-Tertiary 

 rocks. These geologists seem to attach little importance to 

 fluid inclusions, ^^ though I should have thought that on 

 their own hypothesis such inclusions would be valuable as 

 an indication of the amount of the erosion. They grant, 

 however, that this rock contains more fluid inclusions than 

 are usual in the later quartzose volcanics of the Great Basin. 

 Every single slide of the quartz porphyry which I have seen 

 contains fluid inclusions. In many cases they are extremely 

 abundant. I have made no extensive special studies of 

 rhyolites, and cannot therefore state how frequent such oc- 

 currences are. I note, however, that Prof essor Zirkel says^* 

 of a rhyolite f rom the Washoe Mountains: "A remarkable 

 phenomenon, discovered in this genuine rhyolitic rock, was 

 a quartz which contained the most characteristic fluid inclu- 

 sions." If one supposes that fluid inclusions in the quartzes 

 of rhyolites as now exposed are so rare as they appear to be 

 only because the deeper portions of the eruptions are not yet 

 laid bare, then the quartzose rock of Washoe, if it is a rhyo- 

 lite, is a very deeply eroded one. If it is indeed younger 

 than the glassy augite andesites, as Messrs. Ha^iie and 

 Iddings maintain, and as if, as I believe with them, rocks 

 with a glassy groundmass are found only near original sur- 

 faces, it is strange that these andesites have not been eroded 

 as well as the rhyolite. 



Note ^+. — That I regarded the evidence of fluid inclusions as one to be ap- 

 pealed to with caution, may be seen from my memoir, page 50, foot-note. 

 Note i^. — Exploration of the 40th Parallel, Vol. 6, page 197. 



