THE AVASHOE ROCKS. 115 



They do not indeed state that fluid inclusions are confined 

 to, or specially characteristic of the lower portions of rhyo- 

 lite eruptions, but they do make an equivalent statement 

 regarding the andesites, and their description of the pass- 

 age from a glassy to a highly crystalline mass is couched in 

 such general terms that I cannot doubt their holding sim- 

 ilar views with reference to rhyolite. Of course a similar 

 train of reasoning makes it apparently inexplicable that the 

 surface exposures of Mount Davidson should show fluid in- 

 clusions, Avhile glassy rocks still remain on the Mount Kate 

 range, if the augite andesite and the diorite form substan- 

 tially one eruption. There is no reason why the Davidson 

 range should have been deeply eroded while the Kats range 

 escaped degradation. A range may escape erosion while the 

 valley at its base is deeply excavated, but that of two paral- 

 lel ranges, distant a couple of miles, one should be deeply 

 eroded wdiile the other escapes almost entirely, is conceiva- 

 ble only under most extraordinary meteorological con- 

 ditions, if at all. There are no such remarkable conditions, 

 at Washoe. 



Hornhlende andesite intlie tunnel. — The rock laid down as 

 hornblende andesite on my section of the Sutro Tunnel is 

 comparatively fresh at the eastern edge. The remainder of 

 the occurrence in the tunnel is far too thoroughly decom- 

 posed for direct determination either macroscopically or 

 microscopically. Messrs. Hague and Iddings, however, 

 assume that only a narrow dike of this rock is intersected 

 bv the adit, and conclude that the earlier hornblende ande- 

 site of my report is younger than any of the pyroxenic 

 rocks. My determination of the width of this mass was not 

 founded exclusively upon the exposure in the tunnel. The 

 combination shaft is only 400 feet distant from the tunnel 

 section. The top of this shaft is in the typical hornblende 

 andesite figured in my report on plate V. Some of the 

 stations of the shaft were accessible, and I also had access 

 to a private collection of rocks from the shaft which w^ere 



