REGION SOUTH OF GEANITE MOUNTAIN. 695 



McKinney's Pass, is a group of rhyolite hills extending about 4 miles north 

 and south. It will be readily seen that their position occupies almost 

 exactly the plane of what must have been the fault between the easterly- 

 and westerly-dipping Triassic strata. This is only another instance of the 

 rhyolites and other Tertiary intrusive rocks appearing along the old lines 

 of dynamic action, which in recent times must necessarily have been the 

 point of weakest cohesion. This rhyolite group forms a ridge slightly 

 deviating from its south course toward the west, and reaching, at its highest 

 point near the middle, 1,200 feet above the valley. The rock has a yellowish- 

 gray groundmass, made up chiefly of long bunches containing numbers of 

 ferritic needles and more or less sphserulites. The rock is peculiar, how- 

 ever, for the large number of pure crystalline secretions, both of quartz and 

 sanidin, everywhere scattered through it. The sanidin appears exceedingly 

 fresh and glassy, and almost as transparent as quartz, occurring as crystals 

 up to one-eighth of an inch in length, which are noticeable for their fre- 

 quent included glass-cavities. The quartz is present in well-developed 

 dihexahedral forms carrying numerous glass-grains. Some rare black horn- 

 blende crystals and occasional flakes of biotite are seen macroscopically 

 in the rock, but under the microscope do not appear to occur in the ground- 

 mass. It is somewhat uncommon to find an outburst of rhyolite, in which 

 the rock is of so uniform a character throughout its entire mass as here at 

 McKinney's Pass, and, inasmuch as it is a typical variety of this region of 

 Nevada, has been subjected to chemical analysis by Mr. R. W. Woodward, 

 who reports the following composition : 



Silica 74.00 



Alumina 11.93 



Ferric oxide 2.48 



Lime 1.56 



Soda 2.64 



Potassa 5.65 



Lithia trace 



Water 1.24 



99.50 

 Specific gravity, 2.33. 



