REGION SOUTH OP GRANITE MOUNTAIN. 703 



Farther down Pine Nut Canon, below tlie mass carrying the irides- 

 cent sanidin, is a brown, earthy rhy elite, very rich in mica, somewhat por- 

 ous, and of so decomposed a groundmass that only occasional sanidin can 

 be detected, which in turn is overlaid at the base of the range by the rhyo- 

 litic tufas ejected under the lake. 



South of the Chataya Peak group of diorites, rhyolites again cap the 

 summit of the range, extending down on both sides to the plains below, and 

 forming a broad band, crossing the range in a north and south line, about 

 4 or 5 miles in width. The outflows on the summit are tabular in. general 

 form, and closely resemble the sanidin-trachytes. Chemically, many of 

 them appear rather low in silica, and might be classed as trachytes, but for 

 the presence of more or less free quartz scattered through the groundmass, 

 and the characteristic rhyolitic structure revealed by the microscope. There 

 is a remarkable regularity in the position of this rock, without the appear- 

 ance of definite origin. Through a single mass may be traced unusual varia- 

 tions, certain specimens showing almost no quartz, and others running very 

 high in that constituent. In general, where quartz increases, biotite increases 

 also; so that some varieties are typical rhyolites, while others approach very 

 closely to the trachytic forms. They are of a prevailing purplish-gray color, 

 and contain two distinct kinds of feldspar, a very fresh undecomposed sanidin, 

 and an earthy white isotrope substance, much like the feldspar in the decom- 

 posed neighboring diorite. The undecomposed sanidins so often occur in 

 the middle of the earthy porcelainous mass that it seems probable that they 

 represent unacted-upon portions, and that it has been a progressive decom- 

 position from the outside of the crystals toward their middle, which, in some 

 instances, has left considerable portions unattacked. 



North and east of Chataya Peak, the range is overflowed by a broad 

 and thick field of basalt, which has obscured all the underlying rocks. The 

 summit north of the peak for 3 or 4 miles is occupied by a nearly level 

 sheet of basalt, known as Table Mountain, which forms a prominent land- 

 mark, rising 4,000 feet above the valley. A portion of the same massive 

 eruption extends from the base of Chataya Peak in a northwesterly direction 

 quite across the valley, abutting against and overflowing the upturned Ter- 

 tiary beds at the base of Buffalo Peak. This cross-ridge, which falls away 



