3§2 



University of California. 



[Vol. 2. 



Iii the midst of this belt of andesites, at a horizon which is 

 apparently between the flows of the holocrystalline type and those 

 of the porphyritic type, there is a zone of lavas of quite a different 

 character, together with a lens of tuff. These comprise basalt lavas 

 of no great persistence in the line of outcrop and a quite thin flow 

 of a peculiar andesite. This flow is nowhere more than 20 feet in 

 thickness, and its outcrop on the face of the escarpment does not 

 exceed a quarter of a mile in length. The rock is of a pale gray 

 color, of fine, compact texture, but distinctly crystalline, and chiefly 

 characterized by a very perfect and minute lamination parallel to 

 the plane of the sheet. The laminae are for the most part less than 

 a quarter of an inch thick and quite continuous. The partings of 

 the laminae are accentuated by a yellowish color, apparently due 

 to a thin film of a pale yellow crystalline substance. The rock is 

 evidently one in which the feldspathic constituents greatly prepon- 

 derate over the dark ferro-magnesian silicates. 



Beds of Rhyolite Tuff. — Besides these various volcanic forma- 

 tions of minor extent which are included in the belt of the Grizzly 

 Peak andesite, and which are not discriminated on the map, there 

 are two other formations of rhyolite tuff included within the same 

 stratigraphic limits, which, on account of the interest attaching to 

 them, have been especially colored on the map. These also are of 

 quite limited thickness and extent, and outcrop only along that 

 portion of Frowning Ridge escarpment which overlook's the head 

 of Telegraph Canon. 



The lower of the two tuff beds is at the base of the Grizzly Peak 

 andesite belt, intervening between it and the underlying basalts. It 

 resembles the rhyolite tuff already described as occurring lower in 

 the section, but is much thinner, its maximum thickness being not 

 more than about 12 to 15 feet. It is a whitish rock, in which 

 quartz grains abound, and is distinctly stratified. It seems to 

 increase in volume to the southeastward, and is traceable in that 

 direction across the Fish Ranch Canon. The second rhyolite tuff 

 bed crops out much closer to the summit of Frowning Ridge, and 

 it entirely crosses the ridge at a point nearly north of Summit Pass. 

 It has the same general character as the first bed, except that it is 

 occasionally reddish in color, and that it is perhaps even thinner. 



