422 



University of California. 



[Vol. 2. 



some hand specimens a bright green mineral, probably a chlorite, 

 was observed in spots and bunches as a stain in the feldspars. 

 Iddingsite, derived from the olivine, is very sparingly present. 

 The rock thus described is an divine-bearing augite andesite. It 

 is thus classed largely on the basis of its chemical character, 

 analysis showing that it is much less basic than the above minera- 

 logical composition would lead us to expect, and clearly of ande- 

 sitic composition. 



On Grizzly Peak and for about ioo feet below the summit, this 

 facies of the andesite is exceptionally rich in glass in the ground- 

 mass, while not differing greatly in its macroscopic aspect from 

 that just described. In thin sections it shows more abundant 

 phenocrysts. Considerable portions of this glassy rock, however, 

 have been brecciated and extensively altered. The fragments of 

 the breccia have lost their dark color and appear gray or white. 

 This change is due to a process of opalization of the ground-mass. 

 The paste in which the fragments are imbedded is similarly 

 affected. 



Examined in thin section, it is found that the porphyritic crys- 

 tals do not differ from those of the unaltered rock, but the ground- 

 mass, while nearly isotropic, has a totally different appearance from 

 the normal glassy base. It appears to be an impure opal, stained 

 with iron oxide and other foreign matter. The brecciation is but 

 faintly emphasized, the substance of the fragments blending indis- 

 tinguishably with the cementing paste. To test the nature of the 

 isotropic ground-mass, a portion of the rock was finely pulverized 

 and boiled with caustic potash to extract soluble silica if present. 

 By this means the presence of a large amount of soluble silica was 

 shown, and the opaline character of the ground-mass was estab- 

 lished. The substitution of opal for the original ground-mass of 

 the rock must be considered as a result of local solfataric action. 

 No further evidence of such action on an important scale has been 

 observed within the area investigated. 



The Grizzly Peak Andesite, Holocrystalline Facies. — This vari- 

 ety of andesite is not so distinctly characterized as the two pre- 

 ceding, exhibiting a greater variation of texture and a tendency 

 to merge by gradations into the porphyritic facies. It is repre- 



