University of California. 



[Vol 2. 



will be referred to as the holocrystalline variety of the Grizzly Peak 

 andesite; the upper portion of the accumulation is, in contrast to 

 this, a dense or compact rock, frequently glassy, with a prevailing 

 porphyritic habit, and may be referred to as the prophyritic variety. 



The holocrystalline variety of the Grizzly Peak andesite crops 

 out prominently in a series of knobs on the buttresses which rib 

 the Frowning Ridge escarpment from the Canon fault to the head 

 uf Telegraph Canon. These outcrops are generally characterized 

 by a very distinct lamination, simulating the bedding of a sedimen- 

 tary formation, but in reality a kind of jointage. The laminae are 

 from a quarter of an inch to three or more inches in thickness, 

 sometimes forming lenses of limited extent, but more commonly 

 preserving their continuity for considerable distances. They are 

 generally parallel to the dip of the lava sheet, although, in places, 

 they are undulating or sharply curved. An excellent illustration of 

 this structural feature may be seen in the conical knob jutting out 

 from the buttress just south of Grizzly Peak. 



The rock exhibits considerable minor variety of texture and a 

 tendency in places to merge by gradations into a facies resembling 

 that particularly designated the porphyritic variety. Typical speci- 

 mens, however, present the appearance of a massive rock, dark- 

 gray to reddish brown in color, with a rough or uneven fracture, 

 and distinct, rather coarse porphyritic structure. Occasionally 

 amygdules of quartz or natrolite are seen, and on weathered 

 surfaces, which are generally smooth and of a dull gray color, a 

 slightly vesicular or miaralitic structure is apparent. The pheno- 

 crysts comprise glassy feldspars and black pyroxenes, and, under a 

 pocket lens, the ground mass is seen to be distinctly crystalline. 



Certain beds of coarse breccia which are found intercalated 

 between the flows of this lava are regarded partly as flow or klinker 

 breccias, representing the original surface of a lava flow, and partly 

 as formed of the cinders, ashes, and bombs which accumulated on 

 the surface of the lava. These breccias chiefly consist of angular 

 fragments and occasional rounded, bombdike masses of andesite 

 cemented with a fine ash-like material. The fragments are some- 

 times highly vesicular and are frequently thoroughly oxidized, so 

 that they vary in color from gray and rusty brown to bright red, 



