Lawson ~i 



I'M At || eJ 



The Berkeley Hills. 



thus giving the rock a parti-colored aspect, and accentuating its 

 heterogeneous character. 



The porphyritic variety is typically developed in the upper por- 

 tion of the Grizzly Peak andesite belt, forming the latest flows of 

 this accumulation. It occurs in several sheets, the aggregate 

 thickness of which ranges from 175 to nearly 300 feet. The out- 

 cropping edges of these sheets appear on Grizzly Peak and in the 

 line of bold cliffs which form the summit of the Frowning Ridge 

 escarpment. Typical specimens of the fresh rock show it to be 

 deep black in color, massive and compact, containing numerous 

 yellowish phenocrysts of glassy feldspar and rarer olivines embed- 

 ded in a semi-vitreous base. In some specimens the base is less 

 intensely black in color and of less compact appearance, but the 

 porphyritic character is always strongly marked. It weathers 

 variably; in some cases a mere film or crust of gray or reddish 

 decomposition products is formed, beneath which the rock pre- 

 serves its original character intact; more frequently it is extensively- 

 altered, the ground-mass changing to a dull red or brick red earthy 

 material, and the feldspar becoming milk white and earthy. In 

 some of the flows, as on the west face of Grizzly Peak, the vitreous 

 character of the base of the rock is more pronounced than is 

 usually the case, but such variations, due to proportion of glass 

 present, can only be made out satisfactorily by microscopic exami- 

 nation. This more glassy facies is characterized also by an 

 exceptional amount of brecciation, and, when this is the case, by 

 more extensive decomposition. The angular or slightly rounded 

 fragments into which the mass of the rock has been shattered have 

 changed in color to various shades of gray and white, and are 

 cemented by a fine-grained gray paste. The fragments still exhibit 

 their original porphyritic structure, but as a whole the rock has 

 entirely lost the igneous aspect of a fresh lava, and looks like a 

 brecciated mass of impure opal, in which grains and crystals of 

 feldspar and other minerals are embedded. 



To the northwestward these andesites are cut off by the Canon 

 fault and in their southeastward extent along the head of Telegraph 

 Canon they become more basaltic in appearance, and it is not 

 improbable that they actually do grade into basalts. 



3 



