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University of California. 



[Vol. 2. 



and sedimentary, from the base of the Orindan formation to the 

 crest of Frowning Ridge, is here named the Berkeleyan series, and 

 the interval in question is regarded as a break in its accumulation, 

 dividing it into Upper and Lower Berkeleyan. As we proceed 

 with the examination of the ascending sequence on the escarp- 

 ment of Frowning Ridge, this break will be pointed out at the 

 proper place. 



Amygdaloidal Andesite. — The basal member of the volcanic 

 accumulation reposing upon the Orindan conglomerates is an 

 andesite, which is perhaps the most persistent, and, at the same 

 time, most uniform individual sheet of rock we have to deal with 

 in our entire field. This uniformity in character has proved of the 

 greatest value in establishing a stratigraphic datum plane across 

 the Berkeley Hills. The breadth of the sheet indicates that the 

 surface over which the lava flowed was a plain, probably a gravelly 

 flood plane of the river which replaced the Orindan Lake after 

 lacustrine sedimentation had ceased The lateral extent of this 

 andesite sheet is in contrast to that of other lavas which succeed it; 

 the latter seem to have been confined to narrower channels. 



The thickness of this basal andesite varies over its entire extent 

 from about 50 to 200 feet, with an average thickness of probably 

 100 feet. 



The andesite is typically a grayish-black rock with a compact 

 microcrystalline texture and even fracture, generally containing 

 silicious amygdules of various forms and sizes scattered irregularly 

 through its mass. 



The fracture surfaces generally present a somewhat mottled 

 appearance, owing to the presence of anastomosing streaks and 

 spots of a yellowish or purplish color which transverse it. It does 

 not anywhere present a porphyritic fades. Although amygda- 

 loidal, the general aspect of the rock is not vesicular, the amyg- 

 dules not being ordinarily so abundant as to be a conspicuous 

 feature. Locally, however, highly vesicular varieties are found, 

 and the amygdaloidal habit is characteristic of the sheet as a whole. 

 It is for this reason designated on the accompanying map as the 

 "amygdaloidal andesite," the amygdules and the mottled appear- 

 ance serving to distinguish it from other andesites in this field. 



