The Berkeley Hills 



379 



is found in greatly diminished volume, not exceeding 20 or 25 feet 

 in thickness. The diminution of the volume of this tuff to the 

 southward, along this line of outcrop, is regarded as additional 

 evidence of its having been laid down in water, since, had it been 

 deposited under the influence of air currents only, it would not 

 have varied so rapidly in such a short distance. 



Basalt Flows. — Above this threefold accumulation of basic tuff, 

 cloleritic basalt, and rhyolite tuff, is a belt of basalts. These have 

 an aggregate thickness of from 150 to 180 feet, but neither the 

 thickness of the individual flows nor their number can be satis- 

 factorily determined. The rocks are uniformly fine-grained in 

 texture and black in color. A more detailed account of their 

 petrographic characters will be given later. 



These basalts present a prominent outcrop along the entire front 

 of the Frowning Ridge escarpment. To the northwest they termi- 

 nate abruptly against the Canon fault, like the older formations 

 which underlie them. To the southeast the belt extends for many 

 miles. The dip of the formation is uniformly into Frowning 

 Ridge, or to the northeast at angles which do not vary far from 30 . 



Grizzly Peak Andesite. — Continuing the ascent of the Frowning 

 Ridge escarpment above these basalts, we next encounter a thick 

 accumulation of lava flows which for the most part are of a 

 different petrographic type from the underlying basalts. Labora- 

 tory investigation has shown that they should be classed with the 

 andesites. These flows, though rather varied in character, are 

 grouped together and are represented on the map by a single color, 

 under the designation of the Grizzly Peak andesite, and it is under- 

 stood that this color also covers certain subordinate flows of basalt 

 and intercalations of tuff which are not considered of sufficient 

 importance to indicate by a separate convention. The color on the 

 map, thus designated, represents an assemblage of volcanic forma- 

 tions having in their aggregate a certain individuality, rather than 

 a single flow or a set of flows of exactly the same kind of lava. 



Of the andesite there are two rather distinct facies, represented 

 by different flows, which may be readily distinguished even in the 

 field. Of these the lower is a medium-textured holocrystalline 

 rock, showing frequently a pronounced laminated structure, and 



