LAWSON ~| 



The Berkeley Hills. 



383 



A strong interest attaches to the occurrence of these acid tuff 

 beds in the midst of the more basic lavas, on account of the light 

 they throw upon the variation in the character of the magma at 

 the center of eruption from which the lavas and tuffs emanated. 

 They are also of interest, in the same connection, in the meagerness 

 of their volume, as compared with the more basic lavas, and in the 

 total absence of rhyolite lavas corresponding to the tuffs. 



Fresh-water Limestone. — A fresh-water limestone also occurs as 

 a thin bed traceable for only a few hundred feet on the top of 

 Frowning Ridge, at a point northwest of Summit Pass. It 

 resembles closely other fresh-water limestones, of which we shall 

 have more to say later, being a light gray rock of compact, even 

 texture. It is of interest as indicating a condition of fresh-water 

 lakes in the interval between two of these flows, and that the inter- 

 val was not a short one. The stratified rhyolite tuffs above referred 

 to may in all probability have been laid down under similar lacus- 

 trine conditions. 



From the foregoing statements it will be apparent that there is 

 a somewhat varied assemblage of volcanic rocks in the belt of the 

 Frowning Ridge escarpment which we have been discussing, but 

 the andesites, represented in the two chief types, the noncrystalline 

 and the porphyritic, make up the bulk of the entire volume, which 

 is about from 450 to 500 feet. All the formations have the same 

 general dip, to the northeastward, of about 30°. 



EROSION INTERVAL BETWEEN UPPER AND LOWER BERKELEVAN. 



So far as observation goes in the exposures on the face of the 

 Frowning Ridge escarpment, we have no satisfactory evidence as to 

 the length of the interval that elapsed between the extravasation of 

 the underlying basalts and the rocks of the Grizzly Peak andesite 

 belt which repose upon them ; and in the absence of evidence to 

 the contrary, we would naturally assume that the interval was not 

 of exceptional importance. Yet it is this very interval which, as 

 may be seen by an examination of the map, is taken as marking the 

 break between Lower and Upper Berkeleyan. In this interval the 

 rocks of the Lower Berkeleyan epoch were folded and eroded to a 



