AW SON "I 

 ALACHEj 



The Berkeley Hills. 



177 



The weathered surfaces of the andesite are dull gray in color, 

 rough and pitted, the amygdules, when present, standing out in 

 strong relief. The surface of the rock or the light residual soil 

 which it yields are frequently strewn with amygdules which have 

 weathered out of the matrix. 



Mineralogical variations of much interest occur in the composi- 

 tion of these amygdules, but these will be discussed later, in the 

 section of the paper that deals with the detailed petrography of 

 the volcanic rocks, where a description of the microscopic character 

 of this andesite is given. 



The amygdaloidal andesite thus macroscopically characterized 

 extends along the entire base of the Frowning Ridge escarpment, 

 northwestward to the vicinity of Little Grizzly Peak, where it is cut 

 off by the Cafion fault, and southeastward far beyond the Fish ranch 

 cafion. The upper surface of the sheet is much decomposed and 

 in places converted into a brick-red laterite, indicating an exposure 

 to the atmosphere of considerable duration. 



Tuffs and Basalt. — Resting upon this surface lies the second 

 member of the volcanic sequence, a formation partly volcanic and 

 partly detrital in origin. With the exception of a thin sheet of 

 basalt of quite limited extent, the volcanic constituent is wholly 

 pyroclastic and shows abundant evidence of having been sorted and 

 stratified, but whether by air currents at its first deposition or 

 subsequently by currents of water, is debatable. The presence of 

 layers of gravel intercalated with the tuffs and in part mixed with 

 them supports the latter hypothesis, although the water that trans- 

 ported and assorted the gravel was more probably fluvial than 

 lacustrine. In its outcrop on the Frowning Ridge escarpment these 

 beds vary in thickness from about 50 feet to over 200 feet. 



On our line of section and northward to the line of the Canon 

 fault the formation has a threefold character. The basal 50 or 60 

 feet is a basic tuff, in the sense that it contains no quartz, partly 

 fine-grained and partly coarse. It has in it fragments of andesitic 

 lavas and a considerable proportion of water-worn gravel, partly 

 volcanic and partly of the nature of ordinary polygenous stream 

 pebbles, generally of small size. These rock fragments and peb- 

 bles, together with crystals and crystal fragments of feldspar and 



