374 



University of California. 



[Vol. 2. 



Two small patches of fresh doleritic basalt occur low down in 

 the Orindan formation on two of the transverse ridges above Such's 

 ranch. It is not clear whether they are lavas in the Orindan 

 formation or small intrusions of later date, or lava remnants of a 

 later epoch which flowed out over the eroded surface of the forma- 

 tion. They seem to mantle the contact between the Orindan and 

 the Monterey, and so the last interpretation has been placed upon 

 their occurrence in coloring the map. 



VOLCANICS ABOVE ORINDAN. 



Continuing now our section, in the same general direction, we 

 leave Skyline Ridge and climb the steep western slope of Frown- 

 ing Ridge. This is one of the two dominant ridges of the Berke- 

 ley Hills, from which Skyline Ridge extends westerly as a trans- 

 verse spur, forming the divide between Strawberry and Telegraph 

 Canons. Here we find resting upon the Orindan formation a 

 great volume of stratified rocks which are preponderatingly vol- 

 canic. The accumulation comprises varieties of andesitic and 

 basaltic lavas and volcanic tuffs both acid and basic, together with 

 a subordinate proportion of gravels which are chiefly associated 

 with the tuff beds. These volcanic rocks have proved much more 

 resistant to the disintegrating action of erosive forces than any of 

 the formations which we have as yet met with in our section. 

 This fact finds its expression in the steep scarp-like acclivity of 

 Frowning Ridge. Along the face of this escarpment, notched by 

 various high-grade ravines, the constituent rock sheets of the 

 accumulation crop out boldly. Many of these sheets may be 

 traced in unbroken continuity throughout the length of the escarp- 

 ment, as beds of varying thickness, between which are found lens- 

 like masses of less extent. The harder rocks sometimes present 

 almost vertical bluffs 20 to 40 feet in height; and in general the 

 surface is unencumbered with talus and but little obscured by soil. 

 An examination of numerous sections of the escarpment shows 

 that, while the sequence of the rocks and the thickness of the indi- 

 vidual sheets vary much in different places, the total volume of the 

 volcanic material is remarkably uniform, the aggregate thickness 

 being from about 800 to 1,000 feet. The strata thus outcropping 



