3§4 



University of California. 



[Vol. 2, 



noteworthy degree, so that the next succeeding flows of the Upper 

 Berkeleyan period were poured out over the abraded upturned 

 edges of the earlier formations. The evidence upon which this 

 conclusion is based is found, in part, in a portion of the field which 

 lies a little beyond the limits of our map on the north. Here in 

 the bottom of Wildcat Canon, near the "Cave rocks," and also on 

 the county road east of the canon, just beyond where it crosses the 

 summit of San Pablo Ridge, the inclined beds of the Orindan forma- 

 tion, comprising clays, sandstones, tuffs, and limestones, some beds 

 rich in fresh-water ostracods, may be seen uncomformably beneath 

 the lavas of the Grizzly Peak andesite. The lavas repose indiffer- 

 ently upon the eroded surface of the various inclined beds. 



Evidence of the same order is also found within the limits of the 

 map on the north side of Baffling Gulch and on the corresponding 

 northeastern slope of San Pablo Ridge, where the strata of the 

 Lower Berkeleyan for a thickness of between 400 and 500 feet 

 terminate abruptly by abutment upon the thick mass of Grizzly Peak 

 andesite which underlies Ruin Peak. The Lower Berkeleyan strata 

 are here evidently cut off by an old valley of erosion with rather 

 steeply sloping sides, which at the time of the eruption of the 

 Grizzly Peak andesite became filled with lava. The period of 

 erosion which evolved this valley to the depth of over 400 feet and 

 abraded the folded Orindan beds lower down on Wildcat Creek, 

 therefore, clearly intervenes between the upper and lower divisions 

 of the Berkeleyan. 



We may now proceed with the systematic examination of our 

 section across the Berkeley Hills. 



SI EST AN FORMATION. 



Cliangc in Gcomorpliic Profile. — Following the same line of sec- 

 tion as that to which we have been adhering, and which is about 

 the same as that indicated on the map by the line C-D, we find, 

 on arriving at the upper limit of the porphyritic variety of the 

 Grizzly Peak andesite, that we are also at the summit or brink of 

 the Frowning Ridge escarpment, and that we here encounter an 

 abrupt change in the topographic profile. We pass suddenly from 

 a precipitous slope to an almost flat shelf or terrace. We leave a 



