LAWSON "1 



PalacheJ 



The Berkeley Hills. 



447 



together with still lower rocks, a subsidence ensued and the sea 

 once more encroached over the Coast Ranges, and a new cycle of 

 sedimentation was inaugurated for this region. This movement 

 was accompanied by volcanic activity, for the next marine .sedi- 

 ments which we encounter in post Monterey time, those desig- 

 nated the San Pablo formation, are in their lower part chiefly 

 sandstones with admixtures of volcanic ash, and in their upper 

 part almost purely pyroclastic sediments, some of which contain 

 fossil shells. These beds of San Pablo age are lacking in the 

 Berkeley Hills, though well developed on the shores of the Bay 

 of San Francisco a few miles to the northward. And it is there- 

 fore believed that the San Pablo basin was one lying between 

 insular or peninsular land masses, and that the region of the 

 Berkeley Hills was one of those masses which remained unsub- 

 merged in San Pablo time. Portions of the missing Monterey 

 formation in the Berkeley Hills may, therefore, have been removed 

 during this San Pablo epoch. Following the San Pablo, and in 

 part during the latter part of the epoch, fresh-water basins were 

 developed, probably in consequence of orogenic deformation. In 

 one of these basins accumulated the deposits of the Orindan for- 

 mation, which lies imconformably upon the rocks of Monterey age. 

 That there was a close connection in time between the San Pablo 

 and the fresh-water deposits of Orindan age is clearly shown from 

 the fact that the pumiceous tuffs of the upper part of the San Pablo 

 are also found in the basal part of the Orindan in the vicinity of 

 Pinole. The Orindan in its full development is not less than 2,400 

 feet thick, and we seem to have in such an accumulation of fresh- 

 water beds, chiefly lacustrine but probably also in part fluviatile, 

 very clear evidence of depression of the region during their accu- 

 mulation. That such depression was orogenic or local rather than 

 epeirogenic seems also clear, since a general depression would 

 undoubtedly have admitted the sea, and we find no trace of marine 

 conditions in the basin, except to the southeast, where, to the west 

 of Bollinger Canon, we find that the sea at the close of the Orindan, 

 had access to the basin for a time, and marine conditions displaced 

 the lacustrine. The marine beds attain a thickness of about 2,000 

 feet, and their fossil fauna is, in the opinion of Prof. J. C Merriam, 



