I 



448 University of California. [Vol. 2. 



closely allied to that of the San Pablo formation. These beds con- 

 stitute the Trampan formation, so named from Las Trampas Creek, 

 along which they are exposed. Their presence here, signifying 

 the local displacement of the fresh water by the sea, is clearly indic- 

 ative of orogenic deformation of the basin during the progress 

 of its filling up. This marine encroachment did not apparently 

 extend to the Berkeley Hills, for we find no trace of marine sedi- 

 ments between the Orindan formation and the overlying volcanics. 



The accumulation of the Orindan and Trampan formations was 

 brought to a close in the region of the present Berkeley Hills by 

 the outburst of volcanic activity; lavas spread out over the floor 

 of the basin; but the lake was re-established, for we find lacustrine 

 deposits between some of the lavas, notably 30 feet of fossiliferous 

 fresh-water limestone intercalated with the basalts of Eureka Peak. 

 Finally, however, this lake was drained, or more probably dis- 

 placed. The basin was then subjected to deformation, by which 

 the strata were flexed and elevated. The erosion which followed 

 had cut out important channels, when another set of lavas, the 

 Grizzly Peak andesite, occupied these channels and also spread 

 out over the earlier lavas. In the channels we find the lavas of 

 the second epoch, Upper Berkeleyan, are reposing upon the worn 

 upturned strata of the lake beds. The occupation of the drainage 

 channels by these Grizzly Peak andesites re-established lacustrine 

 conditions, and the Siestan lake was the result. 



This lake became filled with a varied assortment of sediments 

 to a depth of about 200 feet. Their fine character and general 

 absence of delta material indicate that the lake had a somewhat 

 prolonged existence. The lignitic character of some of the beds, 

 the occurrence of fossil leaves in others and of well-preserved 

 trunks of trees in others, points to abundant vegetation on the 

 shores of the lake. The two well-defined beds of limestone gen- 

 erally free from detrital admixtures, and having a thickness of from 

 io to 20 feet, with other thinner beds of less importance, contribute 

 to the idea of long duration for the lake. They also afford a 

 picture of clear waters comparatively free from sediment during 

 their accumulation. They, moreover, indicate a non-organic source 

 for the carbonate of lime of which they are composed. 



