LAWSON "I 



PalAcheJ 



The Berkeley Hills. 



449 



The occasional thin beds of volcanic tuff found intercalated 

 with the clays and shales in the Siestan basin, tell us of the spas- 

 modic sputtering- of a volcano in the region, but too distant or too 

 inactive to pour its lavas into the lake. 



Eventually, however, this volcanic energy burst forth in floods 

 of basalt, which destroyed the lake and occupied its basin. 

 Between these flows there were considerable intervals, as is 

 shown by the oxidized surface of the lava and by the presence of 

 tuff beds at the same horizon with thin beds of limestone deposited 

 in a new sheet of water established on the surface of the first flows- 



To what extent these lavas piled up over the Siestan basin we 

 can not now tell, for the surface at which they terminate upwards 

 is one of erosion. 



The occupation of the Siestan basin by these lavas was fol- 

 lowed, after an interval of unknown length, by the deformation by 

 flexure of the entire region, and what remains of the Siestan lake 

 beds and of the overlying lavas occupies the axis of the great 

 synclinal trough. The establishment of this structure was followed 

 by prolonged erosion, whereby the upturned edges of the folded 

 strata were worn down and the corresponding anticlines entirely 

 removed. 



Across the northwest end of what then remained of the syncline 

 there was established the Campan basin, probably by a renewed 

 deformation of the region. The earliest rocks to occupy this 

 basin were clays and shales, with lenses of fresh-water limestone, 

 sandstones, and conglomerates. These were followed by alterna- 

 tions of lavas or tuffs and lake beds, the last of the series known to 

 us being a rhyolitic tuff and agglomerate, of which the succeeding 

 erosion has left only a few remnants capping the hill tops. 



In the midst of this Campan accumulation the basin was 

 converted by the Canon fault into a structural trough, which the 

 later lavas and tuffs filled up. 



After the Campan basin and trough was filled, it was syncli- 

 nally folded and variously faulted, the faults apparently belonging to 

 the same system of movements which in part gave rise in later 

 time to the depression now occupied by the valley of the Bay of 

 San Francisco. But before this valley was evolved, the region had 



