39 2 



University of California. 



[Vol. 2. 



Hills, since their outcrop forms the two parallel ridges which con- 

 stitute their summit; and Bald Peak, the highest in the range, is 

 part of the northeastern limb. These two culminating ridges have 

 between them a well-marked depression in the axis of syncline. 

 It thus appears that the morphological features of this portion 

 of the Berkeley Hills are strictly controlled by the underlying 

 structu re. 



BASALTS ABOVE SIESTAN. 



These uppermost lava beds have an aggregate thickness of 

 about 375 feet, and comprise at least three distinct flows. The 

 lowest of the three is the most extensive, and is an iron-gray, 

 porphyritic, olivine-basalt, which may best be studied in the excel- 

 lent outcrop along the escarpment which extends from Grizzly 

 Peak to Siesta Valley immediately above the Siestan beds. The 

 second flow is scarcely distinguishable from the first except in the 

 detail of surface relief which brings out clearly the presence of 

 two lava sheets. This flow is thus traceable in the northwestern 

 portion of the synclinal trough, about the head-waters of Wild- 

 cat Creek, but appears to thin out to the southeastward in the 

 vicinity of Bald Peak, so that in the Siesta Valley portion of the 

 syncline only two lava sheets are discernible. Lying upon the 

 common surface afforded by these two lower lava sheets is a for- 

 mation of volcanic tuff of variable thickness, and in this horizon of 

 tuff is a thin lens of fresh-water limestone, indicating the presence 

 of lake basins on the surface upon which the tuff was deposited. 



The third lava sheet rests upon this tuff and limestone, and is in 

 general readily distinguished from the two lower flows. It has in 

 places a rude but pronounced columnar structure, which is not 

 apparent in the two other flows, and exhibits a more marked 

 tendency to spheroidal weathering, the spheroids which weather 

 out on the surface having a deceptive resemblance to water-worn 

 pebbles. The rock is also, on the whole, coarser grained than the 

 other basalts, and may be characterized as a dolerite or doleritic 

 basalt. 



The original surfaces of these lavas have been in places exposed 

 by erosion, the overlying and protecting rocks having been stripped 



