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University of California. 



[Vol. 2. 



conditions, in the period of lava accumulation, and indicates that 

 this period was a long one. In other sections of this same series 

 of basalt flows the limestone bed is absent, and a consideration of 

 these sections alone would, perhaps, lead to the erroneous view 

 that the accumulation had been continuous and rapid. 



Asymetry of Syncline. — It has been stated that these basalt 

 sheets dip uniformly beneath the Siestan beds to the southwest, and 

 the general synclinal structure which was recognized in the Siestan 

 formation is here maintained. It is apparent, however, from what 

 has been stated that the syncline beneath the Siestan beds is not 

 symmetrical as regards the petrographical character of the lavas. 

 ( >n the southwest limb of the syncline the Siestan beds rest upon 

 the Grizzly Peak andesite, while on the northeast limb, which we 

 are now considering, they lie upon basalt, and the Grizzly Peak 

 andesite is not represented in our section. The andesite must, 

 therefore, thin out rapidly in the synclinal trough; and the basalts 

 underlying the Siestan on the front of San Pablo Ridge are cor- 

 related with the basalts underlying the Grizzly Peak andesite on 

 the front of Frowning Ridge. These basalts are much thicker on 

 the San Pablo side than on the Frowning Ridge side, indicat- 

 ing that they thin out to the southwest. This petrographically 

 asymmetric character of the syncline is true of other formations of 

 the series and arises from the conditions of accumulation of the 

 strata. Lavas generally flow clown valleys, and, if the valley is a 

 wide one, are likely to occupy one side of the valley, being thickest 

 in the line of flow and tapering off towards the sides. A valley 

 thus partially occupied by a congealed lava stream would have its 

 drainage line displaced to the other side, and a succeeding lava 

 stream would follow the new drainage line. It would not be 

 directly superimposed upon the earlier lava, although it might 

 overlap it to a large extent. Such appear to have been the 

 general conditions attending the accumulation of the lavas in the 

 region now occupied by the Berkeley Hills. It must, therefore, 

 occasion no surprise to find that, although the syncline in a 

 structural sense is perfectly symmetrical, the formations outcrop- 

 ping in the two limbs are not always persistent across the fold. 



