Lawson ~| 



I'AI.Ai HeJ 



The Berkeley Hills. 



395 



Andesite, Conglomerate, and Tuff. — Following our general sec- 

 tion down the San Pablo slope, we find beneath the basalts a sheet 

 of andesite having a southwesterly dip and an average thickness of 

 perhaps 100 feet, although locally much thicker. This sheet is 

 quite persistent in the line of strike, but seems to thin out in the 

 synclinal trough, and does not appear on the Frowning Ridge side. 

 Beneath this we find a formation of conglomerates and tuffs, with a 

 maximum thickness of 500 feet, which seems to correspond with 

 the much smaller volume of tuffs immediately above the amygda- 

 loidal andesite on the Frowning Ridge side of the syncline. This 

 formation yields a heavy soil, rich in well water-worn pebbles of 

 small size, and it is only in certain stream cuts that its composite 

 character can be made out and beds of tuff discriminated from the 

 conglomerate. A portion of the matrix of the conglomerate is 

 probably also tuffaceous. 



Amygdaloidal Andesite. — Beneath these conglomerates and tufts 

 lies a sheet of amygdaloidal andesite, dipping southwesterly or into 

 the ridge. This lava is identical petrographically with the amyg- 

 daloidal andesite at the base of the volcanic series on the opposite 

 limb of the syncline, and is here also the earliest flow. We have, 

 therefore, good reason for correlating it with the amygdaloidal 

 andesite first met with in our ascending section. It is the most 

 uniformly characterized of all the lavas we have to deal with, 

 not only in its petrographical character, but also in its thickness, 

 and it is at the same time the most extensive. The ease with 

 which it may be identified renders it, next to the Siestan formation, 

 the most reliable stratigraphic element in the deduction of our 

 conception of the synclinal structure. 



Orindan Formation. — Just as on the opposite limb of the syn- 

 cline, this amygdaloidal andesite here lies upon the gravels of the 

 Orindan formation. On this side, however, the Orindan formation 

 is much thicker and somewhat more varied in its stratigraphic 

 composition than it is on the upper slopes of Strawberry and 

 Telegraph Canons. In this respect we have another illustration of 

 the lack of symmetry in the formations thus synclinally folded. 

 The maximum thickness of the formation is much greater than that 

 exposed on Skyline Ridge but need not here be stated with pre- 



