366 University of California Publications in Geology [Vol. 10 



the rhyolite, but in two or three places only were any gravels found 

 in such a position, and in these places in very limited amounts. How- 

 ever, this does not preclude the possibility that they underlie the 

 rhyolite, because in only a few places is the base of the rhyolite clearly 

 exposed. 



The field relations and petrographical character of the rhyolite, 

 which will be discussed later, indicate that the lava probably came 

 from one source and was extruded as a single flow. 



AGE OF EHYOLITE 



The age of the rhyolite is one of the principal points to be consid- 

 ered, but after much detailed study of its relation to the adjoining 

 formations the time of extrusion has not been narrowly determined. 

 It is certainly post-Chico, and, judging from the degradation of the 

 surface upon which the rhyolite rests, the degree of its alteration, the 

 amount of its erosion, and the extent of post-rhyolite movements, 

 it seems highly probable that it is as old as Pliocene, as has been 

 suggested. 6 



FAULTING 



The Berkeley Hills orogenie block 7 is a feature of especial interest 

 in the geology of the Coast Eanges in the San Francisco Bay region. 

 The Haywards fault, which traverses this region nearly parallel to 

 the trend of the rhyolite belt, is comparable to the San Andreas fault 

 because of the similar geomorphic features it has produced. It ex- 

 tends through the area mapped, from Berkeley southeast along the 

 base of the steep slope to the vicinity of Temescal Lake, where it be- 

 comes manifest as a rift valley to a point southeast of Leona Heights ; 

 here it turns in a more southerly direction and passes through the 

 low front ridge of the hills out under the alluvium. At Arroyo Viejo 

 Creek the rift again returns to the rhyolite area and passing up this 

 creek in a southeasterly direction through a well defined saddle on 

 the F. C. Talbot ranch connects with the oblique fault west of Lake 

 Chabot. The main fault may be offset somewhat by this oblique fault, 

 but from this point it has been tentatively drawn on the map through 

 the several saddles and breaks in the western slope of the hills to the 

 town of Haywards, where it is lost as it passes out under the alluvium. 



e Op. ext., p. 12. 

 i Op. cit., p. 17. 



