572 



CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



the rocks of the Point Reyes Triangle and those of the San 

 Francisco-Marin Block, states that : "It would therefore seem 

 probable that the earlier movements on this fault zone were 

 pre-Miocene and that they caused a relative upthrow on the 

 southwest side of the fault, in consequence of which the Fran- 

 ciscan rocks were lifted into the zone of erosion and stripped off 

 the underlying granitic rocks. This erosion may have taken 

 place in any part or during the whole of Cretaceous and 

 Eocene time." It must be remembered in this connection that 

 our mapping does not show any Eocene or Cretaceous rocks 

 in this vicinity. Eocene rocks are not found until the vicinity 

 of Carneros Creek in the Napa Quadrangle is reached. Rocks 

 of the Knoxville Cretaceous also occur in this vicinity, so our 

 mapping gives added data in support of Lawson's views. 



The occurrence of great thicknesses of Monterey shale in 

 the Point Reyes Triangle and its absence from the San Fran- 

 cisco-Marin Block was recognized as a significant thing by 

 Lawson,* who states : "The shore line of the sea in which 

 these shales were deposited must have lain far east of Bolinas 

 Ridge, for we can not regard the beds at the base of the ridge 

 as in any sense littoral. It follows that the Monterey beds 

 were laid down not only over the area of Point Reyes Penin- 

 sula but also over a large part of the territory farther north- 

 east, and that they were therefore spread over the trace of the 

 old fault. In post-Miocene time there was probably a recur- 

 rence of movement at the time of the deformation of the 

 Monterey strata, but the effect of this movement can not be 

 satisfactorily differentiated from that of the later post-Plio- 

 cene displacement." The indications of the presence of the 

 bituminous shales beneath the Petaluma formation on the west 

 flank of Sonoma Mountain suggest that Lawson's view con- 

 cerning the distribution of the Monterey is correct. The 

 work in this area under discussion shows that in pre-Pliocene 

 time the San Francisco-Marin Block was uplifted into the 

 zone of active erosion and the northwestern extension of the 

 Berkeley Hills Block which is bounded on the southeast by 

 the Tolay fault was depressed below the base-level of erosion 

 at that time, thus preserving the diatomaceous Monterey 

 shale which in all probability lies beneath the Petaluma lake 



•Idem. p. 16. 



