1907.] CRANDALL— SAN FRANCISCO PENINSULA. 49 



rock. In the first part of the paper the petrography of these serpen- 

 tines was given. There can be Httle doubt but that the serpentines 

 elsewhere in CaHfornia are of the same origin as these in this area ; 

 that is an alteration from olivine and pyroxene-bearing rock. Field 

 observations at New Idria quicksilver mine and other places con- 

 firm this opinion. 



The age of the serpentine intrusions on the San Francisco penin- 

 sula is not determinable, but field evidence elsewhere shows their 

 age to be post-Knoxville and pre-Chico. In his work in the Coast 

 Ranges Dr. Becker reports finding serpentine boulders in the bottom 

 of the Chico/ which makes them pre-Chico. Dr. Fairbanks reports 

 that on Grindstone Creek, Colusa county, the Knoxville shales have 

 been metamorphosed for some distance by the intruded serpentine 

 making the age post-Knoxville.- He also thinks that the uncon- 

 formity between Knoxville and Chico may be due, in part, to the 

 movements caused by the intrusion of these peridotite masses. 



Origin of the Schists. — Whether the schists of the Coast Ranges 

 are due to contact or regional metamorphism is a question that 

 is still open. There is no evidence in the schists of the peninsula 

 to prove this point definitely one way or the other. The schists 

 occur both with and away from the serpentines. The quartzose 

 glaucophane schist, described earlier in the paper, seems a result of 

 dynamic rather than of contact metamorphism. 



In connection with the formation of schists from contact with 

 serpentines it may be said that the fact that sandstone taken from 

 contact with the serpentine shows no noticeable metamorphism indi- 

 cating that the action of the serpentine is not very great. If the 

 schists were formed by regional metamorphism they might be formed 

 along lines which would then be the places where the serpentines 

 could be most easily intruded. 



VII. The Earthquake of April 18, 1906. 



Since the preparation of this paper there has occurred in Cali- 

 fornia, and especially in this district, a seismic disturbance of un- 

 usual violence, resulting in the destruction of much property and a 



^ Monograph XIII, U. S. Geological Survey, p. 186. 

 ^Fairbanks, American Geologist, IX, p. 161. 



PROC. AMER. PHIL. SOC, XLVI. 185D, PRINTED JULY I5, I9O7. 



