48 CRANDALL— SAN FRANCISCO PENINSULA. [January 4, 



from the southwest side of the San Andreas-Stevens Creek fault 

 hne but that remnants are still preserved on the northeast side. 

 There is the possibility that the first movement along this fault line 

 occurred at the time represented by the unconformity shown by the 

 pebbles in the Silver Terrace sandstone. The area containing the 

 unconformity is on the southeast side of this fault line, but the re- 

 moval of the jaspers may be due to the erosion of the hills southwest 

 of the fault during Pliocene times. 



After the deposition of the jaspers conditions changed and the 

 sandstpnes were deposited that appear in the northern end of the 

 peninsula. The writer feels fairly sure that he has seen this Silver 

 Terrace sandstone in other localities with the jaspers. Dr. Becker 

 asserts that the granites underlie the Franciscan Coast Ranges. Then 

 if this uplift took place, as suggested during the period of deposi- 

 tion of Silver Terrace sandstone the erosion would be greater in the 

 Santa Cruz mountains on the southwest side of the San Andreas- 

 Stevens Creek fault and would account for the appearance of the 

 Montara and Ben Lomond granites. 



Besides the large areas of jasper, there are small areas in various 

 places throughout the Coast Ranges that do not seem to belong to 

 the main beds. These may possibly be erosion remnants left by the 

 removal of the larger masses, or it may be that the organisms form- 

 ing the jaspers survive in colonies that form small lens-shaped 

 masses. This latter suggestion receives support by evidence found 

 at a place in the cliffs near Golden Gate Strait, where about six or 

 eight inches of jasper appears in the sandstone several feet below 

 the bottom jasper beds. The small bed appears to show the arrival 

 of the first condition that allowed the development of the organisms 

 followed by a period of slight change with sandstone and final 

 settling to the quiet conditions in which the larger deposits were 

 laid down. 



The writer is of the opinion that both of these conditions may be 

 found in any area where the jaspers occur. 



Origin of the Serpentines. — The field evidence in San Francisco 

 shows that the serpentines are formed by the alteration of olivine 

 and pyroxene-bearing rocks. The large number of exposures has 

 allowed the collection of an abundance of fresh specimens of the 



