16 



CRANDAl.L— SAN FRANXISCO PENINSULA. 



[January 4, 



cene, consisting of a thickness of perhaps one hundred feet of partly 

 consolidated clav. Dr. Ashley mentions a thickness of two hundred 

 feet south of Mussel Rock. In the area mapped the thickness of 

 the Post-Pliocene beds was difficult to determine but is probably 

 not over one hundred feet. 



The question of the conformity or unconformity of the Pliocene 

 and Pleistocene is one that is yet to be decided and is not evident 

 in this area. No paleontologic work was attempted upon these beds. 

 V Serpentines.— In the city the serpentines form a prominent 

 terrane ; the best exposures for study are in the large hills at the 

 Potrero'. The serpentine is found in several places, and is always 



Fig. 7- 

 A conglomeratic effect in serpentine as exposed in a quarry on Potrero Ave., 



near the railroad. 



characterized by the bhie green appearance, and by a lack of soil 

 and vegetation over the surface. In the quarries of the Potrero the 

 original peridotite rock may be seen. This appears as large black 

 boulders in the serpentine, giving the appearance of a coarse con- 

 domerate Each of these boulders is separate from the others and 

 coated with from one to many inches of slickensided serpentine. 

 This shows the breaking up of the peridotite mass and the large 



