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170 



University of California. 



[Vol. 1. 



I. Pyroxenite from Mt. Diablo; analyst, Dr. Melville. 



II. Serpentine from Presidio, San Francisco; analyst, Dr. Easter. 

 The analyses thus shown are of rocks of basic and ultra-basic 



composition, and it may fairly be assumed that they represent 

 approximately certain phases of the Potrero rocks. While no 

 great weight can be attached to the evidence thus obtained, still, 

 such as it is, it corroborates fully the conclusions previously reached 

 from the study of the structural and mineralogical characters of 

 the serpentine and lherzolite of the Potrero. 



Relations of the Serpentine to the San Francisco Sandstone. — The 

 evidence obtained in this area bearing on the relation of the serpen- 

 tine to the sandstone terrane is not wholly conclusive, although the 

 rocks may be observed in direct contact at several points. At the 

 corner of Solano and Connecticut streets the serpentine may be 

 seen resting on a mass of shaly sandstone, the plane of contact dipping 

 to the north at an angle of about 30°. Blocks of sandstone appear 

 to be included in the serpentine near the contact, but the rock is so 

 sheared up and decomposed that this is doubtful. A similar con- 

 tact is exposed at the corner of Tennessee and Napa streets, the 

 sandstone again being beneath the serpentine. No evidence of 

 contact metamorphism in the sandstone is visible in either case. 

 On the top of the hill at the intersection of Iowa and Butte streets 

 is a small body of bedded jaspers associated with sandstone resting 

 upon serpentine. The jaspers are violently contorted, but appear 

 to dip towards a mass of serpentine lying at about the same level. 

 In the large quarry on Potrero avenue, at the extreme northern 

 point, is a block of sandstone about three feet in diameter, appar- 

 ently entirely surrounded by serpentine. At the opposite side of the 

 quarry shaly beds may be seen dipping towards a mass of serpen- 

 tine which crops a few feet away. Two hypotheses may be made 

 as to the relations of these two formations: (1) That the serpentine 

 is the older terrane, the sandstone having been deposited upon it 

 and subsequently tilted and faulted. (2) That the serpentine is 

 younger than the sandstone and intrusive into it. Reviewing the 

 above facts in the light of these hypotheses, it will be 'seen that 

 several of the observed relations, notably the masses of sandstone 

 included in the serpentine, cannot be explained by the first one. 



