1.62 



University of California. 



[Vol. i . 



for the most part slope gently down to the shore level. Although 

 but few houses occupy this area, streets have been run through it 

 in all parts, and numerous cuttings expose large sections of the 

 underlying rocks. The surface of the Potrero is almost wholly 

 occupied by a mass of serpentine, a portion of the more or less 

 irregular belt of that rock which traverses the peninsula in a north- 

 west and southeast direction from Hunter's Point to the Presidio. 

 Associated with the serpentine are shales and sandstones of the 

 San Francisco sandstone formation, thinly bedded jaspers (phthan- 

 ites of Becker), and certain masses of intrusive rock of very 

 peculiar occurrence. 



The unusual opportunities afforded in this area for the study of 

 the structure and for obtaining fresh specimens of the rocks, led to 

 the hope that evidence might here be found bearing on the origin 

 of the serpentine; and with that object particularly in view, the 

 investigations, the results of which are here set forth, were made. 



The details of the petrography of the serpentine which will be 

 found below, presenting, as they do, no new fact in the history of 

 the formation of that mineral, may be considered unnecessarily 

 minute in view of the knowledge of that process at the command 

 of petrographical students. But as the igneous origin of the 

 serpentine of the Coast Ranges has been denied, and as other 

 hypotheses of its formation have been advanced * it was thought 

 proper to give in full the evidence which has been collected of its 



'Becker, G. F. , "Quicksilver Deposits of the Pacific Coast." "Monograph 

 XIII," U. S. G. S., pp. 120-128. 



Note. — Since writing the above there has come to the writer's notice a 

 paragraph by Dr. Becker which it is but justice to him to quote. In "Extract 

 from Mineral Resources of the United States, 1S92," Quicksilver Ore Deposits, 

 p. 9, Dr. Becker says: "After the publication of 'Monograph XIII,' Mr. H. 

 W. Turner, acting as my assistant, proved to his own and my satisfaction that 

 the serpentines of Mt. Diablo are mainly or wholly due to the decomposition 

 of peridotite and other basic eruptives not previously recognized in the Coast 

 Ranges. This investigation convinced me that portions of the serpentines 

 classed as metamorphic in 'Monograph XIII,' in the absence of known evidence 

 to the contrary, are also of eruptive origin. This additional information does 

 not derogate from the importance or the interest of the occurrence of meta- 

 morphic serpentines in the Coast Ranges." 



