316 KRAMM— SERPENTINES OF THE CENTRAL [June 6, 



These serpentines were designated by G. F. Becker as being de- 

 rived from sedimentary rocks, which he beheved had obtained the 

 magnesia necessary for their transformation into serpentine by a 

 process of substitution.^ Previous to his report which appeared 

 in 1888 some Colusa County serpentines had been described by 

 M. E. Wadsworth as Iherzolite serpentines.^ Others, the Mount 

 Diablo,^ Potrero,* Angel Island,^ San Francisco Peninsula^ and the 

 Oak Hiir serpentines have since been shown to be derivatives of 

 eruptives. 



These latter areas aggregate perhaps ten to fifteen square miles 

 which is, according to Becker, about one per cent, of the total ser- 

 pentines in the coast ranges. Their investigation, aside from hav- 

 ing demonstrated that the rock is derived from eruptives, has also 

 shown interesting variations in the serpentine itself. 



Taking this into consideration, a study of the remaining areas 

 seemed desirable, even though such might not produce new and 

 startling facts. 



The accompanying map shows the distribution of the rock in the 

 central coast ranges. Existing maps proved of valuable assistance 

 in the field as well as in the preparation of the map. While some of 

 the data for it were thus obtained, other data were obtained by 

 the writer, who is aware that there may be serpentines especially 

 in Napa, Marin and Solano counties which may have escaped his 

 attention. 



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

 XIII, U. S. G. S., 120-128, Washington, 1888. 



"M. E. Wadsworth,, " Lithological Studies," Mem. Mns. Covip. Zool. 

 Harvard College, 1884, 129-132. 



^ H. W. Turner, " The Geology of Alount Diablo, California," Bull. Geol 

 Soc. Am., II., 388-391, 1891. 



* C. Palache, " The Lherzolite Serpentines and Associated Rocks of the 

 Potrero, San Francisco," Bull. Geol. Dept., Univ. Cal., I., 164-169, 1894. 



° F. L. Ransome, " The Geology of Angel Island," Bull. Geol. Dept. Univ. 

 Cal., I., 219, 1894. 



^A. C. Lawson, "Geology of the San Francisco Peninsula," isth Annual 

 Report U. S. G. S., 445, Washington, 1893-94. 



' E. P. Carey and W. J. Miller, " The Crystalline Rocks of the Oak Hill 

 Area near San Jose, Cal.," Journal of Geol., XV., 160, 1907. 



