I9I0.] 



COAST RANGES OF CALIFORNIA. 



347 



sions, but sometimes reach considerable size as in the case of the 

 .Mount Diablo pyroxenite. 



The age of the serpentine is established by field evidence which 

 agrees with that of Turner and Fairbanks and makes the serpentine 

 post-Knoxville. 



(C) Other facts which are of petrological interest are: 



The Iherzolite is in an advanced state of decomposition. 



The freshest specimen found contained only about twenty per 

 cent, of the original constituents of the rock. 



Olivine readily undergoes decomposition. Enstatite and diallage 

 are more resistant, and the latter seems to be more susceptible to- 

 wards serpentinization than the former. 



Picotite is quite abundant in fresh specimens of the northern 

 areas and appears to give rise to chromite. 



Secondary minerals besides the usual products of decomposition 

 are spinel, tremolite and talc. 



In closing the writer wishes to express his obligation to Pro- 

 fessor A. F. Rogers, of Stanford University, under whose guidance 

 and advice this paper was prepared. 



Table of Analyses. 

 Serpentines. 



I. 11. III. IV. 



Si02 36.57 37-62 37.71 39.60 



AI2O,: 95 1-20 1.81 1.94 



Fe=03 7-29 8.60 ) 



FeO 37 2.15 ( ^^^^ 



MgO 40.27 37-59 35-6o 36.90 



CaO 14 2.49 



Na^O 31 -^7 



K2O trace trace 



H2O 13.37 10-46 12.91 



MnO 10 



Cr203 33 -36 .20 



Ti02 trace .og 



NiO 31 



100.01 100.74 85.68 100.00 



I. Bastite with fine seams of chrysotile from Mount Diablo. W. H. Mel- 

 ville, analyst. 



II. Serpentine from Sulphur Creek, Colusa County. H. E. Kramm, 

 analyst. 



Proc. Amer. Phil. Soc, xlix, 196 w, Printed September 9, 1910. 



