I9I0.] COAST RANGES OF CALIFORNIA. 341 



Another noteworthy place where chromite is found in quantities 

 is at Cedar Mountain in Alameda County. An analysis of chro- 

 mite from this locality corresponds closely with that of a mineral 

 intermediate between picotite and chromite from a dunite from Dun 

 Alountains in New Zealand, which for comparison is appended. 



I. II. 



SiO^ none 



AI2O3 18.79 12.13 



FeO 16.99 18.01 



CaO trace 



]\IgO 8.41 14,08 



Cr^Os 55-74 56-54 



Ni 



no 1 



:a0 J 



P p. r trace trace 



MnO trace 0.46 



H2O 09 



99.82 101.22 



I. Chromite from Cedar Alountain. H. E. Kramm, analyst. 



The mineral was purified by powdering it and passing through Thoulet 

 solution. All iron was determined in the ferric state and Qalculated to fer- 

 rous iron. There was no doubt ferric iron present, but the amount could 

 not be determined on account of the difficulty of getting chromite into 

 solution. 



II. Chromite-Picotite from New Zealand. Mineralogy, 6th ed. Dana, 

 p. 228. Analyst T. Petersen. 



Secondary Minerals. 



The course of hydration and subsequent decomposition of the 

 serpentine necessitates a change of molecular arrangement, and 

 gives rise to a number of secondary minerals. According to 

 Tschermak the conversion of olivine gives rise to serpentine, mag- 

 nesite, limonite and silica. It seems very probable that the excess of 

 magnesia will combine with free silica to form secondary serpentine. 

 This is substantiated by the fact that fissures in the rock are invari- 

 aby filled w4th chrysotile. 



Decomposition and the action of ground waters assist in the 

 development of another series of minerals which are probably oxide 

 of iron, magnetite, hydromagnesite and vein serpentine. It is 

 reasonable that this vein serpentine is not necessarily confined to 

 the serpentine body itself, but may find its way into the surrounding 

 country rock, where under favorable conditions it is deposited. 



