1910.] COAST RANGES OF CALIFORNIA. 343 



Of the different types of structure, the mesh and the bastitic 

 structure are invariably found. The former points towards an 

 origin from olivine and fresh specimens often show remnants of 

 this mineral in the center of the mesh. It consists of bands of 

 serpentine which intersect irregularly, quite often in rectangular 

 position to each other, surrounding aggregates of filled serpentine. 



The bastitic structure is described under bastite, and needs no 

 further comment. 



No characteristic grate structure was observed. It is true, now 

 and then a suggestion of it is seen. 



Magnetite was never observed in fresh specimens of the rock 

 but it is pronounced when the rock is altered. It then occupies 

 cracks and seams in opaque irregular masses. Chemical and mag- 

 netic properties distinguish it from chromite. Large masses in con- 

 nection with the serpentine are to the writer's knowledge not known 

 in the Coast Ranges of California. 



Magnesite is abundantly found as float and in veins of various 

 sizes. It is usually massive, fine-grained, of a beautiful white color 

 and with a conchoidal fracture. Besides being formed in the proc- 

 ess of hydration of the primary minerals which give rise to serpen- 

 tine, it is probably also produced when the serpentine breaks down. 



The following analyses given by F. L. Hess^'' show the chemical 

 character of the magnesite. 



Analyses of Magnesite. 



I. II III. 



Si02 2.15 .30 49.85 



AI2O3 1.22 .16 3.45 



Fe=03 1.16 .38 .18 



CaO 5.28 1.34 .48 



MgO 41.01 45.86 21.53 



CO2 48.72 51.80 23.96 



99-54 99-74 99-45 



I. Magnesite from Chiles Valley, Napa County. P. H. Bates, analyst. 



II. Magnesite from W. W. Burnett's ranch, Coyote, Santa Clara County. 

 A. J. Peters, analyst. 



III. Magnesite from near Morgan Hill, Santa Clara County. A. J. 

 Peters, analyst. 



^° F. L. Hess, "The IMagnesite Deposits of California," Bull. 355, U. -S. 

 G. S., Washington, 1908. 



