320 KRAMM— SERPENTINES OF THE CENTRAL [June 6. 



Analysis of Serpentine, Sulphur Creek, Colusa Co. 

 H. E. Kramm, analyst. 



Per Cent. 



SiOo Z7-^2 



AI0O3 I-20 



Fe.Oa 8.60 



FeO 2.15 



MgO 37-59 



CaO 2.49 



NaiO 27 



K2O trace 



H=0 10.46 



Cr^Oa 36 



Ti02 trace 



100.74 



While the foregoing only treats the massive facies of the ser- 

 pentine it is of interest to study the other facies present. The 

 rock as a whole shows an advanced state of decomposition. It is 

 much shattered and slickensided, due no doubt to the increase of 

 volume of the parent rock in the course of hydration into serpen- 

 tine, and the resulting pressure. Angular fragments, bluish-green 

 to brown in color, crumbling away under slight pressure often en- 

 close rounded boulders of an apple green tough serpentine. These 

 are traversed in all directions by numerous veins of chrysotile, reach- 

 ing sometimes a thickness of two centimeters. The chrysotile being 

 more resistant towards weathering stands out from the main bulk. 

 This facies is especially prominent where the creek cuts the first belt 

 of serpentine. A similar rock is found at the Potrero in San Fran- 

 cisco and has been described by C. Palache.^ 



The decomposition of the serpentine changes its structural fea- 

 tures. The usually massive fresh rock often becomes granular. 

 Numerous veinlets of black magnetite give it a banded appearance 

 and a high specific gravity. Cavities form, which are colored white 

 by magnesite or hydromagnesite. (See Plate XXXIV, Fig. i.) 

 At this stage the serpentine has more or less transformed into a 

 silicious mass, which is tough, and not breaking exhibits the char- 

 acteristic greasy luster of opal, while remnants of the serpentine are 



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

 Potrero, San Francisco," Bull. Gcol. Dcpt. Cai, L, 166, 1894. 



