354: G. F. Becker — Metamorphic rocks of California. 



rocks are best represented by detailed descriptions of special 

 examples for which there is no space here. 



Serpentine in a comparatively pure state occurs throughout 

 the quicksilver belt in irregular areas. As nearly as can 

 be estimated these areas amount to somewhat over 1,000 square 

 miles between Clear Lake and New Idria. Serpentine is also 

 one of the mineral constituents of many of the altered sand- 

 stones and of the granular metamorphic rocks. In order that 

 there might be no mistake as to the identification of this 

 mineral in its various relations, quantitative analyses and quali. 

 tative tests have been made of many occurrences. The optical 

 properties have been studied in the specimens analyzed and 

 microchemical tests have been applied to confirm optical 

 determinations. The result obtained is that the serpentine of 

 the massive occurrences and that of the partially serpentinized 

 sandstones and granular metamorphics is the same. It is 

 a biaxial variety, often just perceptibly dichroitic and rarely 

 shows differences of tint as great as those characteristic of 

 chlorite. It might be called antigorite if it seemed needful to 

 separate the biaxial serpentines. Bastitic developments are 

 common, but these masses do not appear to be pseudomorphic 

 or to differ, except in regularity of structure, from the ordinary 

 forms. Chrysotile and marmolite, which are not infrequent, 

 seem also mere structural modifications. The net structure so 

 usual, though not invariable, in serpentine formed from olivine 

 has nowhere been detected. Where any considerable quantity 

 of serpentine is present it usually shows the grate-structure 

 first studied in the Alpine serpentines by von Drasche and since 

 by others in the Grecian and Swedish occurrences. 



No considerable portion of the serpentine of the Coast Eanges 

 has resulted from the decomposition of olivine. Only pebbles 

 of a single olivine gabbro have been found and these contain a 

 mere trace of serpentine, while the origin of the serpentine has 

 been traced in a great number of cases to rocks containing no 

 olivine. Field observations show most conclusively that the 

 great mass of the serpentine of this area is derived from the 

 sandstones either immediately or through an intermediate 

 granular metamorphic rock. This is evident both fr,pm the 

 larger features of structure and from the details of separate 

 croppings. Highly inclined strata strike into serpentine areas 

 in such a manner as wholly to preclude the supposition that 

 the serpentine is an older mass, and one side of an anticlinal 

 fold is serpentinized while the other is unaltered and carries 

 excellent fossils. These relations are particularly clear at 

 Knoxville and at Mt. Diablo. There are also clear cases of 

 transition from sandstone to serpeutine. In the conversion of 

 more or less metamorphosed saudstone masses to serpentine as 



