Vol. 4] 



Knopf. — An Alteration of Serpentine. 



427 



magnesian carbonates, and in exceptional cases even siderite. The 

 chalcedony is composed of aggregates of beautifully formed 

 minute spherulites. Granular and automorphic magnetite, and 

 grains of chromite occur as accessories. The chalcedony com- 

 monly occupies small lenticular areas surrounded by narrow 

 bands of carbonate, and the structure produced thereby is char- 

 acteristic of the silica-carbonate rock. 



At North Berkeley this type is found grading into a rock 

 whose characteristic feature is the abundance of lamellar pyrox- 

 enes of decidedly bronzy metallic lustre. Veinlets of chrysotile 

 under a millimeter in thickness form a reticulate network tra- 

 versing the rock. 



In thin section large plates of orthorhombic pyroxene become 

 apparent. Their color is brownish yellow, and the deeper colored 

 sections show a faint pleochroism. A strongly colored plate was 

 found to give a decided pleochroism : green parallel to r and 

 yellowish green transverse to c. The pyroxene is therefore, 

 bronzite. Small grains of chromite are included in the bronzite. 



The pyroxenes are partially serpentinized and the remaining 

 bronzite plates represent the survivors of the process of serpen- 

 tinization. Even these show strong variations in the degree of 

 relief, varying from the characteristic high relief of the pyrox- 

 enes down to that of Canada balsam. The latter probably repre- 

 sent the bastite modification, though the position of the optio 

 axial plane could not be tested. The pyroxenes are, moreover, 

 traversed by a ramifying system of carbonate fibres. Certain 

 banded veins of serpentine which intersect the slide are found not 

 merely cut across by the carbonate, but show also a progressive 

 replacement by means of numerous small apophysal tongues. 

 These paragenetic relations show that the carbonate is epigene 

 upon the serpentine, and not merely a residual product of the 

 process of serpentinization. 



In certain sections the carbonate can be seen replacing the 

 pyroxene directly without the intervention of the serpentine 

 stage. The advancement of carbonatization appears easiest in the 

 direction of the vertical axis. 



Often a sort of net structure is produced by capillaries of 

 carbonate encircling areas of serpentine. Prom these, grada- 



