W. N. Benson — Origin of Serpentine. 713 



Knopf (1906) found that the magnesian carbonate 

 rocks of California had replaced peridotites, which were 

 either quite fresh or were more or less serpentinized, and 

 concluded that the carbonation was a process distinct 

 from hydration and subsequent thereto. Some lime and 

 sulphide minerals were introduced during the change, but 

 apparently no talc was formed. In other parts of Cali- 

 fornia, the serpentine associated with the cinnabar 

 deposits has been entirely removed by "solfataric 

 agency," and only the siliceous skeleton remains (Becker 

 1888. See also Lindgren 1895, p. 153). Somewhat sim- 

 ilar features have been observed by Lacroix (1897), 

 as the result of the action on serpentine in Greece, of a 

 fumarole, which exhaled carbonic and sulphuric acids 

 and steam. Ferruginous honey-combed siliceous rocks, 

 containing talc and breunnerite, are associated with ser- 

 pentine in the northern part of the belt of magnesian 

 rocks in Eastern IT. S. A., and the several changes 

 involved are considered to be the result of the action of 

 the same agents as caused the serpentinization (Bascom 

 1902, 1905). In the neighborhood of Madras, there are 

 magnesite rocks derived from peridotites, by the action 

 of magmatic waters bearing carbonic acid but, though 

 Holland (1899a) concluded that the small amount of 

 serpentine present was formed after carbonation, Mid- 

 dlemiss (1896) held that serpentinization had preceded 

 carbonation. 



The writer's observations in New South "Wales (Ben- 

 son 1913) accord with this last. In several regions 

 along the serpentine-belt, the rock is more or less com- 

 pletely replaced by crystalline carbonates, talc, and 

 quartz, chalcedony or opal. Generally this occurs on 

 one side only of the serpentine mass, "the footwail," 

 occasionally in one or more bands within the serpentine, 

 so that it would seem very probable that the carbonation 

 is the effect of solutions moving in the thrust-planes 

 that bound or traverse the serpentine. The location of 

 the -carbonated rocks is completely independent of the 

 present topography, and there is clear evidence at 

 Warialda, where the carbonated rocks are overlain by 

 Jurassic sandstone, that the carbonation occurred before 

 the sandstones were deposited (Benson 1917). The pres- 

 ence of auriferous pyrites among the carbonated rocks 

 gives further support to the hypothesis of the deep- 



