712 W. N. Benson — Origin of Serpentine. 



far as the writer understands it, appears to be similar to 

 Roth's conception (1869) of "complicated weathering,' ' 

 i. e., the production of slow changes at depth by epigene 

 waters, the chemical activity of which had been increased 

 by the presence in solution of products of weathering 

 obtained from the surface layers. Eemarking that the 

 width of the carbonate rock exposed in the St. Gott- 

 hardt Tunnel is less than that appearing on the surface 

 three thousand feet above, Schneider concludes that 

 there is a gradual decrease in width of the carbonate zone 

 with increasing distance from the surface. His observa- 

 tion that the boundary between the serpentine and the 

 carbonate rock is most irregular, weakens this conclu- 

 sion, and, moreover, we have no knowledge of the width 

 of the carbonate zone at intermediate, or lower, levels. 

 (See figs. 1 and 2.) The rocks of this zone contain crys- 

 talline carbonates of magnesia, iron and a small amount 

 of lime, together with talc and chlorite. Redlich (1909), 

 investigating the magnesite deposits of Kraubat, which 

 form on serpentine or peridotite alike, concluded, how- 

 ever, that, while the serpentinization may be the result of 

 secular weathering (though antigorite has been formed) 

 the carbonation is here the result of atmospheric weath- 

 ering, since the magnesite is a gel, not a crystalloid, in 

 this area (cf. Cornu 1910). In this condition the magne- 

 site builds a thick earthy mass adhering to the tongue, 

 and, microscopically examined, is seen to form isotropic 

 flakes, which take a basic stain ; but in the case described 

 by Schneider the magnesite is a crystalloid in the usual 

 rhombohedral form, and cannot be the result of atmos- 

 pheric weathering according to Cornu 's criteria, though 

 it may perhaps be the result of "secular weathering," 

 Schneider's rocks are typical of very many occurrences of 

 carbonates on the margin of serpentine-masses and, more 

 rarely, as in the Bingara District, N. S. W. (Benson 

 1917), in bands within a mass of serpentine. In most 

 of these, there is also quartz or chalcedony, residual or 

 secondary (cf. Rosenbusch 1907) grains of chromite, and 

 fuchsite, as e. g. at Kalgoorli, W. A. (Thomson 1913) ; 

 sometimes sulphides are present. In the last region 

 mentioned the structure of the carbonate rock renders it 

 perfectly clear that the carbonation had occurred after 

 the original rock, a poikilitic harzbergite, had been com- 

 pletely serpentinized. 



