W. N. Benson — Origin of Serpentine. 



703 



bonate rocks, which S tapir* considers the extension of 

 that encountered in the tunnel. It has recently been 

 studied by Schneider (1912). So far as could be learned 

 from the descriptions given and from a personal exam- 

 ination of some of the material in the Museum of the 

 University of Zurich, the rocks that appear on the sur- 

 face are exactly similar to those occurring in the tunnel 

 three thousand feet below. The water-content (loss on 

 ignition) of samples from the center of the surface expo- 

 sure varies from 5-8% to 8-5%, i. e. is the same as that at 

 depth, so that the evidence is clear that hydration does 

 not depend on proximity to the surface. (See fig. 2.) 



The truth of this is shown again in the case of the 

 dunite of the Geisspfad Pass studied by Preiswerk 



Fig. 3. 



Pendotite a,nd Serpentine 



Calc schist en d Dolomite Me&ozoic 

 Two rrucd. Gneiss 



Fig. 3. Occurrence of the Geisspfad serpentine near the Simplon Tun- 

 nel. (After Schmidt and Preiswerk, 1908.) 



(1901), (probably the source of the material from 

 which Schweizer (3840) obtained the original antigorite 

 described by him). In this the center is almost anhy- 

 drous, but is surrounded by a zone of antigorite-bearing 

 peridotite, the relation of the antigorite to the olivine 

 recalling that of plagioclase to ophitic augite, while 

 around this there is a marginal zone of completely 

 hydrated schistose serpentine. The mass of ultrabasic 

 rock. forms an almost horizontal sill in gneiss, and the 

 upper hydrated layer is quite similar to the lower one 

 from which it is separated by the anhydrous rock. (See 

 Zg- 3.) 



Effects that can be definitely referred to atmospheric 

 weathering are very limited in depth. They consist of 

 the formation of a crust sometimes attaining a depth of 



