716 W. N. Benson — Origin of Serpentine. 



iv. Grubenmann, voicing an opinion held by numerous 

 penologists, has stated that while antigorite is a product 

 of hydration under pressure and formed in the upper 

 zone of metamorphism, normal mesh-structure chrysotile 

 serpentine is a product of weathering (Grubenmann 

 1910). One might infer from this that in any rock, con- 

 taining the two forms of serpentine, antigorite would have 

 been formed first at a great depth, and mesh-structure 

 chrysotile would be developed from the residual olivine, 

 when under lesser pressure and at a smaller depth, the 

 rock came under the influence of meteoric waters. 

 Weinschenk (1913), on the other hand, would divide the 

 process into: (a) the formation of primary antigorite in 

 the rare instances in which it occurs; (b) the formation 

 of secondary antigorite by the action of magmatic waters 

 upon the residual olivine ; (c) the formation of veinlets of 

 olivine and antigorite crystallizing from the magmatic 

 water; and (d) finally the formation of mesh-structure 

 serpentine from the residual olivine as last effect of the 

 thermal waters. 



In both hypotheses, antigorite is of earlier formation 

 than mesh-structure serpentine. Nevertheless the writer 

 has found that this is not always the case. It was seen 

 that in certain serpentines from New South Wales that 

 mesh-structure serpentine may recrystallize into anti- 

 gorite serpentine, the long blades of which lie in positions 

 quite without relation to the position of the secondary 

 magnetite, which separated out from the olivine as it 

 formed by its first change into chrysotile-serpentine, and 

 still exhibits the characteristic mesh structure. The pro- 

 cess was traced through a series of slides which exhibited 

 all intermediate stages. In serpentines in which the 

 antigorite comes directly from the olivine (or pyroxene) 

 the magnetite is often in little triangular patches, inter- 

 stitially placed among the laths of antigorite, and recall- 

 ing the intersertal and occasionally the ophitic structure 

 of dolerites (cf. Bonney 1905), but in the serpentines in 

 which the antigorite has formed after the change to 

 chrysotile, the magnetite remains in fine dusty particles. 

 Dr. Flett, who kindly examined the series of slides in 

 question, expressed his concurrence in the interpretation 

 placed thereon. 4 Similar evidence of the replacement of 

 mesh or chrysotile serpentine by antigorite was seen in 

 a specimen from Visp, Switzerland, described by Preis- 



4 (Letter to writer, 2:5:1913.) 



