706 W. N. Benson — Origin of Serpentine. 



mesh-structure and are not schistose, so that their 

 serpentinization must have followed the crushing to 

 which the gneiss has been subjected, during which its 

 water-content was probably greatly reduced. (See also 

 Zirkel 1894.) It must be concluded that in general local 

 diffusion is quite inadequate as a source of the water for 

 serpentinization. 2 



b. Water of the underground circulation. 



That the water of serpentinization maybe derived from 

 the general deep underground circulation (probably the 

 ascending portion) through the upper parts of the crust, 

 has much more in its support, and is the hypothesis 

 adopted by many writers explicitly (Van Hise 1904) or by 

 implication. Since the freest channels for the ascent of 

 the water might well be at the margins of the intrusive 

 masses, the inward decrease of the degree of serpen- 

 tinization might thus be explained, as well as the occur- 

 rence of serpentinization along the fault and contraction- 

 planes occurring within the mass, sometimes causing a 

 banded structure in the serpentine. (Cf. e. g. Graham 

 1917.) Moreover, as Van Hise explained, the upward- 

 moving waters would be enriched in carbonic acid as a 

 result of the silicification of carbonates in the deeper 

 zone a s of metamorphism, and in this we may see the 

 explanation of the strong attack of the waters upon the 

 olivine and the subsequent carbonation of the serpentine. 

 He added, however, that the effectiveness of circulating 

 waters as metamorphic agents must be greatly increased 

 by the addition to the general circulation in the middle 

 zone of metamorphism, of waters of magmatic origin, 

 though these probably form only a small portion of the 

 whole supply. It is to the reactions in this zone of the 

 character indicated that he refers the processes of ser- 

 pentinization and carbonation of the ultrabasic rocks 

 (op. cit. pp. 608-612). Grubenmann's reference of the 

 formation of antigorite-serpentine and of talc-carbonate- 

 schists to the uppermost of his zones of the crystalline 

 schists is perhaps in accord with this view, though he 

 adds that the ordinary massive or fibrous types of ser- 

 pentine are best considered the product of weathering 



2 That serpentine itself may be diffused in solution throughout a rock- 

 mass is assumed by Liesegang (1913) in his very interesting hypothesis 

 of the development by diffusion of Eozoonal structures in magnesian 

 limestones. 



