212 R. A. Daly — Secondary Origin of Certain Granites. 



Loewinson-Lessing points out that when large amounts of 

 foreign rock-material is digested in a magma, there is established 

 a special tendency toward a systematic differentiation of the 

 mixture.* Liquation will then take place when the cooling 

 mixture reaches a certain temperature. The same author also 

 holds that, according to the principles of physical chemistry, 

 a magma becomes actually more fluid as a result of digesting 

 foreign material. Differentiation is thereby facilitated. 

 Vogt's valuable researches tend to corroborate this view.f 



The granites and granophyres of the Moyie sill, of the Pigeon 

 Point intrusive, and of the Sudbury sheet are to be regarded 

 as not directly or merely due to the contact solution of sedi- 

 mentary rocks and schists by gabbro ; they are controlled 

 in their final composition by a common process of differentia- 

 tion supplementary to the gravitative effect. At Pigeon Point 

 the acid rock, whatever its structure and grain, is a rather 

 definite mixture of oxides. This is illustrated in the analyses 

 of granular soda granite, the u quartz keratophyre", and the 

 porphyry of Little Brick Island near Pigeon Point.;}; For lack 

 of sufficient analyses the same statement cannot be made con- 

 cerning the Moyie sill, but within limits it applies to the huge 

 Sudbury sheet.f 



The acid zone may have won some of its soda from the 

 original magma; the gabbro may now hold some of the pot- 

 ash with the silica derived from the micaceous and feldspathic 

 quartzites and other sediments. It is obvious, however, that 

 all the details of the chemical processes engaged in this type 

 of magmatic separation (chemical affinity in magma disturbed 

 by gravitative diffusion currents) cannot be worked out from 

 existing data on the magmatic behavior of silicates. 



The intermediate rock at all three localities may be regarded 

 as occupying zones of incomplete differentiation. 



Special interest attaches to the occurrence of the nickel ores 

 along the lower contact of the Sudbury sheet. Barlow, Cole- 

 man, Yogt and Walker agree that these sulphides are soluble 

 in magmas. The solubility is in inverse proportion to the 

 acidity of those magmas. | The fact suggests that the sul- 

 phides have been precipitated from the norite which has been 

 acidified by assimilation. The concentration of the ore on 

 the lower contact is again the result of differentiation through 

 contrasts of density, the sulphides settling to the bottom of 

 the sheet. Loewinson-Lessing has already suggested this gen- 



*Op. cit., pp. 375 ff. 



f Op. cit. . Part 2. 



JBull. 228, U. S. Geol. Surv., p. 89. 



§ See A. P. Coleman, 1904 report, p. 213. 



|| J. H. L. Vogt, op. cit'., p. 229. 



