R. A. Daly — Secondary Origin of Certain Granites. 213 



eral hypothesis to explain the segregation of sulphide-ores, 

 without, however, connecting the concentration with gravita- 

 tive influence. Coleman has announced the view that the 

 ores have thus settled to the bottom of the sill, but has not 

 connected the action with the digestion of acid rock in the 

 norite."* The whole array of facts connected with the Sudbury 

 intrusive is so accordant with the double theory of assimilation 

 and differentiation through density stratification, as to single 

 out this particular case as perhaps, of all those noted in the 

 present paper, the most convincing and illuminating. 



It is necessary that brief reference be made to an alternative 

 view of all these related phenomena. One may conceive that 

 the granite-granophyre, intermediate rock and gabbroid rock 

 in each of the intrusive sheets may be explained by simple 

 differentiation from an original magma through density strati- 

 fication but without the aid of significant assimilation of the 

 country-rocks. Lack of space forbids that this hypothesis be 

 here discussed at length. The writer believes that the hypoth- 

 esis is untenable or, at least, is much less adapted to explain- 

 ing the facts than the lrypothesis of assimilation accompanied 

 ancl followed by differentiation. Most of the facts on which 

 that belief is founded have been already implied or expressly 

 noted. 



Among the significant facts are the following : 



1. There is a close similarity in composition between the 

 granite-granophyre zone and rims of manifest digestion about 

 xenoliths now surrounded by gabbro. This consanguinity is 

 inexplicable on the theory of mere differentiation within the 

 original magma. 



2. The genetic relationship between the granite-granophyre 

 zone and the invaded sediments is further shown by certain 

 special features already described among the structures of the 

 acid rock in the Moyie sill, and of the overlying, metamor- 

 phosed quartzite. For example, the development of remark- 

 ably poikilitic quartz in the granite-granophyre and in the 

 recrystallized quartzite (the quartz of the latter being largely 

 or wholly indigenous) may be mentioned. This repeated 

 occurrence of a peculiar structure finds no simple explanation 

 on the pure-differentiation theory. 



3. In the period of high temperature preceding the viscous 

 period when the visible xenoliths were frozen in the gabbro, 

 thousands or millions of other xenoliths were completely or in 

 part digested in the gabbro magma. The product of their 

 digestion can be found, apparently, in no other place than in 

 the existing acid zone of each intrusive sheet. 



* 1903 report, p. 277. 



