R. A. Daly — Secondary Origin of Certain Granites. 215 



sill form. The wider and more important question is immedi- 

 ately at hand — does the assimilation-differentiation theory 

 apply to truly abyssal contacts? Do the granites of stocks 

 and batholiths sometimes originate in a manner similar or 

 analogous to that just outlined for the sills? 



The writer has briefly noted general reasons affording affir- 

 mative answers to these questions.* 



Gabbro and granophyre are often characteristically associated 

 at various localities in the British Islands as in other parts of 

 the world.f The field relations are there not so simple as in 

 the case of the Moyie sill, for example, but otherwise the recur- 

 rence of many common features among all these rock-associa- 

 tions suggests the possibility of extending the assimilation- 

 differentiation theory to all the granophyres. Harkers excel- 

 lent memoir on the gabbro and granophyre of the Carrock Fell 

 District, England, shows remarkable parallels between his 

 "laccolite" rocks and those of Minnesota and Ontario.^ 



At Carrock Fell there is again a commonly occurring tran- 

 sition from the granophyre to true granite, and again the gran- 

 ophyre is a peripheral phase. Still larger bodies of gabbro, 

 digesting acid sediments yet more energetically than in the 

 intrusive sheets, and at still greater depth, would yield a thor- 

 oughly granular acid rock as the product of that absorption 

 with the consequent differentiation. This does not imply, of 

 course, that all granites are of this origin, but it is quite pos- 

 sible that most intrusive granites are either of this origin or 

 have been more or less modified through assimilation. 



The difficulty of discussing these questions is largely owing 

 to the absence of accessible lower contacts in the average 

 granite body. All the more valuable must be the information 

 derived from intrusive sills. The comparative rarity of such 

 rock-relations as are described in this paper does not at all 

 indicate the exceptional nature of the petrogenic events signal- 

 ized in the Moyie, Pigeon Point or Sudbury intrusives. It is 

 manifest that extensive assimilation and differentiation can 

 only take place in sills when the sills are thick, well buried, 

 and originally of high temperature. All these conditions apply 

 to each case cited in the present paper. The phenomena 

 described are relatively rare largely because thick basic sills cut- 

 ting acid sediments are comparatively rare. 



On the other hand, there are good reasons for believing that 

 a subcrustal gabbroicl magma, actually or potentially fluid, is 

 general all around the earth ; and secondly, that the overlying 

 solid rocks are, on the average, crystalline schists and sediments 



* This Journal, vol. xv, 1903, p. 269, vol. xvi. 1903, p. 107. 



f See A. Geikie, Ancient Volcanoes of Great Britain, 1897. 



% Quart. Journal Geol. Soc, vol. 1, 1894, p. 311 and vol. li, 1895, p. 125. 



