H. A. Daly — Secondary Origin of Certain Granites. 207 



of the intrusive body, and the relation of the accessible points 

 of its contacts to that form as a whole, must be taken into 

 account. If, for example, differentiation of the compound 

 magma has taken place so as to produce within the magma 

 chamber layers of magma of different density, the lightest at 

 the top, the heaviest at the bottom, the actual chemical com- 

 position of the resulting rock at any contact will depend 

 directly on the magmatic stratum rather than on the composi- 

 tion of the adjacent country-rocks. 



Thirdly, the method of intrusion is of primary significance 

 in the discussion of assimilation in a given instance. There 

 are strong reasons for believing that the subterranean cham- 

 bers of stocks and batholiths have been opened largely or at 

 least in part through magmatic " stoping," whereby magmas 

 have made their way upward through the invaded formations 

 by engulfing suite after suite of blocks shattered off from those 

 formations by the heat of the intrusives.* In such a case the 

 destructive action at the molar contact is chiefly physical, and 

 chemical solution is subordinate. Most of the solution takes 

 place in the complete digestion of the sunken blocks and is 

 therefore abyssal rather than marginal. The conditions are 

 peculiarly favorable for the systematic differentiation of the 

 new compound magma. The chemical composition of the 

 intrusive at any contact will thus depend on the constitution of 

 a (possibly well differentiated) magma containing materials 

 won from all the invaded formations and not simply materials 

 won from the immediately adjacent country-rock. Brogger's 

 argument derived from the low content of lime in the Chris- 

 tiania granite cutting thick limestones (themselves overlying an 

 enormous thickness of crystalline schists, etc.) is clearly incon- 

 clusive until it can be shown that this and the other two 

 factors just noted have not been at work.f 



Magmatic stojDing has, in all probability, taken place to some 

 extent in the great intrusive body at Pigeon Point. The 

 specific gravity of the gabbro varies from 2*923 to 2*970. 

 Molten at 1400 degrees Cent., its specific gravity, at atmos- 

 pheric pressure, would be not far from 2*43 to 2*48. The 

 specific gravities of the intermediate rock and granite are 

 respectively 2'740 and 2 620; molten at 1400 degrees Cent., 

 they would, at atmospheric pressure, be about 2*30 and 2*19 

 respectively. The specific gravity of the invaded sediment 

 varies from 2'70 to about 2 - 75. Blocks of the quartzite and 

 slate immersed in any of the molten magmas and there assum- 

 ing the temperature of 1400 degrees Cent., would at the same 



*Cf. E. A. Daly, The Mechanics of Igneous Intrusion, this Journal, 

 vol. xv, 1903, p. 269, and vol. xvi, 1903, p. 107. 

 f Cf . Loewinson-Lessing, op. cit., p. 368. 



