ON A HILLSIDE IN DONEGAL 359 



in a little altered state, and the " mineralisation " takes place 

 as an emphasis of the original stratification. Again, dynamic 

 metamorphism may work its will on the complex series, and 

 roll out indistinguishably the insidious igneous veins and the 

 invaded sediments. But the vast majority of well foliated 

 gneisses, and especially those with well differentiated bands, 

 may now safely be ascribed to conditions of original flow. 

 Most of them, moreover, will be found to represent the marginal 

 phenomena of intrusive granite domes. 



If absorption on a large scale of the surrounding material 

 goes on we have a rational explanation, and practically the 

 only one, of the relation of such intrusive masses to the 

 sedimentary, metamorphic, or igneous rocks traversed by them. 

 The mere uplift, by arching, of the rocks above them does 

 not explain the phenomena of their mise en place as studied 

 in the field. The features to be expected from plutonic absorp- 

 tion have elsewhere been discussed in connexion with the 

 invasion of a mass of basic lavas and diorites by granite in the 

 county of Londonderry ; * and it was then pointed out that, by 

 a process of internal differentiation, the absorbed basic materials 

 might be carried away into the depths, the marginal granite 

 remaining comparatively pure. Loewinson-Lessing 2 has laid 

 stress on the influence of density in promoting such differentia- 

 tion ; and R. A. Daly 3 has developed the assimilation-theory 

 with skill and, to say the least, unhesitating boldness. As our 

 geological researches in the field do not often allow us to 

 reach the depths to which absorbed matter may be supposed to 

 plunge, 4 we have near the surface the spectacle of a pure 

 igneous mass underlying that which it has invaded. Yet, even 

 then, its viscosity during cooling very often enables it to 

 support numerous blocks in what American writers have called 

 the "shatter-zone," where the final attack on the superincumbent 

 rocks remains recorded ; 5 and movement in this region during 

 the last stages of liquidity will result in the production of a 

 streaky and apparently antique and " fundamental " gneiss. 



1 Cole, " Geology of Slieve Gallion," Sci. Trans. Royal Dublin Soc, vol. vi. 

 (1897), p. 242. 



2 Compte rendu, Congres geol. international, 1899, p. 344. 



3 Amer. Journ. of Science, vols. xv. (1903), p. 269, and xvi. (1903), p. 107. 



4 Compare Doelter, Petrogenesis, p. 123. 



5 R. A. Daly, " Secondary Origin of Certain Granites," Amer. Journ. Sci. 

 vol. xx. p. 208. 



