Cole — On Composite Gneisses in Botjlarjh, West Donegal. 227 



have been formed by tbc same processes ; and even banded gneisses 

 may in certain instances represent stratified materials crystallized 

 and modified in place. This would, at any rate, be the logical 

 deduction from the views of Mr. F. D, Adams/ which would bring 

 us back to some of the oldest and half -forgotten theories respecting 

 metamorphic rocks. Fascinating as the dynamometamoi-phic theory 

 has been, it may be questioned if strongly marked banding can be 

 produced in crystalline rocks by the agencies thereby invoked. 

 Mylonitic destruction, rather than banding, results, as a rule, from 

 earth-pressure combined with movement ; and the distinctions between 

 adjacent layers tend to become obliterated. Professor Judd,- in 1898, 

 called attention to the slow processes of " statical metamorphism," 

 whereby rocks which are kept stationary underground may be modi- 

 fied, not only in mineral constitution, but even in chemical composition. 

 Crystalline layers, their individual characters dependent on those of 

 the successive original strata, might thus eventually arise, and would 

 even produce a banded gneiss. In Boylagh, however, the phenomena 

 of igneous injection and intimate penetration have played by far the 

 most important part ; and there is no particular mystery as to the 

 mechanical or chemical nature of the process, the stages of which 

 are often traceable with the naked eye. 



"While believing with Levy and Lacrois that granite does not 

 come into its final position without a considerable absorption of 

 material from the walls of its caldron,^ I naturally admit, from 

 considerations of geological structure, that the caldron itself most 

 commonly originates in the arch of an anticlinal.* As Salomon^ 

 perceived in the case of the Adamello chain, the position where the 

 igneous rock ultimately manifests itself is determined by the oppor- 

 tunities allowed it during the larger movements of the crust. But are 



1 " Some recent papers on the Influence of Granitic Intrusions upon the 

 Development of Crystalline Schists," Journ. of Geology, vol. v. (1897), pp. 

 293-302. 



^ " On Statical and Dynamical Metamorphism," Geol. Mag., 1889, p. 243, etc. 

 This subject has been greatly developed by Van Hise, " Metamorphism of Rocks 

 and Eock-flowage," Bull. Geol. Soc. America, vol. ix. (1898), p. 269. 



^ Compare Lacroix, op. cit., Bull, de la Carte Geol. de la France, No. 64 

 (1898), pp. 1 and 62. 



* Compare T. A. Jaggar, jr., " The Laccoliths of the Black Hills," 21st Ann. 

 Rep. U.S. Geol. Survey, Pt. iii. (1901), p. 173. 



^ " Ueber Alter, Lagerungsform, imd Enstehungsart der periadriatischen 

 granitisch-kornigen Massen," Tscherm. Mittheil., Bd. xvii. (1898), pp. 173-4. 



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