Cotre— On the Geology of Slieve Gallion, in the County of Londonderry. 241 
relation to the rocks locally in contact with it, were insisted on by Portlock and 
other geologists of his day; and, without reverting to the old and once natural 
view of the direct metamorphic origin of granite, may we not come to regard our 
complex plutonic rocks as in themselves phenomena of contact? The underlying 
magmas, hidden from us, may be of far simpler composition than we commonly 
suppose; and the so-called fundamental rocks of the earth’s crust may be due to 
commingling in what are, after all, its merest surface-layers. Prof. Sollas’s* state- 
ment that ‘‘the heated material of the interior is already in all probability in a 
high state of differentiation” seems to me an expression of belief in the simplicity 
of fundamental magmas, and affords strong support to the view above put 
forward. 
Against this view, however, it may be urged, that the constitution of meteorites 
seems to give us a clue to the internal constitution of the earth, and that these 
bodies are often composed of complex silicates. Yet the great ‘‘ holosiderites,”’ 
consisting of alloys of metallic iron and nickel, point in the opposite direction, 
and indicate the existence of magmas of almost elementary composition. It seems, 
therefore, worth considering whether the mineral complexity of a plutonic rock, 
as we know it near the earth’s surface, does not depend upon the number of times 
that it has been remelted and brought into new chemical environments; or else 
upon the number of times that it has been permeated by new magmas arriving 
from below. In the former case I picture it as absorbing something round about 
it; in the latter case, as being itself practically absorbed. 
The late Mr. Hugh Millert recently observed granites in Upper Strath Brora, 
in Sutherland, which replaced, in his opinion, a group of schists, while preserving 
the structural characters of those rocks. ‘‘ Parts of these granites,” he wrote, 
‘Care, in fact, pseudomorphs, or granite-casts, preserving, as replacement-struc- 
tures, remains of the structure of the pre-existing rock.” Messrs. Horne and 
Greenly’s own paper, cautious as it is, opens up afresh the old field of progressive 
metamorphism. By combining the evidence that we possess as to local absorption 
with that urged in favour of differentiation within molten plutonic masses, we 
may arrive at the following possibilities :— 
(2.) A molten magma may attack a pre-existing and usually overlying series 
with conciseness and lucidity, in 1890 (‘‘ Compte-rendu de l’Excursion du 17 septembre 4 Aydat et a 
Murols,” Bull. soc. géol. de France, 3me sér,. t. xviii., p. 915); and he further emphasised his conclusions 
in 1896 (‘ Sur quelques particularités de gisement du porphyre bleu de l’Esterel ; application aux récentes 
théories sur les racines granitiques et sur la differenciation des magmas éruptifs,” ibid., t. xxiv., pp. 124, 
126, and 137). On p. 137 he writes: ‘‘On constate frequemment . . . une action endomorphe subie par 
les roches granitiques, qui se chargent de certains éléments de leurs salbandes.”’ 
* “ Relation of Granite to Gabbro at Barnayave,”’ Trans. R. I. Acad., vol. xxx. (1894), p. 509. 
+ Quoted by Horne and Greenly, ‘‘ On foliated granites and their relations to the crystalline schists in 
Eastern Sutherland,” Quart. Journ. Geol. Soc. Lond., vol. lii. (1896), p. 635. 
