3 6o SCIENCE PROGRESS 



Is it this fact that led Humboldt to the observation, quoted 

 by Scrope, 1 that the older — i.e. the lower — granites contain least 

 mica ? Fantastic as the statement may appear as a generalisa- 

 tion, I confess that I believe that almost all our granites with 

 two micas, and the amphibole granites practically without 

 exception, result from processes of admixture in the crust, and 

 that the true subterranean granite magma, if such is to be 

 defined, is near in composition to the rock known as aplite, and 

 would crystallise out as a mixture of quartz and acid felspar, 

 with far less mica than we see in the masses that are ordinarily 

 exposed by denudation, Against this we must set Daly's view 

 that the really pure igneous rocks are the basic ones, which move 

 rapidly and thus gather little by the way, and that pure granites 

 may be formed from the fusion of quartzose sediments, any 

 basic material associated with them becoming drained off, by 

 gravitational differentiation, into the depths. 



Here it is indeed time to pause, for our hillside in Donegal 

 has led us far afield. Let us come back from speculation to 

 the sober facts on the surface of this pleasant land. The 

 sunlight gleams on the micaceous rocks above Gartan Lough, 

 and picks out each coarsely crystalline patch that juts forth, 

 well foliated, from the decaying surface of the gneiss. This 

 gneiss is full of muscovite, with a foliation-structure parallel to 

 that of the inclusions of muscovite schist. The granitoid rock 

 and the schist are indeed inextricably involved ; the gneiss is 

 composite beyond a question. This possibility must be faced in 

 every area where " fundamental " masses, affected by serious 

 crushing, are at present asserted to exist. If hundreds of such 

 masses are then found wanting, on a revision of the evidence 

 in the field, it will only increase the interest of those regions 

 which have successfully withstood the test. 



1 Considerations on Volcanos, (1825), p. 233. 



