8 PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 
appear that the plates of the black mica have been loosened from the surfaces 
of the neres, and have become impacted in the adjacent substance of the 
granite, but have remained unresolved into smaller crystals; the metamorphism 
being arrested, or incomplete at these points. 
Of the metamorphism of this granite, however effected, the exfiltrative 
veins would appear to have been one of the last stages. 
How often the grey granite may have been depressed to the zone of 
metamorphism we cannot say, any more than we can how often the aquo- 
thermal agencies had to operate upon it; but, during some one of the sub- 
sidations, it is rational to conceive of its having been rent by pressure, and 
that subsequently—perchance forthwith—the rents were filled up, not by any 
sudden injection from their open terminations, but by a process of transfusion 
Jrom all points of the surface of the rent, a transfusion of the plastic or soluble 
matter of the rock itself—endosmose and exosmose exercising their resistless 
force. The resulting plug is thus—to use JaMEson’s words—“ a secretion from 
the rock ttself.” 
DAUBREE’S experiments have shown that in the presence of water a tem- 
perature of 400° C. sufficed for the alteration of silicates,—crystallisation 
of silica and of felspar,—or for the actual formation of the latter and of mica, 
through the action of alkaline silicates on argillaceous rocks. Considering the 
changes which would result from aquo-thermal action in the light of these 
experiments, and knowing that the repeated action of heated water upon the 
more highly siliceous rocks invariably results in the greater adhesion of water 
for the alkalies,—progressively abstracting them, to leave more and more 
highly-aluminous silicates,—it is not difficult to understand how the amount of 
mica in these exfiltrative or exudation veins should be in excess of its pro- 
portion in the parent rock. For, if the general mass of that rock be held in 
solution through aquo-thermal action,—or if not in actual solution, in a condi- 
tion favourable to chemical change, the rock upon solidification must yield a 
mass which is less alkaline as a whole; and as in the case of granite it is the 
orthoclase which is the alkali-bearing mineral, it is this which would suffer loss. 
The resolidifying material, after the watery abstraction of some of the alkali, 
could no longer attain to the production of so much felspar,—some of which 
would be degraded, so to speak, into an increase of the amount of mica.” 
Now there is what may be called a physical-outcome of this change. 
In virtue of this abstraction of alkali from orthoclase, and consequent 
increase of free silica and formation of mica, there results the production of a 
less alterable, that is, a more enduring material; and if it be the case that 
granite-veins contain more mica than does the rock which they intersect, these 
veins must necessarily be more enduring than the granite itself. 
* The alkali-charged waters are, however, potent, even at low temperatures, to change clay slates 
and argillaceous gneiss into granite, and so to extend the sphere of the metamorphism. 
