ROCKS ALTERED BY GRANITE VEINS. 
559 
to conceive how such volcanic masses may, at a certain 
depth from the surface, pass downward into granite. 
Granitic Veins.—I have already hinted at the close analogy 
in the forms of certain granitic and trappean veins; and it 
will be found that strata penetrated by plutonic rocks have 
suffered changes very similar to those exhibited near the 
contact of volcanic dikes. Thus, in Glen Tilt, in Scotland, 
alternating strata of limestone and argillaceous schist come 
in contact with a mass of granite. The contact does not 
take place as might have been looked for if the granite had 
been formed there before the strata were deposited, in which 
case the section would have appeared as in Fig. 610; but 
the union is as represented in Fig. 611, the undulating out- 
Fig. 610. 
Fig. 611. 
Junction of granite and argillaceous schist in Glen Tilt. (MacCulloch.)* 
line of the granite intersecting different strata, and occasion¬ 
ally intruding itself in tortuous veins into the beds of clay- 
slate and limestone, from which it differs so remarkably in 
composition. The limestone is sometimes changed in char¬ 
acter by the proximity of the granitic mass or its veins, and 
acquires a more compact texture, like that of hornstone or 
chert, with a splintery fracture, and effervescing freely with 
acids. 
The conversion of the limestone in these and many other 
instances into a siliceous rock, effervescing slowly with acids, 
would be difficult of explanation, were it not ascertained 
that such limestones are always impure, containing grains of 
quartz, mica, or feldspar disseminated through them. The 
elements of these minerals, when the rock has been subjected 
to great heat, may have been fused, and so spread more uni¬ 
formly through the whole mass. 
In the plutonic, as in the volcanic rocks, there is every 
gradation from a tortuous vein to the most regular form of 
* Geol. Trans., First Series, vol. iii., pi. 21. 
