362 
GRANITE AND TRAP. 
[Ch. XXV. 
nating the volcanic from the plutonic rocks is sufficiently 
great ; for we must draw an arbitrary line between them, there 
being an insensible passage from the most common forms of 
granite into trap or lava. 
* The ordinary granite of Aberdeenshire/ says Dr. Mac- 
cull och, 4 is the usual ternary compound of quartz, felspar, and 
mica, but sometimes hornblende is added to these, or the 
hornblende is substituted for the mica. But in many places 
a variety occurs which is composed simply of felspar and 
hornblende,, and in examining more minutely this duplicate 
compound, it is observed in some places to assume a fine grain, 
and at length to become undistinguishable from the greenstones 
of the trap family. It also passes in the same uninterrupted 
manner into a basalt, and at length into a soft claystone, with a 
schistose tendency on exposure, in no respect differing from 
those of the trap islands of the western coast*.' The same 
author mentions, that in Shetland a granite composed of horn- 
blende, mica, felspar, and quartz, graduates in an equally 
perfect manner into basalt j-. 
It would be easy to multiply examples to prove that the 
granitic and trap-rocks pass into each other, and are merely 
different forms which the same elements have assumed according 
to the different circumstances under which they have consoli- 
dated from a state of fusion. What we have said respecting the 
mode of explaining the different texture of the central and ex- 
ternal parts of the Vesuvian dikes may enable the reader in some 
measure to comprehend how such differences may originate t. 
The same lava which is porous where it has flowed over 
from the crater, and where it has cooled rapidly and under 
comparatively slight pressure, is compact and porphyritic in 
the dike. Now these dikes are evidently the channels of com- 
munication between the crater and the volcanic foci below; so 
that we may suppose them to be continuous to the depth of 
several hundred fathoms, or perhaps two or three miles, or even 
more ; and the fluid matter below, which cools and consoli- 
* Syst. of Geol., vol. i. p. 157. f Ibid., p. 158. 
| See above, p. 124. 
