Ch. XXV.] 
ORIGIN OF GRANITE. 
363 
dates slowly under so enormous a pressure, may be supposed 
to acquire a very distinct texture and become granite. 
If it be objected that we do not find in mountain-chains vol- 
canic dikes passing upwards into lava, and downwards into 
granite, we may answer that our vertical sections are usually 
of small extent, and it is enough that we find in certain loca- 
lities a transition from trap to porous lava, and in others a 
passage from granite to trap. It should also be remembered, 
that a large proportion of the igneous rocks, when first formed, 
cannot be supposed to reach the surface, and these may assume 
the usual granitic texture without graduating into trap, or into 
such lava and scoriae as are found on the flanks of a volcanic 
cone. 
Theory of the origin of granite at all periods.— It is not 
uncommon for lava-streams to require more than ten years to 
cool in the open air, and a much longer period where they 
are of great depth. The melted matter poured out from 
Jorullo, in Mexico, in the year 1759, which accumulated in 
some places to the height of 550 feet, was found to retain a 
high temperature half a century after the eruption*. For 
what immense periods, then, must we not conclude that great 
masses of subterranean lava in the volcanic foci may remain in 
a red hot or incandescent state, and how gradual must be the 
process of refrigeration ! This process may be sometimes 
retarded for an indefinite period, by the accession of fresh 
supplies of heat, for we find that the lava in the crater of Strom- 
boli, one of the Lipari islands, has been in a state of constant 
ebullition for the last 2000 years, and we must suppose this 
fluid mass to communicate with some cauldron or reservoir of 
fused matter below. In the Isle of Bourbon, also, where there 
has been an emission of lava once in every two years for a long 
period, we may infer that the lava below is permanently in a 
state of liquefaction. 
When melted matter is injected into the fissures of a con- 
tiguous rock at a considerable depth, it may cool rapidly if that 
rock has not acquired a high temperature ; but suppose, on he 
* See vol. i. p. 378, and Second Edition, p. 433. 
