566 
ELEMENTS OF GEOLOGY. 
depth, a much longer period. The melted matter poured 
from Jorullo, in Mexico, in the year 175-9, which accumulated 
in some places to the height of 550 feet, was found to retain 
a high temperature half a century after the eruption.* We 
may conceive, therefore, that great masses of subterranean 
lava may remain in a red-hot or incandescent state in the 
volcanic foci for immense periods, and the process of refrig ¬ 
eration may be extremely gradual. Sometimes, indeed, this 
process may be retarded for an indefinite period by the ac¬ 
cession of fresh supplies of heat; for we find that the lava in 
the crater of Stromboli, one of the Lipari Islands, has been in 
a state of constant ebullition for the last two thousand years; 
and we may suppose this fluid mass to communicate with 
some caldron 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, the lava below 
can scarcely fail to have been permanently in a state of liq¬ 
uefaction. If then it be a reasonable conjecture, that about 
2000 volcanic eruptions occur in the course of every century, 
either above the waters of the sea or beneath them,f it will 
follow that the quantity of plutonic rock generated or in prog¬ 
ress during the Recent epoch must already have been con¬ 
siderable. 
But as the plutonic rocks originate at some depth in the 
earth’s crust, they can only be rendered accessible to human 
observation by subsequent upheaval and denudation. Be¬ 
tween the period when a plutonic rock crystallizes in the 
subterranean regions and the era of its protrusion at any 
single point of the surface, one or two geological periods 
must usually intervene. Hence, we must not expect to find 
the Recent or even the Pliocene granites laid open to view, 
unless we are prepared to assume that sufficient time has 
elapsed since the commencement of the Pliocene period for 
great upheaval and denudation. A plutonic rock, therefore, 
must, in general, be of considerable antiquity relatively to 
the fossiliferous and volcanic formations, before it becomes 
extensively visible. As we know that the upheaval of land 
has been sometimes accompanied in South America by vol¬ 
canic eruptions and the emission of lava, we may conceive 
the more ancient plutonic rocks to be forced upward to the 
surface by the newer rocks of the same class formed succes¬ 
sively below—subterposition in the plutonic, like superposi¬ 
tion in the sedimentary rocks, being usually characteristic 
of a newer origin. 
* See “Principles,” Index^ “Jorullo.” 
t Ibid.. “Volcanic Eruptions.” 
