o occur in a granitic vein passing
through strata of gneiss.

In all these cases we cannot fail to see evidence of igneous eruptions,
taking place, however, under circumstances widely different from those
of our present terrestrial volcanoes, or of the submarine craters of
more remote dates, but which can be readily explained by supposing,
either that the penetration took place when the surface of the earth was
so intensely heated as to admit of the injected veins being slowly
cooled, and therefore more perfectly crystallized; or that the issuing
mass was so great as to retain its heat for a great length of time.

It might at first sight appear difficult to explain how volcanic
energies should still continue in activity, now that the mean
temperature of the earth has become constant, and the outer crust can be
no longer subject to the shrinking, and consequent cracking which it
must have undergone while cooling. The phenomena that attend volcanic
eruptions furnish a full explanation of this, for they are attended in
almost all cases with the evolution of great quantities of gaseous
matters, and steam, which must therefore exist in a state of intense
compression, and at elevated temperatures, in the mass whence the
volcanic flood issues. Their elastic energies are sufficient to account
for all the striking effects that attend the action of volcanoes.

The earthquake is a phenomenon connected with volcanic eruptions, and
arising from the same great cause; but while the latter are confined to
certain mountains, and restricted within narrow limits at the present
day, an earthquake is sometimes found to prevail over a very large
portion of the earth's surface. To omit the more usual phenomena of
earthquakes, we shall speak of but one, which has in some cases been
observed, that throws a great light upon the manner in which the
stratified rocks have had their levels changed, and been dislocated and
distorted in the manner we now find them. We allude to the sudden
raising of countries of greater or less extent. Of this we shall quote
three several instances from a paper of Arago's.

"During the night of the 28th September 1759, a district of
three or four square miles, situated in the Intendency of
Valladolid, in Mexico, was raised up, like an inflated bladder.
The limits where the elevation ceased may still be determined
at the present day, by the fracture of the strata. At these
limits the e