nderlaid by what we may call incipient lavas is
subjected to the peculiar compressive actions which lead to
mountain-building, we should naturally expect that such soft material
would be poured forth, possibly in vast quantities through fault
fissures, which are so readily formed in all kinds of rock when
subject to irregular and powerful strains, such as are necessarily
brought about when rocks are moved in mountain-making. The great
eruptions which formed the volcanic table-lands on the west coast of
North America appear to have owed the extrusion of their materials to
mountain-building actions. This seems to have been the case also in
some of those smaller areas where fissure flows occur in Europe. It is
likely that this action will explain the greater part of these massive
eruptions.

It need not be supposed that the rock beneath these countries, which
when forced out became lava, was necessarily in the state of perfect
fluidity before it was forced through the fissures. Situated at great
depth in the earth, it was under a pressure so great that its
particles may have been so brought together that the material was
essentially solid, though free to move under the great strains which
affected it, and acquiring temperature along with the fluidity which
heat induces as it was forced along by the mountain-building pressure.
As an illustration of how materials may become highly heated when
forced to move particle on particle, it may be well to cite the case
in which the iron stringpiece on top of a wooden dam near Holyoke,
Mass., was affected when the barrier went away in a flood. The iron
stringer, being very well put together, was, it is said, drawn out by
the strain until it became sensibly reddened by the motion of its
particles, and finally fell hissing into the waters below. A like
heating is observable when metal is drawn out in making wire. Thus a
mass of imperfectly fluid rock might in a forced journey of a few
miles acquire a decided increase of temperature.

Although the most striking volcanic action--all such phenomena,
indeed, as commonly receives the name--is exhibited finally on the
earth's surface, a great deal of work which belongs in the same group
of geological actions is altogether confined to the deep-lying rock,
and leads to the formation of dikes which penetrate the strata, but do
not rise to the open air. We have already noted the fact that dikes
abound in the deeper parts of volcanic cones, thou