t, we discover a
means whereby it could have been retained in the liquid condition,
even when forced for long distances through very narrow channels.
Moreover, this explanation accounts for the fact which has long
remained unexplained that dikes, except those formed about volcanic
craters, rarely, if ever, rise to the surface.

The materials contained in dikes differ exceedingly in their chemical
and mineral character. These variations are due to the differences in
Nature of the deposits whence they come, and also in a measure to
exchanges which take place between their own substance and that of the
rocks between which they are deposited. This process often has
importance of an economic kind, for it not infrequently leads to the
formation of metalliferous veins or other aggregations of ores, either
in the dike itself or in the country rock. The way in which this is
brought about may be easily understood by a familiar example. If flesh
be placed in water which has the same temperature, no exchange of
materials will take place; but if the water be heated, a circulation
will be set up, which in time will bring a large part of the soluble
matter into the surrounding water. This movement is primarily
dependent on differences of temperature, and consequently differences
in the quantity of soluble substances which the water seeks to take
up. When a dike is injected into cooler rocks, such a slow circulation
is induced. The water contained in the interstices of the stone
becomes charged with mineral materials, if such exist in positions
where it can obtain possession of them, and as cooling goes on, these
dissolved materials are deposited in the manner of veins. These veins
are generally laid down on the planes of contact between the two kinds
of stone, but they may be formed in any other cavities which exist in
the neighbourhood. The formation of such veins is often aided by the
considerable shrinkage of the lava in the dike, which, when it cools,
tends to lose about fifteen per cent of its volume, and is thus likely
to leave a crevice next the boundary walls. Ores thus formed afford
some of the commonest and often the richest mineral deposits. At
Leadville, in Colorado, the great silver-bearing lodes probably were
produced in this manner, wherein lavas, either those of dikes or those
which flowed in the open air, have come in contact with limestones.
The mineral materials originally in the once molten rock or in the
limy bed