hed out by this action. Yet if the rocks be compact, or if they
have layers of a soft and clayey nature, we may find the construction
water, even in very old deposits, remaining near the surface of the
ground. Thus in the ancient Silurian beds of the Ohio Valley a boring
carried a hundred feet below the level of the main rivers commonly
discovers water which is clearly that laid down in the crevices of the
material at the time when the rocks were formed in the sea. In all
cases this water contains a certain amount of gases derived from the
decomposition of various substances, but principally from the
alteration of iron pyrite, which affords sulphuretted hydrogen. Thus
the water is forced to the surface with considerable energy, and the
well is often named artesian, though it flows by gas pressure on the
principle of the soda-water fountain, and not by gravity, as in the
case of true artesian wells.

The passage between the work done by the deeply penetrating surface
water and that due to the fluid intimately blended with the rock built
into the mass at the time of its formation is obscure. We are,
however, quite sure that at great depths beneath the earth the
construction water acts alone not only in making veins, but in
bringing about many other momentous changes. At a great depth this
water becomes intensely heated, and therefore tends to move in any
direction where a chance fissure or other accident may lessen the
pressure. Creeping through the rocks, and moving from zones of one
temperature to another, these waters bring about in the fine
interstices chemical changes which lead to great alterations in the
constitution of the rock material. It is probably in part to these
slow driftings of rock water that beds originally made up of small,
shapeless fragments, such as compose clay slates, sandstones, and
limestones, may in time be altered into crystalline rocks, where there
is no longer a trace of the original bits, all the matter having been
taken to pieces by the process of dissolving, and reformed in the
regular crystalline order. In many cases we may note how a crystal
after being made has been in part dissolved away and replaced by
another mineral. In fact, many of our rocks appear to have been again
and again made over by the slow-drifting waters, each particular state
in their construction being due to some peculiarity of temperature or
of mineral contents which the fluid held. These metamorphic phenomena,
tho