rict find here and there evidences of this progressive
destruction. Not only do the caves wear out from above, but their
roofs are constantly falling to their floors, a process which is
greatly aided by the growth of stalactites. Forming in the crevices or
joints between the stones, these rock growths sometimes prize off
great blocks. In other cases the weight of the pendent stalactite
drags the ill-supported masses of the roof to the floor. In this way a
gallery originally a hundred feet below the surface may work its way
upward to the light of day. The entrance by which the Mammoth Cave is
approached appears to have been formed in this manner, and at several
points in that system of caverns the effect of this action may be
distinctly observed.

We must now go a step further on the way of subterranean water, and
trace its action in the depths below the plane of ordinary caves,
which, as we have noted, do not extend below the level of the main
streams of the cavern district. The first group of facts to be
attended to is that exhibited by artesian wells. These occur where
rocks have been folded down into a basinlike form. It often happens
that in such a basin the rocks of which it is composed are some of
them porous, and others impervious to water, and that the porous
layers outcrop on the high margins of the depression and have
water-tight layers over them. These conditions can be well represented
by supposing that we have two saucers, one within the other, with an
intervening layer of sand which is full of water. If now we bore an
opening in the bottom of the uppermost saucer, we readily conceive
that the water will flow up through it. In Nature we often find these
basins with the equivalent of the sandy layer in the model just
described rising hundreds of feet above the valley, so that the
artesian well, so named from the village of Artois, near Paris, where
the first opening of this nature was made, may yield a stream which
will mount upward, especially where piped, to a great height. At many
places in the world it is possible by such wells to obtain a large
supply of tolerably pure water, but in general it is found to contain
too large a supply of dissolved mineral matter or sulphuretted gases
to be satisfactory for domestic purposes. It may be well to note the
fact that the greater part of the so-called artesian wells, or borings
which deliver water to a height above the surface, are not true
artesian sources, in