which penetrates deeply into the earth.

Almost all rocks, however firm they may appear to be, are divided by
crevices which extend from the soil level it may be to the depths of
thousands of feet. These rents are in part due to the strains of
mountain-building, which tend to disrupt the firmest stone, leaving
open fractures. They are also formed in other ways, as by the
imperfectly understood agencies which produce joint planes. It often
happens that where rocks are highly tilted water finds its way
downward between the layers, which are imperfectly soldered together,
or a bed of coarse material, such as sandstone or conglomerate, may
afford an easy way by which the water may descend for miles beneath
the surface. Passing through rocks which are not readily soluble, the
water, already to a great extent supplied with mineral matter by its
journey through the soil, may not do much excavating work, and even
after a long time may only slightly enlarge the spaces in which it
may be stored or the channels by which it discharges to the surface.
Hence it comes about that in many countries, even where the waters
penetrate deeply, they do not afford large springs. It is otherwise
where the crevice waters enter limestones composed of materials which
are readily dissolved. In such places we find the rain so readily
entering the underlying rock that no part of the fall goes at once to
the brooks, but all has a long underground journey.

In any limestone district where the beds of the material are thick and
tolerably pure--as, for instance, in the cavern district of southern
Kentucky--the traveller who enters the region notes at once that the
usual small streams which in every region of considerable rainfall he
is accustomed to see intersecting the surface of the country are
entirely absent. In their place he notes everywhere pitlike