egrated by the action of water. We find the spring water
impregnated with it as well as that of the small streams and rivers.
Pure water is a powerful solvent. When the rains fall upon the earth the
water percolates through it and through the limestone strata, which
gradually wears away the limestone and carries it back to the ocean, so
that the process of tearing down and building up is continually going
on. The great caves that are found everywhere in the limestone regions
were formed by the action of water. The great Mammoth Cave of Kentucky,
which is said to have 200 miles of underground passages, has been
entirely worn out by the action of running water.

Some years ago the writer visited this cave and had an opportunity to
study the wonderful eroding or gnawing-out effect of water on limestone.
At some period earlier in the history of the earth there was evidently
an underground river or large stream of water that found its way through
the crevices of the rocks, and gradually wore out a great bed for
itself, which was fed by lateral streams pouring into the main branch,
each one of which lateral branches cut its own channel. A plan view of
the Mammoth Cave presents a picture not unlike that of a great river
with numerous branches emptying into it, all of them showing the
windings such as we see in a river and its feeders upon the surface of
the earth. There are three sets of these channels, one above the other,
and we do not find the water till we get to the bottom of the third
underground story, so to speak. There is one place in this system of
underground channels where the dripping from the roof of the upper
channels has cut a great well hole many feet in diameter perpendicularly
down through the whole system to a great depth. The sides of this great
well hole are fluted into grooves caused by the constant downflow of the
water. Although the amount of water flowing down through this well hole
is very small, it is continually at work. Like interest on money, it
never rests, each minute that passes has eaten away some of the great
rock.

In other portions of the cave the dripping of the water is so gradual
that the carbonate of lime hardens and forms what are called
stalactites, that hang like icicles from the roof of the cave. Sometimes
the water runs down so slowly upon these stalactites that it evaporates
as fast as it appears, leaving behind its little load of carbonate of
lime. If, however, there is a drip,