rocks in a vigorous manner, cutting them down in proportion to
their hardness. As these rocks generally vary in the resistance which
they oppose to the ice, the result is that when the glacier passes
away the surface no longer exhibits the continued down slope which the
rivers develop, but is warped in a very complicated way. These
depressions afford natural basins in which lakes gather; they may vary
in extent from a few square feet to many square miles. When a glacier
occupies a country, the melting ice deposits on the surface of the
earth a vast quantity of rocky _debris_, which was contained in its
mass. This detritus is irregularly accumulated; in part it is disposed
in the form of moraines or rude mounds made at the margin of the
glacier, in part as an irregular sheet, now thick, now thin, which
covers the whole of the field over which the ice lay. The result of
this action is the formation of innumerable pools, which continue to
exist until the streams have cut channels through which their waters
may drain away, or the basins have become filled with detritus
imported from the surrounding country or by peat accumulations which
the plants form in such places.

Doubtless more than nine tenths of all the lake basins, especially
those of small size, which exist in the world are due to
irregularities of the land surface which are brought about by glacial
action. Although the greater part of these small basins have been
obliterated since the ice left this country, the number still
remaining of sufficient size to be marked on a good map is
inconceivably great. In North America alone there are probably over a
hundred and fifty thousand of these glacial lakes, although by far the
greater part of those which existed when the glacial sheet disappeared
have been obliterated.

Yet another interesting group of fresh-water lakes, or rather we
should call them lakelets from their small size, owes its origin to
the curious underground excavations or caverns which are formed in
limestone countries. The water enters these caverns through what are
termed "sink holes"--basins in the surface which slope gently toward a
central opening through which the water flows into the depths below.
The cups of the sink holes rarely exceed half a mile in diameter, and
are usually much smaller. Their basins have been excavated by the
solvent and cutting actions of the rain water which gathers in them to
be discharged into the cavern below. It oft