en happens that after a
sink hole is formed some slight accident closes the downward-leading
shaft, so that the basin holds water; thus in parts of the United
States there are thousands of these nearly circular pools, which in
certain districts, as in southern Kentucky, serve to vary the
landscape in much the same manner as the glacial lakes of more
northern countries.

Some of the most beautiful lakes in the world, though none more than a
few miles in diameter, occupy the craters of extinct volcanoes. When
for a time, or permanently, a volcano ceases to do its appointed work
of pouring forth steam and molten rock from the depths of the earth,
the pit in the centre of the cone gathers the rain water, forming a
deep circular lake, which is walled round by the precipitous faces of
the crater. If the volcano reawakens, the water which blocks its
passage may be blown out in a moment, the discharge spreading in some
cases to a great distance from the cone, to be accumulated again when
the vent ceases to be open. The most beautiful of these volcanic lakes
are to be found in the region to the north and south of Rome. The
original seat of the Latin state was on the shores of one of these
crater pools, south of the Eternal City. Lago Bolsena, which lies to
the northward, and is one of the largest known basins of this nature,
having a diameter of about eight miles, is a crater lake. The volcanic
cone to which it belongs, though low, is of great size, showing that
in its time of activity, which did not endure very long, this crater
was the seat of mighty ejections. The noblest specimen of this group
of basins is found in Crater Lake, Oregon, now contained in one of the
national parks of the United States.

Inclosed bodies of water are formed in other ways than those
described; the list above given includes all the important classes of
action which produce these interesting features. We should now note
the fact that, unlike the seas, the lakes are to be regarded as
temporary features in the physiography of the land. One and all, they
endure for but brief geologic time, for the reason that the streams
work to destroy them by filling them with sediment and by carving out
channels through which their waters drain away. The nature of this
action can well be conceived by considering what will take place in
the course of time in the Great Lakes of North America. As Niagara
Falls cut back at the average rate of several feet a year, it w