ill be
but a brief geologic period before they begin to lower the waters of
Lake Erie. It is very probable, indeed, that in twenty thousand years
the waters of that basin will be to a great extent drained away. When
this occurs, another fall or rapid will be produced in the channel
which leads from Lake Huron to Lake Erie. This in turn will go through
its process of retreat until the former expanse of waters disappears.
The action will then be continued at the outlets of Lakes Michigan and
Superior, and in time, but for the interposition of some actions which
recreate these basins, their floors will be converted into dry land.

It is interesting to note that lakes owe in a manner the preservation
of their basins to an action which they bring about on the waters that
flow into them. These rivers or torrents commonly convey great
quantities of sediment, which serve to rasp their beds and thus to
lower their channels. In all but the smaller lakelets these turbid
waters lay down all their sediment before they attain the outlet of
the basin. Thus they flow away over the rim rock in a perfectly pure
state--a state in which, as we have noted before, water has no
capacity for abrading firm rock. Thus where the Niagara River passes
from Lake Erie its clean water hardly affects the stone over which it
flows. It only begins to do cutting work where it plunges down the
precipice of the Falls and sets in motion the fragments which are
constantly falling from that rocky face. These Falls could not have
begun as they did on the margin of Lake Ontario except for the fact
that when the Niagara River began to flow, as in relatively modern
times, it found an old precipice on the margin of Lake Ontario, formed
by the waves of the lake, down which the waters fell, and where they
obtained cutting tools with which to undermine the steep which forms
the Falls.

Many great lakes, particularly those which we have just been
considering, have repeatedly changed their outlets, according as the
surface of the land on which they lie has swayed up and down in
various directions, or as glacial sheets have barred or unbarred the
original outlets of the basins. Thus in the Laurentian Lakes above
Ontario the geologist finds evidence that the drainage lines have
again and again been changed. For a time during the Glacial period,
when Lake Ontario and the valley of the St. Lawrence was possessed by
the ice, the discharge was southward into the upper Missi