he geological monuments alluded
to, consist of patches of sand and gravel, forty feet thick, containing
fluviatile shells of the genera Unio, Cyclas, Melania, &c., such as now
inhabit the waters of the Niagara above the Falls. The identity of the
fossil species with the recent is unquestionable, and these freshwater
deposits occur at the edge of the cliffs bounding the ravine, so that
they prove the former extension of an elevated shallow valley, four
miles below the falls, a distinct prolongation of that now occupied by
the Niagara, in the elevated region intervening between Lake Erie and
the Falls. Whatever theory be framed for the hollowing out of the ravine
further down, or for the three miles which intervene between the
whirlpool and Queenstown, it will always be necessary to suppose the
former existence of a barrier of _rock_, not of loose and destructible
materials, such as those composing the drift in this district, somewhere
immediately below the whirlpool. By that barrier the waters were held
back for ages, when the fluviatile deposit, 40 feet in thickness, and
250 feet above the present channel of the river, originated. If we are
led by this evidence to admit that the cataract has cut back its way for
four miles, we can have little hesitation in referring the excavation of
the remaining three miles below to a like agency, the shape of the chasm
being precisely similar.

There have been many speculations respecting the future recession of the
Falls, and the deluge that might be occasioned by the sudden escape of
the waters of Lake Erie, if the ravine should ever be prolonged 16 miles
backwards. But a more accurate knowledge of the geological succession of
the rocks, brought to light by the State Survey, has satisfied every
geologist that the Falls would diminish gradually in height before they
travelled back two miles, and in consequence of a gentle dip of the
strata to the south, the massive limestone now at the top would then be
at their base, and would retard, and perhaps put an effectual stop to,
the excavating process.

CHAPTER XV.

TRANSPORTATION OF SOLID MATTER BY ICE.

Carrying power of river-ice--Rocks annually conveyed into the St.
Lawrence by its tributaries--Ground-ice; its origin and transporting
power--Glaciers--Theory of their downward movement--Smoothed and
grooved rocks--The moraine unstratified--Icebergs covered with mud
and stones--Limits of glaciers and iceberg