tain an
elevation of 40 or 50 feet above the present level. As the heaviest
gales of wind do not raise the waters more than three or four feet, the
elevated beaches have by some been referred to the subsidence of the
lake at former periods, in consequence of the wearing down of its
barrier; by others to the upraising of the shores by earthquakes, like
those which have produced similar phenomena on the coast of Chili.

The streams which discharge their waters into Lake Superior are several
hundred in number, without reckoning those of smaller size; and the
quantity of water supplied by them is many times greater than that
discharged at the Falls of St. Mary, the only outlet. The evaporation,
therefore, is very great, and such as might be expected from so vast an
extent of surface. On the northern side, which is encircled by primary
mountains, the rivers sweep in many large boulders with smaller gravel
and sand, chiefly composed of granitic and trap rocks. There are also
currents in the lake in various directions, caused by the continued
prevalence of strong winds, and to their influence we may attribute the
diffusion of finer mud far and wide over great areas; for by numerous
soundings made during Captain Bayfield's survey, it was ascertained that
the bottom consists generally of a very adhesive clay, containing shells
of the species at present existing in the lake. When exposed to the air,
this clay immediately becomes indurated in so great a degree, as to
require a smart blow to break it. It effervesces slightly with diluted
nitric acid, and is of different colors in different parts of the lake;
in one district blue, in another red, and in a third white, hardening
into a substance resembling pipeclay.[338] From these statements, the
geologist will not fail to remark how closely these recent lacustrine
formations in America resemble the tertiary argillaceous and calcareous
marls of lacustrine origin in Central France. In both cases many of the
genera of shells most abundant, as Limnea and Planorbis, are the same;
and in regard to other classes of organic remains there must be the
closest analogy, as I shall endeavor more fully to explain when speaking
of the imbedding of plants and animals in recent deposits.

DELTAS OF INLAND SEAS.

Having thus briefly considered some of the lacustrine deltas now in
progress, we may next turn our attention to those of inland seas.

_Course of the Po._--The Po affords an instructiv