te more severe than that now
prevailing in the western hemisphere, to explain the geographical
distribution of most of the European erratics.

_Deluges._--As deluges have been often alluded to, I shall say something
of the causes which may be supposed to give rise to these grand
movements of water in addition to those already alluded to (p. 9).
Geologists who believe that mountain-chains have been thrown up
suddenly at many successive epochs, imagine that the waters of the
ocean may be raised by these convulsions, and then break in terrific
waves upon the land, sweeping over whole continents, hollowing out
valleys, and transporting sand, gravel, and erratics, to great
distances. The sudden rise of the Alps or Andes, it is said, may have
produced a flood even subsequently to the time when the earth became the
residence of man. But it seems strange that none of the writers who have
indulged their imaginations in conjectures of this kind, should have
ascribed a deluge to the sudden conversion of part of the unfathomable
ocean into a shoal rather than to the rise of mountain-chains. In the
latter case, the mountains themselves could do no more than displace a
certain quantity of atmospheric air, whereas, the instantaneous
formation of the shoal would displace a vast body of water, which being
heaved up to a great height might roll over and permanently submerge a
large portion of a continent.

If we restrict ourselves to combinations of causes at present known, it
would seem that the two principal sources of extraordinary inundations
are, first, the escape of the waters of a large lake raised far above
the sea; and, secondly, the pouring down of a marine current into lands
depressed below the mean level of the ocean.

As an example of the first of these cases, we may take Lake Superior,
which is more than 400 geographical miles in length and about 150 in
breadth, having an average depth of from 500 to 900 feet. The surface of
this vast body of fresh water is no less than 600 feet above the level
of the ocean; the lowest part of the barrier which separates the lake on
its southwest side from those streams which flow into the head waters of
the Mississippi being about 600 feet high. If, therefore, a series of
subsidences should lower any part of this barrier 600 feet, any
subsequent rending or depression, even of a few yards at a time, would
allow the sudden escape of vast floods of water into a hydrographical
basin of enor