rigin of these large
troughs, holding such immense sheets of fresh water, remains still the
subject of discussion and investigation among geologists. It has been
supposed that in the primitive configuration of the globe, when the
formation of those depressions at the poles in which the Arctic seas are
accumulated gave rise to a corresponding protrusion at the equator, the
curve thus produced throughout the North Temperate Zone may have forced
up the Canada granite, and have caused, at the same time, those rents
in the earth's surface now filled by the Canada lakes; and this view
is sustained by the fact that there is a belt of lakes, among which,
however, the Canada lakes are far the largest, all around the world in
that latitude. The geological phenomena connected with all these lakes
have not, however, been investigated with sufficient accuracy and
detail, nor has there been any comparison of them extensive and
comprehensive enough to justify the adoption of any theory respecting
their origin. In an excursion to Lake Superior, some years since, I
satisfied myself that the position and outline of that particular lake
had their immediate cause in several distinct systems of dikes which
intersect its northern shore, and have probably cut up the whole tract
of rock over the space now filled by that wonderful sheet of fresh water
in such a way as to destroy its continuity, to produce depressions, and
gradually create the excavation which now forms the basin of the lake.
How far the same causes have been effectual in producing the other large
lakes I am unable to say, never having had the opportunity of studying
their formation with the same care.

The existence of the numerous smaller lakes running north and south in
the State of New York, as the Canandaigua, Seneca, Cayuga, etc., is more
easily accounted for. Slow and gradual as was the process by which
all that region was lifted above the ocean, it was, nevertheless,
accompanied by powerful dislocations of the stratified deposits, as we
shall see when we examine them with reference to the local phenomena
connected with them. To these dislocations of the strata we owe the
transverse cracks across the central part of New York, which needed
only the addition of the fresh water poured into them by the rains to
transform them into lakes.

I shall not attempt any account of the differences between the animals
of the Devonian period and those of the Silurian period, because th