Professor Dana estimates the thickness of the ice in
southern Connecticut to have been between fifteen hun-
dred and two thousand feet. Attempts to calculate the
thickness of the ice farther north, except from actual dis-
covery of glacial action on the summits of the mountains,
are based upon uncertain data with reference to the slope
necessary to secure glacial movement. In the Alps the
lowest mean slopes down which glaciers move are about
two hundred and fifty feet to a mile ; but in Greenland,
Jensen found the slope of the Frederickshaab Glacier to
be only seventy-five feet to the mile, while Helland found
that of the Jakobshavn Glacier to be only forty-five feet.
It is doubtful if even that amount is necessary to se-
cure a continental movement of ice, since, as already re-
marked, it is unsafe to draw inferences concerning the
movements of large masses of ice from those of smaller
masses in more constricted areas. We have seen, from
the glacial deposits on the top of Mount Washington,
that over the northern part of New England the ice was
more than a mile in depth. We have no direct evidence
of the depth of the stream which surrounded the Adiron-
dack Mountains. Nor, on the other hand, are we certain
that the Catskills were not completely enveloped in ice,
though most observers, reasoning from negative evidence,
have supposed that to be the case. But from the facts
stated concerning the boulders along the glacial boundary

* American Journal of Science, vol. cxxv, 1883, p. 339 et seq.
92 MAN AND THE GLACIAL PERIOD.
