Drift go upon the hypothesis that it was produced by extraordinary
masses of ice--ice as icebergs, ice as glaciers, or ice in
continental sheets. The scientists admit that immediately preceding
this Glacial age the climate was mild and equable, and these great
formations of ice did not exist. But none of them pretend to say how
the ice came or what caused it. Even Agassiz, the great apostle of
the ice-origin of Drift, is forced to confess:

"We have, as yet, no clew to the source of this great and _sudden_
change of climate. Various suggestions have been made--among others,
that formerly the inclination of the earth's axis was greater, or
that a submersion of the continents under water might have produced a
decided increase of cold; but none of these explanations are
satisfactory, and science has yet to find any cause which accounts
for all the phenomena connected with it."[1]

Some have imagined that a change in the position of the earth's axis
of rotation, due to the elevation of extensive mountain-tracts
between the poles and the equator, might have caused a degree of cold
sufficient to produce the phenomena of the Drift; but Geikie says--

"It has been demonstrated that the protuberance of the earth at the
equator so vastly exceeds that of any

[1. "Geological Sketches," p. 210.]

{p. 59}

possible elevation of mountain-masses between the equator and the
poles, that any slight changes which may have resulted from such