Geikie's (and the American) classification.

Before we consider the biological effect of this great of refrigeration
of the globe, we must endeavour to understand the occurrence itself.
Here we enter a world of controversy, but a few suggestions at least may
be gathered from the large literature of the subject, which dispel much
of the mystery of the Great Ice-Age.

It was at one time customary to look out beyond the earth itself for the
ultimate causes of this glaciation. Imagine the sheet of ice, which now
spreads widely round the North Pole, shifted to another position on
the surface of the planet, and you have a simple explanation of the
occurrence. In other words, if we suppose that the axis of the earth
does not consistently point in one direction--that the great ball does
not always present the same average angle in relation to the sun--the
poles will not always be where they are at present, and the Pleistocene
Ice-Age may represent a time when the north pole was in the latitude
of North Europe and North America. This opinion had to be abandoned. We
have no trace whatever of such a constant shifting of the polar regions
as it supposes, and, especially, we have no trace that the warm zone
correspondingly shifted in the Pleistocene.

A much more elaborate theory was advanced by Dr. Croll, and is still
entertained by many. The path of the earth round the sun is not
circular, but elliptical, and there are times when the gravitational
pull of the other planets increases the eccentricity of the orbit. It
was assumed that there are periods of great length, separated from each
other by still longer periods, when this eccentricity of the orbit
is greatly exaggerated. The effect would be to prolong the winter and
shorten the summer of each hemisphere in turn. The total amount of heat
received would not alter, but there would be a long winter with less
heat per hour, and a short summer with more heat. The short summer would
not suffice to melt the enormous winter accumulations of ice and snow,
and an ice-age would result. To this theory, again, it is objected that
we do not find the regular succession of ice-ages in the story of the
earth which the theory demands, and that there is no evidence of an
alternation of the ice between the northern and southern hemispheres.

More recent writers have appealed to the sun itself, and supposed that
some prolonged veiling of its photosphere greatly reduced the amount of
heat emitt