gauge matters of man's history,
this process was doubtless slow. There are reasons, however, to
believe that the coming and going were, in a geological sense, swift;
they may have, indeed, been for a part of the time of startling
rapidity. Going back to the time of geological yesterday, before the
ice began its development in the northern hemisphere, all the evidence
we can find appears to indicate a temperate climate extending far
toward the north pole. The Miocene deposits found within twelve
degrees, or a little more than seven hundred miles, of the north pole,
and fairly within the realm of lowest temperature which now exists on
the earth, show by the plant remains which they contain that the
conditions permitted the growth of forests, the plants having a
tolerably close resemblance to those which now freely develop in the
southern portion of the Mississippi Valley. Among them there are
species which had the habit of retaining their broad, rather soft
leaves throughout the winter season. The climate appears, in a word,
to have been one where the mean annual temperature must have been
thirty degrees or more higher than the present average of that realm.
Although such conditions near the sea level are not inconsistent with
the supposition that glaciers existed in the higher mountains of the
north, they clearly deny the possibility of the realm being occupied
by continental glaciers.

Although the Pliocene deposits formed in high latitudes have to a
great extent been swept away by the subsequent glacial wearing, they
indicate by their fossils a climatal change in the direction of
greater cold. We trace this change, though obscurely, in a
progressive manner to a point where the records are interrupted, and
the next interpretable indication we have is that the ice sheet had
extended to somewhere near the limits which we have noted. We are then
driven to seek what we can concerning the sojourn of the ice on the
land by the amount of wearing which it has inflicted upon the areas
which it occupied. This evidence has a certain, though, as we shall
see, a limited value.

When the students of glacial action first began the great task of
interpreting these records, they were led to suppose that the amount
of rock cutting which was done by the ice was very great. Observing
what goes on, in the manner we have noted, beneath a valley glacier
such as those of Switzerland, they saw that the ice work went on
rapidly, and concluded