rong northward-setting current
in that part of the ocean.

* * * * *

We have now to consider the causes which could bring about such great
extensions of the ice sheet as occurred in the last Glacial period.
Here again we are upon the confines of geological knowledge, and in a
field where there are no well-cleared ways for the understanding. In
facing this problem, we should first note that those who are of the
opinion that a Glacial period means a very cold climate in the regions
where the ice attained its extension are probably in error. Natural as
it may seem to look for exceeding cold as the cause of glaciation, the
facts show us that we can not hold this view. In Siberia and in the
parts of North America bordering on the Arctic Sea the average cold is
so intense that the ground is permanently frozen--as it is, for
instance, in the Klondike district--to the depth of hundreds of feet,
only the surface thawing out during the warm summers. All this region
is cold enough for glaciers, but there is not sufficient snowfall to
maintain them. On the other hand, in Greenland, and in a less though
conspicuous degree in Scandinavia, where the waters of the North
Atlantic somewhat diminish the rigour of the cold, and at the same
time bring about a more abundant snowfall, the two actions being
intimately related, we have very extensive glaciers. Such facts, which
could be very much extended, make it clear that the climate of glacial
periods must have been characterized by a great snowfall, and not by
the most intense cold.

It is evident that what would be necessary again to envelop the boreal
parts of North America with a glacial sheet would not be a
considerable decrease of heat, but an increase in the winter's
contribution of frozen water. Even if the heat released by this
snowfall elevated the average temperature of the winter, as it
doubtless would in a considerable measure, it would not melt off the
snow. That snowfall tends to warm the air by setting free the heat
which was engaged in keeping the water in a state of vapour is
familiarly shown by the warming which attends an ordinary snowstorm.
Even if the fall begin with a temperature of about 0 deg. Fahr., the air
is pretty sure to rise to near the freezing point.

It is evident that no great change of temperature is required in order
to bring about a very considerable increase in the amount of snowfall.
In the ordinary succ