ed by it. More recently still it has been suggested that an
accumulation of cosmic or meteoric dust in our atmosphere, or between
us and the sun, had, for a prolonged period, the effect of a colossal
"fire-screen." Neither of these suppositions would explain the
localisation of the ice. In any case we need not have recourse to purely
speculative accidents in the world beyond until it is clear that there
were no changes in the earth itself which afford some explanation.

This is by no means clear. Some writers appeal to changes in the ocean
currents. It is certain that a change in the course of the cold and
warm currents of the ocean to-day might cause very extensive changes
of climate, but there seems to be some confusion of ideas in suggesting
that this might have had an equal, or even greater, influence in former
times. Our ocean currents differ so much in temperature because the
earth is now divided into very pronounced zones of climate. These zones
did not exist before the Pliocene period, and it is not at all clear
that any redistribution of currents in earlier times could have had such
remarkable consequences. The same difficulty applies to wind-currents.

On the other hand, we have already, in discussing the Permian
glaciation, discovered two agencies which are very effective in lowering
the temperature of the earth. One is the rise of the land; the other is
the thinning of the atmosphere. These are closely related agencies, and
we found them acting in conjunction to bring about the Permian Ice-Age.
Do we find them at work in the Pleistocene?

It is not disputed that there was a very considerable upheaval of the
land, especially in Europe and North America, at the end of the Tertiary
Era. Every mountain chain advanced, and our Alps, Pyrenees, Himalaya,
etc., attained, for the first time, their present, or an even greater
elevation. The most critical geologists admit that Europe, as a whole,
rose 4000 feet above its earlier level. Such an elevation would be bound
to involve a great lowering of the temperature. The geniality of the
Oligocene period was due, like that of the earlier warm periods, to the
low-lying land and very extensive water-surface. These conditions were
revolutionised before the end of the Tertiary. Great mountains towered
into the snow-line, and vast areas were elevated which had formerly been
sea or swamp.

This rise of the land involved a great decrease in the proportion of
moisture in the