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.
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