atmosphere. The sea surface was enormously lessened, and
the mountains would now condense the moisture into snow or cloud to a
vastly greater extent than had ever been known before There would also
be a more active circulation of the atmosphere, the moist warm winds
rushing upward towards the colder elevations and parting with their
vapour. As the proportion of moisture in the atmosphere lessened
the surface-heat would escape more freely into space, the general
temperature would fall, and the evaporation--or production of moisture
would be checked, while the condensation would continue. The prolonging
of such conditions during a geological period can be understood to have
caused the accumulation of fields of snow and ice in the higher regions.
It seems further probable that these conditions would lead to a very
considerable formation of fog and cloud, and under this protecting
canopy the glaciers would creep further down toward the plains.

We have then to consider the possibility of a reduction of the quantity
of carbon-dioxide in the atmosphere The inexpert reader probably has a
very exaggerated idea of the fall in temperature that would be required
to give Europe an Ice-Age. If our average temperature fell about 5-8
degrees C. below the average temperature of our time it would suffice;
and it is further calculated that if the quantity of carbon-dioxide in
our atmosphere were reduced by half, we should have this required fall
in temperature. So great a reduction would not be necessary in view
of the other refrigerating agencies. Now it is quite certain that the
proportion of carbon-dioxide was greatly reduced in the Pleistocene. The
forests of the Tertiary Era would steadily reduce it, but the extensive
upheaval of the land at its close would be even more important. The
newly exposed surfaces would absorb great quantities of carbon. The
ocean, also, as it became colder, would absorb larger and larger
quantities of carbon-dioxide. Thus the Pleistocene atmosphere, gradually
relieved of its vapours and carbon-dioxide, would no longer retain
the heat at the surface. We may add that the growth of reflective
surfaces--ice, snow, cloud, etc.--would further lessen the amount of
heat received from the sun.

Here, then, we have a series of closely related causes and effects
which would go far toward explaining, if they do not wholly suffice to
explain, the general fall of the earth's temperature. The basic cause is
the upheava