emisphere. In the north we have a
preponderance of sea within the Arctic circle, and of lowlands in the
temperate zone. In the south exactly opposite conditions prevail, for there
we have a preponderance of land (and much of it high land) within the
Antarctic circle, and of sea in the temperate zone. Ice, therefore,
accumulates in the south, while a thin coating of snow, easily melted in
summer, is the prevalent feature in the north; and these contrasts react
upon climate to such an extent, that in the southern ocean, islands in the
latitude of Ireland have glaciers descending to the level of the sea, and
constant snowstorms {57} in the height of summer, although the sun is then
actually nearer the earth than it is during our northern summer!

It is evident, therefore, that the phenomena presented by the varying polar
snows of Mars are in no way opposed to that modification of Dr. Croll's
theory of the conditions which brought about the glacial epochs of our
northern hemisphere, which is here advocated; but are perfectly explicable
on the same general principles, if we keep in mind the distinction between
an ice-sheet--which a summer's sun cannot materially diminish, but may even
increase by bringing vapour to be condensed into snow--and a thin snowy
covering which may be annually melted and annually renewed, with great
rapidity and over large areas. Except within the small circles of perpetual
polar snow there can at the present time be no ice-sheets in Mars; and the
reason why this permanent snowy area is more extensive around the northern
than around the southern pole may be partly due to higher land at the
north, but is perhaps sufficiently explained by the diminished power of the
summer sun, owing to its greatly increased distance at that season in the
northern hemisphere, so that it is not able to melt so much of the snow
which has accumulated during the long night of winter.

* * * * *

{58}

CHAPTER IX

ANCIENT GLACIAL EPOCHS, AND MILD CLIMATES IN THE ARCTIC REGIONS

Dr. Croll's Views on Ancient Glacial Epochs--Effects of Denudation in
Destroying the Evidence of Remote Glacial Epochs--Rise of Sea-level
Connected with Glacial Epochs a Cause of Further Denudation--What
Evidence of Early Glacial Epochs may be Expected--Evidences of
Ice-action During the Tertiary Period--The Weight of the Negative
Evidence--Temperate Climates in the Arctic Regions--The Mi