It has been already stated that Belt devoted the scanty leisure of
his last years to the study of the glacial period, entering with
zest into the consideration of its cause, the method of deposition
of its beds, and the time-relationship of man to it--complex
questions on which his imagination had full scope, and which, had
his life been prolonged, his patient accumulation of evidence might
have ultimately led him to suggest answers that would have been
generally accepted by scientific men. But the cause of the
remarkable change of climate during those late Tertiary and
post-Tertiary times known as the glacial period is still without a
completely satisfactory explanation. In Belt's day geologists were
inclined to get over the difficulty of accounting for the phenomena
by any feasible terrestrial change by explaining them as the result
of cosmical causes, and Croll's theory of the increase of the
eccentricity of the earth's orbit was widely received among them.
Belt, on the other hand, held that the cold was due to an increase
in the obliquity of the ecliptic. But these astronomical
explanations have not met with much acceptance by physicists; and
so chemists have been turned to by some geologists for support of
the hypothesis of the variation in the amount of carbon dioxide in
the air, or of other alterations in the atmosphere, while others
have gone back to the idea of geographical changes. That
considerable oscillations of the relative levels of land and sea
took place during the Ice Age has been now clearly established, and
the general result of the investigations favours Belt's opinion
that the land during part of that period stood much higher than now
over the northern regions of Europe and North America. It would,
however, lead us too far away from the present book to enter into
even a cursory examination of his views upon the glacial period,
and those readers who desire to pursue the matter will find