tures as the localization of glaciers, the distribution of
loess, and the mild climate of most parts of geological
time. Nevertheless, because of the great difficulties pre-
sented by the other two possibilities many geologists
still hold that directly or indirectly the greater climatic
changes have been mainly due to movements of the
earth's crust and to the reaction of the crustal movements
on the atmosphere.

The possibility that climatic changes are in themselves
a cause of movements of the earth's crust seems so im-
probable that no one appears to have investigated it with
any seriousness. Nevertheless, it is worth while to raise
the question whether climatic extremes may coöperate
with other agencies in setting the time when the earth's
crust shall be deformed.

As to the third possibility, it is perfectly logical to
ascribe both climatic changes and crustal deformation to
some outside agency, solar or otherwise, but hitherto
there has been so little evidence on this point that such
an ascription has merely begged the question. If heavenly
bodies should approach the earth closely enough so that
their gravitational stresses caused crustal deformation,
all life would presumably be destroyed. As to the sun,
there has hitherto been no conclusive evidence that it is
related to crustal movements, although various writers
have made suggestions along this line. In this chapter
we shall carry these suggestions further and shall see
that they are at least worthy of study.

As a preliminary to this study it may be well to note
that the coincidence between movements of the earth's
crust and climatic changes is not so absolute as is some-
times supposed. For example, the profound crustal
changes at the end of the Mesozoic were not accompanied
by widespread glaciation so far as is yet known, although

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