ALTHOUGH the problems of this book may lead far
afield, they ultimately bring us back to the earth
and to the present. Several times in the preceding
pages there has been mention of the fact that periods of
extreme climatic fluctuations are closely associated with
great movements of the earth's crust whereby mountains
are uplifted and continents upheaved. In attempting to
explain this association the general tendency has been
to look largely at the past instead of the present. Hence
it has been almost impossible to choose among three
possibilities, all beset with difficulties. First, the move-
ments of the crust may have caused the climatic fluctua-
tions; second, climatic changes may cause crustal move-
ments; and third, variations in solar activity or in some
other outside agency may give rise to both types of terres-
trial phenomena.

The idea that movements of the earth's crust are the
main cause of geological changes of climate is becoming
increasingly untenable as the complexity and rapidity of
climatic changes become more clear, especially during
post-glacial times. It implies that the earth's surface
moves up and down with a speed and facility which
appear to be out of the question. If volcanic activity be
invoked the problem becomes no clearer. Even if volcanic
dust should fill the air frequently and completely, neither
its presence nor absence would produce such peculiar fea-

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