Critical geologists may suggest that the
temperature of the Coal-forest has been exaggerated, and the
temperature of the Permian put too low. We are not concerned with
the dispute. Whatever the exact change of temperature was, in
degrees of the thermometer, it was admittedly sufficient to
transform the face of the earth, and bring a mantle of ice over
millions of square miles of our tropical and subtropical regions.
It remains for us to inquire into the causes of this
transformation.
It at once occurs to us that these facts seem to confirm the
prevalent idea, that the Coal-forests stripped the air of its
carbon-dioxide until the earth shivered in an atmosphere thinner
than that of to-day. On reflection, however, it will be seen
that, if this were all that happened, we might indeed expect to
find enormous ice-fields extending from the poles--which we do
not find--but not glaciation in the tropics. Others may think of
astronomical theories, and imagine a shrinking or clouding of the
sun, or a change in the direction of the earth's axis. But these
astronomical theories are now little favoured, either by
astronomers or geologists. Professor Lowell bluntly calls them
"astrocomic" theories.
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