inner 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. Geologists think them superfluous. There is
another set of facts to be considered in connection with the Permian
cold.

As we have seen several times, there are periods when, either owing to
the shrinking of the earth or the overloading of the sea-bottoms, or a
combination of the two, the land regains its lost territory and emerges
from the ocean. Mountain chains rise; new continental surfaces are
exposed to the sun and rain. One of the greatest of these upheavals of
the land occurs in the latter half of the Carboniferous and the Permian.
In the middle of the Carboniferous, when Europe is predominantly a flat,
low-lying land, largely submerged, a chain of mountains begins to rise
across its central part. From Brittany to the east of Saxony the great
ridge runs, and by the end of the Carboniferous it becomes a chain of
lofty mountains (of which fragments remain in the Vosges, Black Forest,
and Hartz mountains), dragging Central Europe high above the water, and
throwing the sea back upon Russia to the north and the Mediterranean
region to the south. Then the chain of the Ural Mountains begins to
rise on the Russian frontier. By the beginning of the Permian Europe was
higher above the water than it had ever yet been; there was only a sea
in Russia and a southern sea with narrow arms trailing to the northwest.
The continent of North America also had meantime emerged. The rise of
the Appalachia and Ouachita mountains completes the emergence of the
eastern continent, and throws the sea to the west. The Asiatic continent
also is greatly enlarged, and in the southern hemisphere there is a
further rise, culminating in the Permian, of the continent ("Gondwana
Land") which united South America, South Africa, the Antarctic land,
Australia and New Zealand, with an arm to India.

In a word, we have here a physical revolution in the face of the earth.
The changes were generally gradual, though they seem in some