1874.] Climate of the Glacial Period. 455 
places only shows that the earth is a rigid body and but 
slowly gives way to great strains. We must, according to 
Mr. Croll’s theory, go back 200,000 years for the height of 
the Glacial period; but not much more than one-tenth of 
that time would be sufficient according to the theory of an 
increase in the obliquity of the ecliptic; and I submit that 
the shorter interval is more in accordance with the con- 
tinuance of the polar movements, the facts connected with 
the progress of civilisation northwards, and the little change 
there has been in the fauna and flora of the world since the 
Glacial period. 
If our Glacial period was merely the heaping up of ice and 
snow around the North Pole that now exists on both hemi- 
spheres, the result would only be a slight shifting of the 
centre of gravity of the earth northwards; but if it was 
‘contemporaneous in the two hemispheres, as would result 
from a greater obliquity of the ecliptic, the figure of the 
earth would be changed, its polar diameter would be 
lengthened, its mean equatorial diameter shortened, and 
a series of strains would be set up tending to restore its 
figure of equilibrium. And if during the Glacial period the 
shape of the earth had approached that of equilibrium 
through the sinking down of the land around the poles and 
the rising of land in the tropics, then, when the ice melted 
away, the polar diameter would be shortened, the mean 
equatorial diameter lengthened, and forces would be set in 
operation, tending to Jower the land of the tropics, and 
raise that around the poles. Therefore I am ready to admit 
that some part of the deepening of tropical oceans as 
evidenced by the growth of coral islands and reefs—and 
especially any now going on—may be due to a sinking of the 
bed of the ocean; but in doing so I by no means admit that 
the whole or even the greater part of the 3000 feet or more of 
depression that has taken place, according to Dana, can be 
ascribed to that movement. Butthe whole of the deepening 
of the sea, both that arising from its surface being raised, and 
that by portions of its bed being depressed, has, I believe, 
been caused by the gradual melting of the ice of the Glacial 
period, liberating the water that had been piled up at the 
poles, and disturbing the equilibrium of the figure of the 
earth. 
I know that eminent physicists ascribe the movements 
of the earth’s surface to its contraction from secular cooling, 
and Mr. Mallet has lately ably argued that volcanoes are 
one of the results of the movement due to that contraction. 
Without wishing to call in question any theories about the 
