1875.] The History of our Earth. ary 
Belt in his delightful ‘‘ Naturalist in Nicaragua.” * This 
eminent investigator urges that in a Glacial epoch a vast 
proportion of water must have been piled up upon the 
continents in the shape of ice and snow; that the level of 
the ocean must have been thus greatly lowered, and that 
wide territories, now submerged, must have been the homes 
of man, and of organic life in general. By this assumption 
he explains certain points in the migrations and distribution 
of plants and animals, and throws a light upon the deluges 
of the ancient world, and upon the myth of Atlantis. 
According to Mr. Croll, a vast amount of water would, 
indeed, be accumulated in the solid form in the glaciated 
hemisphere. He considers that even now the “ antar¢tic 
ice-cap’” extends from the pole down to lat. 70° S., being 
2800 miles in diameter. The whole of this region, he 
concludes, is covered ‘‘with a continuous sheet of ice 
gradually thickening inwards from its edges to its centre. 
A slope of one degree, continued for 1400 miles, will give 
twenty-four miles as the thickness of the ice at the pole. 
But suppose the slope of the upper surface of the cap to be 
only one-half this amount, viz., a half degree—and we have 
no evidence that a slope so small would be sufficient to 
discharge the ice—still we have twelve miles as the 
thickness of the cap at the pole.” As there are in the 
southern ocean icebergs above a mile in thickness, the great 
antarctic ice-cap must be in some places over a mile in 
thickness at its very edge! Were one mile of ice melted off 
the whole extent of this southern cap, it would raise the 
general level of the ocean 200 feet ! 
If the ice-mass at the South Pole is at present so 
enormous, what must the amount have been surround- 
ing the pole of a glaciated hemisphere? Mr. Croll 
does not, however, consider that the total amount of ice 
upon the earth’s surface during a Glacial epoch could 
be greatly in excess of what it is at present. He holds that 
what is now, with some approach to equality, distributed 
between both poles, was then accumulated around one. 
The effect of this upon the general level of the ocean would 
depend not so much upon the direct decrease of water owing 
to its being converted into ice, as to an indirect action. 
Remove all the ice from one pole and transfer it to the 
other, and you affect the centre of gravity of the globe. 
Two miles of ice withdrawn from the southern circumpolar 
continent would displace the earth’s centre of gravity by 
* QUARTERLY JOURNAL OF SCIENCE, vol. iv., p. 320. 
