310 James Croll — On the Glacial Epoch. 



centre above the point reached. But the point reached was 5000 feet 

 above sea-level, consequently the surface of the ice at the centre of 

 dispersion would be 15,000 feet above sea-level, w'hich is about one- 

 fourth what I have concluded to be the elevation of the surface of 

 the Antarctic ice-cap at its centre. And supposing we assume the 

 general surface of the ground to have in the central region an eleva- 

 tion as great as 5000 feet, which is not at all probable, still this 

 would give 10,000 feet for the thickness of the ice at the centre of 

 the Greenland Continent. But if we admit this conclusion in re- 

 ference to the thickness of the Greenland ice, we must admit that 

 the Antarctic ice is far thicker, because the thickness, other things 

 being equal, will depend upon the size, or, more properly, upon the 

 diameter of the continent ; for the larger the surface, the greater is 

 the thickness of ice required to produce the pressure requisite to 

 make the rate of discharge of the ice equal to the rate of increase. 

 Now the area of the Antarctic Continent must be at least a dozen of 

 times greater than that of Greenland. 



Second. That the Antarctic ice must be far thicker than the Arctic 

 is further evident from the dimensions of the icebergs which have 

 been met with in the Southern Ocean. No icebergs over three 

 hundred feet in height have been found in the Arctic regions, 

 whereas in the Antarctic regions, as we shall see, icebergs of twice 

 and even thrice that height have been reported. 



Third. We have no reason to believe that the thickness of the ice 

 at present covering the Antarctic Continent is less than that which 

 covered a continent of a similar area in temperate regions during 

 the Glacial epoch. Take, for example, the North American Conti- 

 nent, or, more properly, that portion of it covered by ice during the 

 Glacial epoch. Prof. Dana has proved that during that period the 

 thickness of the ice on the American Continent must in many places 

 have been considerably over a mile. He has shown that over the 

 northern border of New England the ice had a mean thickness of 

 6500 feet, while its mean thickness over the Canada watershed, 

 between St. Lawrence and Hudson's Bay, was not less than 12,000 

 feet, or upwards of two miles and a quarter (see American Journal of 

 Science and Art for March, 1873), 



Fourth. Some may object to the foregoing estimate of the amount 

 of ice on the Antarctic Continent, on the grounds that the quantity 

 of snowfall in that region cannot be much. But it must be borne 

 in mind that, no matter however small the annual amount of snow- 

 fall may be, if more falls than is melted, the ice must continue to 

 accumulate year by year till its thickness in the centre of the conti- 

 nent be sufSciently great to produce motion. The opinion that the 

 snowfall of the Antarctic regions is not great does not, however, 

 appear to be borne out by the observation and experience of those 

 who have visited those regions. Capt. Wilkes, of the American 

 Exploring Expedition, estimated it at thirty feet per annum. And 

 Sir James Eoss says that during a whole month they had only three 

 days free from snow. The fact that perpetual snow is found at 

 the sea-level at lat. 64° S. proves that the snowfall must be great. 



