18 COSMICAL ASPECTS OF GEOLOGY. [Book lL 
this extreme amount would not necessarily alter the position of the 
axis of figure.’ 
Against any hypothesis which assumes a thin crust enclosing a 
liquid or viscous interior—weighty and indeed insuperable objections 
have been urged. It has been suggested, however, that the almost 
universal traces of present or former volcanic action, the evidence 
from the compressed strata in mountain regions that the crust of the 
earth must have a capacity for slipping towards certain lines, the 
ereat amount of horizontal compression of strata which can be proved 
to have been accomplished, and the secular changes of climate— 
notably the former warm climate near the north pole—furnish 
grounds for inquiry whether the doctrine of a fluid substratum over a 
rigid nucleus, which has been urged by several able writers, would 
not be compatible with mechanical considerations, and whether, under 
those circumstances, changes in latitude would not result from 
unequal thickening of the crust. This question of the internal 
condition of the globe is discussed at p. 49. 
§ 6. Changes of the Earth’s Centre of Gravity.—If the centre 
of gravity in our planet, as pointed out by Herschel, be not co- 
incident with the centre of figure, but lies somewhat to the south of 
it, any variation in its position will affect the ocean, which of 
course adjusts itself in relation to the earth’s centre of gravity. — 
How far any redistribution of the matter within the earth in such a 
way as to affect the present equilibrium is now possible, we cannot 
tell. But certain revolutions at the surface may from time to time 
produce changes of this kind. The accumulation of ice which, as 
will be immediately described (§ 8), is supposed to gather round one 
pole during the maximum of eccentricity, will displace the centre 
of gravity, and, as the result of this change, will raise the level of © 
the ocean in the glacial hemisphere.* Dr. Croll has estimated that, — 
if the present mass of ice in the southern hemisphere is taken at 
1000 feet thick extending down to lat. 60°, the transference of this 
mass to the northern hemisphere would raise the level of the sea 80 
feet at the north pole. Other methods of calculation give different 
results. Mr, Heath puts the rise at 128 feet; Archdeacon Pratt 
makes it more; while the Rev. O. Fisher gives it at 409 feet.* 
More recently, in returning to this question, Dr. Croll remarks 
“that the removal of two miles of ice from the Antarctic continent 
[and at present the mass of ice there is probably thicker than that] - 
would displace the centre of gravity 190 feet, and the formation of a 
mass of ice equal to the one-half of this, on the Arctic regions, 
would carry the centre of gravity 95 feet farther, giving in all a 
' Q. J. Geol. Soc. (1878) xxxiv. p. 41. See also HE. Hill, Geol. Mag. v. (2nd ser.) pp. 
262,479. O. Fisher, op. cit. pp. 291, 551. 
2 O. Fisher, Geol. Mag., 1878, p. 552. 
5’ Adhemar, Iévolutions de la Mer, 1840. 
* Croll, in fteader for 2nd September, 1865, and Phil. Mag., April, 1866; Heath, 
Phil. Mag., April, 1869; Pratt, Phil. Mag., March, 1866; Fisher, Reader, 10th 
Iebruary, 1866. 


