~ he? 
ee ot, 
SF Ae MK re, 
; ee 
x lhe Ane 
\ Pins cg 
16 COSMICAL ASPECTS OF GEOLOGY. [Boon f[. — 
the possibility of any sensible change in the position of the axis of — 
rotation. It has been urged that, since the planet acquired its 
present oblate spheroidal form, nothing but an utterly incredible 
amount of deformation could overcome the greater centrifugal force 
of the equatorial protuberance. It is certain, however, that the axis 
of rotation does not strictly comcide with the principal axis of 
inertia. Though the angular difference between them must always 
have been small, we can, without having recourse to any extra- 
mundane influence, recognise two causes which, whether or not they 
may suffice to produce any change in the position of the main axis 
of inertia, undoubtedly tend to do so. In the first place a widespread 
upheaval or depression of certain unsymmetrically arranged portions — 
of the surface to a considerable amount would tend to shift that axis. 
Tn the second place an analogous result might arise from the denuda- 
tion of continental masses of land and the consequent filling up of 


os 
sea-basins. Sir William Thomson freely concedes-the physical possi- _ 
bility of such changes. ‘We may not merely admit,” he says, “but 
assert as highly probable, that the axis of maximum inertia and 
axis of rotation, always very near one another, may have been in 
ancient times very far from their present geographical position, and 
may have gradually shifted through 10, 20, 30, 40, or more degrees, 
without at any time any perceptible sudden disturbance of either 
land or water.”* But though, in the earlier ages of the planet’s 
history, stupendous deformations may have occurred, and the axis of 
rotation may have often shifted, it is only the alterations which can 
possibly have occurred during the accumulation of the stratified 
rocks that need to be taken into account in connexion with former 
changes of climate. If it can be shown therefore that the 
geographical revolutions necessary to shift the axis are incredibly 
stupendous in amount, improbable in their distribution, and not 
really demanded by geological evidence, we may reasonably with- 
hold our belief from this alleged cause of the changes of climate 
during geological history. , 
It has been estimated by Sir William Thomson “that an eleva- 
tion of 600 feet, over a tract of the earth’s surface 1000 miles square 
and 10 miles in thickness, would only alter the position of the 
principal axis by one-third of a second, or 34 feet.”* Mr. George 
Darwin has shown that on the supposition of the earth’s complete 
rigidity no redistribution of matter in new continents could ever — 
shift the pole from its primitive position more than 3°, but that, if 
its degree of rigidity is consistent with a periodical re-adjustment to 
a new form of equilibrium, the pole may have wandered some 10° or 
15° from its primitive position, or have made a smaller excursion 
and returned to near its old place. In order, however, that these 
maximum effects should be produced, it would be necessary that 
' Brit. Assoc. Rep. (1876), Sections, p. 11. 
* Trans. Geol. Soc. Glasgow, iv. p, 313. The situation of the supposed area of up- 
heaval on the carth’s surface is not stated. 
