Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/231

Rh MOVEMENTS or THE E.tRTH.] the discussion of the question whether the geological evi- dence requires necessarily so important a geographical change, let us consider how far a. shifting of the axis of rotation has been a possible cause of change during that section of geological time for which there are records among the stratified rocks. From the time of Laplace‘ astronomers have strenuously denied 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 rota- tion does not strictly coincide 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, recognize two causes which, whether or not they may suffice to produce any change in the position of the main axis of inertia, un- doubtedly tend to do so. In the ﬁrst place a widespread upheaval or depression of certain portions of the surface to a considerable vertical amount might shift that axis. In the second place an analogous result might arise from the denudation of continental masses of la11d and the consequent ﬁlling up of sea—basins. Sir lVilliam Thomson freely con- cedes the physical possibility 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 dis- turbance of either land or water.”2 But though, in the earlier ages of the planet’s history, stupendous deformations may l1ave 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 completely at variance with geological evidence, we may reasonably withhold our belief from this alleged cause of the changes of climate during geological history. It has been estimated by Sir William Thomson “that an elevation 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-tl1ird of a second, or 31 feet.” 3 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 each elevated area should have an area of depression corresponding in size and diametrically opposite to it, that they should lie on the same complete meridian, and that they should both be situated in lat. 45°. With all those coincident favourable circumstances, an effective elevation of 33-5 of the earth’s surface to the extent of 10,000 feet would shift the pole 11%’; a similar elevation of would move it 1° 46.1_;' ; of GEOLOGY 217 -136, 3° 17'; and of £3, 8° 4.12’. Mr Darwin admits these to be superior limits to what is possible, and that, on the sup- position of intumescence or contraction under the regions in question, the deﬂexion of the pole might be reduced to a quite insigniﬁcant amount.‘ Under the most favourable conditions, therefore, the possible amount of deviation of the pole from its ﬁrst posi- tion would appear to have been too small to have seriously inﬂuenced the climates of the globe within geological history. If we grant that these changes were cumulative, and that the superior limit of deﬂexion was reached only after a long series of concurrent elevations and depres- sions, we must suppose that no movements took place elsewhere to counteract the effect of those about lat. 45° in the two hemispheres. But this is hardly credible. A glance at a geographical globe suffices to show how large a mass of land exists now both to the north a11d south of that latitude, especially in the northern hemisphere, and that the deepest parts of the ocean are not antipodal to the greatest heights of the land. These features of the earth’s surface are of old standing. There seems, indeed, to be no geological evidence in favour of any such geographical changes as could have produced even the comparatively small displacement of the axis considered possible by M r Darwin. In an ingenious suggestion Dr John Evans contended that, even without any sensible change in the position of the axis of rotation of the nucleus of the globe, there might be very considerable changes of latitude due to disturbance of the equilibrium of the shell by the upheaval or removal of masses of land between the equator and the poles, and to the consequent sliding of the shell over the nucleus until the equilibrium was restored. This hypothesis starts on the assumption of a thin crust enclosing a liquid or viscous interior—an assumption which, as will be shown in subse- quent pages, is negatived by considerations in physics. The Rev. 0. Fisher has suggested 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 great 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 in- quiry “whether a fluid substratum over a rigid nucleus would not be compatible with mechanical considerations, and whether, under those circumstances, changes in lati- tude would not result from unequal thickening of the crust.” 5 6. C’/Lcmgcs of the Earth’s Centre of 6'-ravz'(y.—Though no known geological operation seems to have been capable of pro- ducing an effective change in the position of the axis of rota- tion of the earth, there may have been variations in the P051}- tion of its centre of gravity. Any change of that kind must affect the ocean, which of course adjusts itself in relation to the earth’s centre of gravity. The enormous accumulation of ice at one pole during the maximum of eccentricity will dis- place 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. 0. Fisher gives it at 1 Jléca-n2'que Céleste, tome v. p. 14. 2 Brit. Assoc. Rep. (1876), Sections, p. 11. 3 Trans. Geol. Soc. Glasgow, iv. 4 P/Lil. Trans., November 1876. 5 Goal. .l[ag., 1878, p. 552. 6 Adhemar, Revolutions de la Mar, 1840. X. —— 28