mh A 
¥ 
=n 

Parr I. Sucr. iii.] TERRESTRIAL OSCILLATION. 275 
quiet and uniform character, sometimes of an elevatory, sometimes 
of a subsiding nature. So tranquil may these changes be as to 
produce from day to day no appreciable alteration in the aspect of 
the ground affected, so that only after the lapse of several genera- 
tions, and by means of careful measurements, can they really be 
_ proved. Indeed, in the interior of a country nothing but a series 
of accurate levellines from some unmoved datum-line might detect 
the change of level, unless the effects of this terrestrial disturbance 
showed themselves in altering the drainage. It is only along the 
sea-coast that a ready measure is afforded of any such movement. 
It is customary in popular language to speak of the sea rising or 
falling relatively to the Jand. We cannot conceive of any possible 
augmentation of the oceanic waters, nor of any diminution save what 
may be due to the extremely slow processes of abstraction by the 
hydration of minerals and absorption into the earth’s interior. 
Any changes, therefore, in the relative levels of sea and land must | 
be due to some readjustment in the form either of the solid globe or 
of its watery envelope or of both. . Playfair pointed out at the be- 
ginning of this century ‘that no subsidence of the sea-level could 
be local but must extend over the globe. cs 
Various suggestions have been made regarding possible causes 
of alteration of the sea-level. ‘Thus a shifting of the present dis- 
tribution of density: within the nucleus of the planet would affect the 
position and level of the oceans (ante, p. 44). A change in the 
earth’s centre of gravity, such as might result from the accumulation 
of large masses of snow and ice as an ice-cap at one of the poles, 
_ has been already (p. 18) referred to as tending to raise the level of 
the ocean in the hemisphere so affected, and to diminish it in a 
corresponding measure elsewhere. The return of the ice into the 
state of water would produce an opposite effect. A still further 
conceivable source of geographical disturbance is to be found in the 
fact that, as a consequence of the diminution of centrifugal force 
owing to the retardation of the earth’s rotation caused by the tidal 
wave, the sea-Jevel must have a tendency to subside at the equator 
_ and rise at the poles. A larger amount of land, however, need not 

ultimately be laid bare at the equator, for the change of level 
resulting from this cause would be so slow that as Dr. Croll has 
pointed out, the general degradation of the surface of the land might 
keep pace with it, and diminish the terrestrial area as much as the 
retreat of the ocean tended to increase it. The same writer has 
further suggested that the waste of the equatorial land, and the 
deposition of the detritus in higher latitudes, may still further 
counteract the effects of retardation and the consequent change of 
ocean-level.’ 
1 Croll. Phil. Mag. 1868, p. 382. Sir W. Thomson, Trans. Geol. Soc. Glasgow, iii. 
p- 223. ie hi 
2 In a recent communication to the ‘ Geologische Reichsanstalt’ of Vienna, Herr 
Edward Suess has stated his conviction that the limits of the dry land depend upon 
certain large indeterminate oscillations of the statical figure of the oceanic envelopo ; 
ne Doe 
