482 GEOLOGICAL REVOLUTIONS. CHAP. 



of the mountains of North Wales and the North of England ; and 

 lastly, to have recovered its former elevation and climate. 



In considering the causes of these movements it has usually 

 been taken for granted that the sea-level has been constant, or 

 subject only to such limited variations as may be consistent with 



(a] change of position of the axis of rotation of the globe ; or 



(b) change of position of the centre of gravity along this axis. 

 A change of position of the axis of the earth, though not much 

 regarded by geologists or astronomers, is theoretically possible, be- 

 cause every displacement of matter within the earth or at the 

 surface may alter the balance of rotating forces : but the effect can 

 only be of a very small order, and not in any degree effective toward 

 such great changes of oceanic level as the case in hand requires. 



Displacement of the centre of gravity from causes acting below 

 the surface is also to be included among possible events, but of too 

 minute a range to be influential on the mean level of the sea. 



Another cause of such displacement is found by Adhemar in the 

 periodical phenomenon of ' precession of the equinoxes/ whereby 

 first one polar region of the earth and then the other is presented 

 favourably to the sun for 10,500 years; and again in another 

 period of equal length the conditions are reversed. Thus periods 

 of greater and less cold visit the polar regions alternately, and it 

 is supposed that by this means snows gather round one pole and 

 diminish round the other in sufficient quantity to change the place 

 of the centre of gravity of the globe. The reasoning is correct, 

 but the result cannot be sufficient for the present purpose. 



By the researches of Herschel, Croll, and others, another element 

 of astronomical vicissitude is brought to bear on this question; 

 the variation of the mean distance of the earth from the sun. This, 

 subjected to strict calculation, would give beyond a doubt periods 

 of greater and less cold; and alternations of greater and less 

 glaciation round the poles. These variations would occupy long 

 periods of time (expressed in thousands of centuries), and by com- 

 bination with the shorter periods of analogous changes pointed out 

 by Adhemar, would sometimes be greatly augmented, and at other 

 times greatly diminished, in effect. The oscillations thus to be 

 occasioned have been calculated ; the effects are real ; they may 

 be very influential on climate ; but they are not sufficient, nor indeed 

 well fitted, to explain local changes of level amounting to 1000 or 



