596 THE NATURAL HISTORY EEYIEW. 



the northern hemisphere, and the ocean still retaining its former 

 level in relation to this land ; unless the ice-sheet be counter-balanced 

 by one of equal weight placed upon the southern hemisphere. 

 Eut this leads us to another result. The submergence of the land 

 during the glacial epoch leads to the conclusion that the glaciation 

 was not contemporaneous on both hemispheres. If the ice-sheet had 

 covered both hemispheres, the earth's centre of gravity, and, conse- 

 quently, the ocean-level, would have remained unaffected. The sub- 

 mergence of the land is, therefore, another confirmation of the truth 

 of the theory, which attributes the glacial epoch to the excentricity 

 of the earth's orbit ; for, as you are aware, if the glacial epoch had 

 been due to the excentricity, the glaciation could have extended to 

 only one hemisphere at a time. One hemisphere would have been 

 covered with snow and ice, while the other would have been enjoy- 

 ing a perpetual spring. 



A glacial epoch resulting from the excentricity on the earth's 

 orbit would extend over a period of upwards of 100,000 years. But 

 owing to the precession of the equinoxes, and the revolution of the 

 apsides, the glaciation would be transferred from one hemisphere to 

 the other every 10,000 years or so. A glacial epoch extending over 

 100,000 years would, therefore, be broken up with five or six warm 

 periods. A warm period on the one hemisphere would be contem- 

 poraneous with a cold period on the other. Under these circum- 

 stances we ought to have elevation of the land during the warm 

 periods, and submergence during the cold. The land ought to have 

 stood higher than at present during some periods of the glacial e230ch 

 as well as lower. This again, I presume, is in agreement with geo- 

 logical fiicts. That the cold of the glacial epoch was not continuous 

 but was broken up by comparatively warm periods, when the ice, 

 to a considerable extent at least, disappeared, I think, has been 

 clearly proved by Morlot, Geikie, and others, from the stratified beds 

 of sand, clay, and gravel, old water-courses and striated " pavements" 

 which have been found in the true boulder clay. (See Geikie's 

 Glacial Drift of Scotland, pp. 92—94.. Morlot's paper in the Edin. 

 New Fhil. Journal, New series. Vol. II., ].855. Lyell's Antiquity 

 of Man, second edition, p. 320.) 



"We shall now consider whether the relation between the actual 

 extent of the submergence during the glacial epoch and the size of the 

 ice-sheet be something like what it ought to be on the supposition 

 tbat the submergence resulted from the cause which we have as- 



