CHAP. Yiii.] THE CAUSES OF GLACIAL EPOCHS. 



151 



If this argument is valid, then it would follow that, so long as 

 excentricity was high, whatever condition of climate was brought 

 about by it in combination with geographical causes, would 

 persist through several phases of precession ; but this would not 

 necessarily be the case when the excentricity itself changed, and 

 became more moderate. It would then depend upon the pro- 

 portionate effect of climatal and geographical causes in produc- 

 ing glaciation as to what change would be produced by the 

 changing phases of precession ; and we can best examine this 

 question by considering the probable effect of the change in 

 precession during the next period of 10,500 yefirs, with the 

 present moderate degree of excentricity. 



ProhaUe effect of Winter in cqylielion on the Climate of Britain. 

 • — Let us then suppose the winters of the northern hemisphere 

 to become longer and much colder, the summers being propor- 

 tionately shorter and hotter, without any other change whatever. 

 The long cold winter would certainly bring down the snow-line 

 considerably, covering large areas of high land with snow during 

 the winter months, and extending all glaciers and ice-fields. 

 This would chill the superincumbent atmosphere to such an 

 extent that the warm sun and winds of spring and early summer 

 would bring clouds and fog, so that the sun-heat would be cut 

 off and much vapour be condensed as snow. The greater sun- 

 heat of summer would no doubt considerably reduce the snow 

 and ice; but it is, I think, quite certain that the extra accumula- 

 tion would not be all melted, and that therefore the snow-line 

 would be permanently lowered. This would be a necessary result, 

 because the greater part of the increased cold of winter wou.ld be 

 stored up in snow and ice, while the increased heat of summer 

 could not be in any way stored up, but would be largely prevented 



can be increased. The only factor available is the Antarctic ice, and if 

 this were largely increased, the northward-flowing currents might be so 

 increased as to melt some of the Arctic ice. But the very same argument 

 applies to both poles. Without some geographical change the Antarctic 

 ice could not materially diminish during its winter in perihelion., nor in- 

 crease to any important extent during the opposite phase. We there- 

 fore seem to have no available agency by which to get rid of the ice 

 over a glaciated country, so long as the geographical conditions reniamed 

 vnchanged and the excentricity continued high. 



