84 Dr. J. Croll on the Physical Theory 



As to the second of these conditions, great moisture is evi- 

 dently necessary only in order to produce a great snowfall ; 

 a great snowfall is necessary only in order that the snow may 

 become permanent; and the permanent snow in turn is neces- 

 sary only in order to have permanent glaciation. But it has 

 already been shown* that we frequently have permanent snow 

 with a very light snowfall, even where the direct heat of 

 the sun is excessive, as on the summits of lofty mountains. 

 Greenland, for example, has but a very small snowfall, and 

 yet the snow and ice are perpetual. What is necessary is, 

 that the small amount w 7 hich falls should not all melt. If 

 this be the case, the ice will accumulate year by year, and a 

 glacial condition will ultimately result. 



Suppose that the annual precipitation of snow T on a conti- 

 nent is equivalent to only 10 inches of ice, and that at the end 

 of each summer one inch remains unmelted, then, in this case, 

 the ice wall continue to accumulate year by year until the 

 quantity annually discharged by the outward motion from the 

 centre of dispersion equals that annually formed. But in 

 the case of a continent, this condition can be attained only 

 when the sheet at the centre becomes of enormous thickness. 

 Whether high land be necessary to a glacial epoch or not, 

 it is evident that a heavy snowfall is not an indispensable 

 condition. 



As to the second of these conditions, namely High Land, 

 it must be borne in mind that the question is not, Could the 

 causes wrhich are now in operation bring about a glacial con- 

 dition of things without high land ? but, Could those physical 

 agencies brought into operation during a high state of eccen- 

 tricity produce a glacial state of things without high land ? 

 Mr. Wallace's answ r er is that they could not. But I am not 

 satisfied with the grounds on which he bases this opinion. A 

 necessary condition to a glacial epoch is, of course, the exist- 

 ence of perpetual snow; for without perpetual snow there 

 could be no permanent land-ice. The question then is, Could 

 not those physical agencies brought into operation during a 

 high state of eccentricity cover lowlands with perpetual snow 7 

 without the aid of highlands? Mr. Wallace replies, " Per- 

 petual snow nowhere exists on lowlands." Supposing this 

 were true (I have endeavoured to show 7 it is notf), still it does 

 not follow 7 that perpetual snow 7 may not have existed on low- 

 lands, or that, when the present condition of things changes, it 

 may not yet exist. It is not difficult to conceive how 7 , under 



* American Journal of Science, October 1883 ; Philosophical Magazine, 

 October 1883. 



t Philosophical Magazine, November 1883. 



