86 DISCUSSIONS IN CLIMATOLOGY. 



is required only in order to produce a great snowfall ; 

 a great snowfall only in order that the snow may 

 become permanent ; and the permanent snow in turn 

 is only in order to have permanent glaciation. But it 

 has already been shown in Chapter III. 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 also has but a very small snowfall, and 

 yet the snow and ice are there perpetual. What 

 is necessary is, that the small amount which 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 on a 

 continent is equivalent to only 10 inches of ice, and 

 that at the end of each summer one inch remains 

 unmelted, then, in such case, the ice will 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 enor- 

 mous 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 first of these conditions, namely High 

 Land, it must be borne in mind that the question is 

 not,. Could the causes which are now in operation 

 bring about a glacial condition of things without high 

 land ? but, Could those physical agencies brought into 

 operation during a high state of eccentricity produce 

 a glacial state of things without high land ? Mr. 

 Wallace's answer is that they could not. But I am 

 not satisfied with the grounds on which he bases this 



