84 JS. Croll—Examination of Wallace's Modification of the 
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. Green- 
land, for exa ample, has but a very small snowfall, and yet the 
snow and ice are perpetual. What is necessary is, that the 
small amount which falls should not all melt. If this be the 
ease, the ice will accumulate year by year, and a glacial condi- 
tion 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 unm melted, then, in this case, the 
ice will continue to accumulate year by year until the quantity 
annually discharged by the outward motion from the center of 
dispersion equals that annually formed. But in the case of a 
continent this condition can be attained only when the sheet 
at the center becomes of enormous thickness. Whether high 
land be necessary to a glacial epoch or not, 2 is —- that a 
beavy snowfall is not an indispensable conditi 
As to the second of these conditions, avis , high land, it 
must be borne in mind that the question is Ae Could the 
causes which are now in operation bring about a alacial condi- 
tion of things without high land? but, Could thos@ physical 
agencies brought into operation during a high stats of eccen- 
pay produce a glacial state of things without high land? 
allace’s answer is that t seh could not But [ am not 
satisfied with the grounds on which he bases this opinion. 
necessary condition to a glacial epoch is, of course, the exist- 
bee snow nowhere exists on low lands.” Su up sing this 
were true (I have endeavored to show it is nets + still it does 
not follow that perpetual snow 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, unde 
certain conditions, the snow-line st hb in some places have been 
brought to the sea-level. In arc r even in subarctic re- 
gions, an excessively heavy eee followed by piercingly 
cold winds from the north, during t the whole of the summer 
* American Journal of Science, October, 1883; Philosophical Magazine, October, 
+ Philosophical Magazine, November, 1883. 
