CHAP. VII r.] 



THE CAUSES OF GLACIAL EPOCHS. 



135 



with North-eastern Canada seems to have been the source of 

 much of the giaciation of that continent.^ 



The reason why no accumulation of snow or ice ever takes 

 place on Arctic lowlands is explained by the observations of 

 Lieut. Payer of the Austrian Polar Expedition, who found that 

 during the short Arctic summer of the highest latitudes the ice- 

 fields diminished four feet in thickness under the influence of the 

 sun and wind. To replace this would require a precipitation of 

 snow equivalent to about 45 inches of rain, an amount which 

 rarely occurs in lowlands out of the tropics. In Siberia, within 

 and near the Arctic circle, about six feet of snow covers the 

 country all the winter and spring, and is not sensibly diminished 

 by the powerful sun so long as northerly winds keep the air 

 below the freezing-point and occasional snow-storms occur. But 

 early in June the wind usually changes to southerly, probably the 

 south-western anti-trades overcoming the northern inflow ; and 

 under its influence the snow all disappears in a few days and the 

 vegetable kingdom bursts into full luxuriance. This is very 

 important as showing the impotence of mere sun-heat to get rid 

 of a thick mass of snow so long as the air remains cold, while 

 currents of warm air are in the highest degree effective. If, 

 however, they are not of sufficiently high temperature or do not 

 last long enough to melt the snow, they are likely to increase it, 



^ " The general absence of recent marks of glacial action in Eastern 

 Europe is well known; and tbe series of changes which have been so well 

 traced and described by Prof. Szabo as occurring in those districts seems to 

 leave no room for those periodical extensions of ' ice-caps ' with which 

 some authors in this country have amused themselves and their readers. 

 Mr. Campbell, whose ability to recognise the physical evidence of glaciers 

 will scarcely be questioned, finds quite the same absence of the proof of 

 extensive ice-action in North America, westward of the meridian of 

 Chicago." (Prof. J. W. Judd in Geol Mag. 1876, p. 535.) 



The same author notes the diminution of marks of ice-action on going 

 eastward in the Alps ; and the Altai Mountains far in Central Asia show 

 no signs of having been largely glaciated. West of the Rocky Mountains, 

 however, in the Sierra Nevada and the coast ranges further north, signs 

 of extensive old glaciers again appear; all which phenomena are strikingly 

 in accordance with the theory here advocated, of the absolute dependence 

 of giaciation on abundant rainfall and elevated snow-condensers and 

 accumulators. 



