22 WALKS AND TALKS. 



Thus the distribution of Drift materials sustains a relation 

 to altitude similar to that which it sustains to latitude. What 

 factor, or force, or agency exists in altitude which exists 

 identically in latitude? Temperature, certainly. To ascend 

 a high mountain range is the same as to ascend to a high lati- 

 tude. All high mountains support animals and plants related 

 to species farther north. On the summit of Mount Washing- 

 ton are the butterflies and plants of Labrador. Ascending the 

 Andes, you have tropical products at the foot, temperate pro- 

 ducts at ten thousand feet, and arctic conditions at the sum- 

 mit. The distribution of the Drift, then, has relation to heat 

 and cold. Greater cold has been accompanied by larger re- 

 sults. Bowlders are more numerous and more massive in 

 northern and in elevated regions, because the cold is there 

 more intense. 



Now, how does cold act to effect transportation of rock- 

 fragments? Our thoughts run over the world to scrutinize 

 the modes of action of cold. Much cold implies much snow 

 and ice, if moisture and water are abundant. Most far north- 

 ern regions and high mountain summits are covered much of 

 the year, or the whole of it, by a sheet of snow. Winter 

 snow, under the action of thawing and freezing temperatures 

 in alternation, becomes granular, as we often observe in old 

 snow, especially in early spring. With a more advanced 

 stage of granulation, the icy grains coalesce into larger grains, 

 and finally merge completely into a solid mass of ice. This, 

 also, we have often noticed in the last lingering patches of 

 last winter's snow. 



We have many observations of this kind on a large scale. 

 On high mountains broad fields of granular snow come into 

 existence, and at a certain elevation the average annual tem- 

 perature is not sufficient to dissolve it before autumnal snows 

 begin to increase the amount. The old snow becomes a per- 

 manent granular sheet on the high slopes. In the Alps the 

 Germans designate it Firn, and the French, Neve. When the 

 firn-masses are accumulated in valleys, the amount of snow is 

 so great that it may reach to a much lower altitude before 



