164 BOARD OP AGRICULTURE. 



can be tracked, and the way in which we track them is un- 

 mistakable. One who is familiar with the facts can as readily 

 recognize the course which those masses took as the hunter can 

 trace his game by recognizing the footsteps, and knowing the 

 animal which has left his mark upon the soft soil. 



At present we have glaciers only under peculiar conditions. 

 We have them only where the land is above the surface of the 

 ocean, so high that the average temperature is below 32° Fahr., 

 or where, in the northern latitudes, the average temperature 

 sinks regularly below that level ; so that at present there is a 

 cap of ice around the northern pole, another cap of ice around 

 the southern pole, and all the high mountains are capped with 

 ice. And these ice coverings move in definite directions, ac- 

 cording to the amount of moisture which penetrates them. The 

 accumulation of ice or snow is not an agency sufficient to pro- 

 duce a glacier ; it must be accompanied by an infiltration of 

 moisture, otherwise the snow remains loose and produces no 

 mechanical action. It is only when the snow, through partial 

 melting, or through dew or rain, or the condensation of the 

 moisture from the atmosphere, has been softened, and then 

 frozen hard again, that the snow passes into the condition of 

 ice, and then begins to produce a mechanical action, which is 

 all-powerful, as we may see at present in the Alps, in particular, 

 where the glaciers have been studied more extensively than any- 

 where else. Suppose we have a mountain-slope of from nine to 

 fourteen or fifteen thousand feet above the level of the sea. The 

 condensation of moisture from the atmosphere which falls from 

 this surface will remain in the condition of snow over the whole 

 of this extent, for here there is hardly (except perhaps on hot 

 summer days) a condensation of moisture in any other but a 

 frozen condition. It does not rain. The sun is not powerful 

 enough, in the latitude of Switzerland, for instance, to produce 

 a rain-fall at this height ; but the condensation falls always in 

 the shape of snow. When we come to lower levels (and this 

 process is gradual ; the limit is not absolutely nine thousand 

 feet) we have a region where the alternations between snow 

 and rain are frequent, and in consequence of this condensation 

 of the atmosphere in a fluid condition the snow is permeated by 

 water, the cold nights come on, the moisture freezes and grad- 

 ually transforms the snow into a loose kind of ice ; and as we go 



