106 On the Downward Progress 



it does not, as we have mentioned, carry off as an annual 

 mean, much above one metre of the surface of the neves ; in 

 the middle region it carries off two to two-and-a-half metres 

 of the ice ; and in the lower regions of the glaciers, three to 

 three-and-a-half metres ; consequently, all the bodies of ice 

 which the movement carries from above downwards, are suc- 

 cessively exposed to a stronger ablation. 



To recapitulate what has been said regarding the three 

 causes which concur in the mechanism of the formation of 

 glaciers, we may state : 



1*^, The alimentation and movement of themselves would 

 cause an extraordinary extension in the length. 



2<i, Alimentation and ablation alone would produce an in- 

 definite extension in a vertical direction ; the glaciers would 

 not descend into the valleys, but continue in the higher re- 

 gions. 



Zd, Movement and ablation, apart from alimentation, would 

 put an end to their existence. 



It is the combination and influence of these three causes 

 which regulate the glaciers, and maintain them in their pre- 

 sent state ; this is the law of their existence, the condition of 

 their being. 



These preliminaries were necessary to enable us to per- 

 ceive the force of the observations which follow ; the more so, 

 since we know that this equilibrium between the natural 

 forces which confine glaciers within their pi'esent limits, has 

 not always existed on the surface of the earth. At a period 

 of its history comparatively recent, geologically speaking, the 

 equilibrium has been disturbed, the alimentary force has pre- 

 vailed over the melting force, and glaciers have acquired a 

 considerable development. 



In present times, since attention began to be paid to 

 glaciers, we find that they have been subject to perpetual 

 changes : some years they are advancing, in others retreat- 

 ing; or one glacier may be advancing while a neighbouring 

 one is receding. As a general rule, the front of glaciers ad- 

 vances in winter, and i*ecedes more or less during the warm 

 season ; because, in the winter there is no ablation, alimenta- 

 tion and movement alone continuing to exert themselves. 

 Yet we are.'iboutto make an endeavour to shew that glaciers. 



