234 ON GLACIERS IN GENERAL. 



The diminution of temperature as we ascend the slopes of 

 mountains, as indicated by successive zones of vegetation, and 

 finally by the occurrence of perpetual snow, is stated and ex- 

 plained in other articles,* and therefore may be assumed here. 

 Thus, in the high mountains of the Andes and Himalaya, be- 

 tween the tropics, the commencement of perpetual snow is 

 found at from 15,000 to 18,000, or even 19,000 feet, according 

 to circumstances ; whilst in southern Europe the level is from 

 8000 to 9000 feet, and in Norway from 5500 to 3000 feet, ac- 

 cording to the latitude and the distance from the sea. It was 

 first shown by Baron Humboldt and Von Buch that the limit of 

 perpetual snow depends principally on the temperature of the 

 summer, and not upon that of the whole year. 



It has been already explained that an accumulation of snow, 

 even frozen snow, does not constitute properly a glacier. A 

 glacier is a mass of ice, having its origin in the hollows of 

 mountains where perpetual snow accumulates, but which makes 

 its way down towards the lower valleys, where it gradually 

 melts, and it terminates exactly where the melting, due to the 

 contact of the warmer air, earth, and rain of the valley compen- 

 sates for the bodily descent of the ice from the snow reservoirs 

 of the higher mountains. From this it is evident, without 

 any formal measurements, that a GLACIER is ICE IN MOTION 



UNDER GRAVITY.f 



Geographical Distribution of Glaciers. Glaciers are not 

 peculiar to any country or region of the earth- It may be that 

 there are extensive snowy mountains wholly devoid of them, as 

 is supposed to be the case in tropical South America ; but even 

 this exception requires confirmation. There are peculiarities in 

 the form of mountains, and still more in climate, which, as we shall 

 see, favour the formation of glaciers, or may even totally pre- 

 vent it. 



* Encyclopaedia JBritannica, articles "Climate," "Physical Geography," and 

 " Botany," Part III. See also Johnston's Physical Atlas, article on " Glaciers," by 

 the present writer. 



f [The words "under gravity," which give greater precision to the definition by 

 excluding icebergs, are added in this reprint.] 



