348 The Glacial Theory of Prof. Agassiz. 



The heat reflected from the rock fuses a portion of the ice nearest it, 

 or hastens the evaporation, rendering the sides of the glacier a little 

 lower than the middle, and giving the mass a convex shape. The frag- 

 ments rest in the hollows thus produced, and assume the form of the 

 roof of a house, one side sloping down to the rock and the other to 

 the ice. 



e f The terminal moraijie, a line of boulders and gravel at the lower 

 end of the o-lacier, which it pushes before it when advancing, and leaves 

 behind it when retreating. In the latter case it looks like a low mound 

 or barrier across the valley. The terminal moraine is a continuation 

 of the two lateral, but they are not always found united. 

 r r. The rocks forming the flanks of the valley. 

 In the higher parts of the Alps, the perpetual snow forms vast ex- 

 tended masses joining the peaks and ridges, and these, called mers de 

 glace, or " seas of ice," exhibit scenes of grandeur and desolation which 

 have been the wonder of travellers. The glaciers are branches or ofi"- 

 shoots frota these, filling the valleys which descend from the higher 

 regions to the lower. Glaciers pass down sometimes to so low a level 

 as 3000 feet above the sea in Switzerland ; but they do not originate at 

 a lower elevation than 7000 feet, and they rarely exist on isolated 

 mountains, whatever be their height. In the upper part they consist of 

 granular snow, called neve in the Alps, which is changed into minute 

 crystals of ice by the infiltration of water, arising from the outer por- 

 tion of the snow being melted by the sun. As we descend from the 

 higher end of the glacier, the crystals, which are rather irregular 

 fragments, become gradually larger. Towards the lower end they are 

 from half an inch to an inch and a half in diameter, and in some rare 

 cases three inches. If a section of the glacier is exposed, the upper 

 strata (for it is generally stratified) are found to be full of cells, and its 

 substance becomes gradually more compact downward, the lowest part 

 being the most solid. The strata are thick at top, thinner in the mid- 

 dle, and disappear towards the bottom. Glaciers contract in breadth 

 and depth as they descend ; one a league broad at the head will some- 

 times be only 150 or 200 yards at the foot. The thickness varies from 

 80 to 100 feet at the lower, and from 120 to 180 feet at the higher end. 

 M. Agassiz adopts these measures from Hugi, and seems to reject the 

 notion of older writers, that some glaciers are 500 or 600 feet in depth. 

 Glaciers are of all lengths — from 100 yards to 15 miles. 



Every glacier discharges a stream from a vault in its lower end in 

 summer, which disappears in winter, except in some cases, where the 

 water is believed to come from deep springs, with a temperature suffi- 

 ciently high to keep their channels open. 



