ae 
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 moraine, a line of boulders and gravel at the lower 
end of the glacier, 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 uni 
_r r, The rocks forming the flanks of the valley. ce 
In the higher parts of the Alps, the Serpoltinkas 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 off- 
shoots from these, filling the valleys which descend from the higher 
regions to the asin 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 nevé 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 anda 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. A 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 i in 
summer, which disappears in winter, except in some cases, where the 
water is believed to come from deep. springs, with. ene suffi- 
ety high to keep their vacoamien open. 
