
406 DYNAMICAL GEOLOGY.  {Boox IIL 
be effaced by the pressing of their walls together again as the glacier — 
winds down its valley. The glacier continues to descend until it 
reaches that point where the supply of ice is just equalled by the 
liquefaction. There it ends, its place down the rest of the valley 
being taken by the tumultuous river of muddy water which escapes 
from under the melting extremity of the ice. A prolonged augmen- 
tation of the snowfall will send the foot of the glacier further down 
the valley; a diminution of the snowfall with a general rise of 
temperature will cause it to retreat farther up. Considerable 
variations in the thickness and length of glaciers have been observed 
within the last two or three generations. Thus the glacier of La 
Brenva, on the Italian side of Mont Blanc, shrank to such an extent 
in the twenty-four years succeeding 1818, that its surface at one 
place was found to have subsided no less than 300 feet.* 
In a mountainous region, such as the Alps, or a table-land like 
Scandinavia, where a considerable mass of ground lies above the snow- 
line, three varieties of glaciers may be observed. 
(1) Glaciers of the first order come down well below the snow, 
and extend into the valleys. In high latitudes they reach the sea. 
In the Alps such glaciers may be 20 or 30 miles long, by a mile or 
more wide, and 800 feet or more deep. The spiry peaks and sharp 
crests of these mountains everywhere rise through the snow which 
they thus isolate into distinct basins, whence glaciers proceed. The 
total number of glaciers among the Alps has been estimated at 2000, 
covering a total area of 1838°8 square kilometres, A striking 
contrast to the character of Alpine glacier scenery is presented by 
the great snow-fields of Arctic Norway. These accumulate on 
broad table-lands, from which they send glaciers down into the 
valleys (Figs. 137 and 139), 

















Fic. 137.—Vinw or THe Two Guacrers or FonpALEN, HoLanps Fyorp, 
Arctic Norway, 
(2) Glaciers of the second order hardly creep beyond the high 
recesses wherein they are formed, and do not therefore reach as far as 
1 J. D. Vorbes, Travels in the Alps, p. 205. 

