404 .- DYNAMICAL GEOLOGY. ~ [Boox TIT, - 
ment by J. D. Forbes, who found that in the Mer de Glace of 
Chamouni, the mean daily rate of motion in the summer and autumn 
was from 20 to 27 inches in the centre, and from 13 to 194 near the 
side. Helland has observed that on the west coast of Greenland the 
glacier of Jacovshavn has a remarkably rapid motion, its rate for 
r 
twenty-four hours ranging from 14°70 metres (48°2 feet) to 19°77. 
metres (64°8 feet). The consequence of this differential motion is 
seen in the internal banded structure of a glacier, in the downward 
curvature of the transverse fissures (crevasses), and in the arrange- 
ment of the lines of rubbish thrown down at tie termination, which 
often present a horse-shoe shape, corresponding to that of the end 
of the ice by which they were discharged.’ : 
_ Some features of geological importance in the behaviour of the 
ice as it descends its valley deserve mention here. When a glacier 
has to travel over a very uneven floor, some portions may get 
embayed, while overlying parts slide over them. A massive ice- 
sheet may thus have many local eddies in its lower portions, the ice 
there even travelling for various distances, according to the nature of 
the ground, obliquely to the general flow of the main mass. In 
descending by a steep slope to a more level part of its course, a 
glacier becomes a mass of fissured ice in great confusion, It de- 
scends by a slowly creeping ice-fall, where a river would shoot over 
in a rushing waterfall. A little below. the fall the fractured ice, 
with all its chaos of pinnacles, bastions, and chasms, is pressed 
together again into a solid mass as before (Fig. 135). 
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Fic, 185.—SrcTIon or GLACIER WITH IcE-FALLS, FONDALEN, HoLANDS Fyorp, AROTIC 
Norway, 
The body of the glacier throughout its length is traversed by a 
set of fissures called crevasses, which, though at first as close-fitting 
as cracks in a sheet of glass, widen by degrees as the glacier moves — 
on, till they form wide yawning chasms, reaching, it may be, to the 
bottom of the ice, and travelling down with the glacier, but apt to 
* The cause of glacier motion has been a much-vexed question in physics. See 
besides the works cited on the foregoing page, J. Thomson, Proc. Roy. Soc. 1856-7 ; 
Mosely, op. cit. 1869; Oroll, “Climate and Time,” 1875; Hopkins, Phil, Mag. 1845; 
Phil. Trans. 1862; Helmholtz, Heidelberg Verhandl, Nat, Med. 1865, p. 194; Phil. 
Mag. 1806, p, 22; Pfaff, Akad. Bayer. 1876, 
