detritus thus introduced is, for the most part, fresh and angular. 
Its trituration by the glacier reduces the size of the particles, but 
retains their angular character, so that, as Daubrée has pointed 
out, the sand that escapes from the end of a glacier appears in 

‘Fic. 149.—Granirz (c) DECOMPOSING INTO BLocks (a) WHICH GRADUALLY ROLL 
DOWN UPON THE SURROUNDING STRATIFIED Rocks (B.). 
the condition of sharp freshly-broken grains, and not as rounded 
water-worn particles.* 
The surface of a glacier being often strewn with earth and stones, 
these materials are frequently precipitated into the crevasses, 
and may thus reach the rocky floor over which the ice is moving. 
They likewise fall into the narrow space which sometimes inter- 
venes between the margin of a glacier and the side of the valley 
(a in Fig. 150). Held by the ice as it creeps along, they are 
pressed against the rocky sides and 
bottom of the valley so firmly and per- 
sistently as to descend into each little 
hollow and mount over each ridge, yet 
all the while moving along steadily in 
Fic. 150.—Szctron or A Guacer One dominant direction with the general 
iy 17s Rocky CHANNEL, movement of the glacier, Here and 
With a medial moraine at d,a lateral there the ice, with grains of sand and 
moraine partly on the ice and pieces of stone imbedded in its surface, 
parily stranded on a sloping de- ‘ ; 
clivity (b), a mass of rocks fallen CAD be caught in the very act of polish- 
between the ice and the precipi- ing and scouring the rocks. In F ig. 151 
so ae gar a irks ere = a view is given of the “angle” on the 
Lncbee:) wailed cc eat «ee Glace, Chamouni, where blocks 
of granite are jammed between the 
mural edge of the ice and the precipice of rock along which it moves, 
and which is scored and polished in the direction of motion of the 
blocks. Under the slow, continuous, and enormous erosive power of 
the creeping ice, the most compact resisting rocks are ground down, 
smoothed, polished, and striated. The strie vary from such fine 
lines as may be made by the smallest grains of quartz up to deep 
ruts and grooves. ‘hey sometimes cross each other, one set partially 
effacing an older one, and thus pointing to shiftings in the movement 
of the ice. On the retirement of the glacier, hummocky. bosses of 
1 “Géologie Expéiim,” p, 254. 


> = 
414 - DYNAMICAL GEOLOGY. —‘[Boox III. 
