GLACIERS AND ICE. 
49 
the knee, so the glacier breaks up into huge wedges 
of ice when passing over an uneven bed. After a level 
surface has been attained, the broken masses are forced 
together by the pressure from behind, they re-unite, and 
scarcely a trace is left of the gaping chasms above. The 
marginal crevasses, it must, however, be remembered, still 
continue* to be formed from the onward movement of the 
glacier. At a favourable point of view there can be seen 
across the glacier curved streaks of mud, known as dirt-bands , 
faintly shown on the Glacier du Geant in fig. 1. These are sup- 
posed to have their origin mainly in the great ice cascades of 
the Alps ; the debris of the glacier being washed into the 
fissures, the subsequent melting of the ice leaves the silt 
in successive curved stripes. In addition to the marginal and 
transverse, there are also longitudinal crevasses which appear 
at the terminal portions of some glaciers, and are induced by 
a resistance in front, causing a lateral movement where the 
glacier has room to expand. 
Such are the main phenomena of glacier motion. The 
question now arises, liow can so brittle a solid as ice flow 
like a river, or bend hither and thither like a stream of 
molten lava ? Looking at this latter comparison, it suggested 
itself to Principal Forbes that ice was probably not so solid 
as we had hitherto supposed it; but that its nature might 
approximate to that of a plastic body like wax or dough ; or, 
indeed, it might be of a viscous character like tar, honey, and 
Canada balsam. This extremely bold idea gave rise to a 
certain explanation of glacier motion, known as Forbes's, 
the plastic, or the viscous theory. According to this theory 
ice is assumed to be plastic or viscid, and from the possession 
of this property glaciers move through their channels just as 
lava moves down a mountain side, and several glacier- streams 
unite just as several streams of lava would. But imagine a 
stream of dough flowing down the Alps ; it would move without 
rupture and over a moderate declivity it would fall without 
breaking. Indeed, one of the distinguishing characteristics 
of a plastic body is that it can be stretched or drawn out into 
filaments when under tension. But though the passage of 
the glacier through narrow gorges shows that ice can yield 
to pressure, observation has failed to indicate its power of 
stretching even in the smallest degree ; otherwise the crevasses 
would not be formed. The plastic theory fails, therefore, to 
account for all the facts. 
We are not, however, left to bare speculation for an expla- 
nation of the wonderful power of accommodation possessed 
by ice. One of Mr. Faraday's simple but suggestive experi- 
ments put the key of glacier motion into the hands of Professor 
VOL. v. — NO. XVIII. E 
