xv.] FORBES' GLACIER DISCOVERIES. 497 



over the subject, the Dilatation Theory and the (so- 

 called) Gravitation Theory ; both of which, as enunciated 

 by their respective supporters, are inconsistent with 

 simple dynamical principles. Before, however, we ex- 

 plain them, we shall first, as briefly as is consistent with 

 ck'urness, describe the phenomena of glaciers as they 

 appear to the superficial observer. 



The masses of snow which fall on the higher Alps, and 

 on other elevated mountain ridges, are only in small part 

 melted or evaporated where they fall. They descend, 

 sometimes as avalanches, into the higher valleys, which 

 thus become choked with a mass of half snow, half ice, 

 calk-d n<'c<\ far too great to be removed by melting or 

 t-\ a JH .ration in the course of a single season. Thus there 

 \vt u Id be a gradual accumulation from year to year, unless 

 some third mode of escape presented itself. It is observed 

 that at the lower ends of these valleys the neve escapes 

 (often through deep and narrow gorges), but now in a 

 state more resembling solid ice. This gradually works 

 its way towards the plains, carrying with it on its 

 surface sand, gravel, and even immense masses of rock, 

 arranged in what are called moraines. Sometimes such 

 masses are wholly imbedded in the glacier ; sometimes 

 they project from its lower or lateral surfaces, and in 

 this case they grind out its channel as they proceed. 



For it is obvious to the most casual observer that 

 the glacier ice moves down its channel, though not at 

 any great speed, and it was long ago recognized by 

 Playfair ! as the most efficient mode of transport of those 

 ailed erratic blocks which had so much puzzled his 

 lecessors. To the pedestrian on its surface the glacier 

 presents, especially towards its sides, huge cracks called 

 <i88es y whose direction is at 1 a>t a pproximately trans- 

 verse to its bed, and which are often of great breadth, 

 and may sometimes extend downwards to the very bottom 

 of the glacier. 



1 /Ututrationi of the HuUonian Theory, 1802, ">\(l Also Jame- 

 son (1827). See paper by Forbes, Edin. New rinl. Jnunvtl, xl., 

 -p. 99. 



K K 



