SNOW AND ICE. 
I 13 
1. Marginal. 
2. Transverse. 
3. Longitudinal. 
The sides of most glaciers are fissured even when 
the centre is compact. The crevasses do not run in 
the direction of the glacier, but obliquely to it, en- 
closing an angle of about 45 0 (Fig. 29, m m ) and 
pointing upwards, giving an impression that the centre 
of the glacier is left behind by the quicker motion 
of the sides. This was indeed supposed to be the 
cause, until Agassiz and Forbes proved that, on the 
contrary, the centre moved most rapidly. Hopkins 
first showed that the obliquity of the lateral crevasses 
necessarily followed from the quicker movement of 
the centre. 
Tyndall gives the following illustration: — “Let 
A C, in the annexed figure, be one side of the 
glacier, and B D the other; and let the direction of 
the motion be that indicated by the arrow. Let T 
be a transverse slice of the glacier, taken straight 
across it, say to-day. A few days or weeks hence 
this slice will have been carried down, and because 
the centre moves more quickly than the sides it will 
not remain straight, but will bend into the form S' T' . 
“Suppose Ti to be a small square of the original 
slice near the side of the glacier. In its new posi- 
Scenery of Switzerland. /. 8 
