SNOW AND ICE. 
1 1 7 
towards the centre of the glacier. Tyndall points out 
that if a plastic substance, such as mud, be allowed 
to flow down a sloping canal, the lateral portions, 
being held back by the sides, will be outstripped by 
the centre. Now if three circles (Fig. 29) be stamped 
on the mud-stream, the central one will retain its 
form, but the two lateral ones will gradually elongate. 
The shorter axis in 1 n m of each oval is a line of 
pressure, the longer is a line of strain, consequently 
Q 
1 1 
0 
' r ‘ 
0 
P ii 
0 Tzmm 
Fig. 29. 
along the line ni m, or across the tension, we have, 
as already explained, the marginal crevasses; while 
across the line, or perpendicular to the pressure, we 
have the veined structure, which is in fact a form of 
cleavage. Indeed, tension and pressure go together, 
the one acting at right angles to the other. Passing 
to the cases of transverse veining, we find if we walk 
up a glacier presenting this structure that we eventu- 
ally come to an ice-fall or cascade. At the foot of 
the fall the ice is compressed, and this gives rise 
to transverse veining. Longitudinal veining in thd 
