SNOW AND ICE. 
107 
Glacier vary much in size, but the majority are under 
an inch across. 
In some cases they are tolerably uniform in size, 
in others large and small are mixed together. On 
any clean surface of glacier ice they are easily visible, 
as for instance in the ice tunnel which is so often 
cut at the end of glaciers. Their surfaces present a 
series of fine paralled striae , first noticed by Forel. 
Glacier ice then may be said to be a granular aggre- 
gate of ice crystals. By alternately warming and 
cooling snow, and saturating it repeatedly with water, 
Forel found that he produced an ice very similar in 
structure to that of glaciers. There seems no doubt 
that this structure considerably facilitates the move- 
ments of glaciers.* 
Glaciers are generally higher in the middle, and 
slope down at the two sides owing to the warmth 
reflected from the rocks. When the valley runs 
north and south the two sides are equally affected in 
this respect; but when the direction is east to west 
or west to east the northern side is most inclined 
because the rocks lie more in the sun, while those to 
the south are more in the shade. 
* Heim, Gletscherkunde . 
