228 THE FORMATION OF GLACIERS. 
nivalis) well known in the Arctics, where it form) 
wide fields in the summer. 
With the above facts before us concerning the 
materials of which glaciers are composed, we may 
now proceed to consider their structure more 
fully in connection with their movements and the 
effects they produce on the surface over which 
they extend. It has already been stated that the 
ice of the glaciers has not the same appearance 
everywhere, but differs according to the level at 
which it stands. In consequence of this we 
distinguish three very distinct regions in these 
frozen fields, the uppermost of which, upon the 
sides of the steepest and highest slopes of the 
mountain-ridges, consists chiefly of layers of snow 
piled one above another by the successive snow- 
falls of the colder seasons, and which would re- 
main in uniform superposition but for the change 
to which they are subjected in consequence of a 
gradual downward movement, causing the mass 
to descend by slow degrees, while new accumula- 
tions in the higher regions annually replace the 
snow which has been thus removed to an inferior 
level. We shall consider hereafter the process by 
which this change of position is brought about. 
For the present it is sufficient to state that such 
a transfer, by which a balance is preserved in 
tlio distribution of the snow, takes place in all 
glaciers, so that, instead of increasing indefinitely 
