216 
THE FORMATION OF GLACIERS. 
alternate beds varying in thickness according to 
the intensity of the cold, and its continuance be- 
low the freezing-point during a longer or shorter 
period. Singly, these layers consist of irregular 
crystals confusedly blended together, as in large 
• masses of crystalline rocks in which a crystalline 
structure prevails, though regular crystals occur 
but rarely. The appearance of stratification is 
the result of the circumstances under which the 
water congeals. The temperature varies much 
more rapidly in the atmosphere around the earth 
than in the waters upon its surface. When the 
atmosphere above any sheet of water sinks be- 
low the freezing-point, there stretches over its 
surface a stratum of cold air, determining by its 
intensity and duration the formation of the first 
stratum of ice. According to the alternations of 
temperature, this process goes on with varying 
activity until the sheet of ice is so thick that it 
becomes itself a shelter to the water below, and 
protects it, to a certain degree, from the cold 
without. Thus a given thickness of ice may 
cause a suspension of the freezing process, and 
the first ice-stratum may even be partially thawed 
before the cold is renewed with such intensity as 
to continue the thickening of the ice-sheet by the 
addition of fresh layers. The strata or beds of 
ice increase gradually in this manner, their sep- 
aration being rendered still more distinct by the 
