248 
INTERNAL STRUCTURE AND 
ing transformed into pure compact ice. Toward 
the end of August, or early in September, when 
the nights are already very cold in the Alps, but 
prior to the first permanent autumnal snow-falls, 
the surface of these masses becomes frozen to a 
greater or less depth, varying, of course, accord- 
ing to temperature. These layers of ice become 
numerous and are parallel to each other, like the 
layers of ice formed from slosh. Such crusts of 
ice I have myself observed again and again upon 
the glacier. This stratified snowy ice is now the 
bottom on which the first autumnal snow-falls ac- 
cumulate. These sheets of ice may be formed 
not only annually before the winter snows set in, 
but may recur at intervals whenever water accu- 
mulating upon an extensive snow-surface, either 
in consequence of melting or of rain, is frozen 
under a sharp frost before another deposit of 
snow takes place. Or suppose a fresh layer of 
light porous snow to have accumulated above one, 
the surface of which has already been slightly 
glazed with frost ; rain or dew, falling upon the 
upper one, will easily penetrate it ; but when it 
reaches the lower one, it will be stopped by the 
film of ice already formed, and, under a suffi- 
ciently low temperature, it will be frozen between 
the two. This result may be frequently noticed 
in winter, on the plains, where sudden changes 
of temperature take place. 
