PROGRESSION OF GLACIERS. 
247 
one winter melts away from the glacier up to tlie 
so-called snow-line, there will be seen cropping 
out below and beyond that line the layers of the 
preceding years, which are already partially trans- 
formed into ice, and have become a part of the 
frozen mass of the glacier with which they are 
moving onward and downward. In the autumn, 
when the dust of a whole season has been accu- 
mulated upon the service of the preceding win- 
ter’s snow, the extent of the layer which year 
after year will henceforth crop out lower down, 
as a dirt-band, may best be appreciated. 
Beside the snow-layers and the sheets of dust 
alternating with them, there is still another feat- 
ure of the horizontal and parallel structure of the 
mass in immediate connection with those above 
considered. I allude to the layers of pure com- 
pact ice occurring at different intervals between 
the snow-layers. In July, when the snow of the 
preceding winter melts up to the line of perpet- 
ual snow, the masses above, which are to with- 
stand the summer heat and become part of the 
glacier forever, or at least until they melt away 
at the lower end, begin to undergo the changes 
through which all snow passes before it acquires 
the character of glacial ice. It thaws at the sur- 
face, is rained upon, or condenses moisture, thus 
becoming gradually soaked, and aftoi assuming 
the granular character of n6v6-ice, it ends in be- 
