evident that there must be a perceptible difference
in the progress of the successive layers, the lower and older ones
getting the advance of the upper and more recent ones: that is, when the
snow that has covered the face of the country during one winter melts
away from the glacier up to the 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 transformed 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 accumulated upon the surface of the preceding winter'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 feature of the horizontal and parallel structure
of the mass in immediate connection with those above considered. I
allude to the layers of pure compact ice occurring at different
intervals between the snow-layers. In July, when the snow of the
preceding winter melts up to the line of perpetual snow, the masses
above, which are to withstand 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 surface, is
rained upon, or condenses moisture, thus becoming gradually soaked, and
after assuming the granular character of _neve_-ice, it ends in being
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,
according 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 accumulate. These sheets of ice may be formed not only
annually before the winter snows set in, but may recur at intervals
whenever water accumulating upon an extensive snow-surface, either in
consequence of melting or of rain, is frozen under a sharp frost befor