f air caught under the cloth
will throw it into a motion very similar to that which the wind imparts
to the snow-sheets, only that the snow-sheets will run down instead of
up. Under a good head of wind there is a vehemence in this motion
that suggests anger and a violent disposition. The sheets of snow
are "flapped" down. Then suddenly the direction of the wind changes
slightly, and the sheet is no longer flapped down but blown up. At the
line where the two motions join we have that edge the appearance
of which suggested to me the comparison with "exfoliated" rock in
a previous paper. It is for this particular stage in the process of
bringing about that appearance that I tentatively proposed the term
"adfoliation." "Adfoliated" edges are always to be found on the lee side
of the sheet.

Sometimes, however, the opposite process will bring about nearly the
same result. The snow-sheet has been spread, and a downward sweep of
violent wind will hit the surface, denting it, scraping away an edge
of the top layer, and usually gripping through into lower layers; then,
rebounding, it will lift the whole sheet up again, or any part of it;
and, shattering it into its component crystals, will throw these aloft
and afar to be laid down again further on. This is true "exfoliation."
Since it takes a more violent burst of wind to effect this true
exfoliation than it does to bring about the adfoliation, and since,
further, the snow once indented, will yield to the depth of several
layers, the true exfoliation edges are usually thicker than the others:
and, of course, they are always to be found on the wind side.

Both kinds of lines are wavy lines because the sheets of wind are
undulating. In this connection I might repeat once more that the
straight line seems to be quite unknown in Nature, as also is uniformity
of motion. I once watched very carefully a ferry cable strung across
the bottom of a mighty river, and, failing to discover any theoretical
reason for its vibratory motion, I was thrown back upon proving to my
own satisfaction that the motion even of that flowing water in the river
was the motion of a pulse; and I still believe that my experiments were
conclusive. Everybody, of course, is familiar with the vibrations of
telephone wires in a breeze. That humming sound which they emit would
indeed be hard to explain without the assumption of a pulsating blow. Of
course, it is easy to prove this pulsation in air. From certain fur