n a mine,
changing its direction from time to time. I found this to be the case in
one into which I caused myself to be lowered in order to examine the
internal structure of the glacier. For some time my descent was straight
and direct, but at a depth of about fifty feet there was a
landing-place, as it were, from which the opening continued its farther
course at quite a different angle. It is within these cylindrical
openings in the ice that those accumulations of sand collect which form
the pyramids described above.

One may often trace the gradual formation of these wells, because, as
they require certain similar conditions, they are very apt to be found
in various stages of completion along the same track where these
conditions occur. Fissures, for instance, will often be produced along
the same line, because, as the mass of the glacier moves on, its upper
portions, as they advance, come successively in contact with
inequalities of the bottom, in consequence of which the ice is strained
beyond its power of resistance and cracks across. Rivulets are also
likely to be renewed summer after summer over the same track, because
certain conditions of the surface of the glacier, to which I have not
yet alluded, and which favor the more rapid melting of the ice, remain
unchanged year after year. Of course, the wells do not remain stationary
any more than any other feature of the glacier. They move on with the
advancing mass of ice, and we consequently find the older ones
considerably lower down than the more recent ones. In ascending such a
track as I have described, along which fissures and rivulets are likely
to occur, we may meet first with a sand-pyramid; at a certain distance
above that there may be a circular opening filled to its brim with the
sand which has just reached the surface of the ice; a little above may
be an open well with the rivulet still pouring into it; or higher up, we
may meet an open fissure with the two semicircles opposite each other on
the margins, but not yet united, as they will be presently by the
closing of the fissure; or we may find near by another fissure, the
edges of which are just beginning to wear in consequence of the action
of the water. Thus, though we cannot trace the formation of such a
cylindrical shaft in the glacier from the beginning to the end, we may
by combining the separate facts observed in a number decipher their
whole history.

In describing the surface of the glacier,