t be accounted
for by the supposition of gradual thaw of small amount being applied to
a sheet of prismatic ice.

One fact was remarkable from its universal appearance. Wherever an
incision was made in this sheet of ice, the prisms snapped off at the
depth of an inch, and could be mowed down like corn by means of a stout
knife. Although they broke naturally at this constant depth, and left a
surface of limpid ice without any signs of external or internal
division, still the laminae obtained by chiselling this lower surface
carefully, broke up regularly into the shapes to be expected in sections
of prisms cut at right angles to the axis. The roughness of my
instruments made it impossible to discover how far this extended, and
whether it ceased to be the case at any given depth in the ice.

The sea of ice on the floor was in a very wet state at the surface,
being at a lower level than the stones on to which the rain from the
open hole fell; and here the prismatic structure was not apparent to the
eye, nor do I know whether it existed at all. In the Glaciere of La
Genolliere I carried a large block of perfectly prismatic ice into the
outer cave, where it was exposed to the free currents of air passing
from the pit of entrance to the hole newly opened by the falling in of
the ground; and, two days after, the external lines were scarcely
perceptible, while on the occasion of our third visit I found that they
had entirely disappeared, and the whole block was rapidly following
their example. This disappearance of the surface-lines under the action
of atmospheric thaw is probably the same thing as their absence when the
flooring of ice is thinly covered with water. Wherever the flooring rose
slightly towards the edges of the sea of ice, the usual structure
appeared again.

There were no currents of air in the cave, the candles burning steadily
through the whole time of our visit. Excepting for the purpose of
detecting disturbance in the air, there is no need of candles, as the
two holes in the roof supply sufficient light. Some account of the
careful observations made here by M. Thury, at different seasons of the
year, will be found in other parts of this book. We passed, on our
return, by the source of water which springs from the foot of a rock at
some distance from the glaciere, and is supposed to form the outlet for
the drainage of the cave; but it is difficult to understand how this can
be the case, considering the form