to allow any rigid
substance to bridge over the space in its descent from the summit to the
surface below, that we find these conditions; when the knolls are low
and slope gently downward in every direction, they present the
characteristic glacier-surfaces equally on all sides. This circumstance
should be borne in mind by all who investigate the traces of
glacier-action; for this inequality in the surfaces presented by the
opposite sides of any obstacle in the path of the ice is often an
important means of determining the direction of its motion.

The other characteristic peculiarity of these _roches moutonnees_
consists in the direction of the glacier-scratches, which ascend the
slope to its summit in a direct line on one side, while they deviate to
the right and left on the other sides of the knoll, more or less
obliquely according to its steepness. Occasionally, large boulders may
be found perched on the very summit of such prominences. Their position
is inexplicable by the supposition of currents as the cause of their
transportation. Any current strong enough to carry a boulder to such a
height would of course sweep it on with it. This phenomenon finds,
however, an easy explanation in the glacial theory. The thickness of
such a sheet of ice is of course less above such a hill or mound than
over the lower levels adjoining it. Not only will the ice melt,
therefore, more readily at this spot, but, as ice is transparent to
heat, the summit of the prominence will become warmed by the rays of the
sun, and will itself facilitate the melting of the ice above it. On the
breaking up of the ice, therefore, such a spot will be the first to
yield, and allow the boulders carried on the back of the glacier to fall
into the hollow thus formed, where they will rest upon the projecting
rock left uncovered. This is no theoretical explanation; there are such
cases in Switzerland, where holes in the ice are formed immediately
above the summit of hills or prominences over which the glacier passes,
and into which it drops its burdens. Of course, where the ice is
constantly renewed over such a spot by the onward progress of the
glacier, these materials may be carried off again; but if we suppose
such a case to occur at the breaking up of the glacier-period, when the
ice was disappearing forever from such a spot, it is easy to account for
the poising of these large boulders on prominent peaks or ledges.

The appearances about the _roches m