virtually detached masses. A glacier is undoubtedly competent to root
such masses bodily away. Indeed the mere _a priori_ consideration of
the subject proves the competence of a glacier to deepen its bed.
Taking the case of a glacier 1,000 feet deep (and some of the older
ones were probably three times this depth), and allowing 40 feet of
ice to an atmosphere, we find that on every square inch of its bed
such a glacier presses with a weight of 375 lbs, and on every square
yard of its bed with a weight of 486,000 lbs. With a vertical pressure
of this amount the glacier is urged down its valley by the pressure
from behind. We can hardly, I think, deny to such a tool a power of
excavation.

The retardation of a glacier by its bed has been referred to as
proving its impotence as an erosive agent; but this very retardation
is in some measure an expression of the magnitude of the erosive
energy. Either the bed must give way, or the ice must slide over
itself. We get indeed some idea of the crushing pressure which the
moving glacier exercises against its bed-from the fact that the
resistance, and the effort to overcome it, are such as to make the
upper layers of a glacier move bodily over the lower ones--a portion
only of the total motion being due to the progress of the entire mass
of the glacier down its valley.

The sudden bend in the valley of the Rhone at Martigny has also been
regarded as conclusive evidence against the theory of erosion. 'Why,'
it has been asked, I did not the glacier of the Rhone go straight
forward instead of making this awkward bend?' But if the valley be a
crack, why did the crack make this bend? The crack, I submit, had at
least as much reason to prolong itself in a straight line as the
glacier had. A statement of Sir John Herschel with reference to
another matter is perfectly applicable here: 'A crack once produced
has a tendency to run--for this plain reason, that at its momentary
limit, at the point at which it has just arrived, the divellent force
on the molecules there situated is counteracted only by half of the
cohesive force which acted when there was no crack, viz. the cohesion
of the uncracked portion alone' ('Proc. Roy. Soc.' vol. xii.
p. 678). To account, then, for the bend, the adherent of the fracture
theory must assume the existence of some accident which turned the
crack at right angles to itself; and he surely will permit the
adherent of the erosion theory to make a