part of the globe turns
upon it. The _debris_ of which the drift consists are thrown together
pell-mell, without any arrangement according to size or weight, larger
and smaller fragments being mixed so indiscriminately that the heaviest
materials may be on the very summit of the mass, and the lightest at the
bottom in immediate contact with the underlying rock, or the larger
pieces may stand at any level in the mass of finer ones. Impalpable
powder, coarse sand, rounded, polished, and scratched fragments of every
size are mixed together in a homogeneous paste, in which the larger
materials are imbedded, to use a homely, but expressive comparison, like
raisins and currants in a pudding. The adhesive paste holding all these
fragments together is, no doubt, the result of the friction to which the
whole was subjected under the glacier, and which has worked some of the
softer materials into a kind of cement.

The mode of aggregation of water-worn materials is very different.
Examine the shingle along our beaches: we find it so distributed as to
show that the fading tide-wave has carried the lighter materials farther
than the heavier ones, and the successive deposits exhibit an imperfect
cross-stratification resulting from changes in the height of the tide
and the direction of the wind. Moreover, in any materials collected
under water we find the heavier ones at the bottom, the lighter on the
top. It is true that large angular boulders may occasionally be found
resting upon beach-shingle, but their presence in such a connection is
easily explained. They may have been dropped there by floating icebergs,
or have fallen from crumbling drift-cliffs.

I should add, in speaking of drift-materials, that, while we find the
large angular boulders resting above them, we occasionally find boulders
of unusual size mingled with them; but, when this is the case, such
massive fragments are more or less rounded, polished, and marked in the
same way as the smaller pebbles, or as the surfaces over which the
glacier has passed. This is important to remember, because, when we
examine the drift in countries where the ice, during the glacier-period,
overtopped nearly all the mountains, so that few fragments could fall
from them upon its surface, we find scarcely any angular boulders, while
the drift is interspersed with larger fragments of this character,
carried under the ice, instead of on its back. Another distinction
between water-worn deposi