Not infrequently, also, river-pebbles may be found among
glacial materials. This is especially the case where, after the
disappearance of large glaciers, rivers have occupied their beds.
Examples of this kind may be seen in all the valleys of the Alps.
But, besides the special character of the individual fragments, the true
origin of any accumulation of glacier-_debris_, commonly called drift,
may be detected by the total absence of stratification, so essential a
feature in all water-deposits. This absence of stratification throughout
its mass is, after all, the great and important characteristic of the
drift; and though I have alluded to it before, I reiterate it here, as
that which distinguishes it from all like accumulations under water. I
may be pardoned for dwelling upon this point, because the great
controversy among geologists respecting the nature and origin of the
sheet of loose materials scattered over a great 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.
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