ow no time for the abrasion which
produces the smooth surfaces of water-worn pebbles or the polished and
scratched surfaces of glacier-worn ones. In the latter case, we have
seen that the pebbles, being so set in the ice as to expose only one
side, may be only partially polished, while others, more loosely held
and turning in their sockets, may receive the same high polish on every
side. In such a case the lines will intersect one another, in
consequence of the different position in which the stone has been held
at different times. No such appearances exist in the water-worn pebbles:
their blunt surfaces, smoothed and rounded uniformly by the action of
the water in which they have been rolled or tossed about, present
everywhere the same aspect.

The correlation between these different loose materials and the position
in which they are found helps us also to detect their origin. The loose
materials bearing glacier-marks are always found resting upon surfaces
which have been worn, abraded, and engraved in the same manner, while
the water-worn pebbles are everywhere found resting upon rocks the
abrasion of which may be traced to water. It is true that in some
localities, as, for instance, in the gravel-pit of Mount Auburn, near
Cambridge, large masses of glacier-worn pebbles alternate with
beach-shingle; but it is easy to show that there was here a glacier
advancing into the sea, crowding its front moraine and the materials
carried under it over and into the shingle washed up by the waves upon
the beach. 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