{p. 80}

But it would be absurd to suppose that the beds of rivers could have
furnished the immeasurable volumes of gravel found over a great part
of the world in the drift-deposits.

And the drift-gravel is different from the gravel of the sea or
rivers.

Geikie says, speaking of the "till":

"There is something very peculiar about the shape of the stones. They
are neither round and oval, like the pebbles in river-gravel, or the
shingle of the sea-shore, nor are they sharply angular like
newly-fallen _débris_ at the base of a cliff, although they more
closely resemble the latter than the former. They are, indeed,
angular in shape, but the sharp corners and edges have _invariably
been smoothed away_. . . . Their shape, as will be seen, is by no
means their most striking peculiarity. Each is smoothed, polished,
and covered with striæ or scratches, some of which are delicate as
the lines traced by an etching-needle, others deep and harsh as the
scores made by the plow upon a rock. And, what is worthy of note,
most of the scratches, coarse and fine together, seem to run parallel
to the longer diameter of the stones, which, however, are scratched
in many other directions as well."[1]

Let me again summarize:

I. Comets consist of a blazing nucleus and a mass of ponderable,