mechanical work of the ocean is mostly confined to its shores and
soundings, where alone material exists in quantity within reach of
the waves and currents.[2] . . . The eroding action is greatest for a
short distance above the height of half-tide, and, except in violent
storms, it is almost null below low-tide."[3]

But if any one will examine a sea-beach he will see, not a vast mass
of pebbles perpetually rolling and grinding each other, but an
expanse of sand. And this is to be expected; for as soon as a part of
the pebbles is, by the attrition of the waves, reduced to sand, the
sand packs around the stones and arrests their further waste. To form
such a mass of gravel as is found in the Drift we

[1. "Edinburgh Review," October, 1874, p. 202.

2. Dana's "Text Book," p. 286.

3. Ibid., p. 287.]

{p. 79}

must conceive of some way whereby, as soon as the sand is formed, it
is removed from the stones while the work of attrition goes on. This
process we can conceive of in a comet, if the finer _detritus_ is
constantly carried back and arranged in the order of the size of its
particles.

To illustrate my meaning: let one place any hard substance,
consisting of large fragments, in a mortar, and proceed to reduce it
with a pestle to a fine powder. The work proceeds rapidly at first,