until a portion of the material is triturated; you then find that the
pulverized part has packed around and protected the larger fragments,
and the work is brought to a stand-still. You have to remove the
finer material if you would crush the pieces that remain.

The sea does not separate the sand from the gravel; it places all
together at elevations where the waves can not reach them:

"Waves or shallow soundings have some transporting power; and, as
they always move toward the land, their action is landward. They thus
beat back, little by little, any _detritus_ in the waters, preventing
that loss to continents or islands which would take place if it were
carried out to sea."[1]

The pebbles and gravel are soon driven by the waves up the shore, and
beyond the reach of further wear;[2] and "_the rivers carry only silt
to the ocean_."[3]

The brooks and rivers produce much more gravel than the sea-shore:

"The _detritus_ brought down by rivers is vastly greater in quantity
than the stones, sand, or clay produced by the wear of the coasts."[4]

[1. Dana's "Text Book," p. 288.

2. Ibid., p. 291.

3. Ibid., p. 302.

4. Ibid., p. 290.]