boulder beach, where the bits roll to and
fro whenever they are struck by heavy surges. The greater portion of
them remain in this mill until they are ground to the state of sand
and mud. Now and then one of the fragments is tossed up beyond the
reach of the waves, and is contributed to the wall of the beach. In
very heavy storms these pebbles which are thrown inland may amount in
weight to many tons for each mile of shore.

The study of a pebbly beach, drawn from crest to the deep water
outside, will give an idea as to the history of its work. On either
horn of the crescent by which the pebbles are imported into the pocket
we find the largest fragments. If the shore of the bay be long, the
innermost part of the recess may show even only very small pebbles, or
perhaps only fine sand, the coarser material having been worn out in
the journey. On the bottom of the bay, near low tide, we begin to find
some sand produced by the grinding action. Yet farther out, below
high-tide mark, there is commonly a layer of mud which represents the
finer products of the mill.

Boulder beaches are so quick in answering to every slight change in
the conditions which affect them that they seem almost alive. If by
any chance the supply of detritus is increased, they fill in between
the horns, diminish the incurve of the bay, and so cause its beach to
be more exposed to heavy waves. If, on the other hand, the supply of
grist to the mill is diminished, the beach becomes more deeply
incurved, and the wave action is proportionately reduced. We may say,
in general, that the curve of these beaches represents a balance
between the consumption and supply of the pebbles which they grind up.
The supply of pebbles brought along the shore by the waves is in many
cases greatly added to by a curious action of seaweeds. If the bottom
of the water off the coast is covered by these fragments, as is the
case along many coast lines within the old glaciated districts, the
spores of algae are prone to take root upon them. Fastening themselves
in those positions, and growing upward, the seaweeds may attain
considerable size. Being provided with floats, the plant exercises a
certain lifting power on the stone, and finally the tugging action of
the waves on the fronds may detach the fragments from the bottom,
making them free to journey toward the shore. Observing from near at
hand the straight wall of the wave in times of heavy storm, the
present writer has seen