over the bottom to and fro
twice each day, with a maximum journey of a hundred or two feet each
way from a fixed point. This movement would be so slow that it could
not stir the fine sediment; its only influence would perhaps be to
help feed the animals which were fixed upon the bottom by drawing the
nurture-bringing water by their mouths.

Although the divided condition of the ocean perturbs the action of the
tides, so that except by chance their waves are rarely with their
centres where the attracting bodies tend to make them, the influence
of these divisions is greatly to increase the geological or
change-bringing influences arising from these movements. When from the
southern ocean the tides start to the northward up the bays of the
Atlantic, the Pacific, or the Indian Ocean, they have, as before
noted, a height of perhaps less than two feet. As they pass up the
narrowing spaces the waves become compressed--that is, an equal volume
of moving water has less horizontal room for its passage, and is
forced to rise higher. We see a tolerably good illustration of the
same principle when we observe a wind-made wave enter a small recess
of the shore, the sides of which converge in the direction of the
motion. With the diminished room, the wave gains in height. It thus
comes about that the tide throughout the Atlantic basin is much higher
than in the southern ocean. On the same principle, when the tide rolls
in against the shores every embayment of a distinct kind, whose sides
converge toward the head, packs up the tidal wave, often increasing
its height in a remarkable way. When these bays are wide-mouthed and
of elongate triangular form, with deep bottoms, the tides which on
their outer parts have a height of ten or fifteen feet may attain an
altitude of forty or fifty feet at the apex of the triangle.

We have already noted the fact that the tide, such as runs in the
southern ocean, exercises little or no influence upon the bottom of
the sea over which it moves. As the height of the confined waters
increases, the range of their journey over the bottom as the wave
comes and goes rapidly increases. When they have an elevation of ten
feet they can probably stir the finer mud on the ocean floor, and in
shallow water move yet heavier particles. In the embayments of the
land, where a great body of water journeys like an alternating river
into extensive basins, the tidal action becomes intense; the current
may be able to swee