p along large stones quite as effectively as a
mountain torrent. Thus near Eastport, Me., where the tides have a
maximum rise and fall of over twenty feet, the waters rush in places
so swiftly that at certain stages of the movement they are as much
troubled as those at the rapids of the St. Lawrence. In such portions
of the shore the tides do important work in carving channels into the
lands.

Along the shores of the continents about the North Atlantic, where the
tides act in a vigorous manner, we almost everywhere find an
underwater shelf extending from the shore with a declivity of only
five to ten feet to the mile toward the centre of the sea, until the
depth of about five hundred feet is attained; from this point the
bottom descends more steeply into the ocean's depth. It is probable
that the larger part of the material composing these continental
shelves has been brought to its position by tidal action. Each time
the tidal wave sweeps in toward the shore it urges the finer particles
of sediment along with it. When it moves out it drags them on the
return journey toward the depths of the sea. If this shelf were
perfectly horizontal, the two journeys of the sand and mud grains
would be of the same length; but as the movement takes place up and
down a slope, the bits will travel farther under the impulse which
leads them downward than under that which impels them up. The result
will be that the particles will travel a little farther out from the
shore each time it is swung to and fro in the alternating movement of
the tide.

The effect of tidal movement in nurturing marine life is very great.
It aids the animals fixed on the bottoms of the deep seas to obtain
their provision of food and their share of oxygen by drawing the water
by their bodies. All regions which are visited by strong tides
commonly have in the shallows near the shores a thick growth of
seaweed which furnishes an ample provision of food for the fishes and
other forms of animal life.

A peculiar effect arising from tidal action is believed by students of
the phenomena to be found in the slowing of the earth's rotation on
its axis. The tides rotate around the earth from east to west, or
rather, we should say, the solid mass of the earth rubs against them
as it spins from west to east. As they move over the bottom and as
they strike against the shores this push of the great waves tends in a
slight measure to use up the original spinning impulse which