les below the earth's surface. If, then, the rocks contain
an average of even five per cent of water to the depth of one hundred
miles, the quantity of the fluid stored within the earth is greater
than that which is contained in the reservoir of the ocean. The
oceans, on the average, are not more than three miles deep; spread
evenly over the surface of the whole earth, their depth would be less
than two miles, while the water in the rocks, if it could be added to
the seas, would make the total depth seven miles or more. As we shall
note hereafter, the processes of formation of strata tend to imprison
water in the beds, which in time is returned to the earth's surface by
the forces which operate within the crust.

Although the water in the seas is, as we have seen, probably less than
one half of the store which the earth possesses, the part it plays in
the economy of the planet is in the highest measure important. The
underground water operates solely to promote certain changes which
take place in the mineral realm. Its effect, except in volcanic
processes, are brought about but slowly, and are limited in their
action. The movements of this buried water are exceedingly gradual;
the forces which impel it about and which bring it to do its work
originate in the earth. In the seas the fluid has an exceeding freedom
of motion; it can obey the varied impulses which the solar energy
imposes upon it. The role of these wonderful actions which we are
about to trace includes almost everything which goes on upon the
surface of the planet--that which relates to the development of animal
and vegetable life, as well as to the vast geological changes which
the earth is undergoing.

If the surface of the earth were uniformly covered with water to the
depth of ten thousand feet or more, every particle of fluid would, in
a measure, obey the attraction of the sun, of the moon, and,
theoretically, also of all the other bodies in space, on the principle
that every particle of matter in the universe exercises a gravitative
effect on every other. As it is, owing to the divided condition of the
water on the earth's surface, only that which is in the ocean and
larger seas exhibits any measurable influence from these distant
attractions. In fact, only the tides produced by the moon and sun are
of determinable magnitude, and of these the lunar is of greater
importance, the reason being the near position of our satellite to our
own sphere. The so