the derived sediment to more than 1.5
kilometres, or about one mile deep.

The slow accomplishment of results so vast conveys some idea of
the great duration of geological time.

47

The foregoing method of investigating the statistics of solvent
denudation is capable of affording information not only as to the
amount of sediments upon the land, but also as to the quantity
which is spread over the floor of the ocean.

We see this when we follow the fate of the 33 per cent. of
dissolved salts which has been leached from the parent igneous
rock, and the mass of which we calculate from the ascertained
mass of the latter, to be 27 x 1016 tonnes. This quantity was at
one time or another all in the ocean. But, as we saw above, a
certain part of it has been again abstracted from solution,
chiefly by organic agencies. Now the abstracted solids have not
been altogether retained beneath the ocean. Movements of the land
during geological time have resulted in some portion being
uplifted along with other sediments. These substances constitute,
mainly, the limestones.

We see, then, that the 27 x 1016 tonnes of substances leached
from the parent igneous rocks have had a threefold destination.
One part is still in solution; a second part has been
precipitated to the bottom of the ocean; a third part exists on
the land in the form of calcareous rocks.

Observation on the land sediments shows that the calcareous rocks
amount to about 5 per cent. of the whole. From this we find that
3 x 1016 tonnes, approximately, of such rocks have been taken
from the ocean. This accounts for one of the three classes of
material

48

into which the original dissolved matter has been divided.
Another of the three quantities is easily estimated: the amount
of matter still in solution in the ocean. The volume of the ocean
is 1,414 million cubic kilometres and its mass is 145 x 1016
tonnes. The dissolved salts in it constitute 3.4 per cent. of its
mass; or, rather more than 5 x 1016 tonnes. The limestones on the
land and the salts in the sea water together make up about 8 x
1016 tonnes. If we, now, deduct this from the total of 27 x 1016
tonnes, we find that about 19 x 1016 tonnes must exist as
precipitated matter on the floor of the ocean.

The area of the ocean is 367 x 1012 square metres, so that if the
precipitated sediment possesses an average specific gravity of
2.5, it would cover the entire floor to a uniform depth of 218
metres; t