tional support to
the view, that the processes of solvent denudation are
responsible for the saltness of the ocean. The new evidence may
be stated as follows: Estimates of the amounts of sedimentary
rock on the continents have repeatedly been made. It is true that
these estimates are no more than approximations. But they
undoubtedly _are_ approximations, and as such may legitimately be
used in our argument; more especially as final agreement tends to
check and to support the several estimates which enter into
them.

The most recent and probable estimates of the sediments on the
land assign an average thickness of one mile of

44

secondary rocks over the land area of the world. To this some
increase must be made to allow for similar materials concealed in
the ocean, principally around the continental margins. If we add
10 per cent. and assign a specific gravity of 2.5 we get as the
mass of the sediments 64 x 1016 tonnes. But as this is about 67
per cent. of the parent igneous rock--_i.e._ the average igneous
rock from which the sediments are derived--we conclude that the
primary denuded rock amounted to a mass of about 95 x 1016
tonnes.

Now from the mean chemical composition of the secondary rocks we
calculate that the mass of sediments as above determined contains
0.72 x1016 tonnes of the sodium oxide, Na2O. If to this amount we
add the quantity of sodium oxide which must have been given to
the ocean in order to account for the sodium salts contained
therein, we arrive at a total quantity of oxide of sodium which
must be that possessed by the primary rock before denudation
began its work upon it. The mass of the ocean being well
ascertained, we easily calculate that the sodium in the ocean
converted to sodium oxide amounts to 2.1 x 1016 tonnes. Hence
between the estimated sediments and the waters of the ocean we
can account for 2.82 x 1016 tonnes of soda. When now we put this
quantity back into the estimated mass of primary rock we find
that it assigns to the primary rock a soda percentage of 3.0. On
the average analysis given above this should be 3.41 per cent.
The agreement,

45

all things considered, more especially the uncertainty in the
estimate of the sediments, is plainly in support of the view that
oceanic salts are derived from the rocks; if, indeed, it does not
render it a certainty.

A leading and fundamental inference in the denudative history of
the Earth thus finds support: indeed, we may