by measurements of the mass of the sediments, and then the method
becomes more definite. The new method is pursued as follows:

[1] Geikie, _Text Book of Geology_ (Macmillan, 1903), vol. i., p.
73, _et seq._ Sollas, _loc. cit._ Joly, _Radioactivity and Geology_
(Constable, 1909), and Phil. Mag., Sept. 1911.

7

The total mass of the sediments formed since denudation began may
be ascertained with comparative accuracy by a study of the
chemical composition of the waters of the ocean. The salts in the
ocean are undoubtedly derived from the rocks; increasing age by
age as the latter are degraded from their original character
under the action of the weather, etc., and converted to the
sedimentary form. By comparing the average chemical composition
of these two classes of material--the primary or igneous rocks and
the sedimentary--it is easy to arrive at a knowledge of how much
of this or that constituent was given to the ocean by each ton of
primary rock which was denuded to the sedimentary form. This,
however, will not assist us to our object unless the ocean has
retained the salts shed into it. It has not generally done so. In
the case of every substance but one the ocean continually gives
up again more or less of the salts supplied to it by the rivers.
The one exception is the element sodium. The great solubility of
its salts has protected it from abstraction, and it has gone on
collecting during geological time, practically in its entirety.
This gives us the clue to the denudative history of the
Earth.[1]

The process is now simple. We estimate by chemical examination of
igneous and sedimentary rocks the amount of sodium which has been
supplied to the ocean per ton of sediment produced by denudation.
We also calculate

[1] _Trans. R.D.S._, May, 1899.

8

the amount of sodium contained in the ocean. We divide the one
into the other (stated, of course, in the same units of mass),
and the quotient gives us the number of tons of sediment. The
most recent estimate of the sediments made in this manner affords
56 x 1016 tonnes.[1]

Now we are assured that all this sediment was transported by the
rivers to the sea during geological time. Thus it follows that,
if we can estimate the average annual rate of the river supply of
sediments to the ocean over the past, we can calculate the
required age. The land surface is at present largely covered with
the sedimentary rocks themselves. Sediment derived from these
rocks