13

by all the rivers of the world is found to be probably not far
from 175 million tonnes.[1] Dividing this into the mass of
oceanic sodium we get the age as 80.7 millions of years. Certain
corrections have to be applied to this figure which result in
raising it to a little over 90 millions of years. Sollas, as the
result of a careful review of the data, gets the age as between
80 and 150 millions of years. My own result[2] was between 80 and
90 millions of years; but I subsequently found that upon certain
extreme assumptions a maximum age might be arrived at of 105
millions of years.[3] Clarke regards the 80.7 millions of years
as certainly a maximum in the light of certain calculations by
Becker.[4]

The order of magnitude of these results cannot be shaken unless
on the assumption that there is something entirely misleading in
the existing rate of solvent denudation. On the strength of the
results of another and

[1] F. W. Clarke, _A Preliminary Study of Chemical Denudation_
(Smithsonian Miscellaneous Collections, 1910).

[2] _Loc. cit._

[3] "The Circulation of Salt and Geological Time" (Geol. Mag.,
1901, p. 350).

[4] Becker (loc. cit.), assuming that the exposed igneous and
archæan rocks alone are responsible for the supply of sodium to