n of the land may be appraised at their true value by the
ascertained facts of solvent denudation, as the following table
shows.

Tonnes removed in Mean elevation.
solution per square Metres.
mile per annum.
North America - 79 700
South America - 50 650
Europe - 100 300
Asia - 84 950
Africa - 44 650

In this table the estimated number of tonnes of matter in
solution, which for every square mile of area the rivers convey
to the ocean in one year, is given in the first column. These
results are compiled by Clarke from a very large number of
analyses of river waters. The second column of the table gives
the mean heights in metres above sea level of the several
continents, as cited by Arrhenius.[2]

Of all the denudation results given in the table, those relating
to North America and to Europe are far the

[1] See also Poulton, Address to Sect. D., Brit. Assoc. Rep.,
1896.

[2] _Lehybuch dev Kosmischen Physik_, vol. i., p. 347.

17

most reliable. Indeed these may be described as highly reliable,
being founded on some thousands of analyses, many of which have
been systematically pursued through every season of the year.
These show that Europe with a mean altitude of less than half
that of North America sheds to the ocean 25 per cent. more salts.
A result which is to be expected when the more important factors
of solvent denudation are given intelligent consideration and we
discriminate between conditions favouring solvent and detrital
denudation respectively: conditions in many cases
antagonistic.[1] Hence if it is true, as has been stated, that we
now live in a period of exceptionally high continental elevation,
we must infer that the average supply of salts to the ocean by
the rivers of the world is less than over the long past, and
that, therefore, our estimate of the age of the Earth as already
given is excessive.

There is, however, one condition which will operate to unduly
diminish our estimate of geologic time, and it is a condition
which may possibly obtain at the present time. If the land is, on
the whole, now sinking relatively to the ocean level, the
denudation area tends, as we have seen, to move inwards. It will
thus encroach upon regions which have not for long periods
drained to the ocea