does not go to increase the great body of detrital deposits. We
cannot, therefore, take the present river supply of sediment as
representing that obtaining over the long past. If the land was
all covered still with primary rocks we might do so. It has been
estimated that about 25 per cent. of the existing continental
area is covered with archæan and igneous rocks, the remainder
being sediments.[2] On this estimate we may find valuable

[1] Clarke, _A Preliminary Study of Chemical Denudation_
(Washington, 1910). My own estimate in 1899 (_loc. cit._) made as a
test of yet another method of finding the age, showed that the
sediments may be taken as sufficient to form a layer 1.1 mile
deep if spread uniformly over the continents; and would amount to
64 x 1018 tons.

[2] Van Tillo, _Comptes Rendues_ (Paris), vol. cxiv., 1892.

9

major and minor limits to the geological age. If we take 25 per
cent. only of the present river supply of sediment, we evidently
fix a major limit to the age, for it is certain that over the
past there must have been on the average a faster supply. If we
take the entire river supply, on similar reasoning we have what
is undoubtedly a minor limit to the age.

The river supply of detrital sediment has not been very
extensively investigated, although the quantities involved may be
found with comparative ease and accuracy. The following table
embodies the results obtained for some of the leading rivers.[1]