The Birth-Time of the World | Page 6

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.
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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 must be regarded as, for the most
part, purely cyclical; that is, circulating from the sea to the land and
back again. It 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.
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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]
Mean annual Total annual Ratio of discharge in sediment in sediment
cubic feet thousands to water per second. of tons. by weight.
Potomac - 20,160 5,557 1 : 3.575 Mississippi - 610,000 406,250 1 :
1,500 Rio Grande - 1,700 3,830 1 : 291 Uruguay - 150,000 14,782 1 :
10,000 Rhone - 65,850 36,000 1 : 1,775 Po - 62,200 67,000 1 : 900
Danube - 315,200 108,000 1 : 2,880 Nile - 113,000 54,000 1 : 2,050
Irrawaddy - 475,000 291,430 1 : 1,610
Mean - 201,468 109,650 1 : 2,731
We see that the ratio of the weight of water to the
[1] Russell, River Development (John Murray, 1888).
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weight of transported sediment in six out of the nine rivers does not

vary widely. The mean is 2,730 to 1. But this is not the required
average. The water-discharge of each river has to be taken into account.
If we ascribe to the ratio given for each river the weight proper to the
amount of water it discharges, the proportion of weight of water to
weight of sediment, for the whole quantity of water involved, comes
out as 2,520 to 1.
Now if this proportion holds for all the rivers of the world--which
collectively discharge about 27 x 1012 tonnes of water per annum--the
river-born detritus is 1.07 x 1010 tonnes. To this an addition of 11 per
cent. has to be made for silt pushed along the river-bed.[1] On these
figures the minor limit to the age comes out as 47 millions of years, and
the major limit as 188 millions. We are here going on rather deficient
estimates, the rivers involved representing only some 6 per cent. of the
total river supply of water to the ocean. But the result is probably not
very far out.
We may arrive at a probable age lying between the major and minor
limits. If, first, we take the arithmetic mean of these limits, we get 117
millions of years. Now this is almost certainly excessive, for we here
assume that the rate of covering of the primary rocks by sediments was
uniform. It would