[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.