[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]

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).

10

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.