The Birth-Time of the World | Page 6

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 arch?an 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 not be so, however, for the rate of supply of original sediment must have been continually diminishing
[1] According to observations made on the Mississippi (Russell, _loc. cit._).
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during geological time, and hence we may assume that the rate of advance of the sediments on the primary rocks has also been diminishing. Now we may probably take, as a fair assumption, that the sediment-covered area was at any instant increasing at a rate proportionate to the rate of supply of sediment; that is, to the area of primary rocks then exposed. On this assumption the age is found to be 87 millions of years.
THE AGE BY THE SODIUM OF THE OCEAN
I have next to lay before you a quite different method. I have already touched upon the chemistry of the ocean, and on the remarkable fact that