THE BIRTH-TIME OF THE WORLD 



43 



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 



We see that the ratio of the weight of water to the 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 dis- 

 charges, 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 io 12 tonnes of water 

 per annum — the river-born detritus is 1*07 x io 10 tonnes. To 

 this an addition of 1 1 per cent has to be made for silt pushed 

 along the river-bed. 2 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. 



1 Russell, River Deveiop?ne?it (John Murray, 1898). 



2 According to observations made on the Mississippi (Russell, he. at.). 



