358 Prof. J. Joly on the 



These recent discussions chiefly centre round the ascer- 

 tainment o£ the true present rate of supply of sodium to the 

 ocean. The limitations of the method are also discussed. 



My own original estimate of the age of the ocean * was 

 based on the only data then available — the estimates made 

 by Sir John Murray of the average chemical composition of 

 river-water and the probable total annual discharge of the 

 rivers into the ocean. Calculating from its estimated volume 

 and mean chemical composition, the mass of sodium now in 

 the ocean, and dividing this by the calculated amount of 

 sodium entering annually from the rivers, the uncorrected 

 age of 99*4 million years was obtained. To this I applied 

 certain corrections, to some of which I shall refer later. The 

 final result of these corrections left the age as from 80 to 90 

 million years. 



Professor Sollas approaches the question by a recalculation 

 of the average amount of sodium discharged by the rivers 

 annually. He finds that the added results available, as derived 

 from the rivers of North and South America and Europe, 

 give the uncorrected age as 78 million years. After a careful 

 and detailed discussion of the corrections, Sollas concludes 

 that the age lies between 80 and 150 million years ; the latter 

 figure being based on extreme assumptions. 



Clarke bases his discussion of the question upon what he 

 terms the denudation factor, i. e. the number of metric tons 

 annually removed in solution from a square mile of drainage 

 area. This is estimated for a number of important rivers of 

 the world, accounting in this way for a drainage area of 

 28 millions of square-miles out of the total of about 40 millions 

 which drain to the ocean. The mean value found for the denu- 

 dation factor is 68"4 tonnes. Assuming that this denudation 

 factor is a fair average for the whole, the entire matter in 

 solution discharged into the ocean in a year is 2735 millions 

 of tonnes. From the chemical analyses of this saline matter 

 for the several rivers, an average composition for each con- 

 tinent is found. When this is weighted for the quantity of 

 water contributed by each continent, a final weighted mean 

 composition is obtained which may be applied to determining 

 the integral of the sodium passing annuallv from rivers to 

 ocean. In this way it is found that 175,040,000 metric tons 

 of sodium are annually discharged into the sea. Clarke 

 next finds the total amount of sodium in the ocean to be 

 14,130 X 10 12 tonnes. My own results were based on a 

 slightly higher value — 15, 611 x 10 12 tonnes. From his figures, 

 Clarke now gets the uncorrected age as 80,726,000 years. 

 * Trans. R. Dubl. Soc. vol. vii. 1899. 



