272 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 



Prof. 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 correc- 

 tions, 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 winch drain to the ocean. The mean value found for the 

 denudation factor is 68.4 tons. 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 2,735 millions of tons. From the chemical 

 analyses of this saline matter for the several rivers, an average com- 

 position for each continent 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 annually from rivers to ocean. In 

 this way it is found that 175,040,000 metric tons of sodium are annu- 

 ally discharged into the sea. Clarke next finds the total amount of 

 sodium in the ocean to be 14,130 X10 12 tons. My own results were 

 based on a slightly higher value — 15,61lXl0 12 tons. From his fig- 

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



Although the numerous analyses which go to build up this result 

 are not of equal value, there are certain satisfactory features in the 

 computation. 



It is explained by Clarke that in the wonderfully detailed analyses 

 of the Mississippi by Dole and Stabler, taken along with their work 

 on other great rivers of North America and with the observations of 

 Forbes and Skinner for Colorado, data have been obtained for the 

 United States which are not likely to be much altered by any future 

 analysed. Twenty-two river basins enter into the mean for the 

 United States, giving a mean denudation factor of 79 tons. For the 

 rest of North America an estimate only is possible; but, for reasons 

 given, Clarke concludes that "if we assume that 6 millions of square 

 miles of North America lose 79 metric tons in solution per square 

 mile per annum, and that the composition of the saline matter so 

 transported is that found for the United States alone, we shall not bo 

 very far from the truth." Possessing thus a standard based on the 

 drainage of a great continent, we feel confidence in our criticism of 

 other data. The quantity of water thus dealt with is rather more 



