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, 1. 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 
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 14,130X10” tons. My own results were 
based on a slightly higher value—15,611X10” 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 
analyses. 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 be 
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 
