16 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 56 



the}^ exceed, balance, or only in part compensate the other corrections it 

 is impossible to say. The rough quotient at first obtained, 80,726,000 

 years, is as probable as any other value that might be chosen. That value, 

 as will be shown by Mr. Becker's calculations, is certainly a maximum. 

 It represents, in its essential features, Joly's method of computing the 

 age of the ocean, but takes, with one minor exception, no account of 

 changes of rate in the annual additions to the marine salinity. Joly's 

 final estimate of the age is 90,000,000 years, with a possible increase of 

 6 per cent, due to a revision of the mass of the ocean. 



Eeade, in the investigation cited at the beginning of this paper, esti- 

 mated the solvent denudation of England and Wales at 143.5 British, or 

 146 metric tons per square mile per annum, or one foot in 12,978 years. 

 The smaller denudation factors now found lead to different results. For 

 the United States, excluding the Great Basin, the factor of 79 tons gives, 

 for a lowering of one foot, 23,948 years. For South America the figures 

 are 50 tons and 37,751 years; for Europe, 100 tons and 18,875 years and 

 for the Nile Valley, 16 tons and 93,924 years. For the 40,000,000 square 

 miles of the globe, which drain into the ocean, the average values are 

 68.4 tons and 27,660 years, estimates that are subject to large correc- 

 tions, which Eeade did not take into consideration. The foregoing figures 

 only apply to his method of computation. 



On critical examination of the data it is clear that the total apparent 

 amount of solvent denudation is not a true measure of rock decomposi- 

 tion. In the general mean of all the river analyses now under discussion, 

 0.90 per cent, of NOj and 35.15 per cent, of COg appear. The NO,^ came 

 entirely or practically so from atmospheric sources; the CO3 was derived 

 • partly from the atmosphere and partly from the solution of limestones. 

 Dealing now only with the existing discharge of rivers, we must subtract 

 these atmospheric additions from the total annual load of dissolved in- 

 organic matter, before we can compute the real amount of rock denudation. 



The land surface of the earth is covered, nearly enough for present 

 purposes, by 75 per cent, of sedimentary and 25 per cent, igneous and 

 crystalline rocks; ^ and it is on or near this surface that the flowing waters 

 act. The limestones, as shown in my former discussion, constitute only 

 one-twentieth of the sediments, or 3.75 per cent, of the entire area; but 

 the proportion of carbonates derived from them must be very much 

 larger. The composite and average analyses of rocks give, for lime, 4.81 

 per cent, in the igneous, and 5.42 in all the sedimentaries, equivalent to 

 3.78 and 4.26 per cent, of CO, respectively. Assuming that all the sur- 

 face rocks yield lime at an equal rate, which is obviously not quite true, 

 and multiplying these figures by the areas represented as 1 to 3, the 



1 Estimate by A. von Tillo, actually 75.7 and 24.3. 



