22 THE AGE OF THE EARTH 



exact ; at all events such alterations as it is likely to 

 undergo may not greatly affect the final result. When, 

 however, we pass to the last quantity to be determined, 

 the chemical composition of average river water, we find 

 that only a very rough estimate is possible, and this is the 

 more unfortunate because changes in this may very 

 materially affect our conclusions. The total quantity of 

 river water discharged into the sea is, as we have stated, 

 6,524 cubic miles. The average composition of this 

 water is deduced from analyses of nineteen great rivers, 

 which altogether discharge only 488 cubic miles, or 7'25 

 per cent, of the whole. The danger in using this estimate 

 is twofold : in the first place 7'25 is too small a fraction 

 from which to argue to the remaining 92 '75 per cent., 

 and, next, the rivers which furnish it are selected rivers, 

 i.e., they are all of large size. The effect of this is that 

 the drainage of the volcanic regions of the earth is not 

 sufficiently represented, and it is precisely this drainage 

 which is richest in sodium salts. The lavas and ashes of 

 active volcanos rapidly disintergate under the energetic 

 action of various acid gases, and among volcanic exhala- 

 tions sodium chloride has been especially noticed as 

 abundant. Consequently we find that while the propor- 

 tion of sodium in Professor Joly's average river water is 

 only 5'73 per million, in the rivers of the volcanic island 

 of Hawaii it rises to 24'5 per million.* No doubt the 

 area occupied by volcanos is trifling compared with the 

 remaining land surface. On the other hand, the majority of 

 volcanos are situated in regions of copious rainfall, of which 

 they receive a full share owing to their mountainous form. 

 Much of the fallen rain percolates through the porous 

 material of the cone, and, richly charged with alkalies, 

 finds its way by underground passages towards the sea, 



* Walter Maxwell, "Lavas and Soils of the Hawaiian Islands," 

 p. 170. 



