NO. 5 STUDY OP CHEMICAL DENUDATIOX — CLARKE 11 



It is difficult, with existing data, to evaluate the correction for cyclic 

 sodium with any great approach to accuracy. It probably varies for 

 different regions, but from the well-knoAvn " chlorine maps " of New Eng- 

 land and New York, with their " isochlors," a good estimate for the 

 United States can be made. This part of the work lias been done by Dr. 

 Becker, whose discussion of the subject appears elsewhere. Another, 

 cruder estimate may be made as follows: If we assume with regard to the 

 United States that the salt brought down in rain is represented by the 

 Eothamsted figures for a strip 100 miles wide following the coast line, an 

 area, roughly, of about 450,000 square miles, the quantity of sodium thus 

 found amounts to 1,231,300 metric tons. Eegarding the correction for 

 the interior of the United States as zero, an assumption which is justified 

 by a study of the isochlors, the final result may be obtained as follows: 

 The total sodium carried by the rivers of the United States to the sea, 

 after correcting the crude value already given in the table of analyses by 

 Palmer's value for the Na-K ratio, amounts to, roughly, 17,500,000 tons. 

 The cyclic sodium is only 7 per cent, of the latter quantity, whereas Joly 

 allows 10 per cent., but the smaller figure is probably a maximum. De- 

 ducting 7 per cent, from the total sodium carried by the rivers, 175,- 

 040,000 metric tons per annum, the remainder is 162,787,200 tons; which, 

 divided into the sodium of the ocean gives a quotient of 86,800,000 years. 

 Joly's correction of 10 per cent, is very nearly equivalent to the assump- 

 tion that the entire run-off of the globe, 6524 cubic miles, according to 

 Murray, carries on an average one part per million of chlorine. The 

 chlorine maps, so far as they have been made, show this figure to be 

 excessive. 



The foregoing correction for ^'cyclic salt'' is, however, not final. It 

 has already been suggested that the wind-borne salt is only in part re- 

 stored to the ocean, at least within reasonable time. Some of it is retained 

 by the soil, if not permanently, at least rather tenaciously ; and the por- 

 tion which falls into depressions of the land may remain undisturbed 

 almost indefinitely. In arid regions, like the coasts of Peru, Arabia, and 

 parts of western Africa, a large quantity of cyclic salt must be so retained 

 in hollows or valleys which do not drain into the sea. Torrential rains, 

 which occur at rare interv^als, may return a part of it to the ocean, but not 

 all. Some writers, like Ackroyd ' for example, have attributed the saline 

 matter of the Dead Sea to an accumulation of wind-borne salt; an assump- 

 tion which contains elements of truth, but is probably extreme. A more 

 definite, but equally striking instance of the sort is furnished by the 

 Sambhar salt lake in northern India, as studied by Holland and Christie.' 

 This lake, situated in an enclosed drainage basin of 2200 square miles and 



1 rheiii. News, vol. 89, p. 13, 1904. 



" Records Gaol. Survey India, vol. 38, p. 154, 1909. 



