360 Prof. J. Joly on the 



(c) Over-estimation of the legitimate river supply of 

 sodium. 



(d) Decreased river supply of sodium in the past. 



Of these possible sources of error (a) may be at once 

 dismissed. The average depth and area of the ocean, and 

 its average chemical composition, are sufficiently well known 

 to preclude the possibility of any serious error. 



In considering (b) it is necessary to bear in mind the mag- 

 nitude of the quantities involved. The saline matter in the 

 ocean would represent a volume of over 4,800,000' cubic 

 miles, on Clarke's, estimation. X have formerly pointed out 

 that the rock-salt alone would suffice to cover the land area 

 of the Globe to a depth of 122 metres. In comparison with 

 quantities so vast all the salt deposits known sink into 

 insignificance ; nor is it likely that deposits adequate to enter 

 into consideration exist. 



The errors referred to in '(c) must be of the nature of 

 cyclic sodium : that is sodium which circulates from the sea 

 to the land and back through the rivers to the ocean. Cyclic 

 sodium exists in the form of wind-borne spray which de- 

 scending on the land with the rainfall augments that which 

 is truly derived by denudation. In arid regions it may settle 

 as dust, to be, under special circumstances, washed ultimately 

 into the sea. Again, the sodium which the rivers may derive 

 from the ancient salt deposits which have been impounded 

 from the sea is cyclic. 



The influence of wind-borne sodium has been fully dis- 

 cussed by Sollas, Clarke, and Becker. There can be no 

 doubt that it is relatively unimportant. My own original 

 correction was 10 per cent, of the river supply. Becker by 

 examining typical cross-sections of the isochlors, determined 

 for the rainfall of Western North America by the U.S. Geo- 

 logical Survey, finds that an allowance of 6 per cent, is 

 sufficient. Sollas shows that these isochlors indicate that but 

 a small fraction of the sodium chloride of the American 

 rivers can be referred to this source. Clarke, by a somewhat 

 different line of attack, concludes that a correction of 7 per cent, 

 on the sodium conveyed by the rivers of the United States 

 is a maximum allowance. Clarke further considers that a 

 correction for sodium chloride carried as dry dust is 

 unnecessary. 



In a paper contributed by me to the Geological Magazine 

 (May 1900) I considered the possibility of oceanic sodium 

 existing disseminated in the sedimentary rocks. Such sodium 

 would be, of course, cyclic. It was easy to show that, even 



