504 Professor J. Joly — Circulation of Salt. 



forced." Putting aside the possibility of cooling, the effect of 

 chemical action in evolving heat from coal-seams is well known, 

 and an important paper on the subject was read in 1899 by 

 Dr. Haldane and Mr. Meachem before the Society of Mining 

 Engineers. It was clearly shown by these investigators that the 

 heat resulting from the oxidation of marcasite in coal-seams is three 

 times as much as is required to account for the total rise in 

 temperature which the air of the ventilating current undergoes in 

 passing through the mines. Dr. Haldane, who has given great 

 attention to this subject, informs me that he considers the effect of 

 this chemical action has been too little considered, and that he has no 

 doubt it has led to an exaggerated estimate of the mean thermometric 

 gradient in coal borings. While recognizing the great value of the 

 Eeports issued by the Committee of which Professor Everett is 

 Secretary, to whom all geologists must feel grateful for the 

 investigation of a question which is of the first importance to 

 their inquiries, I still consider that, owing to various disturbing 

 factors, the average rate of temperature increase with descent into 

 the crust may have been overestimated, and that divergence from 

 the mean may in some cases be connected with an irregular 

 distribution of molten matter below the ground. 



IV. — ClKCULATION OF SaLT AND GEOLOGICAL TiME. 

 By Professor J. Joly, M.A., D.Sc, F.E.S. 



IN the Geological Magazine for August I gave the major limit 

 to the period of time we can assign to the geological age of 

 the Earth by the solvent-denudation method, when it is assumed 

 that all the chlorine of rivers is derived directly from the ocean, 

 and that all such chlorine (falling, as assumed, in rain) carries its 

 full complement of sodium from the ocean. The major limit with 

 these assumptions is 141 million years. A second estimate is given 

 on the more moderate assumption that one-third the amount of 

 chlorine in rivers is derived from the sea and brings with it its 

 full equivalent of sodium ; this affords 105 millions of years as the 

 age. Finally, there is the original estimate based on a 10 per cent, 

 deduction from the chlorine of rivers as rain-borne, affording 96 

 millions of years. 



Among such numbers we may take our choice. Outside the 

 upper limit we cannot go if we rely on the mean river analyses 

 of Sir John Murray, and of course accept the principle of uniformity 

 involved. It is a perfectly simple matter, which may be stated as 

 follows : — There is a large excess of sodium over chlorine appearing 

 in the mean analysis of 19 chief rivers of the world. The numbers 

 are 157 x 10" tons of sodium and 84 X 10'' tons of chlorine carried 

 to the ocean per annum ; or, dividing by the atomic weights, the 

 relative numbers of ions are as 68 sodium to 24 chlorine. The 

 consequence is that even if the whole of the chlorine be supposed 

 derived from the sea and none at all from denudation, and to reach 

 the rivers fully satisfied with marine sodium, there remains over such 

 an excess of sodium that the age cannot exceed 141 x 10'' years. 



