348 Professor J. Joly — Bait and Geological Time. 



assume so much more chlorine to have been free in the original 

 atmosphere), and thus we are bound to diminish our numerator 

 as well as our denominator. The final result would be 141 X 10^ 

 years. In obtaining this figure we simply assume that no part of 

 tlie chlorine now in the ocean was at any time contained in the 

 rocks, and that in a period of primeval acid denudation it acted 

 as H CI to bring chlorides into the original ocean. In other woi'ds, 

 that the 28,316 x 10'- tons of 01 now in the ocean took part 

 in the primeval denudation, 6'7 per cent, of it uniting with sodium, 

 and thus bringing 1,250 X 10^- tons of sodium into the primeval 

 ocean, the calculation being made on the basis defined in my paper. 

 Deducting this from the 15,627 X 10'^ tons of Na now in the ocean, 

 we have 14,377 x 10^^ tons to be accounted for by the annual river 

 supply. On our present assumptions the annual supply is not to be 

 taken at its full value of 157 x 10^ tons, but this reduced by the 

 sodium equivalent of the whole of the chlorine in rivers (that is, the 

 equivalent of 84 x 10'' tons), i.e. 55 x lO'' tons. The geological 

 age is therefore the quotient of 14,377 x 10^^ by 102 x 10", which 

 is rather less than 141 million years. 



The assumptions made in obtaining this upper limit are, I need 

 scarcely point out, unjustifiable. We are not at liberty to ascribe 

 all the chlorine of the rivers to rain. Whether derived directly 

 from the rocks and soils, extravasated deposits (as when contained 

 in ore deposits), or from accumulations due to past or present inland 

 denudation in ' rainless ' areas, this element, so far as it is a carrier 

 of sodium, must enter our denominator. Nor can we assume the 

 chlorine of rain-water a fair measure of sodium transported from 

 the sea, seeing that such chlorine, if directly sea-derived, should, to 

 the extent of 18 per cent., exist combined with magnesium or other 

 element, and according to observation does so exist to the extent 

 of over 8 per cent. And again, what chloride of sodium is found 

 in rain-water is very surely in part derived from the land. Finally, 

 what we know ot" the percentage of chlorides in inland rains 

 points to a supply inadequate to furnish the quantities appearing 

 in Sir J. Murray's analysis of mean river-water, for of coastal rains 

 by far the greater part finds its way back quickly and directly to 

 the sea by the small rivers and streams, and does not enter into the 

 composition of the larger rivers, the catchment areas of which are 

 either far inland or rise on the land side of the great mountain 

 ranges. The amount of chlorine appearing in the mean river- water 

 is a little over 030 part per 100,000. Contrast this with the 

 estimate 0-04 obtained 300 miles from the sea in India, or the 

 Darmstadt estimate 0'09 part per 100,000, both localities being 

 still relatively close to the sea. On all these grounds I have 

 restricted my allowance for rain-borne chloride of sodium to 10 per 

 cent, of what appears in rivers. 



We might base an allowance on the scanty knowledge we possess 

 as to the chlorine content of rains falling some 300 miles from the 

 sea. Let us accept 040 per 100,000 as the average amount of 01 

 carried by rains to the rivers after evaporation, and assume that all 



