ON THE G-EOLOGICAL AGE OF THE EARTH. 375 



First Approximation to a Numerator. 



According to Professor Dittmar/ if the ocean has a total mass of 

 1-343 X 10''* tonnes, the sodium chloride in it amounts to 36,566 xlO'^ 

 tonnes. However, on Professor Wagner's- estimate of the area of the 

 land surface of the globe as 14,456 x 10' square kilometres, and the ratio 

 of the areas of water and land as 2-54 : 1, and on Sir John Murray's 

 estimate^ of the mean depth as 3 851 kilometres, the total mass is more 

 correctly 1-458x10'** tonnes. On these data we can readjust Professor 

 Dittmar's estimate of the mass of NaCl in the ocean, finding it to be 

 39,703 X 10'- tonnes, and from this finally arrive at the result that the 

 mass of sodium in the ocean is 15,611 xlO'^ tonnes. 



First Approximation to a Denojtiinator. 



On Sir John Murray's estimate ^ the river water annually discharged 

 into the ocean amounts to a volume of 27,191 cubic kilometres, and from 

 his table of the mean dissolved constituents of nineteen rivers — many of 

 them principal rivers of the world — we find that a cubic kilometre of 

 average river water contains as sodium salts 7753 tonnes of NaoSO^, 6534 

 tonnes of NaNOa, and 4061 tonnes of NaCl. Calculating from these the 

 masses of sodium in each case, and multiplying by the total number of 

 cubic kilometres, we arrive at 15,976 x 10' tonnes as the total mass of 

 sodium carried annually into the sea by the rivers. 



The quotient is a first approximation to the age of the earth, and is 

 97-6 millions of years. 



Correction on the Denominator, 



It is convenient to consider this first. 



The mass of sodium chloride carried by the rivers is in part derived 

 from the ocean by means of the atmospheric transportation of this sub- 

 stance from the ocean and its precipitation in rain water. Ten per cent. 

 is allowed as a sufficient deduction for this circulation of the chloride of 

 sodium. The allowance is thus restricted for the reason that while near 

 the coasts a very considerable portion, and even sensibly the whole, of the 

 chloride of sodium may be so derived, in inland areas the amounts of the 

 salt which fall in rain become very minute.'' It is just in these inland 

 areas that the chief rivers of the world derive their supplies. 



Applying this correction to the NaCl of the rivers, the corrected river 

 discharge of sodium is left at 15,542 x 10' tonnes. 



Corrections on the Numerator. 



(a) For the Original State of the Ocean. — We assume that the .sodium 

 as well as most of the metals was silicated in the original crust of the 

 earth. The chlorine, with great probability, was gaseous and combined 



Society, vol, vii. (ser. ii.), 1899, p. 2.S et seq. See also Geological Magazine, 4, 1900, 

 vol. vii et Hcq. 



' ' Challenger' Report, PlujSics and Chemistry, vol. i. 



Scottish Geograj>'>ical Magazine, 1895, p. 185. The measurements throughout 

 have been converted from the British system of units. 



^ Loc. cit., 1888, p. 1 et seg^. ■* Loc. cit., 1887, p. 76. 



^ On the west and east coasts of Scotland 1-19 and 1-26 per 100,000 respectively. 

 In Swiss valleys 0-25 to O'TG per 100,000, and in Ootacamund, India, O'OI per 

 100,000. 



