AN ESTIMATE OF THE GEOLOGICAL AGE OF THE EARTH. 259 



The atomic percentages of Clarke's average are gi\'en by hiui as 

 follows : 



Iron 4. 71 



Calcium 3. 53 



Magnesium 2. 64 



Potassium 2. 35 



Sodium 2.68 



The chlorine taken up may be assumed to be distributed as follows: 

 In the first instance — FegClg, CaClg, MgCl^, KCl, and NaCl. 



The chlorine will be allocated as follows among these elements: 



Four and seventy-one one-hundredths units of weight of iron take 

 up U units of chlorine nearly; 3.53 units of calcium join with 6.3 units; 

 2.64 units of magnesium take up 7.6 units; 2.35 units of potassium take 

 up 2.14 units, and 2.68 units of sodium unite with ^.l units of chlorine. 

 From this it follows that the chlorine taken up b}^ the sodium bears to 

 the total amount of acid neutralized the ratio of 1 to 7.5. If, then, 

 there had not been any suppl}^ of chlorine subsequenth^ from the 

 rocks, as there has been, this would represent the fraction of the pres- 

 ent sodium chloride which was, with comparative rapidity, thrown into 

 the primeval ocean in the first stages of denudation. In other words, 

 of the entile quantity of HCl at that early period neutralized bj 

 reaction with the constituents of the rocks only II per cent can have 

 been expended in bringing the sodium into solution as sodium chloride. 



If, therefore, we estimate the chlorine in the original ocean, we may, 

 on the foregoing basis, take II per cent of this as having existed in it 

 as sodium chloride.' 



In estimating the chlorine in the primeval ocean we have to consider 

 that what is now in it is in excess, to some extent, of what originally 

 existed in it by the amount that has been discharged by the rivers 

 during the subsequent histor}- of the earth. Clarke shows that careful 

 analysis of rocks reveals this element in many rocks wherein it had 

 previously not been looked for. He estimates that it exists to the 

 extent of O.OI per cent of the original crust. ^ In river discharges it 

 will be seen (ante) to amount to no inconsiderable amount, entering 

 chiefly as chloride of sodium, but also as lithium and ammonium 

 chloride. The chloride of sodium is undoubtedly partly derixed from 



^ Note in reference to the calculation respecting the neutralization of free hydro- 

 chloric acid : 



The effect of the aluminum should also be taken into account. This would retluce 

 the estimated percentage of acid neutralized in the formation of sodium chloride, 

 and so raise somewhat the estimate of geological time. 



The margin of error assumed in the final estimate of geological time must, how- 

 ever, cover the oversight, but leaving the balance of probabilities in favor of a dura- 

 tion more nearly ninety than eighty millions of years. 



'■'Bulletin V. S. Geological Survey, No. 148, p. 13. See also Bischof's Chemical and 

 Physical Geology. 



