AN ESTIMATE OF THE GEOLOGICAL AGE OF THE EARTH. 249 



sive marine deposit, and is not laid down sensil)ly upon its floor, and 

 if the amount of uniformity already defined is accepted, eyidently in 

 the rate of annual accretion by the ocean, from the riyers, of this sub- 

 stance and the amount of it now in the ocean, the whole period since 

 the beginning- of its supply can be estimated. 



Such an element is sodium. We take for this calculation the ele- 

 ment alone, thus ayoiding the o))scure question of its ionisation. which 

 does not concern the issue. The quantity of sodium now in the sea, 

 and the annual rate of its supply by the riyers, lead, it will be seen, to 

 the deduction that the age of the earth is OOxlO*^ years. Certain 

 deductions from this are, it will be shown, warranted, so that the final 

 result of this paper will be to show that the probable age is about 

 SOxlC' years. Also, that this is probably a major limit, and that con- 

 siderable departure from uniformity of actiyities could hardh' amend 

 it to less than 80x 10® years. ^ 



In the claims to uniformity here inyolyed, much is ayoided that is 

 most uncertain in those methods of calculation which repose upon a 

 knowledge of the yolume of sediments remoyed from the land and 

 deduced from the geological record. Not only in the latter methods 

 is the rate of denudation of the land surface diflicult or impossible to 

 determine with any degree of accuracy, owing to the difliculties attend- 

 ing determinations of the amount of sediment discharged by riyers, 

 but the bulk of the material which has been acted upon must, to a con- 

 siderable extent, be matter of speculation; for eyen when the best is 

 done to determine the true thickness of the sedimentary deposits, what 

 is missing at the unconformabilities is, in man}" cases, unknown.^ The 

 method used in this present estimate, on the other hand, inyolyes two 

 quantities, the amounts of which are ascertained with an accurac}" 

 depending only on the number of our observations — the dissolved mat- 

 ter in the sea (which is almost homogeneous in composition) and the 

 average dissolved matter in the rivers of the world. That the informa- 

 tion regarding the latter quantity available for the present calculation 

 is not final is very probable. A complete knowledge of the dissolved 

 solids of river discharges nmst involve analyses of all the principal 

 river waters, and these chemical investigations must be combined with 

 volumetric measurements of the discharge. In some cases such obser- 

 vations should be seasonal. Failing such complete knowledge, the 

 data used in this paper may be said to afl'ord an approximation to the 

 nature and amount of river discharge. That no more than an approxi- 



^In Mr. T. Mellard Reade's calculation of a minor limit of the earth's age from the 

 amount of calcium gulphate in the sea (Chemical Denudation in Relation to Geolog- 

 ical Time, Daniel Do<iue: London, 1879), the substance chosen does not possess the 

 requisite qualiti cations to enter into such a calculation as is advanced here. It may 

 be observed, as not altogether immaterial, that the principal calculations of this paper 

 were made independently of Mr. T. M. Reade's interesting views. 



^A good account of the difficulties involved appears in \Vallace's Island Life, 

 Chap. X. 



