Joty—An Estimate of the Geological Age of the Earth. 25 
Such an element is sodium. We take for this calculation the element alone, 
thus avoiding the obscure 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 rivers, lead, it will be seen, to the deduction that the age of the 
Earth is 99 x 10° 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 89 x 10° years. Also, that this is probably a major limit, and that 
considerable departure from uniformity of activities could hardly amend it to 
less than 80 x 10° years.* 
In the claims to uniformity here involved, much is avoided that is most 
uncertain in those methods of calculation which repose upon a knowledge of the 
volume of sediments, removed from the land, and deduced from the geological 
record. Not only in the latter methods is the rate of denudation of the land 
surface difficult or impossible to determine with any degree of accuracy, owing 
to the difficulties attending determinations of the amount of sediment discharged 
by rivers, but the bulk of the material which has been acted upon must, to a 
considerable extent, be matter of speculation; for even when the best is done to 
determine the true thickness of the sedimentary deposits, what is missing at the 
unconformabilities is, in many cases, unknown.+ The method used in this present 
estimate, on the other hand, involves two quantities, the amounts of which are 
ascertained with an accuracy depending only on the number of our observations : 
the dissolved matter in the sea (which is almost homogeneous in composition) and 
the average dissolved matter in the rivers of the world. That the information re- 
garding the latter quantity available for the present calculation is not final is very 
probable. A complete knowledge of the dissolved solids of river discharges must 
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 observations should be seasonal. Failing such complete knowledge, the 
data used in this paper may be said to afford an approximation to the nature and 
amount of river discharge. That no more than an approximate uniformity can 
be claimed for these factors over the past is doubtless true; but this does not 
eliminate the necessity of accurate knowledge where such is available in the 
application of the method, for the possibility of errors occurring of the same sign 
must always be borne in mind. 
* In Mr. T. Mellard Reade’s calculation of a minor limit of the Earth’s age from the amount of 
calcium sulphate in the sea (‘‘Chemical Denudation in Relation to Geological Time.” Daniel Dogue. 
London, 1879), the substance chosen does not possess the requisite qualifications to enter into such 
a calculation as is advanced here. It may 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 Wallace’s Island Life, Chap. X. 
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