The Birth-Time of the World and Other Scientific Essays Part 2

I have next to lay before you a quite different method. I have already touched upon the chemistry of the ocean, and on the remarkable fact that the sodium contained in it has been preserved, practically, in its entirety from the beginning of geological time.

That the sea is one of the most beautiful and magnificent sights in Nature, all admit. But, I think, to those who know its story its beauty and magnificence are ten-fold increased. Its saltness it due to no magic mill. It is the dissolved rocks of the Earth which give it at once its brine, its strength, and its buoyancy.

The rivers which we say flow with "fresh" water to the sea nevertheless contain those traces of salt which, collected over the long ages, occasion the saltness of the ocean. Each gallon of river water contributes to the final result; and this has been going on since the beginning of our era. The mighty total of the rivers is 6,500 cubic miles of water in the year!

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There is little doubt that the primeval ocean was in the condition of a fresh-water lake. It can be shown that a primitive and more rapid solution of the original crust of the Earth by the slowly cooling ocean would have given rise to relatively small salinity. The fact is, the quant.i.ty of salts in the ocean is enormous. We are only now concerned with the sodium; but if we could extract all the rock-salt (the chloride of sodium) from the ocean we should have enough to cover the entire dry land of the Earth to a depth of 400 feet. It is this gigantic quant.i.ty which is going to enter into our estimate of the Earth's age. The calculated ma.s.s of sodium contained in this rock-salt is 14,130 million million tonnes.

If now we can determine the rate at which the rivers supply sodium to the ocean, we can determine the age.[1] As the result of many thousands of river a.n.a.lyses, the total amount of sodium annually discharged to the ocean

[1] _Trans. R.D.S._, 1899. A paper by Edmund Halley, the astronomer, in the _Philosophical Transactions of the Royal Society_ for 1715, contains a suggestion for finding the age of the world by the following procedure. He proposes to make observations on the saltness of the seas and ocean at intervals of one or more centuries, and from the increment of saltness arrive at their age. The measurements, as a matter of fact, are impracticable. The salinity would only gain (if all remained in solution) one millionth part in Too years; and, of course, the continuous rejection of salts by the ocean would invalidate the method. The last objection also invalidates the calculation by T.

Mellard Reade (_Proc. Liverpool Geol. Soc._, 1876) of a minor limit to the age by the calcium sulphate in the ocean. Both papers were quite unknown to me when working out my method. Halley's paper was, I think, only brought to light in 1908.

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by all the rivers of the world is found to be probably not far from 175 million tonnes.[1] Dividing this into the ma.s.s of oceanic sodium we get the age as 80.7 millions of years. Certain corrections have to be applied to this figure which result in raising it to a little over 90 millions of years. Sollas, as the result of a careful review of the data, gets the age as between 80 and 150 millions of years. My own result[2] was between 80 and 90 millions of years; but I subsequently found that upon certain extreme a.s.sumptions a maximum age might be arrived at of 105 millions of years.[3] Clarke regards the 80.7 millions of years as certainly a maximum in the light of certain calculations by Becker.[4]

The order of magnitude of these results cannot be shaken unless on the a.s.sumption that there is something entirely misleading in the existing rate of solvent denudation. On the strength of the results of another and

[1] F. W. Clarke, _A Preliminary Study of Chemical Denudation_ (Smithsonian Miscellaneous Collections, 1910).

[2] _Loc. cit._

[3] "The Circulation of Salt and Geological Time" (Geol. Mag., 1901, p. 350).

[4] Becker (loc. cit.), a.s.suming that the exposed igneous and archaean rocks alone are responsible for the supply of sodium to the ocean, arrives at 74 millions of years as the geological age.

This matter was discussed by me formerly (Trans. R.D.S., 1899, pp. 54 _et seq._). The a.s.sumption made is, I believe, inadmissible.

It is not supported by river a.n.a.lyses, or by the chemical character of residual soils from sedimentary rocks. There may be some convergence in the rate of solvent denudation, but--as I think on the evidence--in our time unimportant.

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entirely different method of approaching the question of the Earth's age (which shall be presently referred to), it has been contended that it is too low. It is even a.s.serted that it is from nine to fourteen times too low. We have then to consider whether such an enormous error can enter into the method. The measurements involved cannot be seriously impugned. Corrections for possible errors applied to the quant.i.ties entering into this method have been considered by various writers. My own original corrections have been generally confirmed. I think the only point left open for discussion is the principle of uniformitarianism involved in this method and in the methods previously discussed.

In order to appreciate the force of the evidence for uniformity in the geological history of the Earth, it is, of course, necessary to possess some acquaintance with geological science.

Some of the most eminent geologists, among whom Lyell and Geikie[1] may be mentioned, have upheld the doctrine of uniformity. It must here suffice to dwell upon a few points having special reference to the matter under discussion.

The mere extent of the land surface does not, within limits, affect the question of the rate of denudation. This arises from the fact that the rain supply is quite insufficient to denude the whole existing land surface. About 30 per cent. of it does not, in fact, drain to the

[1] See especially Geikie's Address to Sect. C., Brit. a.s.soc.

Rep., 1399.

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ocean. If the continents become invaded by a great transgression of the ocean, this "rainless" area diminishes: and the denuded area advances inwards without diminution. If the ocean recedes from the present strand lines, the "rainless" area advances outwards, but, the rain supply being sensibly constant, no change in the river supply of salts is to be expected.

Age-long submergence of the entire land, or of any very large proportion of what now exists, is negatived by the continuous sequence of vast areas of sediment in every geologic age from the earliest times. Now sediment-receiving areas always are but a small fraction of those exposed areas whence the sediments are supplied.[1] Hence in the continuous records of the sediments we have a.s.surance of the continuous exposure of the continents above the ocean surface. The doctrine of the permanency of the continents has in its main features been accepted by the most eminent authorities. As to the actual amount of land which was exposed during past times to denudative effects, no data exist to show it was very different from what is now exposed. It has been estimated that the average area of the North American continent over geologic time was about eight-tenths of its existing area.[2] Restorations of other continents, so far as they have been attempted, would not

[1] On the strength of the Mississippi measurements about 1 to 18 (Magee, _Am. Jour. of Sc._, 1892, p. 188).

[2] Schuchert, _Bull. Geol. Soc. Am._, vol. xx., 1910.

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suggest any more serious divergency one way or the other.

That climate in the oceans and upon the land was throughout much as it is now, the continuous chain of teeming life and the sensitive temperature limits of protoplasmic existence are sufficient evidence.[1] The influence at once of climate and of elevation of the land may be appraised at their true value by the ascertained facts of solvent denudation, as the following table shows.

Tonnes removed in Mean elevation.

solution per square Metres.

mile per annum.

North America - 79 700 South America - 50 650 Europe - 100 300 Asia - 84 950 Africa - 44 650

In this table the estimated number of tonnes of matter in solution, which for every square mile of area the rivers convey to the ocean in one year, is given in the first column. These results are compiled by Clarke from a very large number of a.n.a.lyses of river waters. The second column of the table gives the mean heights in metres above sea level of the several continents, as cited by Arrhenius.[2]

Of all the denudation results given in the table, those relating to North America and to Europe are far the

[1] See also Poulton, Address to Sect. D., Brit. a.s.soc. Rep., 1896.

[2] _Lehybuch dev Kosmischen Physik_, vol. i., p. 347.

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most reliable. Indeed these may be described as highly reliable, being founded on some thousands of a.n.a.lyses, many of which have been systematically pursued through every season of the year.

These show that Europe with a mean alt.i.tude of less than half that of North America sheds to the ocean 25 per cent. more salts.

A result which is to be expected when the more important factors of solvent denudation are given intelligent consideration and we discriminate between conditions favouring solvent and detrital denudation respectively: conditions in many cases antagonistic.[1] Hence if it is true, as has been stated, that we now live in a period of exceptionally high continental elevation, we must infer that the average supply of salts to the ocean by the rivers of the world is less than over the long past, and that, therefore, our estimate of the age of the Earth as already given is excessive.

There is, however, one condition which will operate to unduly diminish our estimate of geologic time, and it is a condition which may possibly obtain at the present time. If the land is, on the whole, now sinking relatively to the ocean level, the denudation area tends, as we have seen, to move inwards. It will thus encroach upon regions which have not for long periods drained to the ocean. On such areas there is an acc.u.mulation of soluble salts which the deficient rivers have not been able to carry to the ocean. Thus the salt content of certain of

[1] See the essay on Denudation.

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the rivers draining to the ocean will be influenced not only by present denudative effects, but also by the stored results of past effects. Certain rivers appear to reveal this unduly increased salt supply those which flow through comparatively arid areas. However, the flowoff of such tributaries is relatively small and the final effects on the great rivers apparently unimportant--a result which might have been antic.i.p.ated when the extremely slow rate of the land movements is taken into account.

The difficulty of effecting any reconciliation of the methods already described and that now to be given increases the interest both of the former and the latter.

THE AGE BY RADIOACTIVE TRANSFORMATIONS

Rutherford suggested in 1905 that as helium was continually being evolved at a uniform rate by radioactive substances (in the form of the alpha rays) a determination of the age of minerals containing the radioactive elements might be made by measurements of the amount of the stored helium and of the radioactive elements giving rise to it, The parent radioactive substances are--according to present knowledge--uranium and thorium. An estimate of the amounts of these elements present enables the rate of production of the helium to be calculated. Rutherford shortly afterwards found by this method an age of 240 millions of years for a radioactive mineral of presumably remote age. Strutt, who carried

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his measurements to a wonderful degree of refinement, found the following ages for mineral substances originating in different geological ages:

Oligocene - 8.4 millions of years.

Eocene - 31 millions of years.

Lower Carboniferous - 150 millions of years.

Archaean - 750 millions of years.