Read Ebook: The Birth-Time of the World and Other Scientific Essays by Joly John

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Algonkian--Keeweenawan - - 50,000 Algonkian--Animikian - - 14,000 Algonkian--Huronian - - 18,000 82,000

Archaean - - ?

Total - - 335,000 feet.

Address to the Geol. Soc. of London, 1509.

In the next place we require to know the average rate at which these rocks were laid down. This is really the weakest link in the chain. The most diverse results have been arrived at, which space does not permit us to consider. The value required is most difficult to determine, for it is different for the different classes of material, and varies from river to river according to the conditions of discharge to the sea. We may probably take it as between two and six inches in a century.

Now the total depth of the sediments as we see is about 335,000 feet , and if we take the rate of collecting as three inches in a hundred years we get the time for all to collect as 134 millions of years. If the rate be four inches, the time is soo millions of years, which is the figure Geikie favoured, although his result was based on somewhat different data. Sollas most recently finds 80 millions of years.

THE AGE AS INFERRED FROM THE MASS OF THE SEDIMENTS

In the above method we obtain our result by the measurement of the linear dimensions of the sediments. These measurements, as we have seen, are difficult to arrive at. We may, however, proceed by measurements of the mass of the sediments, and then the method becomes more definite. The new method is pursued as follows:

The process is now simple. We estimate by chemical examination of igneous and sedimentary rocks the amount of sodium which has been supplied to the ocean per ton of sediment produced by denudation. We also calculate

the amount of sodium contained in the ocean. We divide the one into the other , and the quotient gives us the number of tons of sediment. The most recent estimate of the sediments made in this manner affords 56 x 1016 tonnes.

Now we are assured that all this sediment was transported by the rivers to the sea during geological time. Thus it follows that, if we can estimate the average annual rate of the river supply of sediments to the ocean over the past, we can calculate the required age. The land surface is at present largely covered with the sedimentary rocks themselves. Sediment derived from these rocks must be regarded as, for the most part, purely cyclical; that is, circulating from the sea to the land and back again. It does not go to increase the great body of detrital deposits. We cannot, therefore, take the present river supply of sediment as representing that obtaining over the long past. If the land was all covered still with primary rocks we might do so. It has been estimated that about 25 per cent. of the existing continental area is covered with archaean and igneous rocks, the remainder being sediments. On this estimate we may find valuable

major and minor limits to the geological age. If we take 25 per cent. only of the present river supply of sediment, we evidently fix a major limit to the age, for it is certain that over the past there must have been on the average a faster supply. If we take the entire river supply, on similar reasoning we have what is undoubtedly a minor limit to the age.

The river supply of detrital sediment has not been very extensively investigated, although the quantities involved may be found with comparative ease and accuracy. The following table embodies the results obtained for some of the leading rivers.

Mean annual Total annual Ratio of discharge in sediment in sediment cubic feet thousands to water per second. of tons. by weight.

Potomac - 20,160 5,557 1 : 3.575 Mississippi - 610,000 406,250 1 : 1,500 Rio Grande - 1,700 3,830 1 : 291 Uruguay - 150,000 14,782 1 : 10,000 Rhone - 65,850 36,000 1 : 1,775 Po - 62,200 67,000 1 : 900 Danube - 315,200 108,000 1 : 2,880 Nile - 113,000 54,000 1 : 2,050 Irrawaddy - 475,000 291,430 1 : 1,610

Mean - 201,468 109,650 1 : 2,731

We see that the ratio of the weight of water to the

weight of transported sediment in six out of the nine rivers does not vary widely. The mean is 2,730 to 1. But this is not the required average. The water-discharge of each river has to be taken into account. If we ascribe to the ratio given for each river the weight proper to the amount of water it discharges, the proportion of weight of water to weight of sediment, for the whole quantity of water involved, comes out as 2,520 to 1.

Now if this proportion holds for all the rivers of the world--which collectively discharge about 27 x 1012 tonnes of water per annum--the river-born detritus is 1.07 x 1010 tonnes. To this an addition of 11 per cent. has to be made for silt pushed along the river-bed. On these figures the minor limit to the age comes out as 47 millions of years, and the major limit as 188 millions. We are here going on rather deficient estimates, the rivers involved representing only some 6 per cent. of the total river supply of water to the ocean. But the result is probably not very far out.

We may arrive at a probable age lying between the major and minor limits. If, first, we take the arithmetic mean of these limits, we get 117 millions of years. Now this is almost certainly excessive, for we here assume that the rate of covering of the primary rocks by sediments was uniform. It would not be so, however, for the rate of supply of original sediment must have been continually diminishing

According to observations made on the Mississippi .

during geological time, and hence we may assume that the rate of advance of the sediments on the primary rocks has also been diminishing. Now we may probably take, as a fair assumption, that the sediment-covered area was at any instant increasing at a rate proportionate to the rate of supply of sediment; that is, to the area of primary rocks then exposed. On this assumption the age is found to be 87 millions of years.

THE AGE BY THE SODIUM OF THE OCEAN

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!

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 quantity of salts in the ocean is enormous. We are only now concerned with the sodium; but if we could extract all the rock-salt 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 quantity which is going to enter into our estimate of the Earth's age. The calculated mass 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. As the result of many thousands of river analyses, the total amount of sodium annually discharged to the ocean

by all the rivers of the world is found to be probably not far from 175 million tonnes. Dividing this into the mass 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 was between 80 and 90 millions of years; but I subsequently found that upon certain extreme assumptions a maximum age might be arrived at of 105 millions of years. Clarke regards the 80.7 millions of years as certainly a maximum in the light of certain calculations by Becker.

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

"The Circulation of Salt and Geological Time" .

Becker , assuming 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 . The assumption made is, I believe, inadmissible. It is not supported by river analyses, 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.

entirely different method of approaching the question of the Earth's age , it has been contended that it is too low. It is even asserted 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 quantities 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 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

See especially Geikie's Address to Sect. C., Brit. Assoc. Rep., 1399.

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. Hence in the continuous records of the sediments we have assurance 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. Restorations of other continents, so far as they have been attempted, would not

On the strength of the Mississippi measurements about 1 to 18 .

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. 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 analyses 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.

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

See also Poulton, Address to Sect. D., Brit. Assoc. Rep., 1896.

most reliable. Indeed these may be described as highly reliable, being founded on some thousands of analyses, many of which have been systematically pursued through every season of the year. These show that Europe with a mean altitude 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. 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 accumulation of soluble salts which the deficient rivers have not been able to carry to the ocean. Thus the salt content of certain of

See the essay on Denudation.

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 anticipated 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 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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