The Birth-Time of the World | Page 8

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[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. Assoc. 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 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.[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 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.[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. Assoc. 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 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.[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