One Hundred Years ago in Science Progress …

Abstract

The present short article is a reversion to an aspect of the subject of geologic time which I had thought to be settled, and to require no further research or controversy. In my review of Mr Holmes's book I commented strongly on his ignorance of current literature. I now find that the same imperfect acquaintance with recent discussion and research is shared by the writer who is responsible for putting forward the amount of sodium in the sea as an index of geologic time. I assume, of course, that ignorance is the explanation, for I take it that no man of science of recognised position, when the errors of his research had been pointed out, would deliberately ignore the fact, and proceed as if his work was a valid contribution to the advancement of science. My excuse, therefore, for writing an article containing nothing material which I have not previously published is the following passage, for which Sir Ernest Rutherford and Prof. Joly are jointly responsible: But it is certain that, if the higher values so found are reliable, the discrepancy with estimates of the age of the ocean, based on the now well-ascertained facts of solent denudation, raises difficulties which at present seem inexplicable. The values of geologic time referred to, based on radioactive methods, especially the age of pleochroic haloes, I propose to criticise on a future occasion. There are good grounds, which cannot be stated here, for thinking that all attempts to assess exact times for particular geologic epochs by calculation either of the lead ratios of uranium minerals or otherwise are premature, and are based on an imperfect realisation of the complexity of the subject. The object of the present article, however, is to repeat the arguments which show that the alternative method based on the salt-content of the ocean is of no value whatever. Not only is the discrepancy not inexplicable, there is no discrepancy to explain. So much did I take this for granted that, in my last article on the subject, I did not think it necessary to consider the sea-salt method. I therefore take this opportunity to repeat the arguments, and to remedy what is apparently a deficiency. Prof. Joly's original paper was based on the supposed facts (1) that, as roughly estimated by Sir John Murray, of the solid matter dissolved in river water which reaches the sea 3.47 per cent, is sodium; (2) that nearly all this, hypothetical sodium is obtained by erosion of the rocks; (3) that when this hypothetical sodium reaches the sea, none of it returns to the rocks. On this supposition, dividing the amount of sodium in the sea by the amount which reaches it each year, an estimate of geologic time could be made. The objection is, briefly, that the three supposed facts are merely supposed facts. No single one of them is reliable. For convenience we will take the second point first. Of the sodium which actually reaches the sea, a considerable proportion is associated with chlorine. None of the sodium chloride in the rivers can be attributed to erosion. This is so for two reasons. In the first place, it is well known that the proportion of chlorine in the rocks, igneous or sedimentary, is infinitesimal. In the second place, the sources of the chlorine have been thoroughly well determined. In the main, they are two, cyclic salt, carried by the wind from the sea, and salt due to human contamination. It has been found possible, particularly in New York State, to eliminate the cyclic salt, the amount of which is a function of the distance from the coast, and to show that the residual chlorine in river water is a direct function of density of population. Unless you take the sewage from town and country districts directly out to sea, the salt in it inevitably reaches the rivers. If you obtain an abnormally high chlorine ratio when the sewage is supposed to be carried out to sea, the inference is leakage. The source which would naturally occur to any one, brine-springs, has been shown to be negligible. …