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 booki I commented strongly on his ignorance of current literature. I now find that the same imperfect acquaintance with recent discussion and researeh 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 recognized 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 seem inexplicable.”2 The values of geologic time referred to, based on radioactive methods, especially the age of pleochroic haloes, I propose to criticize 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 realization of the complexity of the subject. The object of the present article, however, is to repeat3 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,4 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 paper5 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 cuclic 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. Even in New York State, where brine-springs are plentiful, there is no appreciable effect on the salt content of the rivers.8 The only known means by which fresh chlorine reaches the sea is volcanic action, and it is a point open to dispute how much of the volcanic chlorine is not ultimately derived from the sea. It follows, therefore, that, of the sodium which actually reaches the sea, only that not associated with the chlorine can be counted. This much Prof. Joly and those who agree with his earlier estimate have been willing to admit. But Prof. Joly maintains that, if the chlorine equivalent of the sodium be substracted, there is still sufficient sodium to necessitate an estimate of geologic time less than 150 millions of years. His reason is merely Sir John Murray's rough tabulation of then current analyses and some more recent results. It does seem strange, however, that Prof. Joly never troubled to inquire whether there were any water analyses sufficiently accurate for this purpose. It is highly probable that Prof. Joly's original paper would never have been written if he had understood why water analyses are undertaken, and the manner in which they are actually performed. Had he been a water analyst, or even a chemist, the first thing that would have occurred to him would have been that these sodium determinations were decidedly hypothetical. Several chemists have expressed doubts as to the validity, but such discussions Prof. Joly has either ignored or failed to understand.7 It may, therefore, surprise Prof. Joly to be informed that it is doubtful whether the sodium content of any single river water has ever been accurately determined. If any such cases have occurred, they are very few. Let us imagine that there is, in a given sample of river water, two parts of sodium per million. Such a proportion would be quite ordinary according to the usual tables. It would be a very interesting problem to try to separate this out and weigh it. To obtain a good weighable quantity (say 0.05 gram of sodium giving about 0.15 gram of sodium sulphate8) would require 25 liters of water, the greater part of a carboy, and the difficulties in the way of isolating it are such as any chemist can understand. As a matter of fact, the accurate determination of the sodium is a form of amusement in which the ordinary water analysts do not indulge. Sometimes the alkalis sodium and potassium are determined together by difference, that is, not determined at all. In a paper9 that has been sent to me recently the analyst describes his methods. In this case, everything possible is got rid of by the usual methods of precipitation,. and the remainder is evaporated and weighed as “Sodium and Potassium Chlorides.” The amount dealt with is only that from 250 cubic centimeters of filtered water, and would, of course, be infinitesimal, and the fact that it amounts to not more than 2 or 3 per cent of the total dissolved solid is a good indication of the general accuracy of the analysis. The residue includes, of course, everything that is not caught by the filter throughout the whole operation. It should be mentioned, also, that the samples usually stand for days in glass bottles. In such cases of water analysis when the sodium and the potassium are separated, the separation is, needless to ' say, a very approximate operation.” It is no reflection on the accuracy of river water analysts to say that the results are of no value whatever for Prof. Joly's purpose. No one, except Prof. Joly and a few geologists, wants to know the proportion of sodium in' river water. It is at the same time the constituent least important for the purpose of the water analyst and the constituent most difficult to determine. The assumption on which Prof. Joly proceeds, that 3.47 per cent of the dissolved matter in river water is sodium, is absolutely unproven. For all the analyses prove, it ' might be less than half that amount. Indeed the principal evidence that there is an excess of sodium over and above its equivalent of chlorine is indirect rather than direct. The results of rock analyses are more reliable and it seems to be established that the sodium content of igneous rocks is greater than that of the aqueous. As a matter of fact it has been pointed out by Prof. Dubois that, when there is any reason to ascribe special accuracy to river-water analysis, the excess of sodium diminishes and tends to vanish. Prof. Dubois” collected a number of good analyses, tabulated them, and inferred from them, according to Prof. Joly's method, a geologic time of 400 million years. The inference he made, as he was a believer in Lord Kelvin's methods, was that the original- sea was salt. The true inference is that the method is of no value. Within the limits of experimental error you can deduce any value you please. Though the previous discussion renders it unnecessary, it is as well to mention one other point. The assumption that no sodium returns from the sea to the rooks is unwarranted. Indeed one instance to the contrary can be mentioned. It is a recognized fact that much of the salt in the salt lakes, and inferentially in salt beds, is windborne and has its origin in the sea. But what would occur when strata containing salt beds are subject to metamorphosis or are absorbed by the magna? Is it not obvious that the sodium would be added to the content of the rocks and that the chlorine would be expelled as some volatile compound? Indeed, is it not probable that some portion of volcanic chlorine has this origin? Again, with regard to the ordinary processes of the formation of sedimentary rock, we do not know enough to say that no dissolved sodium is reabsorbed. This speculation, however, is a side issue, and is not necessary to the argument. Were the analyses of sufficient accuracy, were the method in general valid, such matters would require careful consideration. At present, without taking such remote speculations into account, we can still say that the sea-salt method is absolutely worthless. It is based on a misapprehension of the data on which it rests. It is an instance of the care that is required when results.are transferred from one branch of science to another. With regard to geologic time, the value of radioactive methods is still to be determined. ' The value of the sea-salt method, like the still more famous ones of Kelvin and Tait, is nil.
This article was originally published with the title "Sea-Salt and Geologic Time" in s , , 79-80 (March 2013)