September 13, 1900] 



NATURE 



485 



differences in conductivity of the rocks in which the observa- 

 tions were made, or to the circulation of underground water ; 

 but many cases exist which cannot be explained away in such a 

 manner, but are suggestive of some deep-seated cause, such as 

 the distribution of molten matter below the ground. Inspection 

 of the accompanying map of the British Isles, on which the rates 

 of increase in different localities have been plotted, will afford 

 some evidence of the truth of this view. Comparatively low 

 rates of increase are found over Wales and in the province of 

 Leinster, districts of relatively great stability, the remnants of an 

 island that have in all probability stood above the sea ever since 

 the close of the Silurian period. To the north of this, as we 

 enter a region which was subject to volcanic disturbances during 

 the Tertiary period, the rate increases. 



It is obvious that in any attempt to estimate the rate at which 

 the earth is cooling as a solid body the disturbing influence of 

 subterranean lakes of molten rock must as far as possible be 

 eliminated ; but this will not be effected by taking the accepted 

 mean of observed rates of increase of temperature : such an 

 average is merely a compromise, and a nearer approach to a 

 correct result will possibly be attained by selecting some low 

 rate of increase, provided it be based on accurate observations. 



It is extremely doubtful whether an area such as the British 

 Isles, which has so frequently been the theatre of volcanic 

 activity and other subterranean disturbance, is the best fitted 

 to afford trustworthy results ; the Archaean nucleus of a con- 

 tinent might be expected to afford surer indications. Unfor- 

 tunately the hidden treasures of the earth are seldom buried in 

 these regions, and bore-holes in consequence have rarely been 

 made in them. One ex(;eption is afforded by the copper-bearing 

 district of Lake Superior, and in one case, that of the Calumet 

 and Hecla mine, which is 4580 feet in depth, the rate of 

 increase, as determined by Prof. A. Agassiz, was l° F. for 

 every 2237 feet. The Bohemian "horst" is a somewhat 

 ancient part of Europe, and in the Przibram mines, which are 

 sunk in it, the rate was i" F. for every 126 feet of descent. In 

 the light of these facts it would seem that geologists are by 

 no means compelled to accept the supposed mean rate of 

 increase of temperature with descent into the crust as afford- 

 ing a safe guide to the rate of cooling of a solid globe ; and 

 if the much slower rate of increase observed in the more 

 ancient and more stable regions of the earth has the import- 

 ance which is suggested for it, then Lord Kelvin's estimate 

 of the date of the " consistentior status" may be pushed 

 backwards into a remoter past. 



If, as we have reason to hope, Lord Kelvin's somewhat con- 

 tracted period will yield to a little stretching. Prof. Joly's, on 

 the other hand, may take some paring. His argument, 

 broadly stated, is as follows. The ocean consisted at first of 

 fresh water ; it is now salt, and its saltness is due to the 

 dissolved matter that is constantly being carried into it by rivers. 

 If, then, we know the quantity of salt which the rivets bring 

 down each year into the sea, it is easy to calculate how many 

 years they have taken to supply the sea with all the salt it at 

 present contains. For several reasons it is found necessary to 

 restrict attention to one only of 'he elements contained in sea 

 salt : this is sodium. The quantity of sodium delivered to the sea 

 every yeai by rivsrsis about 160,000,000 tons ; but the quantity 

 of sodium which th ^ sea contains is at least ninety millions of 

 times greater than this. The period during which rivers have 

 been carrying sodium into the sea must therefore be about ninety 

 millions of years. Nothing could be simpler ; there is no 

 serious flaw in the method, and Prof. Joly's treatment of the 

 subject is admirable in every way ; but of course in calculations 

 such as this everything depends on the accuracy of the data, 

 which we may therefore proceed to discuss. Prof. Joly's 

 estimate of the amount of sodium in the ocean may be accepted 

 as sufficiently near the truth for all practical purposes. We may 

 therefore pass on to the other factor, the annual contribution of 

 sodium by river water. Here there is more room for error. 

 Two quantities must be ascertained : one the quantity of water 

 which the rivers of the world carry into the sea, the other the 

 quantity or proportion of sodium present in this water. The 

 total volume of water discharged by rivers into the ocean is 

 estimated by Sir John Murray as 6524 cubic miles. The 

 estimate being based on observations of thirty-three great rivers, 

 although only approximate, it is no doubt sufficiently exact ; at 

 all events such alterations as it is likely to undergo will not greatly 

 affect the final result. When, however, we pass to the last 

 quantity to be determined, the chemical composition of average 



NC 161 I, VOL. 62] 



river water, we find that only a very rough estimate is possible, and 

 this is the more unfortunate because changes in this may very 

 materially affect our conclusions. The total quantity of river 

 water discharged into the sea is, as we have stated, 6524 cubic 

 miles. The average composition of this water is deduced from 

 analyses of nineteen great rivers, which altogether discharge 

 only 488 cubic miles, or 7-25 per cent, of the whole. The 

 danger in using this estimate is twofold : in the first place 7-25 

 is too small a fraction from which to argue to the remaining 

 9275 percent., and, next, the rivers which furnish it are selected 

 rivers, i.e. , they are all of large size. The effect of this is that the 

 drainage of the volcanic regions of the earth is not sufficiently 

 represented, and it is precisely this drainage which is richest in 

 sodium salts. The lavas and ashes of active volcanoes rapidly 

 disintegrate under the energetic action of various acid gases, 

 and among volcanic exhalations sodium chloride has been 

 especially noticed as abundant. Consequently we find that 

 while the proportion of sodium in Prof. Joly's average river 

 water is only 573 per million, in the rivers of the volcanic 

 island of Hawaii it rises to 24 5 per million (Walter Maxwell, 

 '* Lavas and Soils of the Hawaiian Islands," p. 170). No doubt 

 the area occupied by volcanoes is trifling compared with the 

 remaining land surface. On the other hand the majority of 

 volcanoes are situated in regions of copious rainfall, of which 

 they receive a full share owing to their mountainous form. 

 Much of the fallen rain percolates through the porous material 

 of the cone, and, richly charged with alkalies, finds its way by 

 underground passages towards the sea, into which it sometimes 

 discharges by submarine springs. 



Again, several considerations lead to the belief that the supply 

 of sodium to the ocean has proceeded, not at a uniform, but at a 

 gradually diminishing rate. The rate of increase of temperature 

 with descent into the crust has continuously diminished with the 

 flow of time, and this must have had its influence on the tem- 

 perature of springs, which furnish an important contribution to 

 river water. The significance of this consideration may be 

 judged from the composition of the water of geysers. Thus 

 Geyser, in Iceland, contains 884 parts of sodium per million, or 

 nearly 160 times as much as Sir John Murray estimates is 

 present in average river water. A mean of the analyses of six 

 geysers in different parts of the world gives 400 parts of sodium- 

 per million, existing partly as chloride, but also as sulphate and 

 carbonate. 



It should not be overlooked that the present is a calm ancV 

 quiet epoch in the earth's history, following after a time of fiery 

 activity. More than once, indeed, has the past been distin- 

 guished by unusual manifestations of volcanic energy, and these 

 must have had some effect upon the supply of sodium to the 

 ocean. Finally, although the existing ocean water has apparently 

 but slight effect in corroding the rocks which form its bed, yet 

 it certainly was not inert when its temperature was not far 

 removed from the critical point. Water begins to exert a 

 powerful destructive action on silicates at a temperature ot 

 180° C, and during the interval occupied in cooling from 370 

 to 180° C. a considerable quantity of sodium may have entered 

 into solution. 



A review of the facts before us seems to render some reduction 

 in Dr. Joly's estimate imperative. A precise assessment is 

 impossible, but I should be inclined myself to take off some 

 ten or thirty millions of years. 



We may next take the evidence of the stratified rocks. Their 

 total maximum thickness is, as we have seen, 265,000 feet, and 

 consequently if they accumulated at the rate of one foot in a 

 century, as evidence seems to suggest, more than twenty-six 

 millions of years must have elapsed during their formation. 



Obscure Chapter in the Earth's History. 

 Before discussing the validity of the argument on which this 

 last result depends, let us consider how far it harmonises with 

 previous ones. It is consistent with Lord Kelvin's and Professor 

 Darwin's, but how does it accord with Professor Joly's ? Sup- 

 posing we reduce his estimate to fifty-five millions ; what was 

 the earth doing during the interval between the period of fifty- 

 five millions of years ago and that of only 26^^ millions of years 

 ago, when, it is presumed, sedimentary rocks commenced to be 

 formed ? Hitherto we have been able to reason on proljabili- 

 ties ; now we enter the dreary region of possibilities, and open 

 that obscure chapter in the history of the earth previously hinted 

 at. For there are many possible answers to this question. In 

 the first place the evidence of the stratified rocks may have been 



