7i20 REPORT— 1900. 



the accepted mean of olsserved rates of increase of temperature : sucli ah kv^Hg6 

 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 

 80 frequently been the theatre of volcanic activity and other subterranean disturb- 

 ance, is the" best fitted to afford trustworthy results ; the Archteau nucleus of a 

 continent mipht be expected to afford surer indications. Unfortunately the hidden 

 treasures of the earth are seldom buried in these regions, and bore-holes in conse- 

 quence have rarely been made in them. One exception is afforded by the copper- 

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

 mine, which is 4,580 feet in depth, the rate of increase, as determined by Pro- 

 fessor A. Affassiz, was 1° F. for every 223'7 feet. The Bohemian 'horst'isa 

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

 the rate was 1° F. for every 12G 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 affording 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 im- 

 portance 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 contracted period will 

 yield to a little stretching:, Professor 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 rivers 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 the elements contained in sea salt : this is sodium. The quantity of 

 sodium delivered to the sea every year by rivers is about 160,000,000 tons ; but 

 the quantity of sodium which the sea contains is at least ninety millions of times 

 greater than th s. 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 Professor 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, whicli we may therefore proceed 

 to discuss. Professor Joly's estimate of the amount of soHiLuu 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. Ilere 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 Kir John Murray as 

 0,524 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 com- 

 position of average 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 aflect 

 our conclusions. The total quantity of river water discharged into the sea is, as 

 we have stated, G,524 cubic miles. The average composition of this water is deduced 

 from analvses 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 two- 

 fold : in the first place 7'25 is too small a fraction from which to argue to the re- 

 maining 92'75 per cent., and, next, the rivers which furnish it are selected rivers, i.e., 

 they are all of large size. The efliect of this is that the drainage of the volcanic 

 regions of the ea7th is not sufFiciently represented, and it is precisely this drainage 

 which is richest in sodium salts. The lavas and ashes of active volcanoes rapidly 



