EVOLUTIONAL GEOLOGY. 301 



suits. The Aivtuean uuclous of a continent niioht be expected to afford 

 surer indications. Unfortunately, the hidden treasures of the earth 

 are seldom buried in these regions, and bore holes in consequence have 

 rarely been made in theuL One exception is afi'orded 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 Prof. A. Agassiz. was 1° F. for every 223,7 feet. The 

 Bohemian '' horst" is a somewhat ancient part of Europe, and in the 

 Przibram mines, which are sunk in it, the rate was l'^ F. for e\xry 

 12t) 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 arid more stable 

 regions of the earth has the importance which is suggested for it, then 

 Lord Kelvin's estimate of the date of the "consistentior status" may 

 be pushed Ijackward 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 fol- 

 lows: 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 car- 

 ried into it by rivers. If, then, we know the quantity of salt which 

 the rivers bring down each year into the sea, it is easj to calculate how 

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

 pi esent contains. For several reasons it is found necessary to restrict 

 attention to one onl}^ of the elements contained in sea salt. This is 

 sodium. The quantity of sodium delivered to the sea every year by 

 rivers is about 100,000,000 tons; but the quantity of sodium which 

 the sea contains is at least ninety millions of times greater than this. 

 The period during which rivers have been carrying sodium into the 

 sea nuist therefore be about ninety millions of 3^ears. Nothing could 

 be simpler; there is no serious Haw 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, which we may therefore proceed to discuss. Professor 

 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 (luantity 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 6,524 

 cubic miles. The estimate beino- based on observations of 33 great 



