TJic Biochronic llquatioi. 667 



been observed, and near Przil)rani in Bohemia, an in- 

 crease of 1° C. for every 09 meters. Inasmuch as 

 these two latter records have been made in districts which 

 are further removed from local sources of special high 

 temperature than the older mines, we must conclude that 

 the earth has already cooled down much further than 

 was previously thought, and that the pericxl of 20 to 40 

 million years arrived at by Lord Kelvin, is ])y no means 

 too high an estimate. 



George Darwin calculates that the moon separated 

 off from the earth at least 56 million years ago, and 

 Geikie put as the maximum for the existence of the 

 earth's crust, a hundred million years. The general view 

 is that the formation of the sea occurred fairly soon, 

 geologically speaking, after the formation of the crust. 

 and that no great period of time was necessary for a 

 cooling of the water, sufficient to render life possible. 



Further data for similar calculations are furnished 

 by the action of rivers. These carry certain dissolved 

 salts to the sea. From the mean proportion of sodium 

 chloride wdiich they contain and from the total volume 

 of water which is poured into the sea every year by all 

 of them, we can calculate how much the saline contents 

 of the sea must increase from this cause. The total 

 amount of dissolved salts in the ocean can also be cal- 

 culated and we can then estimate the number of years 

 necessary for the accumulation (^f this quantity. From 

 these data Joly calculated the age of rivers to be *^'^ 

 nn'llion years; but it is highly probable that the con- 

 tinents were originally far richer in salt than now. and 

 that the rivers have more or less exhausted them, that is 

 that they carried more salt to the sea in former times 

 than they do now. Applying this qualificatic^n. Som..-\s 



