178 



Avithin some such period as 100,000,000 years."'^' Later he asserted 

 that the allowance of time Avhich he was prepared to grant for 

 the evolution of geological history was " more than twentj^ and 

 less than fortj^ million years, and probably much nearer twenty 

 than fort}'."'-* 



Professor G. H. Darwin, in what has been called " one of the 

 most lieautiful contributions ever made by astronomy to geology,"'^' 

 has estimated that the time M'hich has elajised since the moon 

 occupied a position nine terrestrial radii distant from the earth is 

 at least fifty-six or fifty-seven millions of years, Init may be much 

 more. This period probably corresponded pretty nearly with tlie 

 time at which the earth's crust was formed, for the moon appear a 

 to have been throM-n off while the earth M'as a rapidly rotating 

 molten ball, and it is thought not so long before the solidification 

 of its surface. 



In 1899, Professor Joly contributed to the Transactions of the 

 Royal Dublin Society a remarkable and elaborate paper,''*' in 

 which he endeavoured to estimate from the rate at which salt 

 is being carried down by the rivers into the ocean, and the 

 amount of salt in the ocean at the present time, the date of the 

 formation of the earliest oceans. His conclusion was that the 

 oceans had existed for from 9:J to 100 millions of years. But 

 he only allowed for 10 per cent, of the salt carried down by the 

 rivers being " cyclic salt," that is salt carried by winds from the 

 sea to the land. INIy friend, Mr Ackroyd, of Halifax, has, 

 however, shown pretty conclusively, I tliink, in various papers 

 communicated to the "Chemical News" and the "Geological 

 Magazine," that at least 99 per cent, of the sea-borne salt is 

 cyclic salt. The conclusions of Professor Joly are therefore, in 

 my opinion, entirely invalidated. 



Sir Archibald Geikie, reasoning from the observed rate of 

 denudation, comes to a conclusion comparable with those of 

 Kelvin, Darwin and Joly, and, accordingly, we may take it as a 

 rough estimate according to our present knowledge that from 50 

 to 100 millions of years have elapsed since the oceans were 

 formed, and stratified rocks began to be deposited. The 

 maximum thickness of these stratified rocks is apparently about 

 50 miles. The folIoAving Talilo, Avhich I take from a Paper 



(1) Rep. Brit. Assoc-., 1886, p. 517. 



(2) " The Age of Earth," Presidential Address to the Victoria Institute 



for 18H7, p. 10. See also Phil. Mag., Jan., 1899. 

 (.3) Prof. Sollas. See Geo. Mag., Oct., 1900, p. 451. 

 (4) Trans. Roy. Dub. Soc, ser. ii., vol. vii. 



