284 GEOLOGIC 1 AL CONTEMPORANEITY IX 



On what amount of similarity of their fauna? 

 is the doctrine of the contemporaneity of the 

 European and of the North American Silurians 

 based ? In the last edition of Sir Charles Lyell s 

 &quot;Elementary Geology&quot; it is stated, on the 

 authority of a former President of this 

 Society, the late Daniel Sharpe, that between 

 30 and 40 per cent, of the species of Silurian 

 Mollusca are common to both sides of the 

 Atlantic. By way of due allowance for further 

 discovery, let us double the lesser number and 

 suppose that 60 per cent, of the species an- 

 common to the North American and the British 

 Silurians. Sixty per cent, of species in common 

 is, then, proof of contemporaneity. 



Now suppose that, a million or two of years 

 hence, when Britain has made another dip 

 beneath the sea and has come up again, some 

 geologist applies this doctrine, in comparing the 

 strata laid bare by the upheaval of the bottom, 

 say, of St. George s Channel with what may then 

 remain of the Suffolk Crag. Reasoning in the 

 same way, he will at once decide the Suffolk 

 Crag and the St. George s Channel beds to be 

 contemporaneous ; although we happen to know 

 that a vast period (even in the geological sense) 

 of time, and physical changes of almost unpre 

 cedented extent, separate the two. 



But if it be a demonstrable fact that strata 

 containing more than 60 or 70 per cent, of species 



