x.] GEOLOGICAL CONTEMPORANEITY. 211 



must have been occupied by migration from one of the 

 two, or from an intermediate spot, and the chances 

 against exact coincidence of migration and of imbedding 

 are infinite. 



In point of fact, however, whether the hypothesis 

 of single or of multiple specific centres be adopted, 

 similarity of organic contents cannot possibly afford 

 any proof of the synchrony of the deposits which 

 contain them ; on the contrary, it is demonstrably 

 compatible with the lapse of the most prodigious 

 intervals of time, and with interposition of vast changes 

 in the organic and inorganic worlds, between the epochs 

 in which such deposits were formed. 



On what amount of similarity of their faunaa 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 "Elementary Geology" 

 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 are 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 Britian 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. 

 Beasoning 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 



