382 GEOLOGICAL CONTEMPORANEITY 



On what amount of similarity of their faunae* 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 contem- 

 poraneity. 



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 unprecedented 

 extent, separate the two. 



But if it be a demonstrable fact that strata containing 

 more than 60 or 70 per cent, of species of Mollusca in 

 common, and comparatively close together, may yet be 

 separated by an amount of geological tune sufficient 

 to allow of some of the greatest physical changes the world 

 has seen, what becomes of that sort of contemporaneity 

 the sole evidence of which is a similarity of facies, or the 

 identity of half a dozen species, or of a good many 

 genera ? 



And yet there is no better evidence for the contem- 

 poraneity assumed by all who adopt the hypotheses of 

 universal faunae and flora?, of a universally uniform climate, 

 and of a sensible cooling of the globe during geological 

 time. 



There seems, then, no escape from the admission that 

 neither physical geology, nor paleontology, possesses any 

 method by which the absolute synchronism of two strata 

 can be demonstrated. All that geology can prove is local 

 order of succession. It is mathematically certain that, 



