184 LAY SERMONS, ESSAYS, AND REVIEWS. [x. 



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 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 

 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 un 

 precedented 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 com 

 paratively close together, may yet be separated by an amount of 

 geological time sufficient to allow of some of the greatest physical 

 changes the world has seen, what becomes of that sort of con 

 temporaneity 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 contemporaneity 

 assumed by all who adopt the hypotheses of universal faunae and 

 florae, 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 palaeontology, 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, in any given vertical linear section 



