SYSTEMS AND PERIODS 265 



the first geological map of these islands, which crowned 

 his labours, was the most conclusive testimony to its 

 powers. 



A geologist crossing to America and visiting the Falls 

 of Niagara may pick up fragments of limestone which 

 will at once strike him as remarkably similar to certain 

 rocks he is familiar with at home ; his thoughts will 

 naturally revert to Dudley or Wenlock, classic localities 

 for Silurian limestone, and he will have little hesitation 

 in asserting that the Niagara limestone has a very Si- 

 lurian aspect. But when he looks more closely into it 

 and discovers the fossils it contains, the Dudley " butter- 

 fly," or the Dudley "locust," or the familiar Chain-coral, 

 then any doubt he may have entertained will disappear, 

 and he will exclaim, " This is not merely like, this is Si- 

 lurian limestone." Yet in taking this step from a mere 

 conclusion of similarity to that of identity, he has perhaps 

 been governed more by common sense than logic, and 

 may then easily fall into the assumption that since the 

 limestone at Niagara and at Dudley are the same thing, 

 they must not only have been formed under similar 

 physical conditions, but during the same period of time. 

 This is, indeed, precisely what as a rule geologists did : 

 having identified strata by their fossils, and thus arrived 

 at a Silurian and Devonian and other systems, they next 

 began to speak of a Silurian and Devonian period, mean- 

 ing a period of the earth's history when Silurian or 

 Devonian rocks were in course of deposition. 



Geologists had been so intent in putting fossils to their 

 newly-discovered use as a means of identification, that 

 they had scarcely paused to consider closely what prin- 

 ciples of logic might be involved. This, of course, gave 

 opportunity to the critic, and several candid friends of 

 our science, who had been watching its progress, now 

 became aware of the need for caution. Herbert Spencer, 



