74 PROCEEDTXGS OF THE GEOLOGICAL SOCIETT. 



sent him on his way rejoicing, with the ' Principles of Geology ' in 

 his hand ? 



The second aspect in which Palseontological science presents itself 

 to the geologist is as affording a key to the Chronology of the rock- 

 masses of the globe. We still regard fossils as the "Medals of 

 Creation," and certain types of life we take to be as truly character- 

 istic of definite periods as the coins which bear the image and 

 superscription of a Eoman emperor or of a Saxon king. 



But in the application of the principle that " strata are to be 

 identified by their organic remains," we have now to admit as 

 many limitations, and to exercise as much caution, as when judging 

 of the conditions under which rock-masses must have been deposited 

 from the characters of the fossils which they contain. 



Within the restricted area of the South-west of England, where 

 William Smith achieved his epoch-making discovery, the doctrine 

 which he announced seemed to be absolutely true ; each formation 

 exhibited a peculiar and perfectly characteristic assemblage of 

 organic remains, by means of which it could at once be recognized. 

 The still more detailed studies of strata of the same age, by Hunton 

 and Williamson in Yorkshire, by Marcou in the Jura, and by 

 Quenstedt in Swabia, seemed to indicate that the principle had a 

 wider application than even its author himself could have imagined, 

 and that zones a few feet or even inches in thickness might be 

 followed over considerable districts, everywhere marked by some 

 particular type of Ammonite or other characteristic fossil. 



But the more thorough and systematic study of corresponding 

 formations over wide areas, which was inaugurated by Oppel and 

 has been carried on by many palaeontologists since, has abundantly 

 demonstrated that, striking as is the parallelism of the zones in 

 such a formation as the Lias, when studied in England, Erance, and 

 Germany, yet the species and varieties found on the same horizon 

 at distant points are in many cases not identical, but merely 

 representative ; and, further, that as we pass away from any typical 

 area, the sharp distinction between the several zones seems gradually 

 to vanish. 



The same facts come out very strikingly when we study any 

 other great geological period. In the oldest fossiliferous strata, 

 those of the Cambrian, nothing can be more striking than the 

 similarity of the faunas in j^orth America, Britain, Scandinavia, and 

 Bohemia ; and yet the species which occur at the several different 



