332 HISTORY OF GEOLOGY. 



similar series of events, which have, in some measure, coincided in 

 time and order ; and thus, as we have said, refers us to a theory. But 

 yet, considered merely as a step in classification, the comparison of the 

 geological series of strata in different countries is, in the highest de- 

 gree, important and interesting. Indeed in the same manner in which 

 the separation of Classificatory from Chemical Mineralogy is necessary 

 for the completion of mineralogical science, the comparative Classifi- 

 cation of the strata of different countries according to their resem- 



O 



blances and differences alone, ,is requisite as a basis for a Theory of 

 their causes. But, as will easily be imagined from its nature, this part 

 of descriptive geology deals with the most difficult and the most ele- 

 vated problems ; and requires a rare union of laborious observation 

 with a comprehensive spirit of philosophical classification. 



In order to give instances of this process (for of the vast labor and 

 great talents which have been thus employed in England, France, and 

 Germany, it is only instances that we can give,) I may refer to the 

 geological survey of France, which was executed, as we have already 

 stated, by order of the government. In this undertaking it was 

 intended to obtain a knowledge of the whole mineral structure of 

 France ; but no small portion of this knowledge was brought into 

 view, when a synonymy had been established between the Secondary 

 Rocks of France and the corresponding members of the English and 

 German series, which had been so well studied as to have become 

 classical points of standard reference. For the purpose of doing this, 

 the principal directors of the survey, MM. Brochant de Villiers, De 

 Beaumont, and Dufrenoy, came to England in 1822, and following 

 the steps of the best English geologists, in a few months made them- 

 selves acquainted with the English series. They then returned to 

 France, and, starting from the chalk of Paris in various directions, 

 travelled on the lines which carried them over the edges of the strata 

 which emerge from beneath the chalk, identifying, as they could, the 

 strata with their foreign analogues. They thus recognized almost all 

 of the principal beds of the oolitic series of England. 7 At the same 

 time they found differences as well as resemblances. Thus the Port 

 land and Kimmeridge beds of France were found to contain in abun- 

 dance a certain shell, the gryphcea virgula, which had not before been 

 much remarked in those beds in England. With regard to the 

 synonyms in Germany, on the other hand, a difference of opinion 



De la Beche, Manual, 305 



