582 HISTORY OF GEOLOGY. 



series of strata in different countries is, in the 

 highest degree, important and interesting. Indeed, 

 in the same manner in which the separation of Clas- 

 sificatory from Chemical Mineralogy is necessary for 

 the completion of mineralogical science, the com- 

 parative Classification of the strata of different 

 countries according to their resemblances and dif- 

 ferences 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 elevated 

 problems; and requires a rare union of laborious 

 observation with a comprehensive spirit of philoso- 

 phical classification. 



In order to give instances of this process (for of 

 the vast labour 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 exe- 

 cuted, as we have already stated, by order of the 

 government. In this undertaking it was intended 

 to obtain a knowledge of the whole mineral struc- 

 ture of France ; but no small portion of this know- 

 ledge was brought into view, when a synonymy had 

 been established between the secondary rocks of 

 France and the corresponding members of the Eng- 

 lish and German series, which had been so well 

 studied as to have become classical points of stand- 

 ard reference. For the purpose of doing this, the 

 principal directors of the survey, MM. Brochant de 



