Geological Map of France. 297 



spect to its mineralogical and chemical nature. We might 

 at first sight imagine that the latter way was the best to at- 

 tain the end proposed, as it seems to have most connexion 

 with the action peculiar earths exercise on vegetation ; but, 

 on the other hand, the different states of aggregation of the 

 substances composing the rock, the physical position of the 

 soil, and other circumstances belonging to the epochs of for- 

 mations, often exercise an equally marked influence. Thus 

 the pasturages of the Pennine Alps, the heaths (garrigues) 

 of Languedoc, and the fields of la Beauce, present very great 

 differences in their aspect and productions, though the soil 

 is calcareous in all. It will moreover easily be perceived 

 that the geological mode of consideration is much more ad- 

 vantageous for the progress of science, that it offers much 

 more interest, and that the power it allows of uniting, ac- 

 cording to circumstances, many systems into one group, per- 

 mits us to dispense with the detailed observation which 

 would be required, in the other case, for the frequent 

 changes of the predominant substances, in a formation of the 

 same epoch. 



It may be supposed that, this principle being once adopted, 

 it would be sufficient to take as guides the divisions estab- 

 lished in geological works, and to mark on the map the 

 places where the different formations determined by syste- 

 matic authors occur; but experience soon proved to me that 

 I could not follow this mode, though so simple in appear- 

 ance ; for admitting all the subdivisions established in geo- 

 logical treatises, we should infinitely multiply the obstacles 

 resulting from the Mant of observations sufficient for such a 

 large extent of country, and from the difficulty of referring 

 to common terms the different systems that exist in distant 

 countries. It sometimes happens that rocks which are greatly 

 developed in some countries fine off to such a point in others, 

 that it is no longer possible to mark their existence on a ge- 

 neral map. It has therefore been necessary to sacrifice to 

 uniformity the very natural desire of presenting all the de- 

 tails we possessed on certain countries ; it was also necessary 

 to form a system which, while it left out a great part of the 



