Geological Map of France. 299 



We may deduce two important conclusions from these 

 latter facts : the first is, tliat nature has been able to pro- 

 duce similar rocks at different epochs, and that consequently 

 mineralogical characters are insufficient to determine geolo- 

 gical divisions. The second is, that rocks until now con- 

 sidered as primitive may be posterior to those which contain 

 organic remains : so that, in the actual state of our know- 

 ledge, it becomes very difficult to distinguish the true primi- 

 tive rocks from the transition, and every great division de- 

 parting from this principle is of difficult application. It 

 should not tlierefore be surprising if I propose to unite these 

 tocks into one great class, which I shall name primordial 

 rocks (terrains primordiaux), which has already been em- 

 ployed to indicate a less exclusive property than that attri- 

 buted to the word primitive. 



These rocks possess a very important character, which is 

 that their beds occur in inclined, disturbed, and even verti- 

 cal positions. The most ancient secondary rocks certainly 

 also present circumstances of this nature,* but less generally 

 and not in so decided a manner. 



I was desirous of tracing on the map the principal systems 

 that are distinguished in the primordial rocks ; but after 

 having successively reduced the number of these divisions, 

 I found myself obliged to renounce the project ; for indepen- 

 dently of the rocks presenting the union or rather the con- 

 fusion of many formations, I must confess that I at present 

 find the geological relations of all these divisions to be in the 

 greatest uncertainty. 



It must in fact be confessed, that our means of judging of 

 the relative age of rocks are in the end reduced to the super- 

 position of beds. All the other characters we employ for 

 these determinations are but analogies drawn from the obser- 

 vation of places where the superposition is evident, and 



* And also the newer secondary rocks, as for example the vertical 

 .beds of plastic and London clay in the Isle of Wight, (the latter being the 

 equivalent of the calcaire glossier of the Paris basin,) as also the nearly 

 vertical clmlk of llic same place and Dorsetshire. (Trans.) 



