gD2 M.BnoNGifiAUT on the 



character^ and it is here very different. It appears to me 

 well established, that this magnesite belongs to the serpen- 

 tine formation of the Apennines, consequently to ancient 

 rocks, nearly of the transition epoch. 



There are other examples of magnesitesj but the circum* 

 stances of their geological position are less well known ; yet 

 both what is known, and their composition, still very well 

 agree with what we have stated of the preceding. 



Thus the plastic magnesite of Asia minor, known by the 

 name of Ecume de Mer, has all the exterior characters of 

 that of Piedmont, and even that of Coulommiers, with a 

 composition that very slightly diifers ; it has, like it, the 

 roseate superficial tint which also occurs in the magnesite of 

 Houbricht in Moravia. But in this, the carbonic acid, which 

 is in some quantity, seems to establish a mineralogical differ'- 

 ence, the importance of which is not yet well appreciated ; 

 the presence of silex nodules which pass into the mass, re- 

 minds us of an analogy In the geological circumstances .^ which 

 is rather remarkable. 



CONCLUSIONS. 



We shall confine ourselves to these examples : they are 

 sufficient to prove the relations of formation which we wish 

 to establish between the magnesite of the Paris basin and 

 those we have just mentioned. The magnesite in all, whe- 

 ther it be or be not combined with carbonic acid, contains 

 water and silex : this last substance does not occur only in 

 chemical combination with the magnesia, it also forms iso- 

 lated masses, and whatever the mineralogical differences 

 may be that these varieties of quartz present, not only 

 is its presence all that is necessary to establish the geo- 

 logical resemblances which we desire should be remarked ; 

 but it may be said that these varieties follow without inter- 

 ruption from the oldest to the newest magnesites, as the fol- 

 lowing table will shew : 



