1824.] De la Beche's Selection of Geological Memoirs. 381 



establish between the magnesite of the Paris basin and those we 

 have just mentioned. The magnesite in all, whether 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 isolated masses, and whatever the 

 mineralogical differences may be that these varieties of quartz 

 present, not only is its difference all that is necessary to esta- 

 blish the geological resemblances which we desire should be 

 remarked ; but it may be said that these varieties follow without 

 interruption from the oldest to the newest magnesites, as the 

 following- table will show : — 



D 



Parisian magnesite ....■< Chert 



Crystallized quartz 



Several varieties of opal (silex resinite) 



Magnesite of Salinelle. . Chert 



( Crystallized quartz 



Magnesite of Madrid . . \ C^ (silex corne) 

 6 j Chalcedony 



C Several varieties of opal (silex resinite) 



Maenesite of Moravia. . < ™,- u ± / , , ., :'•'!'% 



b (_ White and green opal (silex resinite) 



r Chert 



Magnesite of Piedmont < v • .. -L . . ., .«. x 



to j Varieties ot opal (silex resinite) 



'Jasper 



" Before geology had acquired in principles and facts the 

 precision to which it has now arrived, the presence of magne- 

 site in the Paris basin had no other result than that of adding a 

 mineral species to the list of those contained in our country ; but 

 this fact now possesses another interest : it has served to unite 

 observations which were, it may be said, isolated. It informs 

 us that the magnesite beds were deposited on the surface of the 

 globe at very different epochs, for some (those of Piedmont) 

 belong to the most ancient sediment rocks, and others (those of 

 Salinelle and Couloinmiers) to the newest sediment (tertiary) 

 rocks ; and yet we see these deposits accompanied by nearly the 

 same geological circumstances. Such a remarkable constancy in 

 the association of silex and magnesia, two bodies between which 

 there is no chemical analogy, will fix the attention of geologists, 

 and may perhaps contribute to show us the origin of these de- 

 posits, as the thermal springs of Italv deposing travertine have 

 pointed out that of the freshwater limestone. It is still apparently 

 iiom the bosom of the earth that the liquid arose which depo- 

 sited these rocks; for we find in certain thermal waters traces of 

 all the ingredients of their composition : the mass of water is at 



