380 Analyses of Books. [May, 



remains are rare in the siliceous limestone of the Paris basin, of 

 which our magnesite forms a part; lastly, if it appears to differ 

 by its position on a saliferous gypsum, much more ancient than 

 our gypsum, and calcaire glossier, it is not covered by any rock 

 which appears more ancient than the latter, and it is like them 

 in horizontal beds. 



" If from Spain we transport ourselves to Italy, to the foot of 

 the Piedmontese Alps, we shall find, at a short distance from 

 Turin, the serpentine hills of Castellamonte and Baldissero, tra- 

 versed in every direction by veins of magnesite which is tenacious 

 yet plastic, light, and with that roseate superficial tint which we 

 have noticed in the preceding magnesites. Its principal or fun- 

 damental and characteristic composition appears to be still the 

 same, i. e. of magnesia, silex, and water. Here however we have 

 carbonic acid, which seems to indicate a different chemical spe- 

 cies ; but its geological circumstances are still the same. I have 

 already noticed them in my memoir on the geological position 

 of the serpentines. 



" The mineral no longer occurs in horizontal beds, or nodules 

 interposed in the beds, but in numerous veins, uniting in every 

 direction in the midst of the serpentine ; chert, common opal, 

 and jasper, presenting many varieties of texture and colours, are 

 constantly and intimately united with it, as at Coulommiers and 

 Salinelle. They have been formed even in the midst of the mag- 

 nesite. This circumstance of geological association is then 

 remarkably constant, even when the geological position has no 

 longer the same character, and it is here very different. It 

 appears to me well established, that this magnesite belongs to 

 the serpentine formation of the Apennines, consequently to 

 ancient rocks, nearly of the transition epoch. 



" There are other examples of magnesites, 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 differs ; 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 difference, the importance of which 

 is not yet well appreciated ; the presence of silex nodules which 

 pass into the mass, reminds 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 



