378 Analyses of Books. [May, 



is difficult to meet with more resemblance between uncrystallized 

 minerals, which occur at more than 100 leagues from each other, 

 and if the niineralogical species cannot be here determined by 

 the form, it is sufficiently so by the composition ; the analogies 

 drawn from its associated minerals, and its position, are the 

 same ; it is mixed with nodules of chertz (silex corne) which 

 resemble our menilite ; it is accompanied and covered by marly 

 limestone containing fresh water shells, consequently it belongs, 

 with that of the Paris basin, to a calcareo-siliceous fresh water 

 formation. 



" But magnesite, i. e. this stone essentially composed of mag- 

 nesia, silex, and water, occurs in many other places dispersed 

 over the surface of Europe, and consequently placed at great 

 distances from each other. Sometimes we are acquainted with 

 its mode of occurrence, and then we know that it is very differ- 

 ent from that I have above described ; sometimes we are igno- 

 rant of it, or at least we do but presume it ; but in all these 

 places and in all these positions we shall see the magnesite to 

 occur accompanied by the Same niineralogical characters and 

 the same geological circumstances (circonstances geologiques);* 

 a consideration that must not be confounded with the geologi- 

 cal position (gisement). 



" The magnesite of Vallecas near Madrid is already known ; 

 for in 1807 I described, in my Traite de Mineralogie (t. ii. p. 

 492), its nature and properties, from the information obtained by 

 the specimens received from Messrs. Sureda, Dumeril, and 

 Mieg, and of its position from the same specimens, and the 

 information of M. Link, who took it for a kind of clay ; a very 

 excusable error at that time. M. de Rivero has however studied 

 the same places, and has sent me an ideal section of this rock, 

 with a detailed description which I shall transcribe almost 

 literally. 



" ' The village of Vallecus is two leagues to the south of 

 Madrid ; it is situated lower than the latter town ; an isolated 

 hill, named the hill of Vallecas, occurs near the village : before 

 we reach the top of this hill, we meet with small hillocks and 

 excavations which arise from the workings of the magnesite ; 

 the tour of this hill may be made in 20 minutes. From observ- 

 ing the locality, an idea is conceived of a gypsum basin on 

 which the magnesian rock rests. 



" ' If we observe the structure of the hill, we observe, com- 

 mencing at the lowest part, gypsum with clay, which belongs to 

 the saliferous formations')- of Villarubia : this gypsum extends 

 from the walls of Madrid to the junction of the river Javama 



* " I have literally translated M. Brongniart's expression, though I should not have 

 used it myself in the same? sense ; M. Brongniart seems only to imply that it is con- 

 stantly associated with certain minerals, without any reference whatever to its geological 

 or relative position. — (Trans.)" 



■t " New red or saliferous sandstone. — (Trans.)" 



