Magnesite of the Paris Basin, Sfc. 2139 



cutnsfances (circonstances geologiques) ; * a consideration 

 that must not be confounded with the geological position 

 (gisement.) 



The niagnesite of Vallecas near Madrid is already known, 

 for in 1807 I described, in my Tralte do Mineralogie (t. 2. 

 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 speci- 

 mens, 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, which I have given, pi. 10, 

 fig. 2. and a detailed description which I shall transcribe 

 almost literally. 



" The village of Vallecas is two leagues to the south of 

 Madrid ; it is situated lower than the latter town ; an iso- 

 lated hill, named the hill of Vallecas, occurs near the vil- 

 lage : before we reach the top of this hill, we meet with 

 small hillocks and excavations which arise from the work- 

 ings of the magnesite ; the tour of this hill may be made ia 

 20 minutes. From observing the locality, an idea is con- 

 ceived 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 (pi. 10. fig. 2.), N° 9, gypsum 

 ■with clay, which belongs to the saliferous formation + of 

 Villarubia : this gypsum extends from the walls of Madrid 

 to the junction of the river Javama with the Manzanarcs; 

 it is very distinctly seen near the hermitage of Notre Dame 

 de la Torre, 150 metres [492 feet] to the west of the hill 

 of Vallecas and near the canal of Madrid ; there then fol- 

 lows a bed of reddish clay (N° 8.) with nodules of flint (silex 

 pyromaque.) Though the magnesite has not been observed 

 immediately on the clay, yet M. de Rivero conceives that it 



• 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 constantly associated with certain minerals, with- 

 out any reference whatever to its geological or relative position. (Trans.) 



t New red or saliferous sandstone. (Trans.) 



T 



