290 M. Broncniakt on the 



rests upon it, because ascending towards the hill, the mag- 

 nesite is found to follow ; and the flint nodules are the same 

 as those of the magnesite. The magnesite (N"?. m.) occurs 

 in very thick beds, coating flints which are disseminated 

 through the beds : these beds are cleft, and in the clefts 

 we find asbestus (asbeste papyriforme), on which crystals of 

 carbonate of lime are observed ; they are also seen on the 

 magnesite. This same deposit re-appears close to Madrid, 

 it may be observed as we leave the barriere by the Portello; 

 the flint is there disseminated in the same manner. M. de 

 Rivero has also met with it on the banks of the river 

 Manzanares, opposite the king's villa; it has also been 

 found at Cabanas, nine leagues to the north of Madrid : the 

 author, not having visited this last place, is unable to describe 

 its situation. A thin bed of greenish clay (N° 6.) contain- 

 ing very little magnesite is observed above the magnesite at 

 Vallecas; then follows a reddish common opal (silex resi- 

 nite) in beds of variable thickness, very fragile, presenting a 

 crust of manganese on some parts of its surface ; this opal is 

 worked for gun flints. A very soft and nearly earthy mag- 

 nesite (N° 4.) is found above this fragile opal." 



" The different beds above noticed by M. de Rivero, oc- 

 cur in the hill of Vallecas. The top of this hill constitutes 

 a platform, on which are found many flints, and pieces of 

 opal, with crystals of carbonate of iron ; crystals of pseudo- 

 morphous quartz have moreover been observed, and have 

 been taken for opal crystals." 



" Shells have never been met with in this formation. The 

 beds, represented in the ideal section, fig. 2, by the Nos. 

 1, 2, and 3, appear on the banks on the Manzanares, as we 

 quit the gate leading to the Escurial. 



The author has above stated that magnesite Is met with 

 on the banks of the river, and if we ascend towards the 

 town, we find beds of greenish and reddish clays (No. 3), 

 of which bricks are made, and above these clays an alluvial 

 formation (No. 2), composed of fine grained sand, and 

 lastly vegetable earth, (No. 1) on the surface." 



Thus the magnesite of Vallecas and Cabanas, near Mad- 



