1824.] De la Heche's Selection of Geological Memoirs. 379 



with the Manzanares ; it is very distinctly seen near the hermi- 

 tage 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 follows a bed of reddish clay with nodules of flint (silex 

 pyromaque). Though the magnesite has not been observed 

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

 rests upon it, because ascending towards the hill, the magnesite 

 is found to follow ; and the flint nodules are the same as those 

 of the magnesite. The magnesite 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 papy- 

 riforme), on which crystals of carbonate of lime are observed ; 

 they are also seen on the magnesite. This same deposit reap- 

 pears close to Madrid, it may be observed as we leave the bar- 

 riere 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 Ihe north of Madrid : the 

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

 its situation. A thin bed of greenish clay containing very little 

 magnesite is observed above the magnesite at Vallecas ; then 

 follows a reddish common opal (silex resinite) 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 magnesite is found above this fragile 

 opal. 



" ' The different beds above noticed by M. de Rivero occur 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 

 three upper beds 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 of which bricks are made, 

 and above these clays an alluvial formation, composed of fine- 

 grained sand, and lastly vegetable earth on the surface. 



" Thus the magnesite of Vallecas and Cabanas, near Madrid, 

 possesses the same tenacity, the same hardness, the same light- 

 in -s, the same superficial roseate tint, as those of Coulommiers 

 and Salinelle. It is equally composed of 23 parts of magnesia, 

 53 of silex, and 20 of water ; it is accompanied, like ours, by 

 chert (flint .'), which also passes into its mass, by common opal 

 (silex resinite), by chalcedony, by crystallized quart/., and calca- 

 reous spar altogether resembling those of our siliceous limestone. 

 It aflbrds, certainly, no organic remains; but wc know that these 



