84 Colonel Silvertop oji the Lacustrine Basins 



hill, in the lower part of which gypsum is again observed. It 

 then enters upon a high plateau-formed district, corresponding 

 in physical character to that which constitutes the plain of La 

 Mancha to the south of the Tagus, which extends northwards 

 to the base of the Guadarama Mountains. Madrid is situated 

 in this district, at the distance of seven leagues from Aranjuez. 

 Between the summit of the last ascent and a village named 

 Valdemoro, several low escarpments of gypsiferous marl strata, 

 in a horizontal position, are seen near the road ; but the general 

 superstratum of this tract is of an argillaceous marly character. 

 In the remaining distance to Madrid, or rather to the banks of 

 the Manzanares, no gypsum was observed ; but the road has in 

 places been cut through a little hillock or low eminence, and 

 the consequent escarpments exhibit a few thin strata of a whit- 

 ish compact limestone, which, for short spaces, occasionally as- 

 sumes a siliceous character, where its surface is sometimes ma- ** 

 millonated by concentric spheres of chalcedony, minute veins 

 of which also penetrate its mass. These strata, often marked 

 by blackish dendritic sprigs or stars, alternate with earthy 

 layers of friable marl. On each side of the road there is an 

 open undulating cultivated plain, whose upper stratum is com- 

 posed of fine diluvial detritus, amongst which innumerable sili- 

 ceous semi-opaline fragments are observed, identical with seve- 

 ral varieties of a similar substance imbedded in the well-known 

 hill of m'agnesite near Vallejas, about 2 miles E.SE. from Madrid. 

 From 'this' ground there is a long gradual descent to the Manza- 

 nares; and the right bank of this stream exhibits an extensive 

 mass of gypsiferous marl, in horizontal laminae or little strata, 

 the gypsum being generally of a laminar, but in some places of a 

 fibrous, structure. • The hill of magnesite, or carbonate of mag- 

 nesia, at Vallejas, rests upon this bed of gypsum. In the im- 

 mediate neighbourhood of Madrid, a compact vesicular whitish 

 limestone is quarried, whose mineralogical character would seem 

 to identify it with that to which allusion has been so frequently 

 made. It would be interesting to ascertain whether or no it 

 contains fresh-water shells, and overlies the gypsum formal ion 

 along the bank of the Manzanares. The Professor of Geology 

 and Mineralogy to the Royal Museum, in Madrid, had the 

 goodness to shew me a fragment of the magnesHe rock^ with a 



