50 On the Mineralogical Geography 



dug in tlic garden of M. Lopez at Fontenay-aux-Roses, 

 which first passed through the gypsum, and afterwards 

 the limcsioue, are ample proofs of the position of the 

 gypsum on the limestone. 



The gypsous hillocks have a peculiar appearance, which 

 renders ihcm conspicuous at a great distance : as they are 

 always placed upon the limestone, they form on the higher 

 eminence a kind of second conical or elongated hillock, 

 always distinctly marked. 



We shall exhibit the details of this formation, by taking, 

 as an example, the mountain which presents the most 

 complete collection of strata; and although Montmartrc 

 has been already well described, it is still the best and the 

 most interesting example that can be selected. 



Wc have recognized, both at Montmartre and on the 

 hillocks which seem to form the continuation of this emi- 

 nence, three masses of gypsum. The lowermost is com- 

 posed of alternate and thin strata of gypsum, frequently 

 selenitous, of solid calcareous marles, and of very scaly 

 argillaceous marles. It is in the former that we chiefly see 

 the coarse crystals of lenticular yellowish gypsum, and it 

 is in the latter that we find the menilite silex. We are not 

 acquainted with any fossil in this mass, which is the third, 

 of the quarries. 



The second or intermediate mass differs from the fore- 

 going only because the gypsous beds are thicker, and the 

 marlcy strata are less numerous. We ought to remark that 

 among these masses, that which is argillaceous, compact, and 

 of a marbled gray appearance, serve* for building-stone. It 

 is chiefly in this mass, that fossil shell-fish have been seen. 

 No other fossils, however, are found in it ; but sulphated 

 sirontian has lately been discovered in it, in scattered frag- 

 ments, in the lower part of the marble-like marie. 



The superficial mass, which the workmen call the first, 

 is in every respect the most remarkable, and the most im- 

 portant. It is, besides, much more extensive than the rest, 

 iince it is m some places 25 metres (S2 feet) thick, inter- 

 rupted only by a small number of marley strata; and in 

 some places, as at Dammartin and Montmorency, it is si* 

 tnated almost immediately under the vegetable earth. 



The lo\v;ermost beds of gypsum in this first mass con- 

 tain silex. The silex and gypsum seem as if mutually 

 dissolved in each other. The intermediate beds arc na- 

 turally divided into coarse prisms, with several pbnes, 

 which M. Desmarets has drawn extremely well. They 

 are called ih^ high pillars^ Finally, the uppermost strati 



are 



