46 On (he %Iineralogkal Geographi/ 



argillaceous formation ; but the lowest calcareous stratum 

 being generally sandy and always iilled with shells, we da 

 not as yet know if this sand be diHerent iVom the first, or 

 if it be the same rormation. What would lead us to sup- 

 pose that it was different iv, that the sand next the clayft 

 which we have seen, is generally very pure, although of a 

 Ted or blueish gray colour; it is refractory, and frequently 

 in very coarse grains* 



The calcareous formation reckcmed from this sand is com- 

 posed of alternate strata of coarse InwestoBe, more or less 

 hard, of argillaceous marie, and even leafy clay in very* 

 thin strata, and of calcareous niarle ; but it must not be 

 supposed that these various strata are placed there by 

 chance, and without following some general law : they 

 uniformly follow the same order of superposition in the 

 tronsiderable extent of sgil which we have examined. Ill 

 some places several of them are entirely wanting, or are 

 very thin ; but that wtucb was tiie lowest in one district 

 4iever becomes the uppermost in any other. 



This constancy in the order of superposition of the thin- 

 nest strata, and over an extent of 12 rnyrivamelrcs (74J- miles) 

 at least, is in our opinion one of the most remarkable facts 

 which we have establi^bel. From this there ought tty result 

 most interesting consequences to the arts and to geology. 



The means which we have employed for ascertaining, in 

 the midst of so great a number of calcareous beds, a stra- 

 tum already observed in a very distant district, istakcft. 

 from the nature of the fossils contained in each stratum, 

 these fossils beins: generally the same in the corresponding 

 strata, and present differences of species sufficiently re- 

 «narkable between one system of strata and the other. Thi^ 

 -is a mark which has not as yet deceived us. 



It must not be thought, however, that the difTercnce- 

 between one stratum and another is equally well marked 

 with that between the chalk and the limestone. If this was 

 the case, we should have an equ^l number of distinct forma- 

 tions ; but the characteristic fossils of one stratum become 

 less numerous in the next superior stratum, and disappear 

 completely in the rest, or are replaced gradually by newfos* 

 «iis wbich had not as yet appeared. 



Wc now proceed to point out (following the-Same comse) 

 ¥he principal systems of strata which may be observed in 

 tbe coarse linjcstonc. In the detailed Memoir about to be 

 Jtublishcd will be found tl'.c complete description, stratum 

 by stratum, of the numerous quarries which we have ob- 

 served in order to procure inatoriuls Lr our publication. 



The 



