MARINE MARL FORMATION. 281 



and throughout its whole extent of the same thickness), 

 but also because it is considered as marking the upper 

 boundary of the first fresh water formation, and the be- 

 ginning of a neAV marine formation. All the shells that 

 occur in the niarl above this bed belong to the ocean. 



A great bed of greenish clayey marl, without petrifac- 

 tions, rests immediately over the yellowish marl, and 

 contains kidneys of clayey calcarious marl, and also of 

 celestine. Immediately over these follows a bed of yel- 

 low clay-marl, which abounds in fragments of marine 

 bivalve shells, cerites, trochites, mactrites, cardites, ve- 

 nites, &c. and fragments of the tail of two species of ray 

 have also been found in it. 



The beds of marl which rest over these contain princi- 

 pally bivalve marine shells ; and in the uppermost bed 

 of calcarious marl, immediately under the clayey sand, 

 there occur two distinct beds of. oysters, of which the un- 

 dermost contains large and thick oysters, and the upper, 

 which is sometimes separated from the under by a thin 

 bed of white marl, without shells, numerous, small, thin, 

 and brown oyster shells. This latter bed of oysters is 

 very thick, is divided into many layers, and is scarcely 

 ever wanting in the hills of gypsum. 



These oysters appear to have lived on the spot where 

 we at present find them, because they are arranged as we 

 find them in oyster-banks in the ocean; and the greater 

 number of them are whole, and with both valves. 

 Lastly, M. Defrance found, near Roquencourt, at the 

 height of the formation of the marine gypseous marl, 

 rounded fragments of marly shell limestone, pierced with 

 :|)holades. and with oyster shells attached to them. The 



36 



