214 Mr. Webster on the Strata lying over the Chalk. 



the Hordwell cliffs. It is very probable therefore that traces of 

 this formation may be found there, if this spot were well examined 

 for the purpose.* 



Land and river shells have been repeatedly discovered in various 

 parts of England ; and often at some depth under beds of sand 

 and gravel. They are then often accompanied by the bones of land 

 animals as those of the elephant, hippopotamus, &c. and may be 

 referred to a very ancient period probably connected with some of 

 these formations. None of them however had as yet been dis- 

 covered imbedded in a stratum of rock. When they have been 

 found under peat bogs they have been most probably produced in 

 some of the later states of the earth. 



It is in this formation, in the Paris basin, that the gypsum beds 

 are placed. Three series of gypseous strata are described : the 

 lowermost consists of thin beds of gypsum, often selenitous, of solid 

 calcareous marls, and foliated argillaceous marls ; and they observe, 

 that these are sometimes deposited on the marine shelly calcareous 

 sand, and then they contain marine shells. In these are also beds 

 of white marl, containing a great quantity of freshwater shells of 

 the genera limneus and planorbis. These lower beds, with the 

 white marl, constitute the oldest freshwater formation ; and, ac- 

 cording to the observations of M. M. Cuvier and Brongnlart, 

 appear to have been formed during the passage or change of the 

 Paris basin from the state of a marine bay to that of a freshwater 

 lake. The second mass of beds differs from the above mentioned 



* Since tlic reading of this paper I have been favoured J)y the Rev. William Buck- 

 land, Prof, of Mineralogy, Oxford, with specimens of freshwater shells which he has 

 collected at Hordwell cliff. They consist of the lymncus, planorbis, eyclostoma, and 

 a bivalve rcsumbiing a freshwater niytiUis. From the state of the fossils, and the 

 nature of the stratum in which they were imbedded, tliey would appear to belong to 

 the lower freshwater formation. 



