LONDON BASIN. 297 



and made into mortar. It contains the following petri- 

 factions, viz. ammonites, scaphites, turrellites, trochites, 

 and madreporites. The middle bed, the hard chalky is in 

 general harder than the bed above it, although Mr. 

 Webster remarks, that it appears from some observations 

 he made in Dorsetshire, that the hardness does not al- 

 ways mark a particular bed, the flint chalk being in some 

 places much harder than that without flints in others. It 

 contains a greater variety of petrifactions than the chalk 

 marl, as appears from the following list of the genera ob- 

 served by Mr. Webster. Several echini of the same fami- 

 lies as those met with in the chalk with flint ; but many 

 of them, particularly the cassides, differing much in their 

 forms from those found in that bed. Spines of echini ; 

 and particularly those described by Brard as resembling 

 the Belemnites. Patellites. Trochites. Serpulites, 

 several species. Belemnites. Lima? Fish, too much 

 mutilated to ascertain the genus. Palates, scales, verte- 

 brae, and teeth of fish. Cancri. The upper bed, the 

 soft chalk with flints, forms the upper part of the formation, 

 and is distinguished from the preceding by its softness, 

 and always containing flints. It also differs from it in 

 the petrifactions it contains, of which the following are 

 enumerated by Mr. Webster. Asterise. Echini of seve- 

 ral families. Spines of the foregoing, resembling belem- 

 nites. Serpulites. Cardium. Spondylus. Ostrea, several 

 species. Pecten, several species. Chama ? Terebretula, 

 many species. Alcyonia, sponges, and numerous un- 

 known zoophytes. A ramose madrepore. Several spe- 

 cies of minute encrini, figured by Mr. Parkinson. 



1 . Lower Marine Formation. 



This formation is separated into two great divisions, 

 1. Sand and plastic clay. 2. London clay. 



38 



