HEADON BEDS. 147 



Every variation in the amount of salt in tlie vv^ater seems to have 

 been marked by a change in the fauna. The purely freshwater beds 

 contain few mollusca except Limnaa, Planorbis, Paludina, Unio, 

 and land-shells. The different species of Potamomi/a, Cyrena, 

 Cerithium {Potamides), Melania, and Melanopsis appear nearly all 

 to have liked water containing more or less salt. So we have a 

 gradual change to beds containing Oysters, and then to beds with 

 Volutes. 



Besides these indications of varying conditions, it is interesting 

 to observe a general tendency in the beds to become more fresh- 

 water towards the south-west, while tufaceous limestones appear 

 in that direction. The land-shells also point to the proximity of 

 land, as do the pebbles of flint.^ Unfortunately at the point where 

 the most rapid changes are taking place — at Headon Hill — the 

 beds have been cut off by denudation. We cannot therefore see 

 whether the beds show any tendency to overlap each other, or to 

 overlap the underlying Eocene. 



* Pebbles of Chalk have been recorded, but they appear to be really white flints . 

 The flint pebbles in the Headon Beds are sometimes weathered to the centre 



K 2 



