CHALK. 157 



which have sunk w gged, more or less de- 



| ed by the borings of the ship worm Ter€< 



e of the remains ui animals, like the 



flying Ornithosaurs and the long-bodied lizard 



indicate proximity to land. 



The larger part of the chalk is supposed to have 



been accumulated in moderate depth oi \v;i! 

 because 5t everywhere shows ev:- 



den< irrents on the sea-bed. 



The chalk is divided into four deposits, partly 

 on the evidence Of its physical characters, and 



partly from the nature of its fusils. The lower 

 part of the rock, often termed Chalk Marl, com- 

 print Ik Of 1 k>ver, which is coloured 

 with clayey matter, and is known as the Chalk 

 . and rotteml 3 lie. 'The bottom bed- 

 contain many univalve shells, and many SUrviv< 



of the remarkable - i Cephalopods, named 



i. TurriliU* (spirally tur- 



reted), i :a!t -lik( whnh are SO 



chai ilt and Upper Greensand. 



The \lk l which succeeds the Chalk 



;, terminates upward in the bed known 

 the rone of // $us. It is thin in 



the eastern counties, but becomes thicker in 

 the south of England. It is covered by the 

 Melbourne rock, a bed formed of chalk boulders, 

 embedded in a paste of chalk, and resting upon 

 a plane Of erosion as marked as that by which 



the Cambridge Greensand rests on the Gault, 



indicating a change of level, which uplifted the 

 underlying deposit. 



The overlying beds are the Middle Chalk. 

 This part of the chalk lies between the Mel- 

 bourne Rock below, and a hard phosphatic bed 

 above, known as the Chalk Rock, which rings 



