THE MATERIALS OF STRATA. 



47 



served at Shell Ness which makes the eastern end 

 of the Isle of Sheppey. Parts of the forest mar- 

 ble in Oxfordshire, consist of accumulated growths 

 of shells; and in Gloucestershire portions of the 

 same deposit show ripple marks which indicate 

 shallow conditions of deposition. 



Coral reefs are also to be classed as shallow- 

 water limestones since the coral grows most vigor- 

 sly where the water is aerated by the movement 

 of the wav^ near to the surface of the sea. The 



Fig. 7. — Carboniferous limestone, the surface dissolved by rain, 

 showing the remains of Encrinite columns, of which it is part- 

 ly formed. 



great brainstone coral Meandrina and the com- 

 pact coral Porites, associated with Nullipores 

 build buttresses which constitute the living foun- 

 dation of the reef. Our English coral limestones 



