chap. x.J CONTINUITY OF THE CHALK. 469 



sedimentary roek, and may be said to be almost pure 

 carbonate of lime. I quote an analysis of the white 

 chalk of Shoreham (Sussex), by Mr. David Forbes. 1 



Calcium carbonate 98*40 



Magnesium carbonate O08 



Insoluble rock debris 140 



Alumina and loss 0-42 



100-00 



Even the grey chalk of Folkestone contains a very 

 large proportion of carbonate of lime, the other sub- 

 stances existing merely as impurities which can 

 scarcely be said to enter into the composition of the 

 rock. The following is an analysis by Mr. Forbes of 

 the base of the Folkestone grey chalk : — 



Calcium carbonate 94 "09 



Magnesium carbonate 0"31 



Insoluble rock debris 3G1 



Phosphoric acid i 



Alumina and loss j ce 



Sodium chloride 1-29 



Water 0-70 



100-00 



The most remarkable point in this analysis is that 

 while white chalk is almost always associated with 

 chert and flints, the chalk itself does not contain a 

 particle of silica. 



The chalk-mud of the Atlantic on the other hand 

 contains not more than GO per cent, of calcium car- 

 bonate, with 20 to 30 per cent, of silica, and varying 

 proportions of alumina, magnesia, and oxide of iron. 

 We must remember, however, that in the English 



1 Quoted in Mr, Prestvvich's Presidential Address, 1871. 



