CHAP. X.] CONTINUITY OF THE CHALK. 469 
sedimentary rock, 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.' 
Calciumicarbonate. 2) 4... 2. . 98:40 
Magnesium carbonate - . . °. . = - 0:08 
Insolublefrock.débris 2. 2 1:10 
/ANOMIMNITE, MMACLNOSS “4 5 5 6 4 6 o 0:42 
100-00 
Even the grey chalk of Folkestone contains a very 
large proportion of carbonate of lime, the other 
substances 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 :— 
Caleiumycarbonate ire. “2905s ee 78 oe E09 
Magnesium carbonate . .-. . . . . Odi 
Imsolubblevnockedebris tu” 2 ye ese a fen) -osOd 
Phosphoric acid 
iumamasamagslosse 00. ts se ee eee nt | PIES 
Pomimmachlormaece ex cele ae ey Uae ee IEZS 
Mice anss (ae etet eee ae hab Og Q 
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 60 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. Prestwich’s Presidential Address, 1871. 
