332 
THE CRETACEOUS ROCKS OF BRITAIN. 
CHAPTER XXIV. 
CHEMICAL COMPOSITION OF THE LOWER CHALK. 
! • 
The beds of the Lower Chalk vary considerably in chemical com¬ 
position, as might he surmised from the obviously different litho¬ 
logical characters which they present at different horizons and at 
different localities. 
The lowest beds, especially where they rest upon greensand, con¬ 
tain a large amount of sand and siliceous matter, and, roughly 
speaking, the proportion of such matter decreases upwards till in 
the higher and whiter paid of the Lower Chalk there is not more 
than 9 or 10 per cent, of such matter. 
The more marly beds, such as are generally known by the name 
of “ Chalk Marl,” usually contain from 66 to 80 per cent, of car¬ 
bonate of lime, with sometimes a small quantity of carbonate of 
magnesia, but there is no regular upward increase in the propor¬ 
tion of carbonates, for the more purely calcareous beds alternate 
with the more argillaceous strata, and the former occasionally 
contain as much as 88 per cent, of carbonates. 
In South Yorkshire, Lincolnshire, and North Norfolk the beds 
which represent the Chalk Marl are pure limestones containing 
from 85 to 95 per cent, of carbonates. 
The upper part of the Lower Chalk, that is, the portion which is 
above the horizon of the Totternhoe Stone, does not vary nearly 
so much in composition, seldom having less than 88 per cent, or 
more than 94 per cent, of calcium carbonate, the average amount 
being apparently from 89 to 90 per cent. 
In the Belenmite Marls at the top of the Lower Chalk We have a 
return to the conditions of the Chalk Marl, the proportion of calcium 
carbonate being reduced by admixture with from 10 to 20 per cent, 
of siliceous and argillaceous matter. 
Analyses of Chalk Marls. 
The following are analyses (x4) of “ grey marl from the lower part 
of the Chalk Mail,” and (B) “ of the harder part of the marl above 
the last ” (A), both from near Farnham, Surrey. * 
* Paine and Way, Journ. Roy. Agric. Soc., Vol. xii. 
