72 
CHALK FORMATION. 
sent, chalk is seldom used in architecture, except 
in the construction of vaults, cellars, and other 
subterranean works. 
The upper portion of the chalk formation in 
Sussex is naturally separated into two divisions ; 
viz. the CHALK with flints, and the chalk with- 
out FLINTS.* The lower strata also form two well- 
marked deposits : the chalk marl, which con- 
tains a large proportion of argillaceous earth ; and 
the FIRESTONE, OF malni-rock, that principally differs 
from the marl in having a considerable intermix- 
ture of green and grey sand. The Gault or Galt, 
(the blue marl of Folkstone), and the Shanklin or 
Lower Green Sand, may likewise be ranked as 
subordinate members of the chalk formation, not- 
withstanding the dissimilarity of their mineralogical 
characters ; for their organic contents correspond 
in so many particulars with those of the marl, 
firestone, and chalk, as to warrant the conclusion 
that they were deposited by the same ocean, and 
that the entire series constitutes but one geolo- 
gical epoch or formation, t We proceed to take a 
* Even this character, so constant and perfect as it is in the south of 
England, is not universally maintained. “ At Havre on the opposite 
coast, the lower chalk contains an abundance of flint and chert 
nodules where it passes into the upper green sand .” — Manual of Geo- 
logy, by Mr. De la Beche. 
f See the excellent observation on the relative value of the mine- 
ralogical and zoological characters of rocks, by M. Brongniart, Essai 
Mineralogique ; note to pp. 327. et seq. 
Messrs. Conybeare and Phillips have separated not only the gait and 
firestone from the chalk, but also the chalk marl, including the whole 
under the general name of “ Chalk Marl.” These gentlemen, however, 
remark, that, in applying the term formation to these subdivisions, they 
merely use it as a convenient designation for a large assemblage of 
similar strata. 
