LOWER CHALK—MICROGRAPHIC STRUCTURE. 269 
It cannot, of course, be assumed that the minerals have been 
derived directly from the parent rocks. They may have done 
duty, so to speak, in pre-existing sedimentary deposits. 
The material obtained from the Cambridge Greensand received 
special attention with the following results : — 
As far as practicable all phosphatic nodules were removed before 
drying and weighing, though a few minute ones were discovered 
after treatment with the acid. 
The remarkable features of this residue are the smallness of the 
mineral grains and the abundance amongst them of mica flakes ; 
some of these were white, silvery, and clear white, others have a 
dark sooty appearance. Except the mica flakes, which have con¬ 
siderable superficial area, no mineral grain measured exceeded 
TO mm. All the quartz-grains are angular and appear little worn. 
The grains of glauconite as a whole are rough and mammillated, 
and vary in colour from a pale to a dark greyish-green ; all have a 
dull weathered or worn exterior though from their aspect in thin 
sections this does not appear to be due to incipient decomposition, 
for their edges are sharp and clear and there is no suggestion of 
alteration of their material. Though no grain of exceptional 
size was noted, the greater part of them are certainly larger than 
usual. The coarse residue was nearly free from particles of iron 
oxide. 
Secondary Minerals.—Glauconite .—This mineral is more abundant 
in the Cliloritic Marl and basement-beds of the Chalk than at any 
other horizon. In the Chloritic Marl of Folkestone it is estimated 
to form CO per cent, of the material examined, and the proportion 
in the Cambridge Greensand is probably about the same; at 
Ventnor there is only 12 per cent.; at Lulworth and Mupe Bay 
13 and 15 per cent, respectively; Maiden Bradley 9 per c mt.; and 
at Shouldham about 3‘5 per cent. The quantity of gla iconite in 
the first-named specimen agrees fairly well with the typical 
glauconitic sands of the Challenger collection, which however, 
“contain 40 to 50 per cent, of Foraminifera and other carbonate 
of lime shells, together with the remains of siliceous organisms.”* 
Foraminifera are not so important a feature in the Folkestone deposit. 
Arenaceous Foraminifera are estimated at only 2 per cent, of the 
coarse residue of this specimen and other forms are by no means 
numerous. 
At Folkestone and Ventnor, in the Cambridge Greensand, and 
in examples of the Chloritic Marl in the West of England the grains 
resemble those of the Gault in their rough, furrowed, mammillated. 
outline, but a larger proportion can be recognised as Foramini feral 
casts. Writing of the Cambridge Greensand in 1872. Professor 
Sollas says :t “ the glauconitic grains are the casts of Foraminifera, 
possibly also of other minute shells. The larger casts, about 
* Deep-Sea Deposits. Report of the “ Challenger ” Expedition, p. 379. 
t See Quart. Journ. Geol. Soc., Vol. xxviii. p. 397. 
