290 
THE CRETACEOUS ROCKS OF BRITAIN. 
or minute spherules. At Speeton it occurred as beautifully-formed 
octahedral crystals or small nodules, consisting of such crystals fused 
together. 
Limonite .—Small nodules or irregularly shaped masses or nodules 
of brown ferruginous material occurred in nearly every specimen. 
The nodules were either minute spheres or cylinders with a brownish 
crust and darker interiors or irregular masses of porous brown 
iron oxide, the latter easily crushed to powder. They are probably 
derived from the alteration of Marcasite, for Dr. Hume* found a 
nucleus of this latter mineral remaining in some of them. Limonite 
not infrequently encrusts the spicules of sponges, and is occasionally 
seen forming casts of Foraminiferal tests. 
Finest Material. 
The finest material of the Chloritic Marl and of the zone of Am. 
V { 
varians is in these pages generally termed the amorphous matrix, 
and by M. Cayeux it is called the “ ciment.” It surrounds and 
includes the recognisable particles contained in the rock. When 
separated from the coarser particles by levigation, it reflects in its 
constitution the varying characters of the marls from different 
localities. Thus in sandy or glauconitic marls minute mineral 
and glauconite grains will be found to be numerous, while in very 
marly examples extremely fine inorganic matter preponderates, 
and the amount of calcareous material is small. That part of the 
amorphous material which is acted on by the acid solution, and 
which appears to be largely, if not altogether, of organic origin, 
is of similar nature both in the Chloritic and Chalk Mail, and may 
be thus described. It consists of (1) particles which behave with 
polarised light as calcite, and are probably of organic origin, (2) 
particles of calcite which seem to be of subsequent formation, and 
(3) exceedingly fine matter, presumably of inorganic origin, the 
pai*tides of which are hardly discernible even with high powers. 
The first division maybe further sub-divided into (a) flat particles 
of irregular shape, their edges frequently angular, usually showing 
an indefinite internal structure and sometimes a fine striation. 
These are presumably minute fragments of shell; (b) very thin 
fragments, not infrequently perforate, which seem to be derived 
from foraminiferal tests. Many washings contain also a number of 
very minute Foraminfera, chiefly Globigerina and Textularia , 
and minute “ spheres ; ” (c) minute calcite particles which as a rule 
give no clue to their derivation, though some can be recognised as 
broken pieces of crystalloids. 
(d) Coccoliths ,—Minute oval bodies which are frequently referred to 
as Coccoliths, sometimes as crystalloids, form a very important part 
of the finest material, though their apparent abundance is not 
so great in this lowest division as in the higher beds of the Chalk. 
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* Micro-Mineralogical Researches, p. 43, 
