MIDDLE CHALK—MICROSCOPIC STRUCTURE. 
515 
parts of this comity. A series of specimens from Cruxton, near 
Maiden Newton, were examined; those from the zone of Rhyn- 
chonella Cuvieri and from the lower part of the Terebratulina zone 
exhibit the usual features, but higher beds disclose an evident 
change of conditions, hard nodules occurring in the usually soft 
chalk of the zone. Thin sections of the nodules show that 
the chalk composing them is exceptionally full of small Foraminifera 
and small fragments of calcareous organisms, amongst which 
portions of the plates of Echinoderms can he recognised. Several 
sponge-spicules which appear to belong to the Monactinellid and 
one to the Tetractinellid group can be seen, and small grains of 
yellowish green glauconite also occur. 
Still higher, but below the Chalk Rock, a well defined “rock bed” 
occurs. Here again the amorphous calcareous matrix is full of 
small foraminiferal cells, small Globigerina and Textularia , 
amongst which occur several larger specimens. There are also 
many calcareous fragments too small and too much broken up 
for identification except pieces of the plates of Echinoderms, but 
which from their diversity of appearance suggest other organisms 
than shells. Sponge-spicules also occur, and one or two grains of 
pale green glauconite. Between the hard beds the chalk is full of 
foraminiferal cells and finely comminuted calcareous particles, 
while glauconite occurs in all specimens from the highest parts 
of the zone. 
In Devonshire also the chalk of the Terebratulina zone 
shows some variation in its structure compared with that of the 
eastern counties. The following is a description of a series from 
Beer Head. 
A yellowish-irhite bed at the base of the zone , Beer Harbour.— 
Foraminiferal cells and Spheres are abundant, but there are very 
few CHobigerina or other Foraminifera; all are well outlined. Sponge 
spicules, chiefly of Monactinellid types, are common, but two 
or three Tetractinellid forms occur, and in one place the nodes 
of some Hexactinellid mesh appear to be cut through. The 
spicular walls are in every case replaced by calcite. The larger 
calcareous fragments are for the most part Inoceramus-prisms, but 
a few pieces show the perforated structure. There are at least a 
dozen circular forms like those of Radiolaria figured as Nos.l and 
2 in the Quart. Journ. Geol. Soc., Vol. 51, page 608. 
Recognisable organic remains may be estimated in this specimen 
at about seventy-five per cent., with one or two mineral grains, but 
no glauconite; the remainder is the usual fine amorphous calcareous 
mud. 
The same locality ten feet higher.— Foraminiferal cells and 
Spheres hardly so abundant; a few Globigerina and Textularia 
occur. There are but few calcareous fragments; and these are 
4219 . L l 
