MIDDLE CHALK—MICROSCOPIC STRUCTURE. 
523 
Hume records sand grains as large as ’88 mm. in tlieir longest 
diameter, while in the same zone at Lulworth the maximum in 
'48 mm. Except in the Swanage specimen, the amount of 
fine clay is slightly larger than in the south-eastern and midland 
counties. 
The characteristic feature of these two zones does not lie with 
the inorganic but with the organic constituents of the rock. 
Thin sections viewed under the microscope show that Spheres 
are extraordinarily abundant, and the deposit is crowded with 
them, no matter from what locality the specimen comes. M. 
Cayeux also notices this feature in the chalk of the Paris basin. * 
In some specimens from the zone of Rhynchonella Cuvieri these 
minute bodies form 70 or 80 per cent, of the rock. Their out¬ 
line is usually though not always well defined, the test a well 
marked ring and their interiors filled either with fine calcareous 
paste or minute caleitic crystals. These organisms, though often 
locally abundant in the Lower Chalk, are never so constant a 
feature as at this horizon. Associated with the Spheres are 
G-lohigerina and Textularia; in the former the walls of the 
tests are seen to be well marked in the thin sections of the rock, 
though they are not so thick as those found in sections of Atlantic 
mud, and the number of specimens occurring in a thin 
slice, though variable, is never so large as in the modern 
oceanic deposit. Another feature of this zone is the abundance 
of the prisms of Inoceramus- shells, which are doubtless derived 
from Inoceramus mytiloides so abundant in this zone. 
There can be no doubt that the lower part of the Middle 
Chalk is built up chiefly of disintegrated Inoceramus-sheMs , 
Foraminifera, and Spheres, though Ecliinoderms, Star-fish, 
Bryozoa, and other calcareous organisms have added their quota 
to the deposit. 
There is no marked divisional line between this zone and that 
of Terebratulina, the conditions of one passing gradually 
to those of the other. Spheres gradually become less numer¬ 
ous, their, shells thinner, their outline less defined, until 
above the lower third a large part of the rock of this zone consists 
of amorphous calcareous material, giving no evidence of its deri¬ 
vation.! 
It is not until quite the upper part of the zone is reached that 
Spheres and Foraminifera again begin to appear conspicu¬ 
ously in thin sections of the rock, and their tests partially regain 
the clear definition and robustness seen in the zone of Rhyncho¬ 
nella Cuvieri. In this part of the zone in some localities minute 
Clobigerina and Textularia are more common than usual. 
* See Contribution a l’etude Micrograpliique des Terrains Sedimentaires, 
by M. L. Cayeux, p. 454. 
t See also Cayeux, op. cit.. p. 456. 
