390 JUKES-BROWNE ', MICROSCOPIC STRUCTURE OF ZONES OF CHALK. 
The Grey Chalk or zone of Holaster subglobosus has the Tottern- 
lioe Stone at the base over a large area ; this stone is a shelly Chalk, 
containing much glauconite and some quartz. The Chalk above it 
consists of fine calcareous dust in which there are some shell frag- 
ments and a little glauconite. In the highest part of the zone, from 
Norfolk to the Thames Valley, there is no visible glauconite or 
quartz, but south of the Thames both minerals occur sparingly up to 
the very top : the grains are, however, small, and other minerals are 
very rare. 
Arenaceous Foraminifera are abundant in some localities, or at 
certain horizons, especially at Folkestone, and resemble those in the 
Chalk Marl. Calcareous spheres are also very abundant in the upper 
half of the zone. 
In the zone of Belemnitella plena the material of the marls 
resembles that of the Grey Chalk, except that the amount of fine 
clay in it is much larger, and that the arenaceous Foraminifera 
belong chiefly to the genera Ammodiscus and Haplophragmium. 
Glauconite grains are very rare even on the south coast. 
The Melbourn Rock is a very pure limestone ; its whiter 
portions consist almost entirely of a fine calcareous paste, through 
which calcareous spheres are abundantly distributed, but the marly 
veins between the white lumps are full of shell fragments. No 
minerals have yet been recorded from it. 
The Melbourn Rock passes up into the zone of Bhynch. Cuvieri 
or Inoceramus mytiloides, both fossils being common in it, and 
broken Inoceramus shells being so distributed through it as to give 
it a gritty character. The Chalk itself, however, is largely made up 
of calcareous spheres, shell fragments becoming less and less abun- 
dant as a microscopic constituent. The most remarkable point is 
the recurrence of mineral grains in the South of England and North- 
east of France ; whether they occur in this zone further north is 
not yet known. The quantity is very small, less than 0'3 per cent, 
of the mass ; the grains are mostly quartz and felspar, and are gener- 
ally of decidedly rounded outline, but are sometimes large. Glau- 
conite seems to be absent, for none has been observed. 
The zone of Terebratulina gracilis is everywhere a purely cal- 
