LOWER CHALK—MlCftOGtRAPHIC STRUCTURE. 
327 
show the strongest traces of the receding shores of the Cretaceous 
Sea, and, as before, the amount both of clay and coarse material 
diminishes to the east. The upper part of the zone, however, 
becomes nearly a pure chalk; nowhere does the amount of the 
residue exceed 10 per cent. 
In the Belemnite Marls there is a sudden increase in the amount 
of fine clay. This increase is most marked at nearly the extreme 
south-west and north-east points of our area—viz., Cerne Abbas, 
Dorset, and South Yorkshire. At Cerne the residue increases 
from 10 per cent, at the summit of the underlying chalk to 31 per 
cent., and in South Yorks from 2 per cent, to 30 per cent.; at 
Swanage also the amount of the residue in the Marl is 37 per cent. 
Although the Marls are well marked in Sussex, Kent and Herts, 
the amount of clayey material is less than at the -above-mentioned 
localities—viz., at Dover the increase is from 6J per cent, to 18f 
per cent. ; at Hitchin (Herts) from 3J per cent, to 19 per cent. We 
have no record of an analysis from the top of the Lower Chalk at 
Eastbourne, but the amount of residue in the Marls is 20 per cent. 
At Eastbourne, Dover and Hitchin Microzoa are common ; but 
in Yorkshire and Dorsetshire it was difficult to find either 
Foraminifera, Ostracods, or shell-fragments, though minute 
Foraminifera came away in the fine washings of the Dorset 
specimen. 
Mr. Teall observes that the detrital mineral grains which have 
been observed in the Lower Chalk, are such as would arise from the 
decay of plutonic and metamorphic rocks, and it is extremely in¬ 
teresting to find them distributed over the whole floor of the Creta¬ 
ceous Sea. In no case have minerals of volcanic origin been detected. 
With regard to the debris of organisms which form very fre¬ 
quently so large a part of the deposit, Inoceramus shells play an 
important part. Prisms of the shell and sometimes larger frag- 
ments are found in every specimen, and many of the minutest 
particles are probably derived from their disintegration. Other 
shells add their quota to the mass of the rock ; fragments of Ostrea 
Plicalula, Pecten , Rhynchonella and Terebratula have all been 
detected, though of less frequent occurrence. 
Echinodermal plates and spines, though frequently recorded, are 
never a feature of the deposit, though the ossicles of star-fish occur 
near the top of the Lower Chalk rather commonly. Fragments of 
Bryozoa, though met with in all zones, are not numerous enough to 
suggest that any large accession to the calcareous mud was obtained 
by their decay. 
Sponge spicules and the results of the decomposition of sponge 
structure play an important role locally, and we have seen in Dorset, 
Wilts, and Berks that these organisms have left marked evidences 
of their former existence. 
It is difficult to say what proportion the Foraminifera contribute 
to the formation of the Lower Chalk. Those species which build up 
their tests with grains of sand are abundant in the lower beds, 
