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THE CRETACEOUS ROCKS OF BRITAIN. 
General Summary. 
In the case of the Lower Chalk our study of the chemical and 
micro-miner alogical aspects disclosed many features of interest. 
Considerable differences were shown to exist in the nature of the 
deposit when specimens from different localities and horizons 
were examined, and it was possible to follow a lateral as well as 
a vertical change from a calcareous marl to a nearly pure cal¬ 
careous rock, with a sudden recurrence of argillaceous matter 
at the top. 
These features are not repeated in the Middle Chalk, and we 
have to deal with a material more homogeneous in its nature, 
the zones of this division showing none of the marked differences 
which occur in the Lower Chalk. The hard nodular Melbourn 
Rock, which makes so well marked a base to this division over a 
large area, seems to have been formed during a time when the 
sea-floor was swept by a steady current. The nodules which are 
so prominent a feature of this bed, are not phosphatic concretions, 
but seem to have been the firmer parts of an original deposit which 
resisted the effects of the current action; they are different 
ir character to the surrounding matrix, which, though vari¬ 
able in its aspect, is usually full of shell-fragments. These 
appear to have been sorted out from the fine mud by a slight 
current, for in several thin sections they have the appearance 
of having been arranged round the nodule as though by the action 
of slowly moving water. The results of chemical analysis of this 
bed do not suggest that the current bore with it terrigenous mate¬ 
rial, for the percentage of the residue is everywhere less (for each 
locality) than in the highest bed of the zone of Holaster subglo- 
bosus. The amount of the coarse residue is everywhere very small, 
and much even of this consists of peculiar organic meshwork 
which may probably be referred to sponges. 
In the overlying chalk the nodular character of the rock 
gradually disappears and analysis of specimens (with one 
exception) shows that the amount of fine inorganic material 
mingled with the calcareous mud diminishes. The exception 
is a specimen analysed by Dr. Hume from Swanage, Dorset, from 
which he obtained a residue of 7‘72 per cent., none of the other 
examples exceeding 2 * 28 per cent. 
Western examples, however, still exhibit the more decided 
traces of adjacent land, and we find, from Dr. Hume’s notes, 
that in the zone of Rhynchonella Cuvieri in Devonshire terri¬ 
genous material is still a feature of the deposit. Though the 
amount of detrital minerals is small, yet it is possible to identify 
amongst them fragments “ such as would be derived from the 
igneous rocks of the south-western counties.” These mineral 
fragments are, however, not so abundant in the base of the zone 
as in the limestone locally known as Beer Stone. In this Dr. 
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