LOWER CHALK—M T CROGR A PHI C STRUCTURE. 
325 
GENERAL SUMMARY OF THE COMPONENT MATERIALS 
OF THE LOWER CHALK. 
The basal beds of the Lower Chalk vary considerably in their 
character. The arenaceous rock of Maiden Bradley differs widely 
from that of Folkestone, and this again from that of Shouldham 
in Norfolk. The basement bed in the Isle of Wight seems to 
present features intermediate between those of South Dorset and 
Folkestone, while that at Cerne, in North Dorset, has a character 
peculiar to itself. 
Viewed as a whole they appear as deposits one would expect to 
find in a sea which gradually deepened from West to East. The 
quartziferous deposit of Maiden Bradley suggests a close approach 
to an adjacent shore line, while the abnormal thickness and sandy 
nature of the Ohloritic Marl of Mupe Bay not only indicates the 
contiguit}^ of land but probably also the action of strong currents. 
It is remarkable to find within a mile of Mupe Bay—viz., at 
Lulworth, the features of the Ohloritic Marl presenting so different 
an aspect ; but even here, though the quantity of the coarser 
detrital minerals is small, the relative size of the grains is large. 
Though the coarse detrital material decreases to the eastward, 
the fine mud increases. None of the Dorset specimens yield more 
than 15 - 8 per cent, of clay, while Bed 5 at Compton Bay, Isle of 
Wight, contained 19’9 per cent., and that at Shouldham in Nor¬ 
folk 28 per cent. The Shouldham deposit seems to indicate 
that in Norfolk there was a closer approach to oceanic conditions, 
though this locality was still within reach of terrigenous material. 
The amount of soluble material in the specimen of the basement 
bed at Cerne is greater than any of the others, but it will be seen in 
the sequel that this locality seems to be out of the reach of the mud¬ 
bearing currents, and that the Lower Chalk at all horizons con¬ 
tains less fine mud and less coarse material than specimens from 
South Dorset and the coast of Devonshire (Beer Bay). 
The glauconitic sand of Folkestone is a greensand which may 
be compared with the greensands described in the Report of the 
Challenger Expedition.* 
The salient features of these beds are so easily seen in the table 
(p. 272), that further comment is unnecessary. 
In the chalk of the Am. varians zone it will be observed by 
reference to the table and general summary of the acid residues that 
fairly large quartz-grains continue to occur most abundantly in 
the specimens from the west of England. 
In the arenaceous beds which represent this zone in Devonshire, 
the coarse; mineral debris is unaccompanied by fine inorganic matter, 
and the matrix is practically a calcareous sand consisting of coarse 
particles, probably derived from the organisms which occur so 
* Deep Sea Deposits. ChCLllengtr Report, p. 230. 
