288 
THE CRETACEOUS ROCKS OF BRITAIN. 
Pontocypris bosquetiana (Jones.) 
Bairdea subdeltoidea (Munster). 
,, harrisiana (Jones). 
Bythocypris Brownei (Jones.) 
Cythereis triplicata (Homer). 
Cytheropteron concentricum 
Oytherella ovata (Homer). 
(Heuss). 
alatum. (Bosquet) 
umbonatum (W}U.) 
auriculata (Cornuel). 
ornamentissima (Heuss). 
obovata (J. and II.). 
Muensteri (Homer). 
Williamsoniana (Jones ). 
var. paupera (J. 
and H ). , 
Examination of the Residues. 
Detrital Minerals.—As before, thedetrital minerals here referred 
to are those which occurred in the coarse portion of the acid resi¬ 
dues, from which the percentages and measurements were taken. 
Quartz is always the most abundant of the detrital minerals. In 
general appearance and characters the grains do not differ from 
those in the basement beds. Referring to the grains in the Chalk 
Marl of Culver Cliff, Isle of Wight, Dr. Hume* says they consist 
of “ highly angular fragments of a beautiful water-clear transparent 
type;” other grains present crystal faces, though it is not usual to 
obtain a perfect crystal. In one case a very delicate prism capped 
by rhombohedral faces has been observed. We also have met 
with one or two such grains. He also notes that some of the grains 
had lost their clear character, having become coated with a fer¬ 
ruginous oxide. In referring to the Chalk Marl of East Wear Bay, 
near Folkestone, he says the quartz differs little from that previously 
described from the Isle of Wight. 
The largest amount of detrital minerals is again found in 
the most westerly specimens. In one sample taken 10 feet above 
the glance nitic marl at Mupe Bay, they are estimated 
to form 6 per cent, of the material examined; further eastward 
they diminish considerably, and in a specimen 10 feet above 
the Chloritic Mail in the Isle of Wight they form only 0 * 8 per cent, 
of the material, end at Dover at the same horizon O'2 per cent. 
At the same horizon at Arlesev, Bedfordshire, the grains in a residue 
from 50 grammes of material could almost be counted on the fingers. 
The quantity of detrital minerals diminishes rapidly upwards. 
At Mupe Bay, 20 feet above the glauconitic marl, they form only 
O'2 per cent, of the material examined; at Dover, at nearly the 
same horizon, only 0'03 per cent. The percentage of detrital 
minerals to the east and-north-east of this is infinitesimal. 
Hie largest grains occur at Mupe Bay, and, as in quantity so in 
size, they diminish rapidly upwards. Their average size in this 
locality 10 feet above the Chloritic Marl was T9 mm. with a 
maximum of 1*2 mm., and 20 feet above it the maximum was 'S3' 
mm., average T3 mm. At Dover, however, while as a whole the 
grains are smaller, averag ng only ' 07 ?ni * 08 respectively 10 feet 
.and 25 feet above the Chloritic Marl, the larger grain occur in the 
higher bed, the maximum being '* oT mm. 25 feet above the 
Chloritic Marl and ' 37 mm. 10 feet above it. 
* Chemical and Micro-Mineralogical Researches by Dr. Hume, p. 25. 
