518 THE CRETACEOUS ROCKS OF BRITAIN. 
east, that from north-west Norfolk giving a residue of only ’776 
per cent., and that from Hessle, Yorkshire, still less, ‘752 per 
cent. 
Though the largest percentage of coarse residue occurs in the 
examples from Beer Cliff, Culver Cliff, and Hessle, this appears 
to he due to the presence of secondary rather than to detrital 
minerals. Thus, in both the Culver and Hessle samples a large 
proportion consists of iron oxide, while Dr. Hume notes that the 
coarse residue in the Beer Head sample consists largely of chalce- 
donic aggregates and limonite. The two specimens which appear 
to contain most detrital minerals are those from Mupe Bay and 
Afton Down. Although in the first of these the coarse residue 
is only ’010 per cent., it consisted chiefly of mineral grains. 
The green-coated nodule bed at Afton Down, from which Dr. 
Hume obtained his specimen, recording it as Chalk Bock, is below 
that horizon, and is included bv us the zone of T erebratulina 
(see p. 410). This, like the Mupe Bay specimen with which it 
may be compared, appears to be rich in detrital minerals.* 
Detrital Minerals. — Quartz is again the most abundant mineral 
in the residues. Dr. Hume recognises Mica (muscovite and bio- 
tite) in the specimen from Beer Cliff*, Devon; and Augite, Horn¬ 
blende, and Tourmaline in that from Afton Down. Amongst 
the detrital mineral grains in the specimen from Bingstead, Nor¬ 
folk, Mr. Teall can identify Ortlioclase. The largest grains occur 
in examples of the zone from the West of England; in that from 
Beer Cliff Dr. Hume records '40 mm. as the maximum size, and 
grains up to '31 mm. in their longest diameter occurred at Mupe 
Bay. The average size, however, scarcely exceeds '10 mm! in any 
locality. 
The quantity of mineral particles which occur in each residue 
is exceedingly small. 
Secondary Minerals. —Many grains of glauconite occur in 
the chalk of this zone from Devonshire, but elsewhere it is 
of rare occurrence in the residues. 
The coarser portion of the residues consists chiefly of frag¬ 
ments of red ferruginous iron oxide, usually porous and easily 
crushed. In the residue from Hessle parts of four separate 
individuals of Terebratulina occurred as casts in this material, 
one of them being nearly perfect. 
Fragments of siliceous network, or fragments of Inoceramus 
shell silicified interprismatically, with a few flat flakes of silicified 
shell were noted in nearly all residues. 
* Chemical and Micro-Mineralogical Researches, p. 53, et seq. 
