504 
THE CRETACEOUS ROCKS OF BRITAIN. 
grains. Besides quartz, Mr. Teall recognises orthoclase in the 
coarse residue of the specimen from Barret Ringstead, Norfolk, 
and three grains of magnetite adhered to the magnet on its 
application to the residue of the same specimen. 
From the base of the nodular chalk at Swanage Dr. Hume 
records felspars “ which differ but little from the quartzes by 
which they are surrounded; they are, however, at once distin¬ 
guished by the kaolinisation which they have undergone.” He 
also notes that at Folkestone zircons occur so large and con¬ 
spicuous as to attract attention on mere inspection of the 
residue, and not only in rounded grains but fragments of well 
preserved crystals. * 
The average size of detrital minerals in our residues varies 
from *10 mm. to T9 mm., the largest grains measuring >53 
and *48 mm., but Dr. Hume found grains measuring 2\ mm. by 
1 1 mm. at Swanage;f 
Secondary Minerals. —Besides masses of a porous iron oxide 
of irregular shape, which occur in all the residues, there is little 
to record under this heading. Glauconite is rare and generally 
absent altogether, but in the rock at the base of this zone at Beer 
there are casts of Foraminifera in a yellowish glauconite. 
In every residue from chalk at this horizon there occurred 
peculiar honeycomb-like structures, apparently of organic origin. 
Dr. Hume thus refers to them J — “ They appear perfectly snow- 
white when viewed by reflected light, and looked at from the upper 
surface present a very definite network, the orifices being fre¬ 
quently circular in outline, and the walls complete and somewhat 
thickened. In transverse section the canals are tubular, and the 
walls are then sometimes seen to be themselves perforated by 
numerous apertures/' Dr. Hinde examined these structures, 
but was unable to refer them to any sponge family. “ The con¬ 
clusion finally arrived at is that the networks were originally 
sponge-mesh, but that subsequent chalcedonic changes have 
obliterated all those characters which would enable their true 
generic position among the Porifera to be determined/' 
There are also other silicified fragments occurring in many 
of the specimens which Dr. Hume describes in his notes on the 
Middle Chalk of Devonshire : —“ These consist of the silicified 
columnar aggregates formed interprismatically in Inoceramus- 
shell.” A few silicified prisms of Inoceramus -shells also occur. 
Besides the fragments noted above there were a few fiat pieces 
in some of the residues, probably shell, but showing no definite 
structure. 
* Chemical and Micro-Mineralogical Researches, p. 49. 
■f This observation requires confirmation ; it is possible that the large 
quartz-grams may have been driven from the shore into the crannies of 
the rock from which the sample was taken, or that some of the white 
chalcedonic lumps, which are from 2 to 3 mm. in size, were mistaken at 
the time for quartz. • 
+ Chemical and Micro-Mineralogical Researches, p. 47. 
