.LOWER CHALK—MICROGRAPHIC STRUCTURE. 
311 
inorganic matter is recognisable in the lower beds of this 
division, but decreases upwards, and its effects are lost in the upper 
part of the Grey Chalk. 
Examination of the Residues. 
Detrital Minerals.— The quantity of detrital minerals in the 
chalk of the zone of Holaster subglobosus is exceedingly small, but 
there is a sensible diminution in the coarse residues when the lower 
parts of the zone are compared with the highest beds. It is prob¬ 
able also, as Mr. Teall remarks, that if a sufficient quantity of chalk 
were treated with acid and enough residue obtained all the 
minerals found in the zone below would occur at this horizon. 
The grains of quartz, which, as usual, are most numerous, are 
angular, subangular, or rounded, some being more worn than 
others. 
The lowest specimen from Mupe Bay again stands first both as 
to the quantity of the coarse residue and the dimensions of the 
grains, though the quantity is estimated at only '12 per cent, of 
the material used. The largest grains measure '55 mm. in the 
longest diameter, the average being *14 mm. Dr. Hume* notes 
a single grain in the Grey Chalk of Culver Cliff, the length of 
which exceeded T5 mm. 
Outside Dorset the maximum length of any grain recorded in 
the residues does not exceed '3 mm. in the Chalk of the southern 
counties, though in Yorkshire they slightly exceed this, the 
largest measuring '38 mm. 
The residues from the Lower Chalk were submitted to Mr. 
Teall, who reports as follows :— 
The foreign clastic material is of the same type as that already 
described from the Chloritic Marl. Quartz is the most abundant 
constituent, and is always present. One specimen, that from 
Mupe Bay, 20 ft. above the top of the Chloritic Marl, was ex¬ 
amined in detail, and the following minerals were recognised ;— 
Quartz, microcline, garnet, zircon, rutile, ilmenite, and tourma¬ 
line. Orthoclase and oligoclase are probably also present. There 
can be little doubt that if equal quantities of the other residues had 
been examined in the same way similar results would have been 
obtained. 
In addition to the foreign clastic material idiomorpliic pyrite 
occurs in the specimen 4 feet above the red chalk and 35 feet below 
the Belemnite Marl at Speeton—in the former as cubes, sometimes 
in combination with octahedra; in the latter as octahedra, some¬ 
times modified by the cube. (J.J.H.T.) 
Secondary Minerals. — Glauconite.— This is a rare mineral in the 
zone of Holaster subglobosus, though it can be recognised here and 
there in the thin sections of specimens obtained along the outcrop 
of the chalk between Wallingford and Devonshire as high as the 
* Chemical and Micro-Mineralogical Researches, p. 36. 
