JUKES-BROWNE : MICROSCOPIC STRUCTURE OF ZONES OF CHALK. 389 
quartz-grains does not prove the deposit to have been formed within 
the influence of shore currents. Again fine volcanic dust, containing 
recognisable particles of felspar, augite, hornblende and olivine, is 
often carried to great distances from land by the wind currents in the 
higher regions of the air. 
3. Structure of successive Zones of the Chalk. 
We now come to a consideration of the composition of the dif- 
ferent zones into which the Chalk can be divided. Much more work 
is required before any such account can be considered complete, and 
the following statements only apply to portions of that part of Eng- 
land which lies to the east of a line drawn from the Wash to the 
South Coast at Weymouth. The Chalk of Yorkshire has not yet 
been sufficiently studied either in its zonal or structural aspects. 
The Chalk Marl, or Zone of Ammonites varians, exhibits great 
differences in minute structure as it is traced from Norfolk to Dorset. 
In the X. W. of Norfolk the beds, which represent the Chalk Marl, are 
hard and highly calcareous ; some are of fine some of coarser grain, 
according to the relative abundance of shell-fragments, but glauconite 
is absent e icept in one bed (the so-called " Inoceramus Bed,") and 
the quartz grains are few and small. As the zone is traced south- 
wards, through Norfolk and Suffolk, beds of soft marl come in 
between the harder layers, and particles of quartz and glauconite 
become more frequent throughout. When traced further south these 
grains become rather larger, as well as more numerous, till in Berks 
and Wilts, where the zone attains its greatest thickness, the material 
is at its coarsest. Wiltshire Chalk Marl consists mainly of shell 
fragments, quartz particles, glauconite grains, and sponge spicules in 
varying proportions ; and some beds contain much globular colloid 
silica. Near the base the grains of quartz are of comparatively large 
size, gradually becoming smaller in the higher beds. On the South 
Coast the following minerals have been found in Chalk Marl : — 
Quartz, Felspar, Zircon, Tourmaline, Apatite, Hornblende, Rutile and 
Garnet. 
The Chalk Marl of the Isle of Wight, and to a less extent that 
of Folkestone, is characterised by the abundance of arenaceous 
Foraminifera, especially species of Tritaxia, Texlular'm dLU&Bulimi?ia. 
