LOWER CHALK—MlCRQGRARHiC STRUCTURE. 
299 
At Folkestone the percentage of coarse particles and fine mud 
seems to decrease progressively from the base to the summit of the 
zone, but the proportionate decrease of coarse particles is greater* 
than that of the clay. This is shown in the three analyses quoted, 
and is confirmed by the examination of thin slices of the rock. 
At Hunstanton the deposit is practically a pure chalk, the in¬ 
soluble residue being only 3*03 per cent., but at Speeton the per¬ 
centage of residue at this higher horizon shows an increase when 
compared with the Hunstanton specimen. 
THE QUARTZIFEROUS LIMESTONES OF DEVONSHIRE. 
ADDENDUM TO P. 274. 
Microscopic Aspects. 
The lowest and coarsest part of the deposit is unfit for slicing, but the 
higher part of No. 10 has been cut and is seen to be a sandy rock, consisting 
of large quartz-grains (mostly well rounded) and shell fragirfents of many 
kinds, with Foraminifera and a sprinkling of glauconite-grains, the whole 
cemented by clear crystalline calcite. Among Foraminifera Textularia , 
Cristellarici , and Rotalia are common, Glohigerina is rare or absent. 
There are many pieces of sponges (? Calcisponges), a few spicules of Siliceous 
Sponges, and some fragments of Bryozoa. 
The succeeding bed (No. 11) resembles the finer and more shelly portions 
of the bed below ; it consists mainly of calcareous particles derived from 
various sources, with Foraminifera and Sponge-spicules, and many grains of 
quartz and glauconite, the latter being more frequent than in Bed 10. The 
whole is cemented by crystalline calcite. There are no Spheres. 
The highest part of the Am. Mavitelli zone (the No. 12 of Meyer) is a 
different rock, parts of it being a true chalk, not unlike an ordinary 
Chalk Marl. The matrix consists of fine amorphous, calcareous matter, 
through which are scattered many Spheres, a few shell fragments, with some 
Foraminifera and Sponge-spicules. It differs from Chalk Marl, however, 
in containing large quartz-grains, large grains of glauconite, and large 
shell-fragments, these often occurring all together in patches, which are, 
nevertheless, integral parts of the rock, and are not sharply marked oft* 
from the finer portions. 
Bed 13 is a similar but less consolidated rock. The matrix is a chalk 
with spheres and small Foraminifera, but with fewer shell-fragments. It 
differs from 12 in being more crowded with large grains of quartz and 
glauconite, the former occurring in rounded, subangular and angular grains. 
Dr. Hume examined a sample of the hard limestone west (1 south) of Beer 
Harbour, and we believe this to have been taken from Bed 12. His other 
sample was from Bed 13, near Lyme Regis. 
