JUKES-BROWNE : MICROSCOPIC STRUCTURE OF ZONES OF CHALK. 391 
careous deposit, consisting of a fine calcareous matrix crowded with 
minute calcareous spheres, and including a few scattered shells of 
large Foraminifera (Globigerina, Rotalia, &c). The insoluble residue 
consists almost entirely of very fine light-brown clay, coloured by 
hydrous oxides of iron. Mineral grains are very rare, but Dr. Hume 
found a few small particles of quartz in one sample from the Isle of 
Wight. 
The Chalk Rock has a remarkable structure. It always contains 
a great variety of organic fragments derived from the shells of Echino- 
derms, Lamellibranchs and Brachiopods, with remains of siliceous 
sponges. Glauconite is abundant in the south central counties, but 
the grains become fewer and smaller to the north and east, and detrital 
minerals have only been found in the Chalk-rock of the southern coun- 
ties. Quartz is there common in small angular grains, and sometimes 
in large rounded grains. Dr. Hume has also observed Augite, 
Hornblende and Tourmaline in the rock of the Isle of Wight. 
The Chalk of the Micraster breviporus and cortestudinarium 
zones is generally rough and shelly ; it consists of a fine calcareous 
matrix, enclosing shell fragments and large Foraminifera in varying 
amounts. In the South of England detrital minerals appear in the 
insoluble residue; these include Quartz, Apatite, Felspar and Tour- 
maline, the quartz grains being of various shapes " from perfectly 
rounded to strikingly angular " (Hume), and some of it is granite 
quartz. Glauconite is either absent or in very small quantity. No 
arenaceous Foraminifera have been observed. 
The Micraster corangnimun Chalk resembles that of the cortes- 
tudinarium zone in general structure, but the insoluble residue is 
almost entirely fine clay, containing minute spherules and rods of 
Glauconite. Only a few very minute particles of quartz have been 
found. 
The Chalk of the Marsupite zone is purer than any of the lower 
zones, the amount of insoluble residue being generally under one per 
cent., and the greater part of this being fine clay ; but, by washing, a 
small residue is obtained, which includes minute angular fragments 
of quartz, grains of brassy iron pyrites, and of a reddish limonitic 
mineral, and rarely of glauconite, all very minute. 
