544 A. J. Jukes-Browne—WNSilica in Chalk. 
Awatysis oF Mippir CHatx, Hircurn. 
Organic matter ... soc see 0-10 
Quartz Le or ats A -0°15 
Colloid silica 000 eae 000 0°35 
Silicates (Clay) ... ase 1°68 
Calcium sulphate ... 0 0°29 
Calcium carbonate 96°16 
Magnesium carbonate 1°34 
100-06 
This closely agrees with Mr. Hume’s analysis of Chalk from the 
zone of Terebratulina gracilis (Isle of Wight), which contained 97:76 
per cent. of carbonates, and he says ‘‘the main residue consisted of 
a light-brown clay, entirely free from siliceous particles.”! He 
found a few Foraminifera, but no sponge spicules nor any indication 
in the shape of glauconite rods that spicules had ever been present. 
This Chalk has no flints, while that at Hitchin has a few. 
3. Upper Chalk.—In this division, as developed in the South of 
England, the Micraster zones are characterized by the extreme 
abundance of flints, the Marsupite zone by their rarity or absence, 
and the zones of Belemnitella quadrata and B. mucronata by a 
moderate number. 
The Chalk of all these zones is very pure and similar in minute 
structure, except that shell fragments are sometimes very numerous 
in the Micraster zones. 
Mr. Hume’s researches show that the carbonates usually amount 
to between 98 and 99 per cent., only two samples yielding less than 
98. He found no sponge spicules nor any kind of colloid silica in 
any of the samples he examined, but glauconite rods, which seemed 
to be the casts of spicular canals, were numerous in the residue of 
the coranguinum Chalk. There are, however, beds in the zone of 
M. cortestudinarium in the South of England which are crowded with 
spicules in which the silica has been replaced by calcite. 
Prof. J. B. Harrison has specially examined two samples of Upper 
Chalk for colloid silica and sends me the following results: (A) being 
from Chalk with many flints near base of coranguinum zone north 
of Salisbury, and (B) from the Marsupite zone, with few flints, 
Bishops Down, eTsntes 
Dried at 100° C. A B 
Moisture O80 gu seeees 0:20 
Quartz OCB 2 ae haat 0:50 
Clay . PSUS | eee 0:45 
Colloid silica. eat ORDO: | 7 eeacke — 
Carbonates by difference ... 97:70 .... 98°85 
100:°00 100:00 
It will be noticed that while the Chalk with many flints (A), has 
an appreciable amount of colloid silica, that with few flints (B), has 
none at all. This is not the result that might be expected if the 
two Chalks had originally equal amounts of disseminated silica, 
1 Chemical and Mineralogical Researches on the Upper Cretaceous zones of the 
South of England, London, 1893, p. 41. 
