542 A. J. Jukes-Browne—Silica in Chatk. 
be left out of consideration, and consequently no analysis of Chalk 
which does not separate the soluble from the insoluble silica will be 
of any use. 
Here arises the question of what is “soluble silica?” In com- 
mercial analyses of soils and rocks the silica recorded as “soluble” 
is only the small amount which is soluble in warm hydrochloric acid ; 
but, properly speaking, soluble silica is that which can be dissolved 
in an alkaline solution of a certain strength. Crystalline silica, such 
as quartz and chalcedony, are not soluble in such a solution, but all 
forms of non-crystalline or colloid silica are. We can therefore only 
use analyses which have been made by capable scientific chemists, 
and unfortunately there are not so many such analyses as geologists 
could desire. 
The microscope, however, here comes to our assistance, for, by 
using the polariscope, we can easily see whether the siliceous 
particles are crystalline or colloid; checking the results by such 
good analyses as exist, we can determine whether flintless Chalk 
always contains soluble silica, and whether Chalk with flints contains 
little or none. 
It will be convenient to consider the case of the Lower, Middle 
and Upper Chalk separately, for it is now known that flints occur in 
each of these divisions in more than one part of England. 
1. Lower Chalk.—It used to be supposed that no flints were to be 
found in this division, but genuine flints, often with black centres, 
are common in the lowest part of the Chalk of Dorset; Mr. W. Hill, 
I.G.S., informs me that they also occur in that of Yorkshire. Sili- 
ceous concretions of the nature of flints also occur in the central part 
of the Lower Chalk of Wilts between Westbury and Urchfont.' 
Mr. W. Hill has cut and examined samples of the Dorset Lower 
Chalk with flints, and finds that it differs from ordinary Chalk-mazrl, 
containing much less quartz and glauconite. There are few spicules, 
and no globular silica has yet been detected in any of the samples 
Mr. Hill has examined. The amount of quartz and glauconite is 
sometimes very small, and one sample of flint-bearing Lower Chalk 
was almost purely calcareous, with only one or two spicules visible 
in the slide. It might, therefore, be supposed that the Dorset flints 
occurred in Chalk which had been depleted of soluble silica. 
The Wiltshire nodules, however, occur under just the opposite 
conditions, the surrounding Chalk being rich in colloid silica. 
Analyses by Mr. J. Brierly, of Southampton, gave over 12 per cent. 
of such silica, and the microscope shows that it exists partly in the 
form of sponge spicules, and partly in that of minute discoid 
and globular particles like those described by Dr. Hinde in the 
Malmstone of Surrey and Hants. Samples taken from just outside 
the cherty nodules showed no difference, except that the globules 
were rather larger than those in the mass of the deposit. The 
nodules themselves are simply portions of this siliceous Chalk 
saturated and cemented by an infiltration of chalcedonie silica.? 
1 Quart. Journ. Geol. Soc. vol. xlv. p. 406. 
2 Quart. Journ. Geol. Soc. vol. xlv. p. 41a. 
