A. J. Jukes-Browne—Silica in Chatk. 545 
Mr. Hume’s analyses also prove the Marsupite Chalk of Margate 
and of Studland Bay (Dorset) to be purer than the Chalk of the 
underlying coranguinum at the same localities. From four analyses 
from each zone given by Mr. Hume (op. cié. p. 74) it would appear 
that the amount of insoluble residue in the coranguinum zone 
averages 1:78 per cent., while that in the Marsupite zone only 
averages 1:02 per cent. 
Conclusions.—The facts and analyses above given are, I think, 
sufficient to prove that there is no definite relation between the 
occurrence of flints and the absence or presence of soluble silica in 
the surrounding Chalk. 
The great abundance of siliceous spicules in certain parts of the 
Lower Chalk and their rarity in the Upper Chalk, cannot be 
taken as evidence of the greater abundance of siliceous sponges in 
the former; as a matter of fact, remains of sponges other than free 
spicules are far more abundant in the Upper. Neither can the 
absence of disseminated spicules in the Upper Chalk be accounted 
for on the supposition that they were once there but have been 
removed in solution after the consolidation of the Chalk, for why 
should they remain in the Lower Chalk, which has been upraised, 
inclined and exposed to the same action of percolating water as the 
higher part of the formation? Moreover, if the spicules or other 
siliceous organisms had existed in the Upper Chalk and had been 
subsequently dissolved, we should find traces of them either as 
empty tubes or as calcite casts. As a matter of fact there are beds 
both in the Lower and in the Upper Chalk where spicules occur 
abundantly in the state of calcite casts, but flints do not occur in 
them, nor are they specially abundant below them. 
Hence the conclusion I have arrived at, from consideration of all 
the facts known to me, is that Chalk which is now destitute of any 
remains of spicules has, since it became Chalk, always been destitute 
of such spicules. It is possible that spicules may have lain for a 
time in the ooze which became Chalk; but if so I believe they were 
eventually dissolved in the sea-water before that ooze was raised or 
compacted into anything like Chalk. 
How then, it may be asked, has it come to pass that spicules in 
the Lower Chalk have so often been preserved? That is a question 
not easy to answer; I have been tempted to suppose that the depth 
of the water had much to do with it, for there is good reason 
to think that the Lower Chalk was formed at depths of from 3800 
to 500 fathoms; while the characters of the Upper Chalk certainly 
indicate a much greater depth of water. The material of the Upper 
Chalk was probably deposited much more slowly than that of the 
Lower Chalk, so that the spicules of decaying sponges might have 
been dissolved before embedment. 
The formation of flints, however, has certainly nothing to do with 
depth of water, because they occur in all parts of the Chalk and in 
the chalky limestone of the Portland series; and yet there may be 
some connection between the solution of spicules, diatoms, etc., and 
the formation of flints. It has always seemed to me that one of the 
DECADE III.—VYOL X.—NO. XII. 35 
