A. J. Jukes-Browne—Silica in Chalk. 541 
its bad state of preservation, but this is fortunately not germane to 
the subject; it lies flat and compressed and is surrounded on three 
sides by masses of Mollusca, in places two and three deep, which 
have doubtless been attached to the wood by their byssi. The 
anterior ends and in many cases the inferior borders are turned 
towards the piece of wood and are in close contact with it, while 
others are again attached to the first row, as in masses of living 
mussels. 
The shells are much crushed ; but in several places the distinctive 
characters of the genus Anthracoptera can be made out :— 
(a) Is one showing the straight hinge-line, the triangular form, 
and oblique swelling. 
(b) Is a specimen showing a typical posterior end. 
(c) Shows a fairly. well-preserved anterior end. 
(d) Is a less crushed specimen than most. 
Microscopically the shell-structure is identical with that of 
Anthracoptera. 
This is the only specimen known which exhibits these shells 
attached by the byssus; but I have seen several slabs of small 
Posidonias similarly attached to calamite stems in the collection 
of Mr. George Wild, of Bardsley, from the Lower Coal-measures 
around Oldham. 
V.—Tse Amount or DisseMInaTED Sriica In CHALK CONSIDERED 
in Renation to Furs. 
By A. J. Juxes-Browne, B.A., F.G.S. 
T has been supposed, though I do not know who was the first to 
suggest it,! that the total amounts of silica existing in the Chalk 
with flints and the Chalk without flints respectively are very nearly 
equal: the idea being that if the quantity of silica disseminated 
through the flintless Chalk could be accurately estimated it would 
be found to be nearly or quite as great as the amount existing in the 
shape of flints in an equal thickness of flinty Chalk. 
Proceeding on this assumption it seems also to have been inferred 
that silica was originally disseminated through the mass of the 
Upper Chalk, and that this silica has in some mysterious way been 
concentrated into nodules and layers of flint, leaving the surrounding 
Chalk in the condition of a nearly pure calcareous material. If this 
supposition were true it could doubtless be used as an argument in 
favour of the theory that flints have been formed by some process of 
‘“‘seoregation ” after the consolidation of the Chalk. I believe, how- 
ever, that it is a pure assumption, made without any due examination 
of the facts, but, as it has never been disproved, I shall probably be 
doing a service to those interested in the knotty question of the 
formation of flints by arraying the evidence against it. 
In the first place, I presume it will be granted that the silica 
which went to make the flints was a soluble form of silica such as 
that of Sponge spicules, Diatoms or Radiolaria. The insoluble silica 
which is present in the shape of quartz-sand and clay must therefore 
1 The late David Forbes, F.R.S., in Proc. Geol. Soc.—_Epir. Grou. Mac. 
