CONSTITUTION OP CHALK AND FLINT. 
155 
exceptions, sucli as Echinoderms, Monomyarians, and some other 
calcareous organisms, which have not become silicified either in 
the Chalk or in any other limestone which has been (as is often the 
case) converted into flint or chert. My own views on this subject 
have been given in full in the 1 Proceedings of the Geologists’ 
Associationwith references supplied for the study of many 
writings on the same matter, f Numerous other observers 
have since then published the results of their experiments and 
researches, notably Dr. Wallich and Prof. Sollas, whose memoirs 
must be carefully looked to by students. The condition in which 
silica (the general name for quartz, chalcedony, flint, jasper, etc.) 
occurs in the fossil Sponges of the Chalk has been carefully defined 
by Sollas and Hinde in their works already alluded to ; and the 
former has suggested that three stages or processes in the silicifi- 
cation of Chalk into flint-nodules are clearly traceable. J Mr. J. 
Allen Brown has worked out, for the Ealing Microscopical and 
Natural History Society, a resume of the views of these and other 
writers down to 1882,§ which is very suggestive for those 
wishing to take up the subject. There are many points to be 
considered—particularly (I) the actual relationship of the visible 
organisms in the flint (and usually these are abundant) to those con¬ 
stituting the chalk,—(II) the actual silicification or non-silicification 
of these several organic particles or small organisms—so also of the 
oolitic and other constituents in other silicified limestones,—(III) 
the condition of the silica with reference to its crystalline or colloidal 
character,—(IV) the successive coatings, layers, or replacements of 
such silica,—(Y) the probable origin of the silica, from (1) siliciferous 
water of springs and worn coasts, (2) from dissolved Polycystines 
and spicules, and (3) from hydrated, colloid, or soluble silica, even 
now found in some rocks,—(YI) its relation to carbonaceous matter, 
either in living organisms or in dead matter,—(YII) how the cal¬ 
careous matter has been removed before or during its replacement 
by the silica,—and (YIII) how much, indeed, of the silicification in 
Chalk and other limestones (for every limestone has its siliceous 
nodules in one form or another) has been due to (1) coating, and 
to (2) replacement by infiltrating water, how much to the (3) 
chemical combination of silica with organic matter, and how much 
to the (4) agglutination (so to speak) of gelatinous or colloidal 
silica, either specially or indiscriminately, to the calcareous and 
other constituents of the mass. 
The frequent occurrence, in hollow flints, of a white siliceous 
meal, consisting mainly of loose silicified Ostracoda and Foramini- 
fera, formerly calcareous, with Polycystina (Wallich, quoted above) 
and spicula which were presumedly always siliceous, has not yet 
been satisfactorily accounted for to my mind. The remarkably 
fine specimens of hollow flints, with mammillated chalcedony and 
* Yol. iv, 1876, pp. 439-458. 
t See also Hudleston on Flint, ‘Proceed. Geol. Assoc.,’ vol. vii, 1881, p. 183. 
f ‘ Brit. Assoc. Beport ’ for 1881, etc. 
§ “ Some Notes on Flint,” etc. 
