273 
1914-15.] Chalk Boulders from Aberdeen, etc. 
globules occur, and here and there they can be seen in a few of the 
spicules. In one of my sections there is, however, a small irregular area 
permeated with silica in the chalcedonic stage, practically a small immature 
flint, the organisms contained in the chalk being clearly outlined therein. 
Viewing the section with crossed nicols, it was at once seen that the whole 
of the recognisable elements — the Foraminifera, cells, spheres, etc. — were 
casts or replacements partly in colloid but chiefly in chalcedonic silica. 
About 150 grams were used in two experiments to test the percentage 
of lime and colloid silica. In the first of these there were two or three 
pieces so permeated with chalcedonic silica that they were unaffected by 
the acid. In the second the whole broke down into a sand, the heavier 
part, consisting of foraminiferal casts, sponge spicules, and Radiolaria, 
small masses of white granular material described above, together with 
comparatively few mineral grains. The casts were partly in colloid and 
partly in chalcedonic silica, both globular colloid and chalcedonic silica 
occurring together in the cells of the same cast. Sponge spicules were 
exceedingly numerous and in great variety of form, many of those 
described by Dr Hinde in his paper on the Horstead flint ^ being present 
as well as others that I have not yet seen in published figures. The 
silica of the walls was in nearly all cases in the chalcedonic stage. Casts 
of Radiolaria in chalcedonic silica were also abundant, the commonest 
being a species of Dictyomitra, the shape of these being often identical 
with those described in B 1. It was evident both in the acid and alkaline 
residues that this specimen contained more amorphous terrigenous matter 
than the preceding one, but it was found impossible to estimate the 
amount in this wreck of silicified organisms. 
A 3. — A small sub-angular fragment of grey chalk, not hard, mealy 
to the touch, seems weathered. The matrix of this specimen is crowded 
with small angular fragments of shell probably derived from Inoceramus, 
many foraminiferal cells, and some spheres ; there are comparatively few 
Globigerinee, Textularians, or sponge spicules, but the casts of Radiolaria 
are a prominent feature throughout the section. 
The matrix is chiefly calcitic, but there is certainly an admixture of 
terrigenous matter, the presence of which is seen in the greyish tint of 
the section. The rock did not break down readily in the acid solution, 
the matrix being permeated with colloid silica, though it is not optically 
visible, some of the original lumps of chalk retaining their shape. They 
broke down, however, on pressure when dried, releasing fragments of 
spicules, casts of Foraminifera, and Radiolaria. Separation of the coarser 
* Fossil Sponge Spicules from the Upper Chalky by G. J. Hinde, 8vo, Munich, 1880. 
VOL. XXXV. 18 
