270 Proceedings of the Royal Society of Edinburgh. [Sess. 
consist of fine material and sand cemented, probably by the silica of the 
Radiolaria, and these with the casts are the opaque grains seen in 
the section. 
The fine material removed by levigation consisted largely of minute 
lath-shaped particles, with clearly defined but somewhat broken outline, 
and very faint double refraction between crossed nicols. The origin and 
nature of these particles is at present uncertain, though no doubt they 
are derived from the disintegration of some rock. The remainder of the 
finer material was amorphous structureless matter, as a whole negative 
to polarised light. 
B 1. — Part of a sub-angular boulder of considerable size. When broken, 
the rock is darkish grey and semi-crystalline in texture. Seen as a thin 
section it appears to consist chiefiy of well-defined calcitic crystals, probably 
formed by the conversion of the calcareous portion of the ooze into 
granular calcite after the deposition of the original material. They mask 
entirely the terrigenous matter which analysis shows is present in con- 
siderable quantity, as well as such organisms as may occur. Three or four 
large fragments of shell can, however, be seen and a few smaller pieces, 
all having the same structure which, though partially obliterated, is not 
that of Inoceramus. Opaque grains are scattered throughout. 
The residue after treatment with acid was large, 22*94 per cent. Of 
this 1*72 per cent, of coarse material was separated by levigation. Three- 
fourths of it consisted of short cylinders of minute nodules of silvery mar- 
casite or brassy-looking pyrites; two casts of Ammodiscus occurred in 
this material. The residue contained also a few Radiolarians, some sponge 
spicules, rodlike shafts or simple Monaxons, with few globate dermal 
spicules of Geodia. There is no silica in the colloid state in this specimen 
beyond that which can be accounted for by the presence of a few spicules. 
The proportion of sand grains was not large. The remainder was inorganic 
matter in an extremely fine state of subdivision, structureless and almost 
invisible when mounted in balsam, negative with crossed nicols. The 
behaviour of this material when analysing it, as well as its optical pro- 
perties, was that of Gault Clay. 
The specimens which follow all answer to the description of the rock 
which we know as chalk — that is to say, they consist of Foraminifera, 
foraminiferal cells, “ spheres,” together with the remains of other organisms, 
mostly calcareous, embedded in an amorphous calcareous matrix. 
The word “ cells ” is used hereafter in contradistinction to ‘‘ spheres.” 
These latter are for the most part round or nearly spherical bodies like 
