1914-15.] Chalk Boulders from Aberdeeu, etc. 
279 
obliterated by the recrystallisation of the calcareous matter in the rock ; but 
treatment with acid discloses Radiolaria spicules of the same species, 
and mineral grains of the same kind that are found in the other boulders. 
It contains 22'94 per cent, of terrigenous material and therefore was 
probably low down in series and its place not far above the Greensand. 
The average size of the sand grains in both deposits is small, but one 
would expect that if the terrigenous material was derived from the same 
land surface, within the same drainage area, and influenced by the same 
currents, that there would be found a similarity in the finer particles 
as well as in the mineral grains. This is not the case ; there are no 
lath-shaped particles in this specimen, and the whole of the lighter portion 
is in a very fine state of division. I know of no Cretaceous strata in 
England, except the Gault, in which the terrigenous material is in so fine 
a condition or exhibits the same physical properties. This curious dis- 
similarity will be noticed in other specimens, and one might infer that 
they came from different localities and do not belong to the same succession 
of beds. It will, however, be noticed that the lath-shaped particles occur 
in the specimens containing the greater proportions of terrigenous material, 
and therefore probably the earliest in the series. It seems to me possible 
that the difference may be due to some alteration in the strength or 
direction of current action as the land gradually became submerged. 
The next boulder, B 13, is a true chalk full of small angular fragments 
of shell probably derived from Inoceramus, Foraminifera, cells, and spheres. 
Radiolaria and sponge spicules are present and silica in the colloid state 
distributed through the rock. The quantity of terrigenous material can 
be fairly estimated, for little else remains after treatment with caustic 
potash. The amount of this, though only 8T9 per cent., places the 
specimen after B 1. Here again the greater part of the finest material 
is in the shape of minute lath-shaped particles. 
The three specimens B 2, A 8, and A 3, and perhaps also A 2 and B 8, 
may be grouped together. They all contain Foraminifera, cells, spheres, 
and shell fragments in greater or less proportions as well as Radiolaria. 
But three-fourths of the residue of A 8 (40‘58 per cent.) consists of sponge 
spicules ; they are also equally numerous in B 2. In this specimen silica, 
probably derived from the spicules, permeates the rock partly in crystalline 
and partly in minutely globular form. Though spicules are less abundant 
in A 3, this rock is also permeated with silica, and its position was probablj^ 
in close proximity to the two previously mentioned. A 2 and A 8 are 
also siliceous chalks containing spicules and more or less permeated with 
silica in the colloid state, though in A 2 much of it seems to have 
