MIDDLE CHALK—MICROSCOPIC STRUCTURE. 503 
the well-marked nodule there is every gradation to one the boun¬ 
daries of which cannot be distinguished from the surround¬ 
ing matrix in a thin section, but which, on the other hand, may 
be detected in a smoothed hand specimen of the rock. 
Nodules are least distinguishable from the mass of the Mel- 
bourn Rock where but few shell-fragments are present and the 
matrix has a finer grain approximating more closely in 
its character to that of the nodule. In such cases the 
presence of Radiolaria will sometime serve to distinguish nodule 
from matrix, for it is only in the nodules of the Melbourn Rock 
that well-marked traces of Radiolaria occur. Bodies of peculiar 
aspect, but otherwise somewhat inconspicuous, were noticed many 
years ago, but it was not until the changes in the mineralization 
and structure which the Radiolarian test undergoes had been 
studied in the rocks of Barbados that their true origin became 
manifest. Radiolarian have been noticed in the nodules of the 
Melbourn Rock from many localities.* Their siliceous skeleton 
has entirely disappeared, and the whole form is filled in and replaced 
by calcite. It is therefore impossible to refer the forms to their 
respective genera with accuracy, but they appeal* to belong to the 
sub-class Spumellaria, and to the families Liosphcerida , Astro - 
sphcerida, and Staurosphcerida , and some to the genera Dory- 
spluera and Amphibrachium . 
Specimens of the basal beds of the Middle Chalk from Lincolnshire 
and Speeton resemble more closely those from Dorsetshire than 
those from the intermediate counties, the nodules being ill-defined. 
The rock is often very full of coarse shell-fragments, while in other 
cases Spheres constitute the chief recognisable ingredient. 
Washings. 
From the hard, rocky nature of the chalk at this horizon, 
washing the specimens would be of little use, unless time allowed 
the examination of a large quantity of material. It was, there¬ 
fore, not attempted. 
Examination of Residues. 
Eight specimens of the Melbourn Rock have been chemically 
treated either by myself or by Dr. Hume, and it will be seen from 
the results given in the table that there is a general uniformity 
in the series, and that the amount of residue obtained from any 
one of them is comparatively small. The largest amount was 
obtained by Dr. Hume from the “grit-bed” of Dover, while that of 
north-west Norfolk is the purest chalk, containing scarcely 1 per 
cent, of residuary matter. 
Detrital Minerals. —The quantity of detrital minerals in the 
Melbourn Rock is very small. Quartz is the most abundant; 
it occurs as usual in minute angular and subangular or rounded 
* Quart. Journ. Geol. Soc., Yol. li., p. 600. (1895.) 
