138 MANGANIFEROUS NODULES IN BOULDER-CLAY.
The manganese peroxide was estimated in the usual manner.
The weighed substance was placed in a small flask with some
hydrochloric acid and heated. The chlorine given off was led
through some bulb tubes containing potassium iodide solution
and the amount of iodine liberated was estimated by deci-normal
thiosulphate solution.
Later, a second sample was taken and subjected to a similar
analysis. The following are the percentage results of the exam-
ination of the two samples :—
From the above figures it will be seen that the nodules
differ in composition, some being richer in manganese, and the
manganese being in some in a more highly oxidised condition
than in others. The nodules appear to be formed of a silicious
material cemented together by the oxides of iron and manganese
with some phosphate and carbonate of calcium and a quantity
of moisture varying in proportion to the iron present, since 5.6
bears the same ratio to 23.9 as 7 does to 29.9.
It is remarkable that the manganese protoxide and lime in
the first sample are exactly equivalent to the protoxide and lime
in the second sample, in fact, the two oxides have evidently been
able to replace each other in the nodules.
This somewhat remarkable material has a great similarity to
that already described to the Essex Field Club in a paper read
by Mr. T. S. Dymond at the meeting held Dec. nth, 1897
(Essex Naturalist, Vol. X., pages 210-12).1 This paper
referred to the hard masses of cemented gravel found in some
parts of Essex, chiefly round Tendring, and which were used for
building purposes. The cement proved to be "ferric oxide
mixed with the peroxide and protoxide of manganese, together
1 Reference should also be made to the paper fay Mr. Johnson on a Manganiferous Seam
in the Drift at Ilford, printed in the present part of Essex Naturalist, which was received
by the Editor previous to the reading of Miss Thresh's paper.—Ed.