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A MANGANIFEROUS CONGLOMERATE IN ESSEX.
A well-section from the Waterworks, Clacton-on-Sea,
reveals the following strata. (G.S. Man., 48 S.W., p. 13) :—
The Conglomerate of Clacton, therefore, immediately over-
lies the London-clay. Now surface soil contains protoxide of
manganese in the form of silicate. This is very slowly dissolved
by water containing carbonic acid and other soil acids. When
such a solution is exposed for a prolonged period to oxygen,
the protoxide undergoes oxidation to the peroxide, which, being
incapable of forming a silicate or other salt, is precipitated in
the solid form either pure or combined with more or less
protoxide according to circumstances.
These facts afford a satisfactory explanation of the occur-
rence of this manganiferous cement, at any rate where it overlies
the London-clay. Water containing manganese dissolved from
the surface soil would percolate through the whole of the Post-
Glacial beds. But on the surface of the London-clay the flow
of water would be arrested, and thus time given for the gradual
oxidation of the manganese which would be deposited in a hard
coherent mass binding together the sand and stones of the gravel,
thus producing a Conglomerate hard enough for building pur-
poses. Where the Conglomerate overlies looser material, it is
possible that the water has risen to some height above the
London-clay, for it is upon the surface of the water-saturated
gravel that the manganiferous cement would be formed.