A MANGANIFEROUS CONGLOMERATE IN ESSEX.
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etc., lie on a spread of London-clay. Near Little Bentley,
Great Bentley, and Wivenhoe Cross the gravel is cemented by
iron into a coherent mass overlying looser material" (Geology of
the Neighbourhood of Colchester, by W. H. Dalton, page 3).1 These
masses are of value for building purposes, being very hard and
durable ; indeed, it is almost the only building stone of which
Essex can boast,2 and not only are many of the churches in
North-east Essex built of it—for instance, the church of Great
Bentley—but it has been conveyed apparently to considerable
distances, the church walls at Danbury, for example, being largely
composed of the same material.
My interest in the substance was aroused by noticing that
when the cementing material was treated with hydrochloric
acid, chlorine was abundantly evolved, indicating that peroxide
of manganese was a constituent. As no other cemented gravel
or sand that I have examined contains this substance, it seemed
to be worth while to examine the Conglomerate more carefully.
The cement proved to be ferric oxide mixed with the peroxide
and protoxide of manganese, together with a large quantity of
sand. The analysis of the air-dried substance gave the follow-
ing results :—
In this there is no simple relation between the proportions
of the constituent oxides, so that the substance has little purely
chemical interest, and the proportion of peroxide is far too small
to render it of any commercial value. The conditions under
which gravel can become cemented with a manganiferous
deposit is, however, a chemical problem of considerable geological
interest.
1 As a matter of fact, the substance is more widely distributed titan is here represented
and throughout the Tendring Hundred it may be found wherever gravel forms the sub-soil.
2 Flints and Septaria nodules are also used for building in Essex. Great Clacton church
is largely built of Septaria nodules. [T. V. H.]