136 NOTES ON A MANGANIFEROUS SEAM.
Like the rest of the gravel, the manganiferous seam con-
tains a quantity of oxide of iron. The amount present in
sample ii. was determined by Mr. Zurcher to be 2.7 per cent.,
while that in sample iii. was estimated by myself as 2.5 per
cent. One would expect the amount of iron in the different
samples to be more uniform than the manganese, as, while the
latter is on the outside of the pebbles only, the former occurs
inside as well, which also explains why the psilomelane, though
really present in smaller quantity than the oxide of iron, should
nevertheless give the colouration to the seam.
The psilomelane was clearly deposited contemporaneously
with, and must have been brought down by the river (Thames)
which deposited, the drift in which it occurs. But there are no
ores of manganese in the rocks over which the Thames and its
tributaries flowed. Nearly all iron ores, however, contain a
minute proportion of manganese, and it is from these iron ores,
of which there are plenty in the Thames Basin, that the
manganese has been derived. It is quite easy to understand
how the manganese was taken into solution together with the
iron and precipitated separately at places where the conditions
were favourable.
During the recent excursion of the Essex Field Club to the
excavations in Tottenham Marshes, in the Lea Valley, I
noticed an extensive manganiferous seam, in the sections in the
Palaeolithic gravel over-looking the new reservoirs.
A good test for the presence of manganese in any material
is to fuse it with sodium bicarbonate, to which it imparts a
green colour.
P.S.—Since writing the above Mr. Cole has drawn my
attention to an interesting paper by Mr. T. S. Dymond, F.I.C.,
on the occurrence of manganiferous conglomerate in gravel at
Tendring (Essex Naturalist, vol. x. (1897), pp. 210-12), in
which another explanation is given of the derivation of the
manganese. The conditions under which the deposit occurs at
the two places would, however, appear to be very different.