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. 



