MANGANESE OXIDES AND MANGANESE NODULES IN MARINE DEPOSITS. 729 



iii the dilute portion (III.) being much more abundant relatively to the manganese 

 than in the other portions consequently offered much more dissolved oxygen, and thus 

 led to a more rapid oxidation. It thus appears that manganese cannot long remain in 

 solution in sea-water even as bicarbonate, but must soon be deposited as dioxide. 



Distribution of Manganese Nodules and Manganese Dioxide Coatings in the 

 Marine Deposits of the Clyde Sea-Area. — The manganese dioxide which coats the stones 

 in the beds of the streams flowing into the Clyde Sea- Area is, as we have shown, rubbed 

 off — especially during floods — and carried to the sea along with ferric oxide and other 

 detrital matters. Soon after reaching the salt-water the greater part of this detrital matter 

 is thrown down not far from the shores, and forms the Blue Muds which cover the 

 bottom in all the deeper reaches of the Clyde basins. In this Blue Mud, as we have 

 seen, active chemical reactions take place, many of them initiated by the action of 

 the decomposing organic matters always associated with these deposits. The sulphates 

 of the sea-water are, in these circumstances, deoxidised, and the sulphur of the hydro- 

 sulphuric acid combines with the iron, forming sulphide of iron, which is unaffected by 

 the presence of the carbonic or humic acids in the mud, and hence remains a permanent 

 constituent of the deposit, giving it, indeed, its blue or black colour. The sulphide of 

 manganese, formed at the same time and in the same manner, is not, however, permanent, 

 but is at once decomposed by the carbonic acid present in the mud-waters, sulphuretted 

 hydrogen being evolved and bicarbonate of manganese formed.* Even the loosely- 

 combined carbonic acid of the bicarbonates in sea- water effects this decomposition. 



There is, however, another way in which bicarbonate of manganese may originate in 

 these mud-waters. The increased alkalinity induced by the above changes gives to the 

 water associated with the mud greater solvent power over silica and silicates, and those 

 mineral particles of the deposit which may contain manganese are slowly decomposed, 

 and, along with other carbonates, carbonate of manganese is formed which, as we have 

 seen, may remain for a short time in solution, but, on meeting the oxygen in the overlying 

 sea- water, is soon deposited as dioxide, t 



A large number of laboratory experiments were conducted with the view of studying 

 these various reactions, among them the following : — 



I. Sulphide of manganese and excess of ferric hydrate were suspended in water 

 through which carbonic acid was passed. The sulphuretted hydrogen, resulting from the 



* See Irvine and Gibson, Proc. Roy. Soc. Edin., vol. xviii. p. 54, 1891. 



t If we take the Blue Mud extending over an area of 1 square mile and 1 foot in depth as containing one half 

 of its weight of water (equal to 867,700,000 lbs.), and holding 1 part of MnC0 3 in 95,000, we will have a total 

 amount of 9134 lbs. MnC0 3 per square mile, and if we take the amount found in the water immediately overlying 

 the mud in Granton Quarry as representing what occurs in the Clyde area, i.e., 1 part in 300,000, we have per 

 square mile of water 1 foot deep (weighing 1,735,400,000 lbs.) 5785 lbs. MnC0 3 , or a total, including that obtained from 

 the mud and from the water overlying it, of 14,919 lbs., or 23 lbs. per acre of surface available for nodule formation. 

 Of course there may be much more, as there must be a continuous removal of MnC0 3 by tidal action ; but, con- 

 sidering the extent of the floor of the Clyde basin, even the amount here estimated is very great. 



Taking the amount of flow at Lanark at 15,000 cubic feet per second, holding 1 part manganese in 28,000,000 of 

 water, this represents a daily quantity of over 1 ton carried by that river to the sea. This is manifestly a low approxi- 

 mation, as the flow is much augmented by tributaries between this point and the sea. 



VOL. XXXVII. PART IV. (NO. 32). 5 S 



