MANGANESE OXIDES AND MANGANESE NODULES IN MARINE DEPOSITS. 735 



mineral particles of the nrnd itself. It is probably derived from both these sources. 

 Further, this bicarbonate, on escaping in solution from the mud into the overlying water, 

 takes up oxygen, and is deposited in a higher state of oxidation on any objects which 

 may lie on or project above the surface of the mud. In this way we may account for 

 the tonsure-like rings of dioxide of manganese which surround many stones, and for the 

 deposits on the shells of the molluscs living in the immediate surface layers of the muds. 



The formation of manganese nodules on the immediate surface of the deposit, on 

 the tops of the barriers, and in the pit-like depressions, is most probably to be accounted 

 for by the more abundant supply of oxygen, or by the diminished amount of decomposing 

 organic matter in these positions. 



In those deep parts of the Clyde basins, where there is little motion from tidal or 

 other currents, the bicarbonate of manganese in the mud-water would gradually ooze 

 out into the overlying water, and be slowly carried along till deposited as dioxide near 

 the tops of the ridges, where motion is more rapid and oxygen more abundant. In the 

 muds of the Clyde Sea- Area there is most probably a continual and very slow shifting 

 of the dioxide of manganese deposits from one position to another, for if a partially 

 embedded stone, covered on its upper surface with a deposit of manganese dioxide, 

 should become more deeply embedded from the accumulation of the deposit, the 

 lower portions of the dioxide would be reduced by this deoxidising mud, and the 

 manganese, passing through the state of sulphide and bicarbonate, would ultimately be 

 transferred to some other point higher up on the stone, or to a still greater distance, 

 before being again laid down as dioxide. There would thus always be a tendency for the 

 manganese dioxide to accumulate in the surface layers of a Blue Mud deposit, as well 

 as at certain favourable points, such as in the little hollows filled with mud, over which 

 the water is continually changing, or where there is an absence of decomposing organic 

 matter. 



When viewed in this light, the amount of the manganese, in relation to the whole 

 mass of the deposits being laid down on the floor of the ocean, may not be so great 

 as some dredgings, at points where accumulation has taken place, would lead us to 

 suppose. 



Manganese Nodules and Manganese Dioxide Deposits in the Deep Sea. — The 

 dredgings, trawlings, and soundings conducted in recent years by the " Challenger " 

 and other deep-sea exploring expeditions, show that deposits containing a large quantity 

 of manganese dioxide are very widely distributed. Over large areas of the Mid-Pacific 

 and Mid-Indian Oceans there is a dark chocolate-coloured deposit, usually in depths 

 beyond 2200 fathoms. Whenever dredgings have taken place on this chocolate-coloured 

 clay, large numbers of manganese nodules have always been procured, associated with 

 sharks' teeth, ear-bones of whales, and the other peculiar substances mentioned in the 

 opening paragraphs of this paper. 



In some few places in the Pacific, the Globigerina Oozes, which occur at lesser 

 depths than the Red Clay, have also this deep chocolate colour due to the minute grains 



