MANGANESE OXIDES AND MANGANESE NODULES IN MARINE DEPOSITS. 727 



mode of accumulation of the manganese dioxide in the deposits dredged from some regions 

 of the sea-bed. The following; details with reference to the manganese in these mud- 

 waters illustrates the difference between these mud-waters and samples of normal sea- 

 water : — 



I. Water filtered from the Blue Mud taken from a depth of 6 fathoms in the sea-water 

 quarry at Granton during February 1892, was found after standing in a bottle for two 

 years to give a deposit of manganese dioxide amounting to 0*045 grammes per kilo- 

 gramme, equivalent to 0*06 grammes MnC0 3 originally in solution, or equal to one part 

 MnC0 3 in 16,600 of water. 



II. In water filtered from the Blue Mud from the same quarry during March 1894 we 

 found manganese in solution amounting to 0'0315 grammes of MnC0 3 per kilogramme, or 

 one part in 31,700 of water. In water immediately overlying this mud we found, at the 

 same time, manganese in solution amounting to 0*0034 grammes of MnC0 3 per kilogramme, 

 or one part in 300,000 of water. In the same place, and at the same date, in water taken 

 1 8 inches above the surface of the mud, there were found distinct traces of manganese in 

 solution. The sea broke into this quarry forty years ago. At the present time there is 

 a distinct black coloration all around the rocky walls of the quarry between high and 

 low water marks, due to a deposit of manganese dioxide — the deposit being apparently 

 more abundant on a built portion, where the lime pointing is exposed at the joints 

 to the action of the water. At some places the stones and shells in the channel, by 

 which the sea-water flows in and out of the quarry at each tide, are likewise coated 

 with a thin deposit of manganese dioxide. The salt-water was examined as it entered 

 the quarry with the flowing tide, and it was not found to contain any manganese in 

 solution. 



The mud at the bottom of the quarry is made up of a mass of fine detrital matter, in 

 which are small crystals of quartz, felspar, hornblende, augite, mica, magnetite, and 

 other minerals. When dried in the air this mud was found to contain about 0*1 per cent, 

 of manganous oxide (MnO), j>resent partly as carbonate and partly as silicates. It 

 therefore seems evident that the manganese dioxide on the walls of this quarry and on 

 the stones in the tidal entrance has its origin in the carbonate of manganese in solution 

 in the sea-water associated with the mud, and that this carbonate, in its turn, is derived 

 from the decomposition of the minerals contained in the mud. 



III. Sea- water filtered from mud from Granton Harbour (depth 2 fathoms) in March 

 1894 contained manganese carbona/te in solution amounting to 0*008 grammes per kilo- 

 gramme, or 1 part in 120,000 of water. 



IV. Sea- water filtered from a grey-coloured mud obtained in a depth of 20 to 25 

 fathoms, off the Tan Buoy, Cumbrae, in the Clyde, gave distinct indications of manganese 

 in solution, but not nearly in such abundance as in the waters from the muds in Granton 

 Quarry and Granton Harbour. 



V. Sea-water filtered from a brownish-black mud obtained in a depth of 22 to 

 29 fathoms, off Castle Bay, Little Cumbrae, contained manganese in solution amount- 



