728 DR JOHN MURRAY AND MR ROBERT IRVINE ON THE 



bag to 0'0114 grammes of MnCO per kilogramme, or 1 part MnC0 3 in about 94,000 

 of water. 



V f. Sea-water filtered from a Blue Mud obtained off Balloch Pier, Cumbrae, in a depth 

 of 25 fathoms, contained manganese in solution as MnC0 3 amounting to 0*0105 grammes 

 per kilogramme, or 1 part of MnC0 3 in about 95,000 parts of water. This mud, as well 

 as that obtained off the Little Cumbrae, contained, in addition to quartz, man}^ volcanic 

 minerals and rock fragments, some of them much altered. 



All our experiments seem to show that in the oozy pulp formed by the sea-water, 

 organic matter, and the mineral constituents, active chemical changes must necessarily be 

 taking place. This mud, when completely freed from adherent sea- water salts by washing 

 with distilled water, gave no indication of manganese peroxide by the Bunsen test ; but, 

 on fusing the washed mud with alkaline carbonates, the characteristic colour of the mass 

 showed manganese to be present in the silicates. It may be taken for granted, then, that 

 the carbonate of manganese in these mud-waters is derived cither (1st) from the direct 

 decomposition of the rock-fragments in the mud by the alkaline carbonates in the sea- 

 water, or (2nd) from the reduction of the higher oxides of manganese by the organic 

 matter in the muds ; for, as we will show later on, in the presence of decomposing organic 

 matter, deoxidation takes place in the muds, and soluble bicarbonate of manganese is 

 formed. Both these processes, doubtless, take place in most marine deposits. The great 

 increase of alkalinity owing to the formation of sulphides must give these mud-waters a 

 great power in decomposing silicates and setting free the bases to combine with the 

 carbonic acid always present in excess in mud- waters.* The influence of carbonate of 

 lime in rendering silicic acid soluble through the formation of calcium silicate must also 

 be important in initiating chemical changes in many deep-sea deposits. 



Rate of Oxidation of the Bicarbonate of Manganese in Sea- Water. — In the foregoing 

 paragraphs we have frequently referred to the oxidation of the carbonate of manganese 

 and the subsequent deposition of the dioxide. A number of laboratory experiments 

 were undertaken with the object of arriving at some idea of the rate at which these 

 changes take place : — 



I. One part of bicarbonate of manganese was added to 10,000 parts of sea- water. 

 II. „ „ 30,000 



III. „ „ 100,000 



The most dilute portion (III.) was the first to change by oxidation of the bicarbonate 

 into dioxide of manganese, which attached itself to the walls of the vessel. The other 

 portions (I. and II.) separated more slowly; but, in all three experiments, the oxidation 

 of the whole of the bicarbonate was completed in the course of a few months. There can 

 be little doubt that the separation of the whole of the dioxide would have taken place 

 much more rapidly had the water been aerated by constant agitation. The water 



* See MURRAY and IRVINE, Trim*. Hoy. Soc lidin., vol. xxxvii. p. 485. 



