740 DR JOHN MURRAY AND MR ROBERT IRVINE ON THE 



amount of decomposing organic matter in the deposits at the bottom. In this way there 

 may have been a gradual transference of manganese from the continents towards the 

 remoter recesses of the abysmal regions from the earliest geological times, for, in the 

 abysmal regions, where there is relatively a small amount of organic matter, and where the 

 rate of accumulation of the deposit is slowest, the manganese dioxide would be more 

 stable than in other areas. 



Wherever manganese dioxide has once been deposited, a relatively rapid nodular 

 formation there takes place in the clay or ooze, owing to the acidiferous properties 

 of this dioxide, which, decomposing the carbonate of manganese in solution, produces 

 manganous-manganic oxide, it may be Mn 3 4 or a mixture of MnO and Mn0 2 . This 

 manganous-manganic oxide in the hydrated condition gradually becomes fully oxidised 

 into dioxide ; in this way we may account for the fact that the inner layers of some 

 nodules are found to be more highly oxidised than the outer layers. 



To the same action of manganese dioxide as an acid we may probably attribute the 

 presence of calcium, nickel, cobalt, copper, and other metals in the nodules, for this 

 dioxide would decompose the carbonates of any of these substances if present in the oozy 

 Blue Mud or clay by uniting with their protoxides. In these soft oozes and clays the 

 most favourable conditions are likewise present for the deposition of the manganese in 

 concentric layers around a nucleus, similar in many respects to the urinary calculi found 

 in the organs of many mammalia. 



Theories concerning the Origin of Manganese Nodules in Marine Deposits. — Ever 

 since the discovery of large numbers of manganese nodules by the "Challenger" Expe- 

 dition on certain parts of the floor of the ocean, there has been much discussion both with 

 regard to the source of the manganese and the mode of formation of these concretionary 

 bodies. 



Gumbel, # after an analysis of some of the " Challenger " specimens, referred the for- 

 mation of the nodules to the action of submarine springs holding manganese in solution, 

 which would be precipitated upon contact with sea-water. The rounded form of the 

 nodules he believed to be due to repeated turnings and rollings on the bed of the ocean. 

 It is almost certain that this distinguished geologist would not have held these opinions, 

 could he have seen the large number and variety of nodules procured by the " Challenger " 

 Expedition in many regions of the ocean. The form and distribution of the various 

 nodules and coatings would have convinced him that submarine springs could not have 

 had anything to do with their formation. 



A cosmic or meteoric origin has been assigned to the oxides of manganese and iron in 

 the manganese nodules. t Murray and Renard have shown that magnetic spherules con- 

 taining native iron and nickel, and certain spherules, called chondres, composed largely of 

 silicates are present in deep-sea deposits, and have brought forward almost conclusive 



■•■' Gumbel, SUzb. d. I:. Bayer. Akad. ii Wise., Btl. viii. p. 189, 1878; Forschungsreise S.M.S. "Gazelle," Th.ii, 

 p. 103. 



LiOCKTER, Nature, vol. xxxviii, p. 521, 1888 ; HlCKSON, The Fauna of the Deep Sea, p. 38, London, 1894. 



