418 



where volcanic material Forms a large proportion of the constituents 

 of the deposit on the bottom of the sea. 



In the Kara-sea, highly ferruginous nodules of manganese have 

 been brought to the surface from terrigenous muds, at a moderate 

 depth by the Netherlands Arctic expedition in the years 1882/83. 



Daring the Siboga-expedition, Weber, in the deep-sea basins of 

 the Netherlands East-Indian archipelago, has found manganese 

 nodules on one spot only between the islands of Letti and Timor, at 

 a depth of 1224 metres, in mud containing a strong proportion of 

 terrigenous material, being in no way a true pelagic deposit; man- 

 ganese forming an incrustation on a fragment of dead coral, has 

 moreover been observed in a sample dredged from a depth of 1633 

 metres, between the islands of Misol and Ceram. ^) 



As to tlie fossil occurrences, I have found in Upper-Triassic deposits, 

 on the eiland of Timor, roots of Crinoids which certainly did not 

 grow on the bottom of an ocean of abysmal depth, heavily incrus- 

 stated with a coating of concretionary manganese. 



Nodules and concretions of manganese therefore are not charac- 

 teristic of abysmal deposits in this way, that from the occurrence 

 of such concretions in a certain deposit, one would be justified in 

 concluding that the deposit could be nothing else than an abysmal 

 deposit and could only have been formed on the bottom of a very 

 deep ocean. On the contrary, concretions of manganese have been 

 formed on the bottom of all oceans in varying depths when the 

 conditions for their formation were favourable. 



Murray and Renard maintain — and I have no reason to diverge 

 from this opinion — that these favourable conditions are afforded 

 by the presence of basic volcanic material in an easily decomposable 

 form. As soon as this condition is fulfilled the possibility is realized 

 for the formation of concretions of manganese, but the chemical 

 process of their growth is a very slow one, as has been amply 

 proved by the researches of the Challenger-expedition. In shallow- 

 seas, especially at small distances from the mainland, sediments derived 

 from land or from a planctonic and neritic fauna accumulate rapidly, so 

 rapidly indeed, that thei'e is only a remote chance of finding by 

 dredging, concretions of manganese, which in the mud in odd places 

 grow very slowly. In abysmal seas far from land very different 

 conditions prevail, the rate of accumulation of sediment is an 

 extremely slow one there, the afflux of terrigenous material is 

 reduced almost to nil, whereas from the plankton only the siliceous 



1) Siboga-Expeditie 1, M. Weber. Introduction el description de l'expédition, 

 p. 81 and p. 137. Leiden 1902. 



