427 



Polished slabs of nodules, examined in reflected light, in manj 

 cases proved to be better fitted for microscopic stud}' than slides, 

 the coherence in Ihe nodulus being often not sufficient for the 

 preparation of thin slides. 



In cases where slides of sufficient thinness could be made, tiie 

 effect was striking, the perforated tests of the radiolaria, which are 

 composed of silica, and are distinctly pellucid, contrasting strongly 

 with the completely opaque manganese both without and within. 



On the mode of accumulation of manganese in the deep-sea ooze. 



From the mutual relations between grains and nodules of manganese 

 in mesozoic deep-sea deposits and the pecularities of the occurrence 

 of radiolaria therein, deductions may be made regarding the mode 

 of accumulation of ore in deep-sea ooze. 



Manganese is precipitated on numerous spots as minute grains in 

 the deep-sea ooze, which is a siliceous and somewiiat argillaceous 

 colloid in which tests of radiolaria are found suspended. In some 

 places the precipitation is evidcnily more rapid than in others, and 

 thus grains of different size are formed, all of them floating in a 

 similar manner to the tests of radiolaria in tlie siliceous colloid. It 

 appears that by mutual attraction ') the grains pack together and 

 thus form stronger and stronger centra of attraction for other grains, 

 forming eventually clouds, which on closer packing together, are 

 gradually transformed into concretions or nodules, composed of pure ore. 



During this process of gradual concentration of the ore into nodides, 

 the tests of radiolaria are surrounded by the ore, without being shifted 

 from their position, and finally are found in the nodules just at these 

 places where they had been floating, suspended in the ooze. As long 

 as the ooze remains viscous, a nodule once formed, continues to be 

 a centre of attraction and collecting more and more minute grains of 



not be expected in Jurassic deposits, but sharks' teeth and remains of other animals 

 as e. g. belemnites could be expected to occur as nuclei in the Jurassic nodules of 

 manganese This difference, certainly, is remarkable, and as yet cannot be explained, 

 but not too much importance ought to be attached to it, according to the author's 

 opinion, firstly because many nodules from recent deep-sea deposits in a similar manner 

 do not show a concentrical structure, and secondly because the fossil nodules, 

 which hitherto have been examined microscopically, come from three localities 

 only, not far distant from each other, all from the island of Rotli. 



1) The existence of this attraction is deduced by the author from the observed 

 facts, without giving an explanation of its cause ; a fair proportion of iron always 

 entering into the composition of the nodules of manganese it might be suggested 

 that magnetic forces could be the cause of tiie mutual attraction of the small grains, 



28 



Proceedings Royal Acad, Amsterdam Vol. XVtll. 



