425 



so to say the pi'ocess of tlie accinnulatioii of the iTianganese in fail 

 progress but fixed or petrified at a certain moment. 



Rocks from certain localities on the island of Rotti, which proved 

 to be suitable for microscopical examination, have been studied bj' 

 the author, and therefore a few words on the mode of occurrence 

 of these rocks may serve as an introduction, before the results of 

 this study will be dealt with. 



On the island of Rotti deep-sea deposits, both of triassic and of 

 Jurassic age occur, but concretions of manganese in their original 

 position, have as yet only been found in sediments the Jurassic age 

 of which has been determined in more than one locality. These 

 sedimentary rocks are characteristically exposed in the rocky 

 cliffs of Sua Lain near Termanoe situated on the north coast ') of 

 the island. 



They are well stratified here, and the strata folded and tilled, but 

 not so disturbed, that the original secpieuce of the beds could not be 

 determined with cevtainty. 



The bulk of this com|)lex of strata is composed of true abysmal 

 deposits in which exclusively tests of radiolaria occur, but in the 

 same complex also limestones are found which contain both I'adio- 

 laria and belemnites of Jurassic") age. The geological age of these 

 deep-sea deposits has thus been proved beyond doubt. 



The deep-sea deposits are here represented by siliceous and slightly 

 calcareous red clay shales, which pass into reddish marls and lime- 

 stones in proportion as the content of lime in the rock increases, 

 the latter containing numerous coiu-entrations of silica in nodules 

 grouped together in more or less distinct layers. The shales, luarls, 

 limestones and cherts are completely studded with tests of radiolaria. 

 Manganese is concentrated irregularly in fairly equal proportions, 

 however, in the calcareous clay shale, in the siliceous limestone 



1) H. A. Brouwer. 1. c p. 614. 



-) WiCHMANN, wiio in Ills journey to the island of Rotti in the year 1889 visited 

 Sua Lain, reports as follows on the geological structure of tliis groups of rocks: 

 "Der Fels besteht aus einem wahrscheinlicli lertiaren Kaikstein, die sehr reich an 

 Foraniiniferen, namentlich Globigerinen ist und ausserdem von zahlreichen Kalk- 

 spalhlrümniern durchzogen wird." (A. Wichmann, Tijdschr. Kon. Ned. Aardr. 

 üenootsch. 2, IX, p. '231, 1892). This statement is erroneous. Verbeek has proved 

 that the rock is not filled by tests of Globigerina but of Radioloria and Hinde takes 

 them to be of triassic age. (compare: R. D. M. Verbeek I.e. p. 317 and G. J. 

 Hinde, ibid. p. 696) moreover, in the numerous samples, taken by Brouwer from 

 the strata of these rocks, no Globigerina, but exclusively Radiolaria are found, whereas 

 it follows from the Belemnites, occurring in the same complex of strata, that 

 these rocks cannot possibly be of tertiary age." 



