430 



the aocniiuilalioii of tlie manganese, must have reached ils final 

 stage, (jefore llie second commenced. This is quite in iiarmony 

 with the testimony given by modern deep-sea deposits. Nodnles of 

 manganese are found in abundance in the deep-sea oozes, but concre- 

 tions of silica e. g. as nodules of hornstone, have not yet been met 

 with. Obviously, in the recent deep-sea oozes (especially in the red 

 clay and the radiolarian ooze), the process of accumulation of man- 

 ganese partly has been completed, partly is still in full progress, 

 but the process of concentration of silica into hornstone, chert, jasper 

 etc. has not yet commenced. 



It might be cpiestioned, whether possibly the concentration of the 

 manganese and a fortiori of the silica, might have taken place after 

 the deep-sea deposits, by diastrophism, had been brought into the 

 position, where they take part now in the formation of mountain- 

 ranges. This question has to be answered in the negative : the con- 

 cretions of manganese and those of silica have been influenced by 

 the mountain-building processes precisely in the same way as the 

 rocks in which they are found enclosed, and it is easy to prove 

 that before the mountain-making processes came into operation they 

 had already been solidified, and had attained their full size. 



It is only the last of the possible modes of accumulation mentioned 

 on p. 421 — 423 i.e. the infiltration of manganese in the cracks of the 

 rocks, which according to the opinion of the author has taken place 

 entirely, or almost entirely after the deep-sea ooze had been solidified 

 into firm rock, and had been crushed more or less by pressure. 



Iron and manganese are generally found together in cracks of 

 fossil deep-sea deposits, especially in cherts. In some places, as is 

 the case in West-Borneo, iron predominates, in other places, as in 

 East-Borneo, manganese prevails. In case of strong pressure the chert 

 is often converted into a crush breccia cemented by manganiferous 

 iron-ore. Frequently the chert is then found altered into white amorphous 

 silica, in which case beautiful rocks originate, being composed of a 

 mosaic of pure white angular fragments, cemented by chocolate-brown 

 films of iron-ore. ') 



1) G. A. F. MoLENGRAAFF, Geoioglcal explorations in (Jentral-Borneo, p. 92, 1902. 



