419 



tests i. e. these of radiolaria and diatoms reach the bottom, the 

 calcareous test being dissolved by the cold water of the deep seas 

 witli its high ratio of oxvgeninm and carbonic acid held in solution, 

 before they reach (he bottom. Tiie growth of the concretions of 

 manganese, however, is not hampered in these depths; on the contrai'3" 

 it even appears as if in abysmal deptiis in water of a temperature 

 very near the freezing point and containing much oxygenium in 

 solution, the conditions for the formation of concretions ofoxydesof 

 manganese, are more favourable than in shallow seas, provided that 

 traces of vokianic material occur as a source of manganese from 

 whence the manganese could have been derived. Thns, concretions 

 of manganese, slow as ihey are in their process of formation, and 

 inconspicuous as they are in sediments in places where the i-ate of 

 accumulation is rapid, can become an important constituent where 

 the rate ot accumulation of a deposit is extremely slow, as is the 

 case in the abysmal areas. 



Consequently concretions of manganese are in this manner charac- 

 teristic of abi/snm/ deposits that they may form an important per- 

 centarje in proportion to other constituents exclusively in such deposits. 



And from this it is easy to conclude that concretions of manga- 

 nese are characteristic of abysmal deposits in the same manner as 

 the tests of radiolaria. The latter sink to the bottom of the ocean 

 from the plankton everywhere witiiin the limits of their geographical 

 distribution, just as well near the mainland as far from the shore. 

 Near the land the.se tests, owing to their miiuitcness, iiowever, dis- 

 appear being incorporated in enormous quantilies of otiier chiefly 

 tei-rigenous material which there comes to deposition ; far from land, 

 on the contrary, at the bottom of the very deep ocean-basins at 

 depths over 5000 metres, where calcareous tests sinking down are 

 dissolved before reaching tiie bottom of the ocean, these siliceous 

 tests, small as is their individual mass, may form a great, sometimes 

 a preponderating pcrrtion of whatsoever is deposited. 



It is therefore quite Justifiable to maintain, that radiolaria and 

 concretions of manganese, form part of the most characteristic con- 

 stituents of abysmal oceanic deposits, and further that nodules of 

 manganese containing radiolaria almost with certainty must have 

 been forme<J in the deeper portions of the ocean basins. 



Localities where concretions of manganese have been found 

 in deep-sea deposits of mesozoic aye. 



Concretions of manganese have been discovered by the geological 



