!9 18 ] Davis: The Radiolarian Cherts of the Franciscan Group 357 



with the cherts of Alaska and the East Indies. It appears then from 

 the evidence of the lime content that radiolarian cherts may be de- 

 posited outside of the extremely deep portions of the ocean. 



Manganese Oxide. — Professor G. A. F. Molengraaff, 119 who 

 regards the radiolarian cherts as abyssal radiolarian oozes, and the 

 shale partings as the equivalents of the red clay, points to the pres- 

 ence of managanese in these cherts as additional evidence in favor of 

 his view. He believes that the mode of occurrence of manganese in 

 the Borneo cherts is identical with that in which manganese occurs in 

 the present, deep sea oozes. He finds it in the radiolarian cherts of 

 Borneo in three distinct forms. It occurs in minute scattered grains 

 throughout the chert, in nodules in the cherts and associated shales, 

 and in beds, interlayered with shale in a manner similar to that in 

 which the chert occurs. From his descriptions, the nodules do not 

 appear to be abundant. Molengraaff points out that under ordinary 

 conditions, manganese oxide accumulates with extreme slowness, and 

 only in the very slowly accumulating, deep sea oozes does it become 

 abundant enough to make up an appreciable part of the formation. 

 He believes that the manganese aggregates itself into nodules in the 

 deep sea oozes through the processes of solution and precipitation, 

 and also by the movement of minute grains of manganese oxide 

 through the colloidal clay to the centers of accumulation. He believes 

 that there is an aggregation of silica in much the same way, but that 

 the aggregation of silica takes place at a later stage than that of the 

 manganese. 



The presence of manganese in a sediment does not appear to be 

 a proof of its abyssal origin. Manganese oxide is found in other than 

 abyssal deposits though, according to Agassiz, 120 the nodules are only 

 met witli in the deeper sea waters. One example, which is sometimes 

 referred to, is described by J. Y. Buchanan, 121 who writes of the find- 

 ing of a large number of manganese nodules in the Firth of Clyde in 

 a depth of only 104 fathoms. Later these nodules were found in 

 abundance in other parts of the Firth of Clyde at a depth of ten 

 fathoms. This fact, however, should not be pressed too strongly as 

 contradictory evidence for the reason that these nodules are stated 



119 On the Occurrence of Nodules of Manganese in Mesozoic Deep-sea Deposits 

 from Borneo, Timor, and Eotti; their Significance and Mode of Formation, E. 

 Acad. Sei., Amsterdam, vol. 24, p. 415, 1915. 



120 Three Cruises of the "Blake," vol. 1, p. 141, Cambridge University Press, 

 1888. 



121 Proc. Koy. Inst, of Great Britain, vol. 17, p. 363, 1903. 

 Nature, vol. 18, p. 628, 1878. 



