USEFUL METALS 



247 



deposits. It appears as grains in the clay or ooze, som.-t im.-s as a 

 coating upon pumice, coral, shells, or fragment* of bones, often 

 in (he form of nodular concretions made up of concentric layers 

 about some other substance as a nucleus. Even in shallow 

 waters, as in Loch Fyne, in Scotland, these nodules have been 

 found, but they seem to be more characteristic of the deeper ocean 

 abysses, whence the dredge often brings them up in great 

 numbers. 



Some doubt still exists as to the origin of these manganese 

 nodules. C. W. Gumbel ascribes them to submarine springs 

 holding manganese in solution, which is precipitated on contact 

 with sea water. J. B. Boussingault considers that the manganese 

 was derived from carbonates carried in solution by oceanic waters. 

 According to L. Dieulafait the oxidation of the carbonates takes 



FIG. 122. Section in the manganese region of north Arkansas, show- 

 ing the formation of manganiferous clay by the decay of the St. Clair lime- 

 stone. (After Penrose.) 



place at the surface through atmospheric contact after which the 

 precipitated oxide falls to the bottom of the sea. According to 

 J. Murray the manganese is derived, like the well-known red clay, 

 from the subaqueous decomposition of volcanic debris. 



According to F. W. Clarke the manganese is derived from 

 rock fragments as it is by weathering on the land. It goes into 

 solution as a carbonate, is oxidized by the dissolved oxygen of the 

 sea water, and precipitated near its point of derivation around any 

 nuclei that may happen to be at hand. These nodules occur in 

 close association with altered volcanic materials, and most abun- 

 dantly in connection with the red clay of similar origin. Their 

 impurities are those which this method of formation would lead 

 one to expect (Fig. 122). 



Manganese is almost invariably present in small quantities in 





