426 



and ill llie chert. Manganese is present in small grains and in 

 nodules, wliicli are either spheroidal (PI. I, fig. 1) or possess various 

 irregular, often flat cake-like shapes, but are always rounded. 



Microscopical examination shows that (he ore is found as black 

 dust all through the rock, and that it is, moreover, concentrated on 

 numerous spots in larger grains, which tend to cluster together. In 

 some spots these grains are so congregated together that with the 

 naked eye the presence of a concretion of pure manganese is sur- 

 mised, but the microscope reveals that in such a case the grains, 

 though very closely packed together and thus i-esembling a cloud, 

 still remain isolated from each other. 



In otiier s[)Ots tiie accumulation is still more compact and a true 

 concretion oi- nodule is thus formed, composed exclusively of man- 

 ganese and tests of radiolaria. 



Surrounding such a nodule or concretion, there is generally a 

 concentration of the grains of ore, forming an opaque halo or border, 

 which however rapidly diminishes in density with increasing distance 

 from the nodule. 



The larger and smaller nodules are more or less arranged and con- 

 nected together in layers, thus tending to form beds or Hat deposits 

 of manganese. In recent deep-sea deposits flat concretions forming 

 a kind of cake or slab of ore are similarly found. ^) 



A great number of slides of nodules have been examined under 

 the microscope in order to determine whether, in the interior of 

 the nodulus, particles of minerals or remains of organisms were 

 present, that had acted as a centre or nucleus, around which the 

 ore had grown, thus giving rise to a concentrical structure of 

 the nodule around one or more nuclei. As a rule no nuclei and no 

 arrangement in concentric layers have been found VAathin the nodules. 

 Sometimes the manganese is first deposited within the tests of 

 radiolaria, and the author has found cherts in which the accumu- 

 lation of manganese has I'einained strictly limited to the interior of 

 the tests of radiolaria. In some cases the nodules may grow from 

 such filled tests as centres, and thus polynucleal concretions may 

 be formed. This is, however, rather of rare occurrence, and as a 

 rule no nucleus whatever, and no concentric arrangement could be 

 detected in the fossil nodules"). 



1) J. Murray and A. E. Renard. 1 c. PI. HI fig. 3. 



-) In this respect there is a difference between the fossil nodules of manganese 

 and those of the existing deep-seas, (or the latter very often, although not always, 

 show a concentric arrangement around a nucleus as e.g. around a crystal of phil- 

 lipsite, a shark's tooth or an otolith of a cetacean. It is clear that otoliths could 



