THE nonmetallk; minerals. 255 



the same as that of l)raiiiiito. The composition of the commercial ore 

 is given in the analyses on p. 250. 



PsiLOMELANE.— This is, with the. i)ossibh- exception of pyrolusite, 

 the commonest of the man<4am\st> minerals. Th(» usual form of occur- 

 rence is that of irreoular nodular or Itotryoidal masses embedded in 

 residual clays. It is readily distinguished from manganite or wad by 

 its hardness, and from hausmannite, ])raunite, or polianite by yielding 

 an abundance of water when heated in a closed tube. The sample 

 (Specimen No. 66722, U.S.N.M.), from the Crimora mines in Virginia, 

 is characteristic. See Plate 11. The composition of the commercial ore 

 is given in analyses V, VI, and VII on p. 256. 



Wad or bog manganese (Specimen No. 66602, U.S.N.M., from Cuba) 

 is a soft and highly hydrated form of the ore, as a rule of little value, 

 owing to impurities (analysis VIII). Asbolite is the name given to a 

 variety of wad containing cobalt (see p. 187). See further Rhodonite 

 and Rhodochrosite, pp. 280, 314. 



Origin. — The deposits of manganese oxides which are of sufficient 

 extent to be of commercial importance are believed to be in all cases 

 of secondary origin; that is, to have resulted from the decomposition 

 of preexisting manganiferous silicate constituents of the older crys- 

 talline rocks and the subsequent deposition of the oxides in secondary 

 strata. Indeed, in many instances the ore has undergone a natural 

 segregation, owing to the decomposition of the parent rock and the 

 accumulate of the manganese oxide, together with other difficult sol- 

 uble constituents in the residual clay. Thus Penrose has shown ^ that 

 the deposits of the Batesville (Arkansas) region result from the decay 

 of the St. Clair limestone, the various stages of which are shown in 

 the accompanying Plate 10. The fresh limestone, as shown by analy- 

 sis, contains but 4.30 per cent manganese oxide (MnO), while the 

 residual clay left through its decomposition contains 14.98 per cent of 

 the same constituent. 



Occurrence. ^As above noted, the ore is found in secondary rocks, 

 and as a rule in greatest quantities in the clays and residual deposits 

 resulting from their breaking down. The usual form of the ore is that 

 of lenticular masses or nodules distributed along the bedding planes, 

 or heterogeneously throughout the clay. Penrose describes the Bates- 

 ville ores as sometimes evenly distributed throughout a large body of 

 clay, but in most places as being in pockets surrounded by clay itself 

 barren of ore. These pockets vary greatly in character, being some- 

 times comparatively solid bodies separated by thin films of clay, and 

 containing from 50 to 500 tons of ore; sometimes they consist of large 

 and small masses of ore embedded together, and again at other times 

 of small grains, disseminated throughout the clay. In the Crimora 



1 Annual Report of the Geological Survey of Arkansas, I, 1890. 



