GEOLOGICAL POSITION AND DESCRIPTION OF THE ORES 477 



It should be noted, however, that the ore deposits in large measure are 

 on and not in the rocks of this age — that is, they are residual deposits 

 and have been formed since the uplift of these beds in late or post- 

 Permian time. The process of formation is going on at present and has 

 been presumably more or less continuous since Carboniferous time. 



Description of the Ores 

 associated minerals and classification 



The bulk of the ores consists of the mineral limonite, associated with 

 which are variable quantities of the other hydrous oxides, turgite and 

 goethite, and the anhydrous oxides, hematite and magnetite.* 



The ores occur in mahy diversified forms, which might be variously 

 classified, but for convenience the following group names, based on form, 

 are used by the author : nodular ore, pipe ore, brecciated ore, flake or 

 sheet ore, fragmental ore, and yellow ocher. 



NODULAR ORE 



The nodular ore consists of irregularly rounded masses, varying in 

 size from a fraction of a pound to several hundred pounds in weight. 

 The masses are frequently hollow (see numbers 2 and 3 in plate 50, 

 figure 1), but some inclose a rounded or subangular rock fragment (see 

 number 1 in figure 1), which is sometimes sandstone, as in the illustra- 

 tion, sometimes chert, sometimes slate, and sometimes clay. Some of 

 the shells are filled with clay or sand, and workmen report finding many 

 of them filled with water. * Some are filled with clay, which still retains 

 the laminated structure and appearance of the original slate from which 

 the clay was derived. Furthermore, this slaty structure was found to 

 extend through the ore shell, which showed, besides the plain lamination 

 of the slate, a faint concentric structure as well. This shell, which is 

 illustrated in figure 1, was not found imbedded in the slate, but in the 

 loose material on an outcrop of it where it occurred along with many 

 similar ones. It was clearly a concretionary form in the slates, an ad- 

 vanced stage of the concentric structure that maybe seen in almost any 

 shale bed. While only one shell was found still retaining the lamina- 

 tions in the clay, there were many others containing clay and sand. 

 Some of the shells are but thin crusts, while others are quite thick, 

 almost solid; some have a rough irregular inner surface, while others 

 have a smooth, velvety or bright mammillated inner surface, frequently 



*The extensive deposit of magnetite at Cornwall, Pennsylvania, while occurring at about the 

 same geological horizon, is so intimately associated with igneous dikes that it will not be consid- 

 ered with the limonite deposits. 



