480 T. C. HOPKINS — CAMBROSILURIAN LIMONITE ORES 



ciated with partly rounded chert and sandstone pieces. These are local 

 in occurrence! and form a comparatively small part of the ore deposits. 

 They may lie caused in part by surface and in part hy subterranean 

 streams. An especially favorable point for them is at the bottom of a sink- 

 hole, where the surface waters pouring in during the wet seasons wear the 

 materials at the bottom like the pebbles in a brook. The subsequent 

 decay of the limestone buries the rounded materials in the residual clay 

 and obscures the manner of their formation. These sink-holes are quite 

 abundant at the present time, and in the few places where the bottom is 

 accessible the rounded fragments of ore and chert are quite numerous. 



YELLOW OCHER 



Yellow ocher, while not properly an iron ore, is probably the most 

 common form of the iron oxide in the region considered. It is asso- 

 ciated with the ores in all the deposits, and in many places occurs free 

 from lump ore. It represents the diffused iron oxide which has not been 

 segregated. In several places it is prepared for market as ocher, but 

 generally no attempt has been made to save it. 



Chemical Composition 



The following analyses of carefully selected samples show the chemi- 

 cal nature of the ores. Commercial anal} r ses of carload lots give a much 

 lower per cent of iron and a corresponding increase of silica and alumina, 

 as shown in analysis number 8 of the table, which gives the average of 

 29 analyses, each of which represents 150 to 500 specimens of ore. The 

 commercial analyses include considerable mechanical impurities in the 

 form of clay and sand that were eliminated as far as possible in the other 

 analyses. Doctor Genth, who made most of the analyses, says the ores 

 are mechanical mixtures of limonite with hydrous ferric silicate and mi- 

 nute traces of hydrous ferric phosphate. It is impossible, he says, to 

 state whether the hydrous ferric silicate is anthosiderite or degeroite.or, 

 he might have added, one or more of several other hydrous ferric silicates, 

 of which chloropal is a common form. There is a possibility also that 

 some of it may occur as grunerite or some other iron, or iron-magnesian, 

 amphibole, or pyroxene. Bischof* says that hydrate of iron will de- 

 compose silicate of alumina. The deposition of the ores in contact 

 with the clays and cherts would furnish opportunity for such chemical 

 reaction. That the waters carrying the iron have a chemical action on 

 the silica is shown by the dissolution of the chert, shale, and sandstone 

 fragments inclosed in the shells and the breccias. 



* Elements of Chemical and Physical Geology, vol. 2. 



