MATERIALS OF THE EARTH 71 



Surface concentration. The simplest of all modes of concen- 

 tration takes place in the formation of mantle-rock. An insoluble 

 or slightly soluble metallic substance sparsely distributed through 

 a rock may be concentrated to working value by the decay and 

 removal of the principal rock material, leaving the metallic matter 

 in the residuary mantle. The tin ores of the Malay peninsula 1 

 are examples. Crystals of tin oxide were originally scattered 

 sparsely through granite and limestone, but by the decay and 

 partial removal of the rock, the crystals have accumulated in work- 

 able quantities. Certain gold fields and certain iron ores have 

 acquired higher value in the same way; also certain ores of man- 

 ganese, as those of Arkansas. Such residuary ores may be further 

 concentrated by wash into gulches or alluvial flats, in the course 

 of which the lighter parts of the mantle-rock are largely carried 

 away, and the heavier, including the metal or its compounds, 

 are mainly left behind. Gold placers are the best example. Such 

 concentrates in past ages have in some cases been buried by later 

 deposits, and hence certain ancient sandstones, conglomerates, 

 and mantle-rocks have become ore-bearing horizons. 



Purification. A somewhat different mode of concentration 

 and purification has affected certain of the great iron deposits. As 

 already explained, the iron compounds were originally parts of a 

 sedimentary formation, and in beds. In some cases they were suf- 

 ficiently pure, as first deposited, to be worked profitably; but in 

 most cases they were seriously affected by undesirable mineral 

 associates. After such impure deposits have been subjected to the 

 percolation of waters from the surface for long periods, the impuri- 

 ties, under favorable conditions, are dissolved and taken away, 

 and at the same time, new and valuable materials may be added. 

 The great Bessemer ore-deposits of Lake Superior are examples. 

 Originally impure carbonates or silicates, they have been converted 

 into rich and phenomenally pure ferric oxides by ground-water. 

 Vast quantities of lean ores lie in the tracts not thus purified and 

 enriched. The waters seem to have brought in ferric oxide while 

 they carried away the impurities, especially silica. 2 



l Penrose. Jour, of Geol., Vol. XI, pp. 135-155, 1903. 



2 Van Hise & Leith, various monographs of the U. S. Geol. Surv. 



