478 GEOLOGY. 



into sheets, lodes, veins, and similar aggregations by ground-water 

 circulation, or else in the purification of the ores by the removal of use- 

 less or deleterious material, or in both combined. 



Surface residual concentration. — The simplest of all modes of con- 

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

 or slightly soluble metalHc substance sparsely distributed through a 

 rock may be concentrated to working value by the decay and removal of 

 the main rock material, leaving the metallic material in the residuary 

 mantle. The tin ores of the Malay peninsula* are especially good 

 examples. The crystals of tin oxide were originally scattered sparsely 

 through granite and hmestone, but by their decay and partial removal 

 it has accumulated in workable quantities. Certain gold fields and 

 certain iron ores have acquired higher values in the same way. Such 

 residuary material may be further concentrated by wash into gulches 

 or alluvial flats, in the course of which the fighter 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 ex- 

 ample. The mining of placers by hydraulic processes is but a further 

 extension of the natural process of concentration. 



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. The Rand of 

 South Africa appears to be of this type. 



Purification and concentration. — A somewhat different mode of 

 concentration and purification has affected certain of the great iron 

 deposits. As already explained, the iron compounds were originally 

 dissolved from the iron-bearing constituents of the primitive or of 

 igneous rocks, or their derivatives, and were deposited in beds as chem- 

 ical stratiform deposits. In some cases they were sufficiently pure, 

 as first precipitated, to be worked profitably, but in most cases they 

 were seriously affected by undesirable mineral associates. When, 

 however, such impure deposits are subjected for long periods to the per- 

 colation of waters from the surface under favorable conditions, the 

 impurities are often dissolved and the ores concentrated. The great 

 Bessemer ore-deposits of Lake Superior are examples. Originally impure 

 carbonates or silicates, they have been converted into rich and phenom- 



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



