44 GROUND-WATER 



quantity and in industrial value, and (2) a lean enrichment of 

 the sediments of certain other areas which, after subsequent 

 processes of concentration of the metals by ground-water, became 

 productive. 



Origin of ore regions. From these considerations it appears 

 that the fundamental explanation of many " mining regions" is 

 to be found in (i) magmatic segregation, so far as the country rock 

 is igneous, and (2) enrichment during sedimentation, so far as the 

 rock is secondary. Either of these processes may, in rare cases, 

 give rise to ores directly; but in most cases, further concentration 

 of the metallic substances is necessary. This concentration is 

 effected in various ways by the help of ground-water. 



1. Surface concentration. The simplest of all modes of con- 

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

 soluble or slightly soluble metallic substance sparsely distributed 

 through 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 penin- 

 sula 1 are examples. Crystals of tin oxide were originally scattered 

 sparsely through granite and limestone. By the decay and partial 

 removal of the rock, the crystals have accumulated in workable 

 quantities. Certain gold fields and certain iron ores have acquired 

 higher value in the same way; also certain ores of manganese, as 

 those of Arkansas. Such residuary ores may be further concen- 

 trated by running water, because the greater weight of the metals 

 causes them to be left behind when the lighter substances are 

 washed away, or because their greater weight causes them to be 

 partially separated from the other sediments, in deposition. Gold 

 placers are the best example. 



2. Purification. A different mode of concentration and puri- 

 fication has affected some of the great iron deposits. As already 

 stated, the iron compounds were originally parts of a sedimentary 

 formation, and in beds. In some cases they were sufficiently pure, 

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

 were affected by impurities. From such deposits the impurities 

 have been dissolved by the percolation of waters, and at the same 

 time, more of the valuable metal has been added. The great Bessemer 

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

 silicates or carbonates, they have been converted into rich and 



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



