CHEMICAL CONCENTRATION 13 



as do the combined physical and chemical properties of all the substances 

 entering into the composition of the earth. 



RESIDUAL CONCENTRATES 



Owing to the selective power of water, the universal solvent, certain 

 substances under prescribed conditions, especially of temperature and 

 pressure, are taken into solution and carried away, leaving in many in- 

 stances other substances as a residue. The world-wide importance of this 

 process is forcibly shown by the abundance throughout many estensive 

 regions of residual earth. The familiar terra rossa, consisting essen- 

 tially of iron-stained clay, no matter what may have been the nature of 

 the rock from which it was derived, is evidence that the process of select- 

 ive solu.tion carried on by percolating water aided by organic acids is a 

 most important agency leading to the disintegration of the rocks form- 

 ing the superficial film of the lithosphere and the concentration of a part 

 of the resultant debris. 



The principle illustrated by the mode of origin of terra rossa applies 

 to many other substances, some of them of great industrial value. Among 

 the numerous examples that might be selected, none, perhaps, will serve 

 better than the so-called fossil iron ore of the Clinton formation in the 

 Appalachian region, which in certain important instances, if not in all 

 cases, has resulted from the leaching of ferruginous limestone ; the more 

 soluble constituents of the rock having been removed in solution, leaving 

 the less soluble ferric oxide as a residual concentrate. Again, in the lead 

 and zinc mining districts of the Mississippi valley and the silver and gold 

 fields of the Pacific mountains and elsewhere, natural concentration pro- 

 duced by the removal of the more soluble constituents from mineralized 

 rocks, mineral veins, etcetera, has led to the concentrating of residual 

 minerals and ores of great commercial value. 



"Knowing the process by which residual concentrates are produced and 

 the nature of the material from which they are derived, guides are fur- 

 nished which aid in the search for rock residuums. Important princi- 

 ples in this connection are : The process goes on most actively where rain 

 is abundant, the temperature high, and plant life luxuriant. Each of 

 these dominant conditions, as a further analysis would reveal, is depend- 

 ent on other conditions, which we denominate as secondary, none of 

 which could be eliminated withoiit seriously impairing the results, any 

 more than the smaller wheels or cogs in a watch could be removed with- 

 out stopping the action of the mainspring or other, as they seem, more 

 essential portions of the mechanism. Geographically, the process of 

 residual concentration through the action of chemical agents is at its 



