14 I. C. RUSSELL CONCENTRATION AS A GEOLOGICAL PRINCIPLE 



maximum, degree of efficiency in warm, humid regions, and diminishes 

 when toward the region of less mean annual temperature or of summer 

 warmth and toward regions of little or no rainfall. A like graduation, 

 but dependent on variation in temperature and humidity, is also present 

 from densely plant-clothed areas to either hot or cold deserts. 



Concurrently with the production of residual concentrates as with all 

 other products of concentration, in order that important accumulations 

 may result, is the fact that concentration must progress at a greater rate 

 than the process of removal or redistribution. The fact that under given 

 conditions material of a particular kind is concentrated carries with it the 

 conclusion that at the same or other localities depletion is in progress; 

 concentrates in general represent the net result or algebraic sum of the 

 results of agencies tending to bring like kinds of matter together in a 

 given locality over the results of the same or other agencies tending to 

 dissipate or redistribute material. 



Eesidual concentrates, therefore, in order to be extensive or commer- 

 cially important, can only occur in regions where they are to a conspicu- 

 ous degree shielded from, or are beyond the reach of, destroying or di- 

 minishing agencies. Of the agencies which remove surface material, 

 streams and glaciers are the most important. As the production of 

 residual concentrates goes on most actively in regions of abundant pre- 

 cipitation, they are especially liable to removal through the denuding 

 operations of streams, but derive protection from the vegetation usually 

 present in such localities. In going from regions of heavy precipitation 

 to regions of little or no rain, rock decay diminishes, but the efficiency of 

 the rain wash and streams in promoting denudation also diminishes, on 

 account of diminution in the amount of water available to do the work, 

 although other factors, as decrease in density of vegetation, also enter 

 into the problem. Although the conditions are highly complex, the 

 result of observation seems to be that in warm, humid regions rock decay, 

 and consequently the concentration of residual material, is most apt to 

 be in excess of depletion, owing to the denuding action of streams. It is 

 in such regions that the prospector looks for the most extensive residual' 

 concentrates, although even at present the most arid regions hold out 

 promises of rich rewards. 



Of the agencies tending to remove residual concentrates, glaciers work 

 most thoroughly and most efficiently. So generally is this fact known 

 that no well informed prospector would expect to find commercially 

 valuable surface concentrates in regions scoured by glaciers during geo- 

 logically recent times. 



