12 I. C. RUSSELL CONCENTRATION AS A GEOLOGICAL PRINCIPLE 



accTinmlation of solids, liquids, and gases with certain like physical 

 properties. 



SPHERE OF INFLUENCE 



The field of operation, so to speak, of the various processes which 

 bring about mechanical concentration is essentially the earth's surface. 

 Included in it are the lower portion of the atmosphere, with an indefi- 

 nitely defined outer limit, the waters of the earth's surface, or what may 

 be termed the free hydrosphere, and the outer film of the lithosphere. To 

 some small extent the processes in question perform their functions in 

 caves and fissures, but below a depth of a few score meters from the sur- 

 face of the lithosphere the conditions on which their activities depend 

 cease to exist. Former land surfaces and ancient lake and ocean beds, 

 however, are now in many instances buried beneath subsequently formed 

 aqueous or igneous terranes, and ancient mechanical concentrates are 

 present far below the present surface of the earth. 



Chemical Concentration 



the agencies involved 



The processes of concentration which are characteristically of a chemi- 

 cal nature result principally from solution and precipitation, but others, 

 which are dominantly chemical changes, such as sublimation, crystalliza- 

 tion, replacement, etcetera, play important roles. 



SOLUTION 



Concentration by solution results in two classes of products : First,, 

 residue or the fractional parts of substances acted on by solvents which 

 remain when their more soluble associates are removed in solution, and,, 

 second, the material entering into solution which may or may not be 

 more highly concentrated than during its previous state, but which in 

 many and probably most instances is in a condition favoring later con- 

 centration, owing to precipitation or some other process. 



The process of concentration by solution under natural conditions 

 includes solvent action of gases, liquids, and solids ; but for the present 

 onljf the part taken by liquids, and still more strictly, only the action of 

 water, will be considered. No change known to have occurred in the- 

 material composing the earth is, perhaps, more important or carries with 

 it such far-reaching geological results as the familiar process of solution. 

 The underlying principle is that a selective action is brought into play, 

 since the solubility of various substances, as it seems, has as wide a range 



