22 I. C. RUSSELL CONCENTRATION AS A GEOLOGICAL PRINCIPLE 



certain results of which have already been cited and others will receive 

 attention later. After recognizing the results of these various processes, 

 there remain certain precipitates that result from the commingling of 

 waters with different chemical constitutions, which may be termed con- 

 centrates due to chemical reactions. 



The surface waters of the earth are dilute chemical solutions, and by 

 their mingling, as when two streams join, springs are tributary to lakes, 

 etcetera, no recognizable precipitates are formed. In inland lakes and 

 the ocean, direct chemical precipitation not dominated by concentration, 

 by evaporation, by vital action, or other processes recognized in our 

 schedule are of such minor importance that no characteristic examples 

 seem recognizable. In the laboratory of the outer film of the lithosphere, 

 or the zone of weathering through which weak chemical solutions are 

 passing, but linger for shorter periods than is the case at the surface 

 above or the zone of cementation below, precipitation on account of. 

 chemical reactions is in general of minor importance. Such substances 

 as certain salts of iron, copper, zinc, etcetera, and silica and a few sili- 

 cates, etcetera, which are formed are closely associated with the residues 

 left from solutions and in general may be classed with them. In the 

 portion of the lithosphere below the zone of weathering, as will be noted 

 later, chemical reactions under the influence of energetic physical con- 

 ditions become of paramount importance. 



3. The union of liquids, or, more precisely, of water with solids, finds 

 its chief field of activity in the zone of weathering, and is illustrated by 

 the hydration of certain minerals, as, for example, the change of anhy- 

 drite to gjrpsum and the alteration that certain of the feldspars experi- 

 ence. Belief of- pressure accompanied by the pressure of water, which 

 rocks experience when they pass into the zone of weathering, owing to the 

 removal of previous incumbent terranes, seems to be the leading factor 

 in this change. Although hydration is of importance in the study of the 

 disintegration of rocks and is an agency in preparing material for me- 

 chanical concentration, as a direct process leading to the transfer of 

 material from one place to another which is in more or less complete 

 isolation — that is, as true concentration — it does not seem to play an im- 

 portant role. 



Physical-chemical Concentration 



The zone of weathering has been likened to a chemical laboratory in 

 which great results are produced. The operations carried on in it, how- 

 ever, are such as require for the most part low and nearly unif onn tem- 

 peratures and correspondingly small and nearly constant pressures. The 



