PRIMARY DIVISIONS OF CONCENTRATION PROCESSES 5 



field of action of tlie three groups of processes under consideration. The 

 field of action of the processes of concentration in which either physical 

 agencies or chemical agencies are in a large majority of instances clearly 

 dominant is the outer portion of the earth and includes the zone of 

 contrast of the atmosphere with the lithosphere and the zone of weather- . 

 ing on the outer earth. The portion of the earth in which physical 

 and chemical agencies favoring concentration go hand in hand and are 

 to a conspicuous degree mutually interdependent has as its superior or 

 outer limit the base of the zone of weathering and includes the zone of 

 concentration and the centrosphere, or the inner earth. 



In reference to what is termed above the outer earth, temperature 

 and pressure are low in intensity and have but small yearly fluctuations, 

 but within this narrow boundaries are subject in each instance to rapid 

 fluctuations both at a given locality and from place to place. In the 

 inner earth both temperature and pressure are of a high order of inten- 

 sity, in each instance increase with depth, and soon reach what may be 

 termed incomprehensible values, but are without yearly variations and 

 during even secular changes are modified but slowly. 



In the first of the provinces thus defined, namely, the outer earth, 

 the part played by dominantly chemical and dominantly physical proc- 

 esses of concentration may be differentiated and chemical concentrates 

 given equal rank with the mechanical concentrates. 



In the second province, namely, the inner earth, physical conditions, 

 and principally temperature and pressure, are, as nearly as can be judged, 

 equal and perhaps of greater importance than the conditions usually 

 designated as chemical, which bring about concentration. The two, how- 

 ever, are so intimately associated and so thoroughly introactive that 

 their combined influence only can be consistently used as a basis of 

 classification in the present connection. Physical-chemical forces are 

 therefore given equal rank with the dominantly mechanical and domi- 

 nantly chemical forces which lead to concentration. 



Again, under the infiuence of life, physical and chemical changes are 

 guided by a force not recognizable in the inorganic world, and through 

 its action matter is given an organic structure comparable in rank with 

 the crystalline structure in the inorganic world. Through the dominant 

 action of vital force certain substances are differentiated from other 

 substances and concentrated to form the tissues of plants and animals. 

 Still further concentration by the organic bodies themselves may result 

 by the action of mechanical agencies or from chemical changes, but the 

 accumulations of solid organic debris or of liquids or gases arising from 

 the decomposition of such material when accumulated so as to form a 



