4 I. C. RUSSELL CONCENTRATION AS A GEOLOGICAL PRINCIPLE 



have been formerly active. With this idea in mind, the diverse and 

 frequently highly complex processes of concentration may be conveniently 

 classified as mechanical, chemical, physical-chemical, and vital processes. 

 To be sure, in attempting to classify the processes whereby concentra- 

 tion is effected, as in most other similar efforts to tabulate natural 

 phenomena, the divisions which it is convenient to establish are largely 

 empirical. The dividing lines are not in all cases definite and sharply 

 defined, but resemble rather the recognition of species among organisms 

 or the partitioning of prairie lands among various owners. In many and 

 perhaps most instances, however, in which natural methods of concen- 

 tration are in operation, some one physical, chemical, physical-chemical, 

 or vital process is dominant and determines the major characteristics of 

 the resultant product. In the recognition of the dominant factors 

 opinions may differ, as they do concerning species among plants and 

 animals, but this qualification need not detract from the utility of the 

 general principle which it is the purpose of this address to emphasize. 



Primary Divisions 



It is obvious from current geological knowledge concerning the various 

 processes of erosion, transportation, and deposition that an extensive 

 group of concentrates have resu.lted mainly and essentially from the 

 mechanical changes which take place on the earth's surface. A typical 

 example is furnished by the way in which dry, heterogeneous debris 

 is separated by the winnowing action of the wind into residual material, 

 too heavy for the Avind to move, and graded accumulations formed by 

 the deposition of the transported material. In this process mechanical 

 action, or force, is the dominant agency leading to the results referred 

 to, and all similar processes may be conveniently grouped under the term 

 mechanical concentration. 



In a similar manner in many instances chemical changes are in control 

 and lead to the isolation of residual material and the accumulation of 

 graded precipitates or other deposits. In many of the operations of 

 nature, however, which result in concentration the part played by the 

 physical processes and by chemical processes are so equally balanced 

 that no plane of cleavage between the two can be detected. In a manner 

 analogous to the recognition of the science of physical chemistry to 

 embrace the borderland between two long recognized sciences, it seems 

 permissible to differentiate physical and chemical processes of concen- 

 tration on the one hand from physical-chemical processes on the other. 

 Assistance in this direction may be had by recognizing the dominant 



