b I. C. RUSSELL CONCENTBATION AS A GEOLOGICAL PRINCIPLE 



geological deposit may justly be termed an. organic concentrate. Vital 

 force may thus be ranked with mechanical, chemical, and physical- 

 chemical forces as a primary agency leading to the concentration of 

 matter. 



Mechanical Concentration 



effects of oravitt 



Gravity acting singly, as when talus slopes are formed and avalanches 

 occur, or with assistance of such a transporting agency as is furnished 

 by glaciers, may lead to the concentration of heterogeneous accumula- 

 tions. When, however, gravity works in conjunction with a transport- 

 ing agency in a liquid or gaseous condition it is enabled to record a select- 

 ive power holding certain objects in place while their associates are 

 moved, and during the time the material moved is in transit causing 

 certain portions to come to rest while others are carried on. The media 

 with the aid of which gravitation is enabled to separate more or less 

 perfectly the component parts of heterogeneous mixtures of mineral and 

 rock fragments are principally air and water currents. It is in this 

 connection that the best illustrations of the principles governing mechan- 

 ical concentration occur. 



AIR AND WATER CURRENTS 



With the aid of air currents, gravitation causes the separation of 

 lighter from heavier material, of smaller from larger fragments, of 

 rounded from angular grains, etcetera. The material which the wind 

 is capable of removing is more or less completely carried away and 

 a residuum usually remains. Two important phases of the general 

 process of concentration are thus illustrated: first, the concentration 

 of material that is left as residue, owing to the removal of certain 

 varieties of material previously present, and, second, the accumulation of 

 the material removed in more or less perfectly graded deposits. 



Examples of mechanical concentration in which air currents are the 

 controlling agency are furnished: first, by the stones, gravel, etcetera, 

 on wind-denuded areas, as desert plains, and, second, accumulations of 

 gravel, sand, and dust, which are formed where vrind-transported debris 

 comes to rest. This comprehensive and important process, which has 

 been in operation since debris first became dry on the earth's surface 

 and was moved by the wind, corresponds to the winnowing of chaff from 

 grain. In seeking to supply a knowledge of the principles governing 

 eolian concentration as an aid in discovering products of commercial 

 importance, not only the piles of chaff composing dust, soils, sand dunes, 



