MECHANICAL CONCENTBATION 7 



etcetera, should be examined, but search made for the grain left on wind- 

 swept areas, with the hope of discovering gems and flakes and nuggets 

 of gold, platinum, and other heavy metals and ores. 



An important adjunct to the process of eolian concentration is fur- 

 nished by explosive volcanic eruptions. During such eruptions the ratio 

 of the size, weight, and shape of the fragments blown into the air to the 

 propelling force leads to an assorting of the debris showered on the 

 earth's surface, but assisting this process, and, as it seems, usually assum- 

 ing the leading role, is the transporting power of air currents. During 

 the operations of either the propelling or carrying agency, however, the 

 selective power of gravity may be credited such assorting as takes place 

 of the material discharged. 



"With the aid of water currents, gravitation leads to the selection from 

 heterogeneous debris of certain classes of material for transportation and 

 the leaving of other classes of material as residues. Concentration of 

 material, owing to the removal of other kinds of material previously 

 mingled with it, through the combined action of water currents and 

 gravitation, is of the same nature as the similar process already noted in 

 the case of air currents, but from a geological point of view is vastly 

 more important. Examples of residual material concentrated through 

 the agency under consideration are furnished by the boulders and stones 

 in stream beds and on the beaches of the ocean and of lakes, in numeroxis 

 now abandoned waterways and along ancient coast lines. 



In the process of stream and current transportation, mineral and rock 

 fragments carried in suspension or rolled and pushed along the bottom 

 are assorted with reference to size, gravity, and shape and are finally de- 

 posited in more or less perfectly assorted accumulations. Witnesses to 

 the importance of this process are furnished on every continent by exten- 

 sive beds of assorted debris, like gravel, sand, clay, etcetera, and by thick 

 and extensive strata composed of consolidated material of like character. 

 A special phase of the process is illustrated by the manner in which 

 grains and nuggets of gold, platinum, cerussite, etcetera, are concen- 

 trated in depressions and crevices in the beds of streams and the current- 

 swept shores of standing water bodies. So well known is the process 

 referred to that it is employed in many ways in the arts, as, for example, 

 the separation of heavy minerals from crushed rock on the concentrating 

 tables used in certain metallurgical operations. 



A knowledge of the laws governing transportation by water currents 

 and the secondary or associated conditions which modify their operation 

 is of prime importance in searching for localities where fractional parts 

 of former heterogeneous stream loads have been laid aside. The practical 



