8 I. C. RUSSELL — CONCENTRATION AS A GEOLOGICAL PRINCIPLE 



phases of this far-reaching principle have been worked out in detail by 

 the placer miner. 



Modifying the action of gravitation in its selective work when aided by 

 air or water currents is at times the physical condition of the surface 

 over which the currents carry debris. Degree of roughness is here an 

 important and frequently a controlling factor, as is illustrated by the 

 natural riffles which catch and retain grains of gold, but other conditions 

 may favor the exercising of a selective preference. Variations in molecu- 

 lar attraction or adhesion and in chemical affinity between the various 

 fragments transported, or between the material carried and the surface 

 with which it comes in contact, are, as it seems, minor phases of what is 

 in the main a mechanical process. The selective power manifested by 

 mercury for gold in the troughs of the sluices used by placer miners and 

 the peculiar selective property that grease has for the diamond, as illus- 

 trated on the concentrating tables used at the South African diamond 

 ■ fields, should lead geologists to look for analogous processes under 

 natural conditions. 



ICE CURRENTS 



While air and water currents furnish the chief determining or qualify- 

 ing conditions which cooperate with gravitation in bringing about the 

 mechanical concentration of debris, at least analogous results are pro- 

 duced by ice currents. Glaciers, however, have but little, if any, selective 

 or assorting power, but 'both the residue they leave — if such a term is 

 permissible in reference to the debris they sometimes pass over without 

 securing — and the deposits they make are characteristically heterogene- 

 ous. An assorting of the debris composing surface and marginal mo- 

 raines does occur, however, owing to the rolling and sliding of the larger 

 or more spherical stones present on steep slopes, and a rude sort of con- 

 centration is brought about during the accumulation of moraines; but 

 these minor results of the work of sluggish and inflexible ice currents do 

 not require consideration at this time. 



FILTRATION 



Associated with the transportation of debris in suspension by water 

 currents is the process of natural filtration. Although this process has 

 received but scant attention from geologists, its functions are evidently 

 of vast importance. The direct influence of filtration in concentrating 

 debris or in leading to the accumulation of material of economic im- 

 portance is less obvious than the restraining infiuence it exerts on the 



