MECHANICAL COKCENTRATION 9 



action of agencies tending to remove and perhaps redistribute accumula- 

 tions of debris brought about in various other ways. 



The function of filtration is the separation of solid substances in sus- 

 pension in liquid from the containing liquid. In all geological processes 

 of this nature the containing liquid is water. Land areas on which rain 

 falls may be considered as filters of vast extent. Eain drops on striking 

 the ground in many instances disturb fine particles on which they im- 

 pinge and take them into suspension. In fact, the accumulated rain 

 drops gather into surface streams which bear away this freight of silt, 

 but in part the water sinks into the ground and is filtered. When the 

 ■filter is coarse no material of sufficient fineness to form soil remains, and 

 barren areas of angular rocks, boulder pavements, form the surface. 

 When the filter is of fine texture, however, and when plants assist in 

 binding its particles together, efficient filtration results, as is abundantly 

 proven not only by the conservation of the soil present, but by the clear- 

 ness of the springs formed when the filtered water emerges once more at 

 the earth's surface. It is to the conservative influence of filtration that 

 the preservation and fertility of soils is largely due. By this same 

 process, also, the beds of streams when of an open texture are rendered 

 impervious. In such instances the meshes of the filter become clogged 

 with the material separated from the water which percolates through 

 them. 



Downward percolation favors the solution of debris previously in sus- 

 pension at the surface of the land. Lateral filtration, especially through 

 mats such as are present in swamps, is also an important process leading 

 to the concentration of silt and of the fractional solid products arising 

 from the maceration of organic bodies. 



While the direct infiuence of filtration in assisting in the concentra- 

 tion of mineral substances of commercial value awaits detailed investi- 

 gation, the suggestion is pertinent that in certain instances the retention 

 at a given locality of surface precipitates, as, for example, bog iron ore, 

 may be due in a determinable measure to lateral filtration through vege- 

 table mats. Possibly, also, when chemical precipitates are formed in 

 fissures or other openings in rocks, filtration in certain instances may be 

 a part of the process of which the solid particles produced are prevented 

 from becoming widely disseminated. 



In studying the influence of filtration, it needs to be remembered that 

 there is but an indefinite boundary between well characterized examples 

 of the process and the influence of obstruction on more or less completely 

 debris-charged air and water currents. The rushes growing in a stream 



