10 I. C. RUSSELL CONCENTRATION AS A GEOLOGICAL PRINCIPLE 



may cause sedimentation by checking the current or producing eddies, 

 while a mat of sphagnum might lead to the complete classification of 

 percolating water; but there is no definite boundary between the two 

 agencies, so far as their physical action is concerned. 



SEDIMENTATION 



In the brief outline just given of the functions of air and water cur- 

 rents in promoting the concentration of either inorganic or organic 

 debris, the selective process performed by gravity was made prominent. 

 This same group of changes, visible in a different way, has in part been 

 termed sedimentation. The operation of currents, however, is not strictly 

 essential in the process of sedimentation, although under natural condi- 

 tions almost always present, as an assorting of debris in suspension in 

 still water, takes place. In still water, substances of less specific gravity 

 than the water, whether fresh or saline, rise to the surface, thus indicat- 

 ing concentration, which, should currents be present, leads to still further 

 concentration, as in the eddies of streams, in ocean currents, etcetera. 

 Material of greater specific gravity than the water in which they are sus- 

 pended descends under the pull of gravity and is assorted, in reference to 

 the ratio of volume and weight. A like result follows when debris is 

 suspended in still air and graded layers of dust and of dust-like particles 

 result. 



In the case of heterogeneous fragments in a minute state of subdivi- 

 sion, in suspension in water, other conditions, as the molecular attraction 

 of particle by particle, the chemical nature of the substances in solution 

 in the water, and their relative proportions, and yet other and still less 

 well understood conditions, as, for example, in reference to electricity, 

 become important functions. An illustration is furnished when streams 

 deliver fine silt in suspension to the ocean and flocculation on account of 

 the substances in solution in sea water occurs, which quickens the rate of 

 sedimentation. 



The importance to the geologist of the process of sedimentation in 

 leading to the concentration of fractional parts of previously heteroge- 

 neous debris, whether inorganic or organic, is attested by the accumula- 

 tions that are present on the floor of streams, lakes, and the ocean. The 

 fact that like results have been attained throughout geological history is 

 made evident by the extensive and frequently thick strata of shale, sand- 

 stone, limestone, etcetera, and by similar metamorphosed sediments in 

 various geological terranes. 



