930 E. W. SHAW SORTING IN SEDIMENTARY ROCKS 



Fundamental Cause of Sorting 



If one attempts to evaluate deductively the factors affecting nature and 

 degree of sorting, and particularly to classify them according to whether, 

 on the whole, they probably favor or retard sorting, he may arrive at the 

 conclusion that the fundamental cause of most sorting is the friction 

 which tends to retard the relative movement of a solid body through a 

 fluid medium. In other words, we may regard the causes of sorting as 

 relatively simple and the retarding factors numerous and complex. The 

 friction increases as the square root of the diameter of the grains — shape, 

 specific gravity, and other factors being equal. If a handful of unsorted, 

 well rounded coarse sand and fine gravel be dropped into a deep and very 

 slowly moving stream of water of uniform velocity and cross-section, the 

 coarsest particles will reach the bottom in the shortest distance, those of 

 half the diameter of the coarsest in four times the distance, those of one- 

 fourth the diameter in sixteen times the distance, and so on. If some 

 very fine particles had been included, they would, on account of this geo- 

 metric ratio, require an almost infinite distance to reach the bottom, and 

 thus remain in suspension almost indefinitely. 



If, after most of the grains have reached bottom, the channel is tilted 

 a little, so as to give the current sufficient velocity to move the particles 

 along the bottom, the same law of friction again becomes operative, 

 though now the water is moving faster than the grains. 



On account of these considerations it seems highly desirable to make 

 the sizes separated in mechanical analyses bear a fixed ratio to each other, 

 and this ratio should be 4, 2, the square root of 2, or the fourth root of 2. 

 If an arithmetic progression is used, or a variable ratio, the result is a 

 distorted picture of the sediment, and one which can not be readily inter- 

 preted in the light of the fundamental principle of sorting. 



Attempts to analyze the factors modifying the effect of sorting pro- 

 cesses raise repeatedly the query : Even though the factors are numerous 

 and complex and the analyses show unlimited variety, may it not be that 

 each factor, or at least some of them individually or by groups, have some 

 obscure and heretofore unnoticed effect on mechanical composition which 

 can be discovered by careful study and comparison ? 



Perhaps, on account of the fact that mechanical analyses seem to throw 

 little light on the genesis of a sediment, only a few hundred of them are 

 available. Among these, some kinds of deposits, as, for example, dune 

 sand, are much more abundantly represented than others. A few weeks' 

 study of the available analyses, including their representation in various 



