926 E. W. SHAW SORTING IN SEDIMENTARY ROCKS 



positing waters around Florida and Yucatan, for example, sweep the 

 deposits about over the bottom, and are commonly so loaded with sedi- 

 ment in suspension that one exiclaims here is transportation on such a 

 scale as to make the Mississippi's annual 470 million tons seem small in 

 comparison. 



The Factors affecting SoPtTiNG 



A brief consideration of the probable causes of the sorted or sized na- 

 ture of an unmodified sedimentary deposit leads to the inference that the 

 causes are very complex and that the result depends on such factors as the 

 following : 



1. Character, particularly granularity and degree of sorting, of parent 

 materials from which the grains composing the deposit came. The ero- 

 sion product of an even-grained rock is likely to be uniform in grain, 

 whatever the nature of the transporting and depositing agent. If a sea 

 cuts into a hill having a layer of small pebbles and a layer of larger ones, 

 it may build a beach in which pebbles of two sizes preponderate. 



2. Method and ease with which the grains yield to processes of crack- 

 ing, attrition, and solution, and the vigor of these processes. 



3. The nature of the current, including its velocity, variations in veloc- 

 ity, variations in direction, and length of time it acts. 



4. The absolute and relative amount of sediment delivered to the de- 

 positing agent per unit of time. 



5. The area over which this agent operates — its average depth and 

 variations in depth. 



6. The presence or absence of eddies or departures from uniformity in 

 velocity of threads of depositing agent. 



7. Specific gravities of depositing agent and grains of sediment. 



8. The viscosity of the depositing agent. 



9. Shapes of grains, particularly if grains of one size tend to have one 

 shape and those of another a different shape. 



Molecular attractions play an appreciable part in very fine-grained ma- 

 terial, particularly in the sorting of sticky clay flakes. A current may 

 carry away all material below a certain size, and the relict deposit will 

 thus have a peculiar nature. 



Mechanical analyses of sediments of known origin seem to confirm the 

 inference that the causes of sorting are very complex, for they seem to 

 show infinite variety, even within a single class, such as river deposits. 

 An analysis of one river deposit may be more like that of an ocean deposit 

 than that of anotlier river deposit. A well washed and sized beach gravel 



