CONDITIONS OF SEDIMENTARY DEPOSITIONS. 481 



by the relation of volume of sediment to the force of waves or 

 currents. If the analysis be based on the sub-processes and 

 conditions which favor them, it may be stated and discussed as 

 follows: 



SORTING. 



The conditions under which sorting is more or less efficiently 

 carried on are three in number. 

 Favorable conditions: 



[a\ Vigorous wave action accompanied by strong 



undertow. 

 (b) Prolonged transportation in consequence of 



deep water and continuous currents. 

 (<:) Moderate volume of sediments. 

 The conditions under which sorting is not accomplished are 

 the reverse of these, namely: 

 Unfavorable conditions: 



{a) Feeble or diffused wave action. 



(b) Concentrated deposition. 



(c) Excessive volume of sediments. 



It will be profitable briefly to discuss these positive and neg- 

 ative conditions. 



[a) Vigorous wave-actio?i. — The force of waves is determined 

 by their fetch and the strength of winds. In the study of mod- 

 ern beaches the latter is important, since it controls the form and 

 the greatest storm 1 fixes the maximum size of detritus moved; 

 but in considering fossil beaches as strata we deal with sands 

 which have been so rearranged during submergence that the 

 beach form is lost. However the former condition, the fetch 

 of the waves is more constant, and the force of the waves 

 determined by it may be inferred from the nature of the beach 

 deposits. 



The efficiency of waves of a given force is determined by the 

 concentration of their blows, and this is conditioned by the slope 

 against which they break. If relatively deep water prevails 



1 For full discussion of wave erosion and deposition, see Lake Bonneville, by G. K. 

 Gilbert. Monograph 1, U. S. C. S. 



