736 J. A. UDDEN COMPOSITIOK OF CLASTIC SEDIMENTS 



which any deposits are made^ it is believed that the indices given will be 

 found to be fair approximations of the mechanical composition of such 

 sediments in general. 



The ratios maintained by the percentages in all grades in the averages 

 were found arithmetically between first the maximum and the average 

 of the pair of admixtures of the first order^ then between the average of 

 this pair and that of the pair of admixtures of the second order, and so 

 on. The sorting index was then obtained by averaging these ratios. 



LAW OF THE SECONDARY MAXIMUM 



In drifting and blowing, the finest particles are carried away from the 

 place of deposition of coarser materials. Momentary vertical currents 

 prevent the finest material from settling. Particles of somewhat larger 

 size are alternately lifted and let down; the smaller of these are carried 

 in long leaps, the larger in shorter leaps. With increasing sizes a limit 

 of elements is reached that will not allow them to be lifted even momen- 

 tarily. In the case of a material where all elements would have nearly 

 equal size, and where the current w^ould be too weak to momentarily 

 move any of them, there could be no transportation; but if the same 

 current were to attack a bed of more heterogeneous composition, the 

 smaller grains would soon be partly removed from around the larger. 

 These would then project up into the current. These larger elements, 

 say pebbles, might still be too heavy to be lifted up momentarily at any 

 time. The impinging current might nevertheless be strong enough to 

 roll them on the surface of the other material. Working in this manner, 

 the transporting power of the medium varies more nearly in approxi- 

 mation to its erosive force than to its transporting power. With changes 

 in velocity the latter varies as the sixth power, while the former varies 

 as the square. With variations in the speed of a drifting current it hap- 

 pens, therefore, much more frequently that coarser elements are rolled 

 along on a supporting bed than that finer drifting material is lifted up 

 and carried away with still finer elements. AYith increase of size of the 

 clastic elements of the load, the transporting medium much more rapidly 

 ceases to pick up elements near, but exceeding, the sizes it has the power 

 to carry by momentarily lifting them than it ceases to roll elements of 

 considerably larger size. 



The operation of this law is the cause of the occurrence again and 

 again in the analyses made of a secondary maximum among the coarse 

 admixtures. The decrease in the quantities of the grades in the direc- 

 tion of the coarsest material is not continuous in such sediments; but 

 there is a rise mostly in the fourth, fifth, and sixth grade of the coarse 

 admixtures in drifted sediments and in the second, third, and fourtli 



