DISCUSSION OF DATA 735 



348. Magnetite sand in sample number 341. 



349. Magnetite sand in sample number 342. 



350. Magnetite sand in sample number 343. 



351. Magnetite sand in sample number 344. 



LAW OF THE SORTING INDEX 



With their complexity and variation in continuity^ speed, duration, 

 and mode of application, the currents in the same element may form 

 sediments differing greatly in their ratio of sorting. Most eolian de- 

 posits are better sorted than most water deposits; but not a few beach 

 sands are better sorted than many dune sands. The condition producing 

 the perfect sorting of beach sands is the prolonged continuation of re- 

 peated currents of subequal speed washing the deposit, as already ex- 

 plained. To account for every variation of sorting existing in nature is 

 impossible, owing to the complexity of the factors involved; but it is 

 quite practicable to differentiate one or two variations of sorting, due to 

 some conditions of work that have general prevalence in the atmosphere 

 as well as in water, 



3. If the series of group separations are so placed that any group will contain 

 clastic elements having diameters a-^eraging half the length of the next coarser 

 group and twice the length of the next finer group, then the index of sorting for 

 aqueous sediments produced by drifting and silting is near 2.5:1, and for eolian 

 sediments produced by the analogous modes of blowing and dusting it is near 

 4.5:1. 



This is the most generalized statement that 'can be made for the me- 

 chanical composition of the two principal classes of sediments repre- 

 sented in the analyses made. The analyses used in obtaining the general 

 averages of the contents of the grades of water deposits were those of 59 

 samples of known drifted materials and of 19 samples of presumably 

 drifted materials, of 49 samples of known silted deposits and of 18 sam- 

 ples of presumably silted materials, namely : 



Known drifted materials, numbers 9-21, 24-33, 35, 36, 56-69, 75-81, 84-89, 93-98, 



100. 

 Presumably drifted materials, numbers 134, 137-141, 143, 145, 150, 155, 158, 



171-173, 178-180, 184, 185. 

 Known silted materials, numbers 37, 46-55, 70-73, 101-104, 111-113, 121-133. 

 Presumably silted materials, numbers 151-154, 157, 159-164, 174, 175, 181, 



187-190. 



For eolian deposits the general averages were obtained from the con- 

 tents of the grades in 73 samples of blown material and 74 samples of 

 dust, including all analyses of this class of sediments. 



As these materials are fairly representative for most conditions under 

 LII — Bull. Gkol. Soc. Am., Vol. 25, 1913 



