738 



A. I'DDEX COMPOSITION OF CLASTIC SEDIMENTS 



DIFFERENCES IN SEDIMENTS DUE TO DIFFERENT MODES OF WORK 



Washed sediments. — As already stated, washing is an aqueous mode 

 of sedimentation which has no analogue in the work performed by the 

 atmosphere. In a separate average of 50 samples of beach materials it 

 is found that nearly 66 per cent of the samples are contained in their 

 maxima, and percentages equal to or exceeding 0.1 per cent are scattered 

 through nine grades. The sorting index arerages 4.5 : 1. the same as for 

 atmospheric sediments. A curve drawn to show the quantities in the chief 

 ingredient and tlie admixtures in this average coincides very closely with 

 a similar curve drawn for blown material. The 50 samples of averaged 

 washed materials were numbers 22, 23, 34, 38-45, 82, 83, 90-92, 99, 

 105-110, 114-120, 135, 136, 142. 144. 146-149, 156, 165-170, 176, 177, 

 182, 183, and 186. 



It does not seem likely that such deposits as these could be recognized 

 from their sorting as water deposits. Only one or two samples show a 

 decided aqueous characteristic and these are numbers 177 and 176. The 

 former has two secondary maxima, one of which occurs in the sixth 

 grade from the maximum. Blown sand with its bulk as well sorted as 

 this sample would be very unlikely to also contain as much as this of 

 coarse material. The same may be said of sample IT 6. 



Tal)U showing Averages of the Marimum and Admixtures in Fifty Samples 



of Washed Materi<ils 





Cc::.. ;;- 



^:-:xmre5. 





Maximum. 



Fine admixtures. 



4 



3 



. ! 



1 



1 



I ^ 



3 



4 



•1 



.3 



O X 



17.0 



6.5.9 



11.6 



i ! 



1.9 i .2 



.1 



Drift and blown material, silt, and dust. — The difference between de- 

 posits formed by drifting and blowing on the one hand and bv silting 

 and dusting on the other is most evident in the general difference in the 

 size of the elements in the chief ingredient. The two modes of trans- 

 portation and sedimentation (drifting and silting in water and blowing 

 and dusting in the atmosphere \ overlap, of course, in each medium, with 

 different speed of currents. A very fine sand may drift in a slow cur- 

 rent of water and may be carried as silt in a fast current. It may be 

 blown by a moderate wind and be lifted high, like dust, in a strong wind. 

 The two modes change, one into the other, through imperceptible grada- 

 tions. They are, as already stated, merely the opposite extremes of the 



