740 



J. A. TDDEX- 



'OMPOSITIOX OF CLASTIC SEDIMENTS 



dental ingredients operates to conceal them. If we select and examine 

 deposits made under the most uniform conditions, these show better the 

 features mentioned. The averages of four samples of sand obtained from 

 single layers in dunes show such an excess in the coarse admixtures, and 

 27 samples taken from single layers of drift do the same; also nine sam- 

 ples of dust collected under conditions of small range of wind velocity 

 and seven samples secured from single layers in glacial silt in each case 

 show an excess of fine admixtures. Sand collected in the lee of high 

 dunes nearly always has a considerable excess in the fine ingredients. 

 Overloading and rapid sedimentation no doubt favor flocculation. 



DJFFEEEXCES BETWEEX WATER AXD WIXD SEDIMEXTS 



GenercU discusswn. — The general differences in mechanical composi- 

 tion between water and wind deposits are clearly shown in several ways 

 by the analyses presented. The total range of size of clastic elements in 

 water deposits here examined is from boulders measuring 128 milli- 

 meters in diameter to clay particles measuring 1/2048 of a millimeter. 

 In the wind deposits the coarsest ingredient observed was 16 millimeters 

 in diameter, and dust particles less than 1/256 millimeter in diameter 

 were practically absent. Sedimentation by the atmosphere, generally, is 

 limited to a smaller range of sizes in both directions — that is, sedimenta- 

 tion resulting in appreciable deposits. 



Difference in grade distribution.- — The same is true if we consider the 

 range of effective working limits of any particular local current. The 

 i-ange in this case <'an be more definitely given. The best characteriza- 

 tion to be made for this limitation is expressed by the averages of quan- 

 tities in allgrades. For a general comparison we take averages, as be- 

 fore, by superimposing all the maxima on each other and all the admix- 

 tures of the same order on both sides in the greatest number of analyses 

 available of sediments made by each medium. Such averages for both 

 water and wind deposits are shown in the following table: 



Average Mechanical Composition of 190 Samples of Water Deposits and of 

 147 Samples of Wind Deposits 



Names of 





Coarse adm 



ixtuies. 



Maxi- 



Fine admixtures. 



jrrades. 



6 



■5 



i 



3 



mum. 

 - 1 



1 



2 3 4 



5 



6 



7 



8 



9 



Water depos- 

 its 



Wind depos- 

 its 



tr. 



.1 



.5 

 tr. 



1.0 

 .2 



1.4 



.9 



5.2 16.9 47.9 

 4.4 20.3 53.1 



16.5 

 16.9 



6.12.0 



3.3 .5 



1 



1.0 

 tr. 



.4 



.1 



tr. 



tr. 



tr. 















