DISCUSSION OF DATA 



74S^ 



Table showing the average Number of Grades containing Minimum 



Percentages 





Trace. 



.1 



1. 



5. 



lo; 







Water deposits 



Wind deposits 



7.93 

 6.02 



7.9 

 5.7 



5.9 

 4.6 

 1.3 



4.0 

 3.3 



3. 



2.7 







Differences . ... 



1.91 



2.2 



.7 



.3 







Difference in uniformity of composition. — The simplest and, in onQ, 

 sense, the clearest way of appreciating the difference between wind and 

 water deposits is to see the analyses arranged side by side, showing the 

 actual distribution of material of different grades in each sample. The 

 general uniformity shown in the composition of the wind sediments, ex- 

 cepting the coarsest, which are of small importance geologically, shows 

 that in many cases it should be possible to distinguish between these two 

 classes of deposits in any formation on the basis of their mechanical 

 composition when this has been determined for a sufficient number of 

 samples. 



RESTRICTIONS IN THE APPLICATION OF SOMp CHARACTERISTICS 



The sorting index for the 50 samples of washed water deposits given 

 in the tables is 4.5 : 1, which is the same as found in eolian deposits. In 

 the two best sorted samples of washed water deposits the index is 9.3 : 1, 

 much higher than the average for washed water deposits. Most washed 

 sands have a high index, as has already been seen. For classifying clastic 

 rocks correctly the sorting index can evidently not be used indiscrimi- 

 nately. Individual samples of any sediment may represent an extreme 

 in its class. Averages should be obtained from typical samples. In this 

 way mechanical analyses will no doubt be found helpful in distinguish- 

 ing between wind deposits, water drift, and silted deposits. Water drift, 

 blown materials, and washed water deposits may all be well sorted, but 

 the sorting of the former seldom approaches that of dune sand and very 

 rarely that of the best sorted washed deposit. In silt and dust the index 

 is much more constant for each and approximates more closely the fig- 

 ures given. But even here exceptional cases exist. In 11 samples of 

 loess from Pottawattamie County, in low'a, analyzed in 1900, I find that 

 the sorting index averages 4.9 : 1 and varies only slightly.^*^ 



"Geology of Pottawattamie County. Iowa Geological Survey, vol. xi, p. 257, 

 nil — Bull. Gbol. See. Am., Vol. 25, 1913 



