y.yy 









99 



- 





^ / 



95 



_ 





/ 



90 



- 



Sample i—^J 



/ 



84 







/ 







~ / " 



g 70 

 § 60 



- 





cj- Sample 2 



o50 

 £40 



- 







■g^30 



- 







16 















10 



- 







5 

 1 



- ^ 







Ql 



- 



1 1 1 



1 1 1 



I 2 

 Grain Size i^ 



Figure 4. Cumulative size frequencies on phi 



probability plot (data from Table 2). 



through the 84th and 16th percentile intercepts of the observed curve. 

 The comparison can either be made qualitatively by noting the size of 

 the "gap" between the curves along the phi size equal to the mean, or 

 quantitatively by computing an estimate of the skewness parameter. 



SK, 



- Md.) 



(5) 



In both cases, the difference between the mean and median sizes is re- 

 flected by the observed as>'Tnmetry. For example, a negative skewness 

 exists when the, observed distribution lengthens or tails out toward the 

 coarser, negative phi sizes. In this case, the mean (center of gravity) 

 is more affected by the long, coarse tail than by the position of the 

 median. Positive skewness arises when the curve tails toward the finer, 

 positive phi sizes. 



Skewness differences among sediment samples are frequently used 

 to compare sediment-size distributions to characterize sedimentary 



14 



