Total 94.0 gms 99.9 per. 



the curve. The most commonly used statistical values are the median (Ma), 

 the first quartile, (Q x ), and the third quartile (Q 3 ). The median diameter (M d ) 

 is the diameter at the intersection of the 50-percent line and the cumulative 

 frequency curve. The first quartile (Q ± ) is the diameter at the intersection of 

 the 75-percent line and the curve and the third quartile (Q s ) at the intersection 

 of the 25-percent line and the curve. From these values the sorting coefficient 

 (S ) and skewness (S k ) introduced by Trask (1930) are computed. 



The sorting coefficient (S ) , a geometric quartile deviation, is computed by 

 the equation S — \/Q 3 /Qi. As Q 3 is always the larger value, the value of S„ 

 will always be greater than 1.0. A sediment having a sorting coefficient equal to 

 1.0 would be perfectly sorted. The greater S becomes, the more poorly sorted 

 the sediment. Trask proposed that sediments with an S less than 2.5 are well 

 sorted, a value of 3.0 normally sorted, and values greater than 4.5 are poorly 

 sorted. It should be noted that Trask's sorting coefficient applies only to the 

 central 50 percent of the curve. The coarser and finer grades have little influence 

 on the value. 



The skewness (S k ) , which is a measure of asymmetry of the frequency 



curve, is obtained by the formula, S k = ( ^fl 2 • This value, which is in effect 



the square of the geometric quartile skewness, has the advantage that, if the 

 value is less than unity, the maximum sorting or spread lies to the left or coarse 



101 



