Skj^ - (phi quartile skewness). Is that statistical param- 

 eter which is a measure of half the sum of the first and 

 third quartile values less the median and is expressed in 

 phi units with the given value computed from the formula: 



Sk ^ = J£ 3g _ Md ^ 



Mdjzf - (phi median). Is the middlemost member of the 

 distribution curve above which 50 percent of the 

 diameters in the distribution are larger and below which 

 50 percent of the diameters are smaller and is expressed 

 in phi units. 



The following table is presented for the conversion of phi 

 units to millimeters: Zr ^ = log2 diameter (millimeters \J 



Phi ($) Millimeters Geological Classification 



-2 U.O 



-1 2.0 Granule 



U 1.0 



1 0.50 



2 0.25 



3 0.125 



h 0.0625 Sand 



5 0.0313 



6 0.0156 



7 0.0078 



8 0.0039 



9 0.00195 Silt 



>9 Clay 



20. Subsample Dry Weight (gm.) . Total dry weight of the sub- 

 sample . 



21. Sphericity ( avg . ) . A measure of the approach of the 

 grain to the form of a sphere and expressed as one of the following: 

 high, medium high, medium, medium low, or low. The laboratory 

 procedure was to take an average sample from the sand and larger 

 size grains that passed through a 35 mm but remained on a 230 mm 

 sieve. This sample was viewed under a microscope. To determine 

 the sphericity, a comparison was made of the viewed sample with a 

 chart based on Wadell's method. 



22. Roundness ( avg . ) . A function of the sharpness of the 

 grain edges and recorded as one of the following: very angular, 

 angular, subangular, subrounded, rounded, or well rounded. De- 

 termination and laboratory procedures were the same as for 

 Sphericity . 



184 



