19. Size Analysis and Statistical Measures . Sample size fraction values are 

 based on dry weight and given in phi (^ units to the nearest whole percent. An 

 American Instrument Company sieving machine and U.S. standard sieves were used 

 for determining sand and larger size fractions (> 1/16 mm). The pipette method, 

 based on Stokes Law (for computing settling rates of spherical particles), was used 

 to determine silt size (1/16 to 1/1256 mm) and clay size particles (1/1256 to 1/4096 

 mm). Any colloidal material (< 1/4096 mm) was included with the clay. 



QD^ - (phi quartile deviation). Is that statistical parameter which is 

 a measure of one half of the spread of the quartiles and Is expressed in 

 phi units with the given value computed from the formula: 



00^ = ^3^.^^ 



Sorting referred to In the text was determined In accordance with the 

 relationship between QD^ and Trask's Sorting Cofficient (So) 



Sk/i^ - (phi quartile skweness) . Is that statistical parameter 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: 



Md^ - (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 ex- 

 pressed In phi units . 



* 



Reference to Sorting 



Krumbeln, W. C, and F. J. Pettijohn, 1938, Manual of sedimentary petrography, 

 Appleton-Century-Croft, New York, 549 pp. 



Page 235, Fig. 110. Conversion chart for So and QD/. logoSo^ QD/ 



QD/ <1.32 = well sorted 



1 .58 = normally sorted sediment 

 >2 , 18 = poorly sorted 



59 



