192 



Subsurface Geologic Methods 



and the degree of asymmetry are compared qualitatively. A majority of 

 workers, however, use the cumulative curves to read statistical values. Gen- 

 erally three values are read — the median (Md) and the first and third 

 quartiles (Qx) and { Qs) [fig- 80]. The median diameter is found by reading 

 the diameter value at the point where the cumulative curve is intersected by 

 the fifty-percent line. The first and third quartiles are determined as the 

 diameter values corresponding to the intersections of the curve with the 25- and 

 75-percent lines. Qj is assigned the larger value. 



+ 20 mesh 



+ 40 mesh 



+ 60 mesh 



+ 80 mesh 



- 80 mesh 



1 I — r 



Percent 



Figure 81. Graphic method of compihng size-analysis data of several samples. 



Applying these data, Trask '^^ has defined the geometric coefficient of 

 sorting as : 



As mentioned by Krumbein, the geometric measures are essentially 

 ratios between quartiles, or quartiles and median, thus eliminating both 

 the size factor and the units of measurement. Trask states that if the So 

 value is less than 2.5, the sediment is well sorted; if greater than 4.5, it 

 is poorly sorted; and if 3.0, it has normal sorting. A study of a number 

 of sandstones of the Rocky Mountain region shows these values to be too 

 high. Hough ^^ points out that the coefficient of sorting for most near- 

 shore marine sediments lies between 1 and 2. 



From the foregoing it can readily be seen that, if this general ap- 

 proach is followed, statistical values may be recorded and compared for 

 various granular clastic sediments. 



For more detailed information concerning this subject the reader is 

 referred to chapter 6 of the "Manual of Sedimentary Petrography" by 

 Krumbein and Pettijohn.^^ 



" Trask, P. D., Origin and Environment of Source Sediments of Petroleum, pp. 71-72, Houston, 

 Tex., Gulf Publishing Co., 1932. 



"* Hough, J. L., Sediments of Buzzaris Bay, Massachusetts: Jour. Sedimentary Petrology, vol. 10 p. 

 26, 1940. 



'^ Krumbein, W. C, and Pettijohn, F. J., Manual of Sedimentary Petropraphy, New York, Appleton 

 Century, 1938. 



