188 



Subsurface Geologic Methods 



tor equipped with an automatic clock for time control. Results are more 

 complete if the material first is placed on the "Ro-Tap" and the final sep- 

 aration then completed by hand. After the separation stage, the weight 

 of material retained on each screen is determined, and the results are 

 tabulated as shown in table 6. 



TABLE 6 



Results of Size Analysis Graphically Shown in Figure 80 

 Sample No.: P-64 Lithology: Clear quartz sand Formation: Dakota 



Date: Nov. 26, 1948 



Sample weight: 500 grams 



Analyzed by: W. Stuart 



Plotting of Data 



Three methods of graphic representation are followed in illustrating 

 size-analysis data, the histogram, the simple-frequency curve, and the 

 cumulative-frequency curve. In each, weight percentages of the various 

 grades are plotted against dimension with the former represented on the 

 vertical axis and the latter on the horizontal axis. 



Histogram Plot 



The histogram method requires cross-section paper, with the largest 

 grade size being placed on the left (figs. 78 and 80) . Each grade percent 

 is designated by rectangular blocks, the height of which represents the 

 percentage by weight of the respective grade. Histograms are useful for 

 rapid visual comparison. The shape of the diagram is controlled by the 

 number and percentage of grade fractions involved. For comparative work 

 a uniform scale should be employed. Another type of histogram involves 

 various percentage blocks laid end on end along the horizontal axis with 

 each grade block graphically symbolized (fig. 81). 



Simple-Frequency Curve 



The simple-frequency curve is commonly constructed on cross-sec- 

 tional paper (fig. 80). The weight percentage is plotted on the vertical 

 axis and the grade value on the horizontal. Points are connected by a 



