190 



Subsurface Geologic Methods 



smooth curved line. This plot is a more accurate representation of sedi- 

 ment-grade relationships than is shown by the histogram. 



Cumulative-Frequency Curve 



The cumulative-frequency curve is constructed as follows (fig. 79) : 

 The weight percentage of the sample retained on the coarsest sieve is 

 plotted first, then the percentage by weight retained on the coarsest sieve 

 plus that retained on the next finer sieve, etc. Each point represents the 

 total weight percentage of material that would be retained if only the 

 sieve represented by that particular point were used in the analysis. 



The cumulative-frequency curve is practically independent of the 



80 « 

 75 5 



o o o o o 



Q Q O Q O. 



Diometer In mm. 



Figure 79. Cumulative-frequency curves of several sandstones of Rocky Mountain 

 region; note variation of slope and coefficient of sorting. 



grade scale used and is thus a more reliable index of the nature of the 

 particle distribution in sediments than a histogram or simple-frequency 

 curve. 



Two-, three-, or four-cycle semilogarithmic paper is used in plotting. 

 Data from numerous samples may be plotted on the same base to permit 

 direct comparisons (fig. 79). 

 Computations 



Krumbein "^^ comments that 



Some workers have used cumulative curves in a purely descriptive manner, 

 similar to the use of histograms. That is, the slopes of the curves, their spread, 



"^ Krumbein, W. C, Graphic Presentation and Statistical Analysis of Sedimentary Data in Recent 

 Marine Sediments, p. 564, Am. Assoc. Petroleum Geologists, 1939. 



