136 Subsurface Geologic Methods 



The three lines of four grains each in figure 55 are representative of 

 three roundness classes that the writer has used for rapid roundness studies. 

 Roundness varies with grain size, and consequently roundness measure- 

 ments must be made on grains of the same size. Because differences and 

 similarities in roundness have been found to be most significant for the 

 very fine sand size, that size is recommended for initial study. 



Roundness of detrital mineral grains must be measured on the origi- 

 nal grains. In many indurated or partly indurated sandstones the original 

 shape of the quartz, feldspar, and carbonate grains has been modified by 

 the deposition of quartz, feldspar, or carbonate on them. Consequently, 

 the present shape and roundness of such grains have no significance. 

 When the original grain outlines can be recognized in these sections, the 



Figure 55. Representative round, subangular, and angular grains. 



roundness can be determined. When an indurated or partly indurated rock 

 is crushed, the heavy minerals, being of different composition, tend to 

 break out of the rock along their original boundaries. Consequently, 

 heavy minerals can be used for roundness measurements in many casRS 

 in which the major constituents of the rock are badly broken. 



The number of grains on which roundness, measurements should be 

 made depends on the difference in roundness in the samples being studied. 

 If the differences are large, fewer grains need be measured than if the dif- 

 ferences are small. Probably a minimum of 25 to 50 grains should be 

 measured in any case. The errors due to the size of the final sample can be 

 determined from figure 56. Roundness studies are subject to the same types 

 of errors as are heavy-mineral analyses. 



When quantitative measurements are made on a number of samples 

 of a formation and the results are plotted on a triangular diagram, the 



