Subsurface Laboratory Methods 



137 



data commonly will occupy a restricted part of the diagram. Other forma- 

 tions that differ in grain roundness will be restricted to other parts of the 

 diagram. Then the identity of any unknown sample can be determined. 

 The roundness of tourmalines of very fine-sand size in three formations 

 in Ohio is shown in figure 57. 



The rounding of tourmaline, quartz, and other hard minerals of very 



Figure 56. Curves for determining probable errors in heavy-mineral, shape, and 

 roundness studies. The probable errors are expressed as percent of the total 

 number of all grains; for example, with 20 percent frequency and 50 grains 

 counted, the probable error is 3.8 percent.) 



fine-sand size appears to be very slow. In the Appalachian Basin the 

 sands have the same roundness over thousands of square miles. There is 

 no observable gradation of the type that would indicate progressive change 

 in roundness due to wear as the sediment was transported along a river 

 or a beach. Along certain lines the roundness changes abruptly. The 



