514 



CHESTER K. WENTWORTH 



deviation of convexities from a mean convexity. Each of these 

 values or coefficients of roundness reaches a minimum in the case 

 of a sphere and has a maximum in the case of a Hne or a plane 

 without volume. 



Considerable computing and some experimenting was done to 

 test out these methods for the definition of roundness. None was 

 found entirely satisfactory, especially for field use. In order to 



290 



3fO 



.740 



.380 



Fig. 7. — Profiles of cobbles showing differing degrees of roundness and giving 

 values of R. 



be of use in the field study of a deposit, the method of measure- 

 ment should be simple and rapid enough to permit the calibration 

 of one or two hundred cobbles in a day's time and to allow using 

 the average of the results to characterize the deposit. In (i) the 

 measurement of surface area with sufficient accuracy, while not 

 impossible, was too laborious to use with more than one or two 

 specimens. Likewise in (2) and (3) the need for many measure- 

 ments of diameters and convexities rendered them liable to the 

 same objection. The method of plotting different kinds of diame- 

 ters used in the study of cube rounding reported below is of no 

 value for various general shapes. As a practical solution of the 



