Si8 



CHESTER K. WENTWORTH 



Fig. 23. — Stage 16, 263 miles, R= .823 



an ellipsoid of equilibrium for wear by abrasion toward which the 

 cobble approaches, which depends on the hardness, or more, cor- 

 rectly, durability along 

 various axes. 



For a perfectly 

 homogeneous rock this 

 ellipsoid is a sphere; 

 for a non-homogeneous 

 rock it is an ellipsoid 

 of greater or less eccen- 

 tricity as the case may 

 be and has a greater 

 or less value for R in 

 the notation here used. 

 Further, and this is 

 the essential point, this 

 ellipsoid of equihbrium 

 is of different eccen- 

 tricity for different 

 sizes, more eccentric 

 for smaller sizes than 

 for larger. Thus it is 

 conceived that at 3 . 29 

 grams ' the marble 

 cobbles had nearly or 

 quite reached the ellip- 

 soid of durability for 

 that size and from then 

 on were practically fol- 

 lowing the equilibrium 

 figure in its decreasing 

 values of R as the sizes 

 grew less. This would 

 mean that slight varia- 

 tions in durability 

 between different direc- 

 FiG. 25.— Stage 18, 307 miles, i2=. 797 tions come out more 



Fig. 24. — Stage 17, 286 miles, R = 



