A STUDY OF COBBLE ABRASION 



517 



Fig. 20. — Stage 13, 209 miles, R= .862 



problem, I have used the ratio of the radius of curvature of the 

 most convex part of the surface to half of the longest diameter 

 through that point. 

 This radius is meas- 

 ured with a gauge such . 

 as shown in Figure 6 

 and the diameter with 

 a caliber or usually 

 with a scale with suffi- 

 cient accuracy. 



Figure 7 shows in 

 profile various cobbles 

 with their correspond- 

 ing values of R. 



Figures 8-25 show 

 the history of four 

 white marble cubes 

 with the corresponding 

 values of R. It will be 

 noted that the four 

 pieces reached a maxi- 

 mum roundness of .862 

 at an average weight of 

 3.29 grams and then 

 systematically became 

 somewhat less round 

 as they decreased in 

 size. 



Figure 26 is a graph 

 of the roundness as a 

 function of the distance 

 traveled. For this 

 curious increase in an- 

 gularity with travel I 

 suggest the following 

 interpretation: For 

 any given rock there is 



2 



Fig. 21. — Stage 14, 223 miles, R= .838 



Fig. 22. — Stage 15, 244 miles, R = .826 



