512 



CHESTER K. WENTWORTH 



variation in rocks of different sorts. The curve of size history would 

 therefore be notably different in the scale along the abscissa, though 

 the form would be similar for different rocks. 



2. Relation of wear to size of associated cobbles. — Not yet studied. 



Fig. 5. — Effect of amount of mixture in compartment of drum 



3. Relation of wear to number of associated cobbles. — That the 

 total amount of material in a compartment of the drum was a 

 very critical factor in the rate of wear was recognized from the first, 

 and to aid in interpreting and correcting for unavoidable variations 

 in the amount, series of runs were made with varying numbers of 

 cobbles in the drum and measurements made on a number of test 

 pieces. The results are shown in Figure 5. 



