5o8 CHESTER K. WENTWORTH 



(3) the number of such associated cobbles; (4) its angularity; (5) the 

 violence of the motion; (6) the kind of rock (this factor might 

 be subdivided into elasticity, hardness, etc.). The change of shape 

 depends on: (7) distance traveled; (8) kind of rock; (9) violence 

 of motion; (10) kind of motion; (11) size of cobble; (12) size 

 of associated cobbles. The change of surface texture depends 

 on: (13) kind of rock; (14) violence of motion; (15) kind of 

 motion; (16) size of cobble; (17) size of associated cobbles. 



APPARATUS 



The apparatus employed consists of two metal drums lined with 

 soft wood, 25 inches in internal diameter and 13 inches long. These 



^^ 



Metal ^^ 



Fig. I. — Diagram showing in cross-section relations of cobbles to water and to 

 lining of drum while in motion. 



are situated at the two ends of the supporting axis and, for all the 

 work to date, have been driven at 27 revolutions per minute. 

 The drums are kept flushed with water so that the finer detritus up 

 to the size of coarse sand is carried out and collects in the settling 

 tubs. 



