724 THE JOURNAL OF GEOLOGY. 



able also to carry large blocks of rock, and preferably hard rock, 

 great distances, at the same time that they frequently carried a 

 large proportion of the fine materials deposited at any given point, 

 trivial distances only. 



Size of bowlders in relation to distance from their source. Bowl- 

 ders of a given type can sometimes be referred to a single 

 source of very limited area. In such cases, it has been 

 repeatedly noticed that the average size of the bowlders becomes 

 less with increasing distance from their source, though individ- 

 ual exceptions to this rule are sometimes striking. This might 

 be accounted for, conceivably, on either of two suppositions. 

 Either the drift forces were not able to move the large bowlders 

 so far as the smaller ones, or their size was reduced in the proc- 

 ess of movement. If the latter conjecture be the correct one, 

 the smaller bowlders, which are farther from their source, should 

 show evidence of greater wear than tb.e larger ones, which have 

 not been carried so far. This might also be the case if the first 

 supposition be the true one. For even if the drift agents could 

 carry only the smaller bowlders considerable distances, these 

 smaller bowlders might be worn more in their longer journey, 

 than the larger ones, in their shorter. Between the two hypotheses 

 the greater wear of the smaller bowlders might riot therefore be 

 decisive. But the extreme physical heterogeneity of the drift 

 clearly indicates that the drift agents were, on the whole, very 

 independent of the size of the materials handled. Bowlders 

 many tons in weight were sometimes carried scores of' miles. 

 In view of this, the conclusion that the decrease in size of bowl- 

 ers with increasing distance from their source is the result of 

 wear suffered during the transit, seems the more probable. If 

 this conclusion be correct, the drift force or forces must have 

 been able to wear effectively the materials carried, during the 

 process of their movement. 



Rollin D. Salisbury. 



[To be continued.] 



