TEANSPORTATION OF DEBRIS. 



2)7 



the water is sufficient to move it when subject to no other resistance than 

 its own weight* 



Thus whatever a stream receives it carries along, whether it be water 

 or solid rock. Certainly much of the matter rolled into it is in the form of 

 coarse fragments, but it urges them onwards, grinding thein to silt as they 

 move. Nothing which it receives does it retain, except in places here and 

 there where its current is suddenly checked, and here for a time coarser 

 materials accumulate. But in the secular life of the river even these local 

 accumulations may in turn be removed by subsequent changes of relative 

 level along different portions of its course. 



The distance which a fragment may ultimately travel is independent of 

 its original size. Large stones, being moved with difficulty, are detained at 

 numerous halting places and subjected to long attrition until they are suffi- 

 ciently reduced to be within the power of the current, and at length become 

 no bigger than those which were originally smaller. In truth, all frag- 

 ments, in a certain sense, travel the same distance ultimately, for they all 

 pass the mouth of the river in the form of silt and dissolved constituents. 

 Viewed in another aspect, however, the size of the fragment determines in 

 a general way its amount of progress. The larger ones have at any given 

 stage moved a shorter distance and the smaller ones a greater distance — 

 on the average 



The action of a current upon rocky fragments, then, is to sweep them 

 along and to grind them to powder as it sweeps. It never accumulates 

 them except in a limited way and under circumstances which will be here- 

 after described at some length. Whether the detritus which a river dis- 

 charges shall be in the form of pebbles, gravel, or silt, depends upon the 

 length of the stream and the power of its current. A long stream with a 

 low .slope and sluggish current along its lower course, but with more rapid 

 tributaries above, will have dissipated its fragments and discharge nothing 

 but silt. A short stream with a rapid descent may readily discharge coarse 



•Where a sudden retardation of the velocity of a stream occurs, as by the snddeu widening or 

 deepening of a channel, and where this change predominates over all other changes from maxima to 

 minima, there will occnr a persistent accumulation of coaracr debris without any great admixture of 

 finer. » • • Concerning the power of water to move pebbles, it will be merely necessary to refer 

 to Dr. Hopkins's well-known theorem. 



