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Show TEANSPOKTATION OF DEBEIS. 217 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 them 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 sufficiently reduced to be within the power of the current, and at length become no bigger than those which were originally smaller. In truth, all fragments, 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 distanceâ€" 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 hereafter described at some length. Whether the detritus which a river discharges 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 sudden widening or deepening of a channel, and where this change predominates over all other changes from maxima to minima, there will occur a persistent accumulation of coarser 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. |