CLIMATE AND TERRESTRIAL DEPOSITS 367 



in the same place will only be moved a few centimeters and therefore remains 

 behind. Such unceasingly repeated shoving must necessarily lead to a classifi- 

 cation along the stream course of the rolled material according to its size. So 

 soon as the volume of a stream is altered through climatic change or tectonic move- 

 ment the boundaries are immediately shifted to which large or small pebbles, 

 sand, or clay are carried and laid down.^ 



Chamberlin and Salisbury^ state that: 



Under certain circumstances, a stream may overload itself. Thus if a stream 

 loaded with coarse detritus reaches a portion of its valley where fine material is 

 accessible in abundance, some of the velocity which is helping to carry the coarse 

 may be used in picking up and carrying the fine. This reduces the velocity and 

 since the stream already had all the coarse material it could carry, reduction of 

 velocity must result in deposition. It follows that when a stream fully loaded 

 with coarse material picks up fine, it becomes overloaded, so far as the coarse 

 material is concerned. 



DISCUSSION OF THE DATA REGARDING RIVER TRANSPORTATION 



Preliminary inductions. — From the preceding experiments, obser- 

 vations, and well-founded conclusions on the characteristics of river 

 action the following inductions may be drawn. 



First.^ the coarse material is not usually sufi&ciently strong or hard 

 to stand long transportation by rivers, quartz, chert, or quartzite 

 most effectively resisting the wear and alone being able to withstand 

 transportation for distances of a hundred miles or more. It has 

 sometimes been stated that the composition of a conglomerate of 

 vein-quartz and quartzite pebbles indicates the erosion of a deeply 

 and thoroughly decayed regolith, all but these constituents having 

 been previously destroyed by weathering in situ. It is seen, however, 

 that such a hypothesis is not necessary, since, if rivers existed suffi- 

 ciently strong in current to sweep pebbles long distances, the quartz 

 and quartzite would alone survive. 



Second, rivers normally carry a large amount of finer material 

 and a small amount of coarser, due to the disintegration and decompo- 

 sition of the rock before it reaches the river, as well as the inherent 

 weakness of the partially decomposed material which does reach the 

 river in fragments. This loading up with fine material results in a 

 lagging behind of the coarser more than if the fine was not present. 



1 op. cit., pp. 644, 645. 



2 Geology (1904), Vol. I, pp. 169, 170. 



