144 GEOLOGY 



it is prepared tor transportation. Weathering is the first step 

 in most erosion, but it is not the only one, and erosion may take 

 place without it. 



Transportation 



A second element of erosion is transportation. The Iran- 

 tation of sediment is to be distinguished from the transportation 

 of materials in solution. In so far as mineral matter is dissolved. 

 it becomes a part of the fluid of the stream. If the quantity 

 dissolved were large, it might influence the mobility of the water; 

 but the amount is usually too slight to influence flow sensibly. 



The sediment transported by a stream is either rolled along its 

 bottom, or carried in suspension above the bottom. The coarser 

 materials (gravel and sand) are carried chiefly in the former posi- 

 tion, and the finer (silt and mud) largely in the latter. 



Transporting power and velocity. The transporting pow 

 running water depends on its velocity. Swift streams have enor- 

 mously greater power of transportation than sluggish ones, but 

 transportation does not always keep pace with transporting power. 

 The Niagara at its rapids is a stream of great transporting power, \ 

 but it carries little sediment, because there is little to be had. 



The velocity of a stream depends chiefly on three elemen 

 its gradient, its volume, and its load. The higher the gradient,: 

 the greater the volume, and the less the load, the greater the 

 velocity. The relation between gradient and velocity is evident:. 

 that between volume and velocity is illustrated by every st 

 in time of flood, when its flow is greatly accelerated. The relation 

 between velocity and load is less obvious, but none the less definite 

 Every particle of sediment carried by a stream makes a draught 

 on its energy, and energy expended in this way reduces the velocity. 

 A muddy stream is never so swift as a clear stream of the samcj 

 size would be, flowing in the same channel. 



How sediment is carried. Coarse materials, such as gravel 

 stones, are rolled along the bottom of the swift streams which earn 

 them. Their movement is by the impact of the water. The samd 

 is true to a large extent of sand grains. So far as concerns tin 

 material rolled along the bottom, it is to be noted that a stream' 1 

 transporting power is dependent on the velocity of the water ai< 



