124 GEOLOGY. 



has tools to work with. Since high dechvity greatly increases both 

 the transporting and the corrasive power of running water, and favors 

 certain elements of weathering, it is clear that the aggregate effect 

 of high declivity is to favor erosion, whether in the channel of the 

 stream or on the general surface of its drainage basin. 



The Influence of Rock. 



The physical constitution, the chemical composition, and the stratig- 

 raphy of a rock formation, influence the rate at which it may be broken 

 up and carried away. Clastic or fragmental rocks are usually stratified 

 and made up of cemented pebbles (conglomerate), sand grains (sand- 

 stone), or particles of mud (shale). Igneous rocks, such as granite, 

 are massive instead of stratified, and are usually made up of great 

 numbers of interlocking crystals which bind one another together. 

 Some crystalline rocks, such as schists, though not stratified, possess 

 cleavage, v/hich has much the effect of stratification, so far as erosion 

 is concerned. All rocks are affected by systems of more or less nearly 

 vertical cracks called joints. All these structures have their influence 

 upon the rate of degradation. 



Physical constitution. — Clastic rocks may be firmly cemented, 

 or their constituents may be loosely bound together. The less the 

 coherence the more ready the disintegration, and the finer the particles 

 the more easily are they carried away. When the particles in transpor- 

 tation are angular they effect more wear on the bed over which they 

 move, and on one another, than when they are round. The difference 

 is great where the particles are large, and little where they are very 

 small. If the materials carried be harder than the bed over which 

 they pass, corrasion of the latter is favored. 



Chemical composition. — Something also depends on the chemical 

 composition of the rock, since this affects its solubility, and therefore 

 its rate of decomposition. The more soluble the rock the larger the 

 proportion of it which will be taken away in solution; but it does 

 not foUow that the most soluble rock will be most rapidly eroded, 

 since the rate of erosion depends on abrasion as well as solution, and 

 a rock which is readily soluble, as rocks go, may be less easily abraded 

 than a rock which is made of discrete and insoluble particles bound 

 together by a soluble cement. In such rocks, for example a sand- 

 stone in which the grains are cemented together by hme carbonate, 



