114 A MANUAL OF TOPOGRAPHIC METHODS. 



with steep slopes on both sides. The mouths of the secondary streams are 

 rapidly lowered, and thereby their work is greatly increased. 



There is therefore a distinction to be observed between superficial 

 erosion or erosion by the petty rain-water streams on the one hand and 

 that by the larger streams on the other. The first forms, as a rule, convex 

 slopes; the last, concave slopes. Between them, however, no sharp line can 

 be drawn. In general, the former erodes soil only, the soft superficial bed, 

 while the latter, if swift, is at work chiefly on rock. The energy of the 

 former is widely dispersed, that of the latter is concentrated. The general 

 reduction of the surface is done by the former, while the latter is confined 

 to deepening narrow stream beds. Where the main streams are near base 

 level, superficial erosion goes on more rapidly than stream corrasion, since 

 the slope and velocity of the streams are near a minimum. Where the 

 streams are still corrading rapidly, their beds are usually lowered faster 

 than the terrain, and the balance is more and more on the side of the 

 streams, the greater the range of elevation. In a mountain region, as has 

 just been stated, the gorges are cut far below the spurs and summits. 

 Hence, where stream corrasion predominates over surface erosion, the con- 

 cave curve predominates, and where surface erosion is more rapid than cor- 

 rasion by the streams, the convex curve is the ruling one. 



In an arid region, where the rain-fall is not only scanty, but spasmodic 

 in character, coming mainly in sudden showers of great volume, but short 

 duration, the stream beds are few in number. The drainage system is 

 scanty and imperfectly developed. The weathering of rocks goes on slowly, 

 and consequently the soil bed is thin. The soft material which the 

 streamlets can erode is not abundant. Consequently the scanty rains do 

 little surface erosion, but as they collect in large volume in the few water 

 courses, they deepen them at a rapid rate. Erosion of the terrain in an arid 

 region is therefore slow, while stream corrasion is proportionally rapid. 



It is frequentlv the case that streams collect their waters from high 

 mountains, and on their way to the sea pass down through arid regions. 

 The action of such streams upon the arid region is the same as above 

 described from streams originating within this region, except that it is more 

 intense. Little or none of the waters of such a stream flows over the ter- 



