The wear effected in this way is not proportional to the number of blows struck. Since every pebble and every grain of sand carried diminishes the velocity of a stream, and since with diminished velocity the force of the blows struck is diminished, it follows that the blows may become so weak, as the result of their multiplication, as to be ineffective. The larger the load, therefore, which the stream carries, the more the tools with which it has to work, but the less effectively can it use them; and the load may be so far increased as to destroy its corrasive power altogether. On the other hand, the smaller the load of the stream the greater its velocity and the more effectively will its tools be used; but their number may be so far reduced that their aggregate effect is slight. To accomplish the greatest results on a bed of solid rock a stream must have tools to work with, but must not be so heavily burdened as to interfere with its effective use of them. Whatever the cause of their unequal velocities swift and slow streams corrade their valleys differently. The erosion of a swift stream is chiefly at the bottom of its channel. The sluggish stream lowers its channel less rapidly, while lateral erosion is relatively more important. The result is that slow streams increase the width of their valleys more than the depth, while swift streams increase the depth more than the width. It follows that slow streams develop flats, while swift ones do not. Not only is a slow stream more likely to have a flat, and therefore a better chance to meander, but it is more likely to take advantage of opportunities in this line, for a slow stream gets out of the way for such obstacles as it may encounter, while a swift stream is much more likely to get obstacles out of its way. Special phases of corrasion are introduced where waterfalls and other peculiarities dependent on inequalities of rock resistance occur. =Solution.=—In most cases the solution effected by a stream is much less important than its mechanical work. Only when conditions are unfavorable to the latter, is solution the chief factor in the excavation of a valley. This may be the case where a stream’s bed is over soluble rock, such as limestone, and where the stream is clear, or its gradient so low that its current is sluggish. The solvent power of water is not influenced by the presence of sediment, though the presence of sediment offers the water a greater surface on which to work. CONDITIONS AFFECTING THE RATE OF EROSION. In considering the rate of erosion, both the work of the stream in its valley and that of the general run-off are to be considered. The conditions which favor the most rapid erosion in a stream’s channel are not necessarily those which determine most rapid degradation in the basin outside of the valley. _The Influence of Declivity._