slight or too infrequent to support abundant vegetation. [Illustration: +Fig.+ 108.—Bad-land topography developed under conditions of aridity and unequal distribution of rainfall. Slope of Pinal Mountains, Ariz. (Ransome, U. S. Geol. Surv.)] During its fall, and immediately after, rain is more effective than an equal amount of snow; but the snow may be accumulated through a considerable period of the year, and then melted rapidly, when it has an effect comparable to that which would be produced by the concentration of the rainfall into a limited period of the year. If the ground beneath be frozen when the snow melts (and this is often the case) the erosion accomplished by the resulting water will be diminished. Except in dry regions, where wind-work sometimes exceeds water-work, the movements of the atmosphere are of less importance directly than precipitation in determining the rate of erosion. But even in regions which are not arid the winds have much to do with the rate of evaporation and the distribution of rainfall, so that their indirect effect is great. Even their direct effects in moist climates are not to be lost sight of, for even here the surface is sometimes dry enough to yield dust and sand, and the uprooting of trees so disturbs the surface as to make earthy débris more accessible to wind and water. Where trees gain precarious footholds on steep slopes, as they often do, they are likely to be overturned as soon as they are large enough to offer considerable resistance to the wind, and in the overturning, large quantities of rock are sometimes loosened and carried down the slope by gravity. This phase of destructive work is seen at its best on the walls of gorges, where trees often flourish until their tops project above the rim of the valley. Through vegetation, climate influences erosion in ways which are easily defined qualitatively, but not quantitatively. Both by its growth (wedge-work of roots) and by its decay (supplying CO₂, etc., to descending waters) it favors certain phases of weathering; but, on the other hand, it retards corrasion and transportation both by wind and water. This is well shown along the banks of streams and on the faces of cliffs, in clay, sand, etc. Its aggregate effect is probably unfavorable to erosion by mechanical means, and favorable to that by chemical processes. [Illustration: +Fig.+ 109.—Characteristic cliffs of high arid regions. Right wall of Snake River canyon, nearly opposite the mouth of Salmon River, Id. Two spring-formed coves, with “Castle Rock” between. (Russell, U. S. Geol. Surv.)]