U. S. Geol. Surv. Fig. 4. NEBRASKA. U. S. Geol. Surv.] Sand drifted over loose stones lying on the surface often develops flat or flattish faces or facets on them. These facets are likely to be three in number, and the exposed portion of the stone is likely to develop a sort of pyramidal shape, the three flattish surfaces being mutually limited by tolerably well-defined lines (Fig. 24). Thus arise the three-faceted stones (_Dreikanter_ of the Germans) commonly seen where sands have been long in movement. [Illustration: +Fig.+ 22.—Wind-carved hillock of cross-bedded sandstone. Missouri River, Montana. (Calhoun, U. S. G. S.)] Not only does the drifting sand wear the surface over which it passes and against which it strikes, but the grains themselves are worn in the process. They are liable to be broken as they strike rock surfaces, and they are likely to strike one another in the atmosphere. In both cases they are subject to wear, and so to reduction to a finer and finer state. The erosion accomplished by the wind is therefore of various sorts. The impact of the wind itself picks up the fine materials which are already loosened, thus wearing down the surface from which they are removed; the materials picked up wear the rock surfaces against which they are blown, and the transported materials themselves suffer reduction in transit. =Effects of wind on plants.=—Another effect of wind work is seen in the uprooting of trees (Fig. 25). The uprooting disturbs the surface in such a way as to make loose earth more readily accessible to wind and water. The uprooting of trees on steep slopes often causes the descent of considerable quantities of loose rock and soil. Again, organisms of various sorts (certain types of seeds, germs, etc.), as well as dust and sand, are extensively transported by the wind. While this is important biologically its geological effects are remote. [Illustration: +Fig.+ 23.—Figure showing details of wind-carving on rock surface (rhyolite). Mono Valley, California.] [Illustration: +Fig.+ 24.—Wind-worn stones (_Dreikanter_).] [Illustration: +Fig.+ 25.—Shows the disturbance of surface earth and rocks by upturning of trees. (Darton, U. S. Geol. Surv.)] =Indirect effects of the wind.=—Other dynamic processes are called into being or stimulated by the atmosphere. Winds generate both waves and currents, and both are effective agents in geological work. The results of their activities are discussed elsewhere. _II. The Chemical Work of the Atmosphere._ The chemical work of the atmosphere (including solution and precipitation from solution) is principally accomplished in connection with water, a dry atmosphere having relatively little direct chemical effect on rock or soils.