The Matterhorn of the Alps is a famous example of a mountain peak whose carving by the frost and other agents is in active progress. On its face "scarcely a rock anywhere is firmly attached," and the fall of loosened stones is incessant. Mountain climbers who have camped at its base tell how huge rocks from time to time come leaping down its precipices, followed by trains of dislodged smaller fragments and rock dust; and how at night one may trace the course of the bowlders by the sparks which they strike from the mountain walls. Mount Assiniboine, Canada (Fig. 20), resembles the Matterhorn in form and has been carved by the same agencies. "The Needles" of Arizona are examples of sharp mountain peaks in a warm arid region sculptured chiefly by temperature changes. Chemical decay, especially when carried on beneath a cover of waste and vegetation, favors the production of rounded knobs and dome-shaped mountains. =The weather curve.= We have seen that weathering reduces the angular block quarried by the frost to a rounded bowlder by chipping off its corners and smoothing away its edges. In much the same way weathering at last reduces to rounded hills the earth blocks cut by streams or formed in any other way. High mountains may at first be sculptured by the weather to savage peaks (Fig. 181), but toward the end of their life history they wear down to rounded hills (Fig. 182). The weather curve, which may be seen on the summits of low hills (Fig. 21), is convex upward. [Illustration: Fig. 20. Mount Assiniboine, Canada] [Illustration: Fig. 21. Big Round Top and Little Round Top, Gettysburg, Pennsylvania] In Figure 22, representing a cubic block of stone whose faces are a yard square, how many square feet of surface are exposed to the weather by a cubic foot at a corner _a_; by one situated in the middle of an edge _b_; by one in the center of a side _c_? How much faster will _a_ and _b_ weather than _c_, and what will be the effect on the shape of the block? [Illustration: Fig. 22] =The coöperation of various agencies in rock sculpture.= For the sake of clearness it is necessary to describe the work of each geological agent separately. We must not forget, however, that in Nature no agent works independently and alone; that every result is the outcome of a long chain of causes. Thus, in order that the mountain peak may be carved by the agents of disintegration, the waste must be rapidly removed,--a work done by many agents, including some which we are yet to study; and in order that the waste may be removed as fast as formed, the region must first have been raised well above the level of the sea, so that the agents of transportation could do their work effectively. The sculpture of the rocks is accomplished only by the coöperation of many forces.