=The mechanical breakdown of the desert rocks.=—The chemical changes of decomposition within desert rocks are, as we have seen, largely due to the action of concentrated solutions of salts at high temperatures. That there is a certain mechanical rending of these rocks, due to the “freezing” of salts within the capillary fissures, has been already mentioned. A further strain effect arises in rocks like granite, which are a mixture of different minerals. Heated to a high temperature during the day and cooled through a considerable range at night, the different minerals alternately expand and contract at different rates and by different relative amounts, so that strains are set up, tending to tear them apart. The effect of these strains is thus a surface crumbling of rocks. But rock is, as already pointed out, a relatively poor conductor of heat, and hence it is a relatively thin skin only which passes through the daily round of wide temperature range. This outer shell when heated is expanded, and so tends to peel off, or exfoliate, like the outer skin of an onion. The process is therefore described as _exfoliation_. In all rocks of homogeneous texture the continued action of this process results in convexly spherical surfaces, the material scaled off in the process remaining as a slope or talus which surrounds the projecting knob (Fig. 210). Naked, these projecting domes rise above the rim of débris at their bases. Not a particle of dust adheres to the fresh rock surface—no dirt interferes with its glaring whiteness. Yet close at hand lie masses of débris into which wells may be carried to depths of more than six hundred feet without encountering either solid rock or ground water. The bare walls of granite sometimes mount upwards for thousands of feet into the air, as steep and as inaccessible as the squared towers of the Tyrolean Dolomites. [Illustration: FIG. 210.—Smooth granite domes shaped by exfoliation and surrounded by a rim of talus. Gebel Karsala, Nubian Desert (after Walther).] Rock is such a poor conductor of heat that special strains are set up at the margin of sunlight and shade. This localization of the disintegration on the margin of the shaded portions of rock masses is known as _shadow weathering_ (see Fig. 215, p. 206).