Think of the hottest day in August you ever saw, and then multiply by two. That will give you an idea of how hot a desert gets in the day-time--something like 200 degrees; and 212 degrees boils eggs, you know! But how cold do you suppose it gets at night? Fifteen minutes after sunset the temperature drops to freezing. The reason of this is that there are no clouds over the desert to keep the heat of the sand wastes and the burning rocks from passing off rapidly into space. The days are so hot and the nights are so cold that the rocks get a kind of fever and ague, which makes them pull themselves to pieces. THE "GOOSE-FLESH" ON THE ROCKS It is the same process we have just read about in the story of the stones of our fields, only it goes on much faster in the desert on account of the more rapid changes of temperature. You know how your skin will pucker up into goose-flesh when you are cold. The desert rocks do something similar. Because rock is a poor conductor, the heat of the day and the cold of the night penetrate only a little way--only through the skin of the rock, as it were; so this skin, stretching in the day-time and puckering up at night, becomes loosened and shells off bit by bit. Then it is blown about and in time ground into sand by the desert winds. Some rocks have an additional way of getting picked to pieces. Granite is one of these. It has several different kinds of mineral in it, and some of these minerals contract and expand faster than others; some more than others. As a consequence, the particles of the rock keep pulling and hauling at each other. This helps to break it up into little pieces, which soon become sand. The darker the rock, other things being equal, the greater the changes, because anything dark--a suit of clothes, for instance--absorbs heat faster than a light object. [Illustration: _From Norton's "Elements of Geology." By permission of Ginn and Company_ HOW RAIN-DROPS HELP SPLIT BOULDERS A big boulder in western Texas split, just as you see it here, by rain-drops, with the help of the sun, and under the conditions described in the text, sat for this photograph. A friend of mine who has been all over that country says that on blistering-hot days you can see little pieces pop out of the granite boulders, like chips from an invisible chisel struck by an invisible hammer. This is why: We Granites are made up of particles--little bits--of several different minerals, and some of these minerals expanding much faster than others pop themselves out. ] The great mountain rocks of the desert, bare of all protecting soil and verdure, are always crumbling as a result of all these causes, and so the winds are constantly blowing them away, piece by piece. HOW LITTLE RAIN-DROPS SPLIT BIG BOULDERS