=Precipitation from solution.=—The water in the soil is constantly evaporating. Such substances as it contains in solution are deposited where the water evaporates, and where evaporation is long continued without re-solution of the substances deposited, the surface becomes coated with an efflorescence of mineral matter. Conspicuous examples are found in the alkali plains of certain areas in the western part of the United States. Since the alkaline efflorescence is the result of evaporation it is connected with the atmosphere, but the material of the efflorescence was brought to its present position by water. The principle involved is illustrated by the white efflorescence which frequently appears on brick walls during the dry days which follow a drenching rain. The water penetrates the brick and mortar and dissolves something of their substance, and when it is evaporated from the surface the material in solution is left behind. In arid regions the deposition of substances other than alkali is common. The percolating waters dissolve whatever is soluble, and when they evaporate their mineral content is left. The pebbles and stones of the arid plains have in many places become heavily coated with mineral matter deposited in this way, and not infrequently cemented into conglomerate. One of the commonest mineral substances found in such situations is lime carbonate. In some cases it was doubtless derived by solution from limestone beds beneath the surface, but this is not always the case. It often encrusts the bits of lava on lava plains where it can hardly have been derived from limestone. The faces of cliffs of granite or gneiss, hundreds and even thousands of feet above all other sorts of rock,[19] are sometimes spotted with patches of lime carbonate. In the first case the lime carbonate was derived by chemical change from the lava, and in the second, from the granite or gneiss (see _Carbonation_ below), but its present position is the result of evaporation. =Oxidation.=—In the presence of moisture the oxygen of the air enters into combination with various elements of the soil and rocks. This is _oxidation_. No other common mineral substance shows the results of oxidation so quickly and so distinctly as iron. The oxidized portion is loose and friable, and a mass of iron exposed to a moist atmosphere will ultimately crumble away. This change is comparable to other less obvious changes taking place in many minerals at and below the surface. Oxidation generally involves the disintegration of the rock concerned. Its effects in this direction will be referred to in other connections.