42 GEOLOGY. 



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 fre- 

 quently appears on brick walls during the dry days which follow a 

 drenching rain. The water penetrates the brick and mortar and dis- 

 solves 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 Avaters dissolve whatever is soluble, and 

 when the}^ 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 wa}^, and not infrequently 

 cemented into conglomerate. One of the commonest mineral substances 

 found in such situations is lime carbonate. In some cases it was doubt- 

 less 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 haA^e been deriA^ed from limestone. The 

 faces of chffs of granite or gneiss, hundreds and even thousands of 

 feet above all other sorts of rock,^ 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 Carhonation 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 generafly involves the disintegration 

 of the rock concerned. Its effects in this direction will be referred 

 to in other connections. 



* For example, in the Big Horn Mountains of Wyoming. 



