THE SOIL. 31 



So, if lime is present in an unusually large amount, 

 tlie soil may be spoken of as limey or calcareous. 



The decayed vegetable matter in soils is called hu- 

 mus. If this is in large supply the soil is called peat, 

 swamp muck, or more rarely, vegetable mold. 



Formation and Distribution of l^oils.— 

 All the mineral matter in soil was .once solid rock. 

 The breaking up or disintegration of rocks and the 

 addition of vegetable matter has formed the soil as 

 it now exists. Not only are fragments of rock found 

 in many soils by even careless inspection ; not only 

 does chemical analysis show that soil is composed of 

 the same elements as those which are found in rosks, 

 but the process of soil formation may be seen and 

 studied in many places. It is said that in less than 

 100 years lava rock thrown from volcanoes, in such 

 quantity as to thickly cpver the mountain sides or the 

 valleys below, has been changed into fertile soil. 



Air and water have been the chief agents in soil 

 formation; not pure air and pure water, but air and 

 water as they are ordinarily found in nature. In 

 the air there 13 always a very small percentage of car- 

 bonic acid. This is dissolved in or absorbed by 

 water and has a powerful effect in causing rocks to 

 disiategrate into sand or clay. It works slowly but 

 Burely. The oxygen of the air also unites with the 

 iron or manganese compounds in the rocks, thus hast- 

 eniag the breaking up of the rocks. 



Water has even a greater influence on rocks than 

 has the air. Pure water has a considerable power of 

 dissolving mineral compounds. This power is much 

 greater when the water contains carbonic acid. The 

 freezing of water aids greatly in this work. Water 



