X6 ENGINEERING FOR LAND DRAINAGE. 



valleys are thus deposited the gravel, sand, and rock 

 dust detached from the slopes of the neighboring 

 mountains. Lakes and gulfs become filled with silt 

 brought into them by streams. Alluvium is found be- 

 low as well as above the drift, and recent alluvium in 

 the drift region is very often composed of drift material 

 rearranged by water currents. 



Organic Matter of tlie Soil, — As before noted, vege- 

 tation plays an important part in the conversion of 

 rocks into soil. The lower orders of plants, such as 

 the lichens and mosses, prepare the way for grasses and 

 forests. The decay of vegetation adds to the soil a 

 brown or black friable substance commonly known as 

 humus which gives off gases and aids in the further 

 conversion of inert material into productive soil. The 

 gases such as carbonic acid and ammonia are largely 

 held in the soil, their volume depending upon the quan- 

 tity of vegetable matter which the soil contains, and 

 the supervention of warm wet weather. 



The proportion of carbonic acid contained in the 

 pores of different kinds of soil compared with that found 

 in the ordinary atmosphere is strikingly shown in the 

 following analysis made by Boussingault and Lewy. 



CARBONIC ACID IN 10,000 PARTS OF AIR BY WEIGHT. 



Ordinary atmosphere 6 



Air from sandy subsoil of forest . 38 



'' '' loamy ^' ^^ ** ...124 



*' '* surface soil ** ** 130 



Air from surface soil of vineyard 146 



'' *' ** '' old asparagus bed.. . 122 



*^ ** '* '* newly manured land. 23^ 



'* ** i< << pasture land ....... 270 



rich in humus 543 



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