30 



AGRONOMY 



deposit formed by the decay of plants under water, and may 

 be seen in the process of formation along the shores of many 

 lakes and ponds. It also occurs in extensive deposits called 

 peat bogs, which mark the site of old lakes that have been 

 filled by such accumulations. When pure, peat contains 

 enough carbon to make it useful as fuel. Most of our coal 



beds have been 

 formed from peat. 

 ffumus, sometimes 

 called vegetable 

 mold or leaf mold, 

 is, like peat, formed 

 from plant remains, 

 but in this case the 

 decay has taken 

 place in or on the 

 soil. It is a black, 

 loose substance 

 usually abundant 

 on the forest floor 

 and ig an indispen- 



FIG. 15. 



Section of an abandoned quarry partly 

 tilled with water 



The dark deposit at the top of the exposure is peat, 



the light material below is marl, heneath this is the 



mantle rock which merges into the bed rock 



sable element in all 

 fertile soils. Muck 

 is a black deposit, 

 midway between 

 humus and peat, that occurs in swamps and low grounds. The 

 words " peat " and " muck " are often used rather loosely to 

 designate the same substance. Marl is a deposit of lime and 

 clay, like a whitish mud, which forms at the bottom of ponds. 

 The lime is derived from the decay of the shells and bones 

 of water animals. Marl is valued for adding to soils deficient 

 in lime. Clay is a soft powder or rock flour usually resulting 

 from the weathering of feldspar rocks. Clay consists of parti- 

 cles less than one five thousandth of a millimeter in diameter, 



