GEOLOGICAL STUDY OF SOILS 65 



a part or nearly all of the soda. If the mineral contains 

 soda as the alkaline constituent, it is known as albite, 

 or if mainly potash, it is called potash feldspar or 

 orthoclase. 



The members of the feldspar group are insoluble in 

 acids, and before disintegration takes place are not 

 capable of supplying plant food. Potash feldspar con- 

 tains from 12 to 15 per cent of potash, none of which is 

 of value as plant food until disintegrated. When feld- 

 spar undergoes disintegration, it produces kaolin or clay. 

 A soil formed from feldspar is usually well stocked with 

 potash. Feldspar containing lime readily yields to the 

 solvent action of water in which there is carbon dioxide. 



Orthoclase, AlKSi 3 O 8 Potash feldspar 



Albite, AlNaSi 3 O 8 Sodium feldspar 



69. Hornblende. The hornblende and augite groups 

 are formed by the union of magnesium, calcium, iron, 

 and manganese, with silica. As a rule none of the 

 members of the alkali family are present in hornblende. 

 The augites are double silicates of iron, manganese, cal- 

 cium, and magnesium. Quite frequently, phosphoric 

 acid is in chemical combination with the iron. The 

 members of this groyp are readily distinguished by 

 their color, which is black, brown, or brownish green. 

 The hornblendes which contain lime are quite readily 

 decomposed when subjected to weathering and the 

 action of water charged with carbon dioxide. They are 



