THE MAJOR SOIL-FORMING MINERALS. 35 



ficient in potash, of course the soils formed from them are apt 

 to lack an adequate supply of this substance for plant use. 

 This is markedly true of hornblende schist or amphibolite 

 rocks. 



Mica, commonly known as isinglass, is so conspicuous 

 wherever it occurs that it is more readily recognized than any 

 other mineral. It occurs in glittering scales in soils and 

 sands, and in rocks it sometimes forms sheets of sufficient size 

 to supply the small panes for the doors of stoves, lamp 

 chimneys, etc., which being flexible are not liable to break, but 

 only gradually scale into very thin films, into which it can also 

 be split by hand. When white, (muscovite, phlogopite) its 

 scales are sometimes mistaken for silver by mine prospectors; 

 when yellow, for gold ; but their extreme lightness should 

 soon remove these delusions. The composition of mica is not 

 widely different from that of the two preceding minerals ; like 

 these it sometimes contains much iron, and is then dark bottle- 

 green (biotite) ; this variety in weathering becomes bright 

 yellow, and soon disintegrates. 



This mineral is so abundant an ingredient of many rocks and 

 soils, that one naturally looks for it to play some definite or im- 

 portant part in soil formation. By its ready cleavage it favors 

 the disintegration of rocks ; but it seems that owing to the ex- 

 tremely slow weathering of its smooth, shining cleavage sur- 

 faces, it exerts no notable effect upon the chemical composi- 

 tion of the soil, although, owing to its peculiar character of 

 fine scales, it sometimes adds not immaterially to the facility 

 of tillage in otherwise somewhat intractable soils. So far as 

 is known at present, its presence or absence does not constitute, 

 in itself, any definite cause or indication of the quality of any 

 soil. It may nevertheless be said that the rock in which it 

 usually occurs most abundantly mica-schist, a mixture of 

 mica and quartz is known to form, as a rule, lands of poor 

 quality. On the other hand, the soils derived from granites 

 and gneisses, even when rich in mica, are usually excellent, on 

 account of their content of feldspars, and frequently of other 

 associated minerals. 



Hydoniica differs from the preceding mainly in containing 

 a larger proportion of combined water; but it hardly de- 



