THE OEIGIN OF SOILS 21 



Olivine. Essentially a silicate of magnesia, but part of 

 this base is often replaced by iron, and it is generally 

 represented by the formula MgFeSiC^. The name is de- 

 rived from its olive green colour. It occurs in the volcanic 

 rocks, and also, to some extent, massive. 



Mica. A familiar mineral, easily recognised by its 

 translucent appearance, pearly lustre, softness, and tend- 

 ency to split into flakes. The commoner of the two 

 varieties known as white, or potash, mica is a hydrated 

 (acid) silicate of potash and alumina ; the formula 

 KH 2 Al 3 Si30i2 has been ascribed to it. It is a common 

 constituent of granitic rocks, and is found also in some 

 of the sedimentary rocks. The large proportion of potash 

 (about 9 per cent.) it contains gives it a certain agricultural 

 importance. Micaceous sand has been used as a dressing 

 for land deficient in potash. The other variety, called 

 black, or magnesia mica has a somewhat different com- 

 position, the alumina being in part replaced by magnesia. 

 It contains about the same proportion of potash but is 

 harder than the white variety. 



Leucite. A characteristic ingredient of many of the 

 more recent lavas. The formula K 2 Al 2 Si40i2 is ascribed to it. 



Aucjite. Essentially a silicate of lime and magnesia; is 

 a very complex mineral ; it enters largely into the composi- 

 tion of volcanic rocks and imparts a calcareous (limey) 

 character to them. Diallage has much the same composi- 

 tion and properties. 



Hornblende. A. silicate of the same type as augite ; it 

 contains lime, magnesia, iron and alumina as bases, and is 

 found chiefly in volcanic rocks. It is usually very dark 

 in colour and exhibits a metallic lustre. 



Felspars. The commonest of all minerals ; are essen- 

 tially silicates of alumina and potash, soda or lime. Ortho- 

 clase, the potash felspar, is represented by the formula 

 KAlSi 3 8 . It is white or greyish in colour, enters largely 



