18 THE MTNEEALS CONSTITUTINa BOCKS 



Leucite. — Composition: Silica, 55.0%; alumina, 23.5%; pot- 

 ash, 21.5%. 



Leucite occurs as an original and essential constituent of 

 many volcanic rocks, but is not an abundant mineral except in 

 rare instances. Its chief interest, from the present standpoint, 

 lies in its high percentage of potash which must become available 

 as plant food on decomposition. Leucite is a common constitu- 

 ent of certain lavas of Vesuvius, and it is not improbable that 

 this may account in part for the well-known fertility of the soils 

 of that region, though naturally climatic influence has much 

 to do. 



Nepheline; Elseolite. — These names are given to what are 

 varietal forms of one and the same mineral. In composition 

 they are silicates of alumina, soda, and potash of the formula 

 (NaK)2Al2Si20s = silica, 41.24; alumina, 35.26; potash, 6.46; 

 soda, 17.04. 



Nepheline occurs in Tertiary and post-Tertiary eruptive rocks, 

 and is an essential constituent of phonolite, tephrite, and nephe- 

 linite. The variety eteolite occurs only in older rocks, and is an 

 essential constituent of eteolite syenite. 



Both nepheline and elgeolite gelatinize readily with hydro- 

 chloric acid, and the powdered rock when treated on a glass slide 

 with this acid yields abundant microscopic cubes of sodium 

 chloride. This is one of the easiest of microchemical tests for 

 the determination of the mineral. Nepheline occurs as a rule 

 in well-defined short and stout hexagonal prisms, which in longi- 

 tudinal sections show up as short, colorless rectangular areas. 

 Elseolite differs in being more opaque and occurring in less well- 

 defined, more granular forms. When occurring in sufficient 

 abundance in a rock mass it is readily recognized by its char- 

 acteristic greasy appearance. The mineral undergoes a ready 

 alteration, giving rise to zeolitic minerals and on ultimate de- 

 composition through weathering, yielding a rich and fertile soil. 

 (See p. 196.) 



The AmpMboIes. — Composition: Two principal varieties are 

 recognized. (1) Non-aluminous, consisting mainly of the meta- 

 silicates of magnesium and calcium, with 55 to 59% of silica, 

 21 to 27% of magnesia, 11 to 15% of lime, and small pro- 

 portions of protoxides of iron and manganese. Under this head 

 are included the white, gray, and pale green, often fibrous forms, 

 as tremolite, actinolite, and asbestos. (2) Aluminous, contain- 



