480 Frederick Guthrie on Eutexia. 



eleven per cent, of water. By its union with alumina it gives 

 rise to clay, slate, and the minerals staurolite, andalusite, 

 kyanite ; with lime to Wollastonite ; with magnesia (and iron) 

 to bronzite ; with lime and magnesia (and iron) to augite and 

 diallage ; with magnesia and iron (and lime) to hypcrsthene 

 and olivine ; with magnesia, lime, and iron to hornblende ; with 

 iron to Fayalitc, and so on. 



If, now, we adopt the often-asserted theory that variations 

 in minerals belonging to the same class are brought about 

 by the exchange of more or less of one base by another, the 

 analyses land us in a difficulty, which seems insuperable. 

 Let us suppose, for instance, that we have pure Wollastonite, 

 consisting of 



Silica 51-73, 



Lime 48*27. 



Replacing part of the lime by magnesia and iron, say 23*51 

 by 18*55 magnesia and 0*99 protoxide of iron, the silica 

 rises to 55*15, and gives us an excellent example of augite 

 (neglecting the manganese and alumina of that body). 



V. BonsdorfF. 



Silica 54*83 



Lime 24*76 



Magnesia 18*55 



Protoxide of iron . . . 0*99 

 Protoxide of manganese 0*32 

 Alumina 0*28 



But next let us examine hornblende, an analysis of which 

 by V» BonsdorfF is given. 



Silica 60*10 



Magnesia 24*31 



Lime 12*73 



Protoxide of iron . . . 1*00 



Protoxide of manganese 0*47 



Alumina 0*42 



Hydrofluoric acid . . »83 



Water -15 



100*01 

 These bases, neglecting the water and hydrofluoric acid, 

 require together 51 *83 of silicic acid, instead of 60. No pro- 

 cess of exchange of one base for another can make hornblende 

 out of Wollastonite; It is essentially acid, unless in those 

 varieties of amphibolo which contain a considerable proportion 

 of alumina ; these, indeed, may be basic, acid, or neutral, 

 according as the alumina is considered as a base saturating 



