GEOLOGY. 



87 



at others it is in laminated leaves or scales like slate, of various 

 thickness. 



TALCOSE SLATE. 



Talc is the principal ingredient in this variety of slate, it 

 is sometimes in a pure state and sometimes mixed with quartz, 

 feldspar, mica, hornblende or limestone : it is a softish stone, 

 valuable for building purposes. There are several varieties. 



Composition of Feldspar. 

 Silica, - - 65.21 



Alumina, - - 18.13 



Potash, 16.66 



100.00 



Composition of Basaltic Hornblende. 

 Silica, - 42.24 



Alumina, - - 13.92 



Lime, - 12.24 



Magnesia, - 13.74 



Protoxide of Iron, - 14.59 



Oxide of Manganese, - - - p. 3 3 



Composition of Mica. 

 Silica, - 



Alumina, - - 



Protoxide of Iron, - 

 Potash, - 

 Oxide of Magnesia, 

 Fluoric Aeid, - 

 Water, - 



CHALK. 



97.06 



46.10 

 31.60 

 8.65 

 8.39 

 1.40 

 1.12 

 1.00 



98.26 



Chalk is similar in composition to carbonate of lime, viz: 



