agricultural geology. 9 



§3. Composition of simple minerals. 



The composition of felspar and alhite^ together with that of 

 me of the other more common rocks, it may be well to state in 

 is place. The two first named consist respectively of 



Felspar. Albite. 



Silica, 65.21 69.09 



Alumina, 18.13 19.22 



Potash, 16.66 



Soda, 11.69 



100.00 100.00 



In attempting to distinguish these minerals from quartz, or flint 

 i5 it is often called, we are to notice their hardness. Felspar and 

 Ibite just scratch common window glass, but quartz does not. 

 Ibite is always white ; felspar is white or flesh-colored, and each 

 I ves a strong reflection of light from the planes of the crystal ; 

 hile quartz has the lustre of glass, or more of a vitrified appear- 

 ice in the mass. 



Another kind of felspar is the labradoritc, which abounds in the 

 cks of the Adirondack mountains. The rock itself, as already 

 itecl, is termed hypersthene rock^ from a small quantity of this 

 iiieral which it contains. The whole mass is mostly labradorite ; 

 id by decomposing, it has formed in some places an imperfect 

 prcelain clay. Its composition is as follows : 



Labradorite. 



Silica, 55.75 



Alumina,.. 26.50 



Lime, 11.00 



Iron, 1.25 



Soda, 4 . 00 



98.50 Klaproth. 



This species is usually smoke-grey, though the exposed surface 

 cthe rock is grey or greyish w^hite : it appears to be bleached. 



Mica^ another mineral found in granite, gneiss and mica slate, 

 1 > a composition much like that of the felspars, or at least is 

 ailogous to them, as containing two alkalies, potash and magne^ 



6 ; thus, 



PoTAssic mica. Magnesian mica. 



Silica, 46.10 40.00 



Alumina, 31.60 12.67 



Protoxide of iron, 8.65 19.03 



Potash, 8.39 



Magnesia....... .... 15.70 



rOL. II. — KO. I. B 



