GEOLOGY, BY WM. D. MACPHERSON. 81 



So it is quite complicated in its makeup, though hornblende itself is only 

 a subordinate part of the granite. Occasionally we see a mass of hornblende 

 by itself and it makes a handsome mineral. 



Mica is that peculiar mineral that separates so easily into thin sheets. 

 As children, we saw it in stove doors and often wondered why it was not 

 used for window panes also. A mass of mica has the form of a crys- 

 tal before it is split up into sheets. There are two principal varieties, 

 muscovite, generally in light colors, and made up largely of silica, and bio- 

 tite, a darker colored mica, containing more iron and magnesia and less 

 silica. Each of these micas forms a strongly marked family by itself, mus- 

 covite being an acidic mica and biotite, a basic mica. The basic rocks have 

 a preponderance of the dark, heavy minerals like iron etc., while the acidic 

 rocks have mainly silica, potash, etc. Aside from stoves, mica of late 

 years, is largely used as an insulator by electricians, and they generally 

 choose the light colored variety, probably because it has less iron in it. 



Feldspar. This constituent of granite is quite easily distinguished from 

 the other three. It is generally of a pink or flesh color, though may be red, 

 brown or greenish. It is softer than quartz and may be scratched and cut by 

 quartz and it also has a cleavage, while quartz has not. Its color is not so 

 glassy or vitreous, either. Though the little ieldspathic junks in granite look 

 innocent enough, feldspar is really the name of a family of minerals and they 

 are very important in geology, composing a great part of the earth's crust. 

 Five principal varieties of feldspar are now recognized. They all have sixty or 

 more per cent of silica, together with several other minerals, and are hence called 

 silicates. 



Orthoclase is a silicate of aluminum and potassium and is called a potash 

 feldspar. 



Albite is a silicate of aluminum and sodium and is called a soda feldspar. 



Oligoclase in like manner is a soda-lime feldspar, and Labradorite is a lime- 

 soda, the last one having more lime than soda, and the one before it, more soda 

 than lime. On account of containing so much potash, soda and lime, the feld- 

 spars are easily destroyed or disintegrated by the weather, when exposed to the 

 rain, frost and sun, hence it is not so durable as quartz. Orthoclase, the first 

 one given above is the most common feldspar, and the one that our readers 

 would be most apt to see, Commercially, feldspar is used in large quantities all 

 over the world in the manufacture of crockery and china ware. The product 

 of the decomposition of feldspar is Kaolin or common clay, a silicate of alum- 

 inum, which is made into bricks and tiles. The aluminum is also extracted, or 

 separated from the silica, and made into sheet metal. While most metals have 

 to be mined out of the solid rock, at great depths, aluminum, one of the most 

 beautiful and useful of all metals is found on every hillside, roadside and by-way 

 about us, in the clay over which we walk. 



