80 NATURE STUDY IN SCHOOLS. 



GEOLOGY. 



BY 



Wm. D. Macpherson. 



• DIFFERENT VARIETIES OF GRANITE. 



Last month we discussed the three very important rocks, granite, sand- 

 stone, and limestone, giving our reasons for doing so. Before departing 

 finally from them, to learn of other geological questions, let us spend a 

 while investigating the constituents of the three rocks which we considered be- 

 fore only in a general way. While sandstone is simply sand of the seashore 

 compressed into big cakes, and limestone is only lime rock or calcium car- 

 bonate, granite, on the other hand, is a good deal like mince pie and has 

 four principal constituents, mixed in as though with a spoon, showing little 

 angular junks all through it. Each little junk, moreover, has its own pe- 

 culiar chemical composition, so that while a piece of marble is nothing more 

 than calcium carbonate, crystallized, granite has at least eight, and often more, 

 elements in it, and they are combined in a complex manner. These four con- 

 stituents of granite are quartz, feldspar, hornblende, and mica, and all but 

 quartz vary in composition according to what rock they are in. 



Quartz is a very important compound in granite rocks. The conventional 

 granite, and also gneiss and syenite, are profusely speckled all through with 

 these glassy grains, which are white 'and without cleavage. There are also 

 many ledges of pure quartz. I knew a farmer once who thought he had a 

 marble quarry on his farm and was surprised when I explained to him chem- 

 ically that it was only quartz. Quartz is an oxide of silica, which is one of 

 the non-metallic elements, and when not subjected to pressure, as most rocks 

 are, makes beautiful hexagonal crystals. Quartz is nearly colorless, but may 

 take on almost any hue if impurities are in it, these colors, as smoky quartz, 

 rose quartz, etc. being very beautiful sometimes. It will scratch glass, show- 

 ing that it is quite hard, and it is one of the few rocks that refuses to be 

 melted or dissolved in the strongest acid. Quartz is the hardest of the con- 

 stituents that we have under consideration to-day. 



Hornblende ( amphibole ) is the black, or greenish black mineral that 

 we see in the granite, being just the opposite in color to the snow white 

 quartz, but shining with the same glassy lustre. Occasionally the biotite 

 form of mica would have the same dark color in the same rocks with the 

 hornblende, but the mica is in little thin sheets, while the hornblende is 

 not. All these little fine black specks of hornblende that we see in the 

 granite, are composed of silica, alumina, magnesia, lime, iron oxide and soda, 



