Geology 



The minerals, felspar and mica, belong to the class 

 of chemical compounds called silicates, while quartz is 

 free silicic acid (SiO 2 ). It will be seen from the above 

 analysis that the granite contains a very large percentage 

 of the acid silica, in comparison to the other oxides in 

 the list, which are all basic in chaarcter. For this reason 

 granite is spoken of as an acid rock. 



The other rocks named above differ from granite 

 chiefly in containing less silica. Thus a syenite would 

 contain about 58, a diorite 53, and a gabbro not more 

 than 45 per cent. 



It has already been stated that igneous rocks have 

 been produced by the cooling of a molten mass, and it 

 may be of some interest to see whether we can produce 

 igneous rocks in this way. 



Let us take some of our granite and melt it in a 

 crucible furnace, and then allow it to cool, so that we 

 can examine the material produced and ascertain if it 

 resembles the original granite. On turning out the 

 cooled mass we shall find that instead of a coarsely 

 crystalline rock consisting of three different minerals, we 

 have a homogeneous material resembling green bottle 

 glass, and on submitting it to certain optical tests we 

 shall find that it is in reality a glass. This might be 

 taken as evidence against the view that granite is formed 

 in the manner supposed, but experiments with other 

 mixtures of a more basic character show that, if the cool- 

 ing is rapid, a glass results, but that, by prolonging the 

 cooling, a finely crystalline mass is produced. Still slower 

 cooling produces larger crystals, and thus we see that the 

 rate of cooling has much to do with the nature of the 

 rock. 



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