The Lithosphere 



If now we break up some of the granite in a mortar 

 we shall be able to pick out pieces which consist entirely 

 of one kind of material, and if these are examined 

 chemically it will be found that the pink is different from 

 the glassy or the dark brown matter, and that each of these 

 possesses a definite and constant composition of its own. 



These constituents of constant composition are called 

 minerals, and we can say, therefore, that our specimen of 

 granite is made up of three minerals, which have received 

 names. In granite the three principal minerals are : 

 Quartz, the clear, glassy material ; Felspar, which in our 

 particular specimen was pink ; and Mica, the dark brown 

 shining specks. 



Other igneous rocks which look very like granite, 

 and are easily mistaken for it by the uninitiated, on 

 examination will prove to be made up of different 

 minerals, though possessing a crystalline structure similar 

 to that of granite. A chemical analysis of the powdered 

 rock would readily show that they differ widely from 

 granite in composition, and various names are given to 

 them according to the nature of their constituent minerals, 

 e.g. Syenite, Diorite, and Gabbro. 



An analysis of our granite specimen shows that it 

 is constituted somewhat as follows : 



Silica (SiO 2 ) .... 74-0 per cent. 



Alumina (A1 2 O) 3 . . . 14-0 



Ferric Oxide (Fe 2 O 3 ) . . . 0-5 



Ferrous Oxide (FeO) . . . 1-5 



Lime(CaO) . . . . ro 



Potash (K 2 O) .... 4-5 



Soda(Na 2 0) .... 4-5 



IQO'O it 



59 



