282 ELEMENTARY STUDIES OF SOME COMMON ROCKS. 



quartz, felspar and mica, and show them to the class as 

 individual mineral types. Point out that a mineral is a 

 single substance, with uniform * characters throughout its 

 mass, such as colour, hardness, transparency, lustre, and 

 so on. On the other hand, rocks like granite are mix- 

 tures of minerals. This is more especially true of igneous 

 rocks. 



2. This leads to an enquiry as to how many con- 

 stituents are present in the granite that is being studied. 

 In normal granites the characteristic ingredients are three. 

 They can be identified, and their leading characters be 

 determined by the pupils themselves, if they are guided to 

 look for the following points : 



(a) Quartz glassy, shapeless, colourless or less com- 

 monly brownish, not scratched by pin-point or 

 knife however hard pressure be applied, trans- 

 parent, broken faces rather rounded or shell-like, 

 unaffected by a drop of acid laid on it. 



(&) Felspar frequently two differently coloured fel- 

 spars are present, the commonest tints being a 

 flesh-red and a white or bluish- grey, shape less 

 rounded than quartz leaning to four-sidedness, 

 lustre more pearly than glassy, opaque, very hard 

 but not quite so hard as quartz, breaks in some 

 directions with nearly flat faces, unaffected by a 

 drop of acid. 



(c) Mica a mineral familiar to children as " sheep's 

 silver." In a granite there may be two micas, a 

 black and a silvery white ; both are very soft, easily 

 scratched with a pin, they split (" cleave ") into 

 very thin scales which are flexible. The flat faces 

 are very bright and shining. The shape is six- 

 sided, but it is only sometimes possible to see this 

 in a rock. 



* This is only approximately correct. 



