ELEMENTARY STUDIES OF SOME COMMON ROCKS. 283 



Besides these three normal constituents of granite, other 

 minerals will at times be detected, which may be recorded 

 but they need not be discussed in detail, unless the teacher 

 himself happens to know them. 



After a careful study of the individual constituents, the 

 general characters of the rock may be discussed the 

 colour as a whole, generally determined by the felspars, 

 the coarseness or fineness of grain, the size of the indi- 

 vidual constituents, the relative proportions of the in- 

 gredients in a square inch surface, and any other points 

 the teacher may see fit to raise. The general crystalline 

 character of granite may be emphasised by comparing it 

 with a bit of clay or chalk or other sedimentary rock. 



The question of the origin of granite is one that must 

 be left to the discretion of the teacher. Perhaps it is too 

 difficult a subject to discuss in any detail. But the atten- 

 tion of the children may be called to the industrial uses of 

 the ingredients of granite and of granite itself. Thus 

 mica, which is mined in huge sheets in Russia, India and 

 elsewhere, is used in lamp-chimneys, in glossing wall papers, 

 and so on. Quartz is used in making "pebble " spectacles 

 and in other ways, and its coloured varieties form the 

 amethyst, the cairngorm, and other gem stones. The uses 

 of granite itself will be familiar to most pupils. 



Attention may be directed to the weathering of granite 

 on hillsides into artificial-like masses " tors." 



CLAY AND SLATE. 



Material. One or more specimens of clay for each pupil. 



slate ,, 



An indoor lesson on rocks may eitlieu take the form of a 

 purely observational exercise, or the exercise may be made 

 the text of a subsequent lesson in which the history or 

 the uses of the rock may be explained to the pupils. The 

 time at the teacher's disposal will determine the method 

 he adopts. But the pupils' interest will be best secured if 



