266 NEW ZEALAND NATURE-STUDY BOOK 
al 
would to a large extent, if not entirely, be confined to 
surface operations. These, however, would prove full of 
interest and would help more than all available diagrams 
to make the subject real. 
No attempt has been made to detail the operations carried 
on in Coal-mining, partly because lack of space forbids that 
the matter be further dealt with and partly for the reason 
that such descriptions can only be general and hence lose 
much of the interest which ought to surround them. A 
detailed description of the kind should have reference to 
some special mine with which the teacher and class are ac- 
quainted. If this cannot be done it will be sufficient (by 
the aid of diagrams and pictures) to lead pupils to follow a 
general account of mining operations. 
IIl.—ROCK SALT. 
As in the case of Coal, pupils will examine the specimens 
under review, and discover for themselves their physical 
properties. The same method of procedure may be adopted 
as that employed when dealing with Coal. 
When pure the mineral is clear and colourless, but 
specimens of this kind are rare. An average piece of 
Rock-Salt is either reddish in colour from the presence of 
iron oxide, or tinged with green or blue showing the 
presence of copper. It is frequently mixed with impurities, 
usually clay or sand, which serve to modify its clearness 
still further. Its lustre is glassy, and its transparency 
varies considerably, the average being translucent. 
Though not so brittle as School Chalk and certain kinds 
of Coal, it is not difficult to break. 
