256 NATURE STUDY 



made up of pebbles instead of the smaller grains of 

 sand. 



Slates a^std Shales : 



These are simply compressed clay, and can be easily 

 recognized by the characteristic odor obtained by moist- 

 ening them slightly. 



They are of aqueous origin, and in consequence often 

 contain fossils. Sometimes ripple marks and the im- 

 press of raindrops are to be seen. 



Their principal commercial use is for blackboards, 

 slates, roofing, flagging, tiles. 



Method : 



None of these families of stones should be taught, 

 unless the children themselves furnish the pretext by 

 bringing in the specimens. 



Aside from the observation and tests to which, by this 

 time, they should have learned to subject all minerals, 

 endeavor to make tlie children reason out the origin. 



A bottle of acid and a glass dropping rod should be 

 always ready for use. 



