284 ELEMENTARY STUDIES OF SOME COMMON ROCKS. 



the latter aspect is not entirely omitted. For an isolated 

 specimen of a rock may not have much to arrest attention, 

 but its history or its use may at once supply the required 

 stimulus to intellectual interest. " Clay " is not, to most 

 people, a particularly inspiring theme, but the history and 

 the uses of clay and slate are both interesting and in- 

 structive. 



(1) An exercise in observation. 



The pupils may be put through a series of simple 

 tests as follows : 



Clay its colour its fine grain its crumbly , meagre 

 feeling in the fingers its earthy odour when breathed 

 upon its power of adhering to the tongue its soft- 

 ness under the knife or any sharp edge its glossy 

 surface when cut its absence of lustre as compared 

 with glass its willingness to split or cleave its degree 

 of porosity, how far it admits water its plasticity 

 when wet its purity, that is, how far it contains 

 foreign minerals like " sand " and mica its behaviour 

 with hydrochloric acid its rate of sedimentation in a 

 jar or test tube, as compared with sand. 



Slate as compared with clay : Same fine texture 

 often same colour but relative hardness under knife 

 or pin-point, though easily scratched its greater com- 

 pactness its flatness its remarkable proneness to 

 split (" cleave ") in one direction the varying glossi- 

 ness of its surface the bands or stripes sometimes 

 seen on flat faces. A slate may be powdered down 

 and the characters of the powder compared with those 

 of clay. 



These are all points which the pupils, with judicious 

 guidance, may be led to find out for themselves. But 

 there are some things a child may wish to ask about, 

 which cannot be readily answered from an examination of 

 the specimens before him. 



