ELEMENTARY STUDIES OF SOME COMMON BOOKS. 285 



(2) The origin of clay. 



The question is one that cannot be fully discussed 

 with children. But the teacher can easily procure, 

 say from one of the clay pits in Cornwall (St. Austell), 

 a specimen of granite, showing the decomposition of 

 the felspar into white Kaolin (China clay). 



This will demonstrate to the class clearly enough 

 that clay, or at any rate some kinds of clay, arise 

 from the decomposition of one of the constituents of 

 granite. 



The cause of the decomposition is an inquiry be- 

 yond the limits of the school course. 



(3) The origin of slate. 



It has been abundantly proved that slate is just 

 clay hardened and altered by pressure. Slates are 

 commonly found on the flanks of mountain ranges 

 where tangential pressure has been great. 



It can be shown experimentally that if wax, for 

 example, be mixed with iron filings and subjected 

 to severe tangential pressure, the filings take up a 

 position with their flat faces perpendicular to the 

 direction of pressure. This illustrates what has hap- 

 pened in the production of a slate. 



The pressures of mountain-making cause the par- 

 ticles of an original clay or shale to rearrange them- 

 selves perpendicular to the direction of pressure and 

 therefore with their flat faces all parallel, and hence 

 slates split or "cleave" readily parallel to these flat 

 faces. 



Of course in nature the process takes place on 

 a great scale and extends over long periods of time, 

 and the slate may undergo other changes. 



The teacher will find the origin of slate described 

 in any elementary text-book of Geology. 



Again, attention might be directed to the different 



