Slate anD Sbale. 33 



a slight extent. It is easy to see, then, if we 

 could change the lines of cleavage in the shale, 

 so as to run at right angles with their present 

 lines, the face of a slab would show bands 

 of different colors or shadings, such as we 

 often see in slate. If you take a piece of clay 

 that has been thoroughly mixed, and subject 

 it to a very great pressure, and then examine 

 the piece that has been submitted to pressure 

 under a microscope and compare it with a 

 piece of the clay after it has been thoroughly 

 mixed, but has not been submitted to pressure, 

 you will find that the two are very different in 

 structure. The pressed clay will show that 

 the particles of which it is made up have all 

 turned, so that their longest dimensions are in 

 a line at right angles with the direction of 

 pressure. Here is an interesting fact that we 

 must remember. And it is in this that we 

 find the reason for the structural difference 

 between shale and slate. The lines of cleav- 

 age in shale are not formed necessarily by 

 pressure, but because in the disposition of the 

 material of which it was formed the particles 

 naturally laid themselves down so that their 

 longest dimensions were on a horizontal line. 

 Ages after, when other rock and other for- 

 mations had been laid down on top of the bed 

 of deposited mud, the upheavals of the earth 

 have so changed the lines of pressure upon 

 this material and the pressure is so great that 



