Slate anfc Sbale* 35 



that of the first experiment we should find that 

 the lines of cleavage had also changed and that 

 the particles had all been re-arranged. Apply 

 the principle to the formation of slate, and we 

 can understand how it happens that what we 

 call the grain runs crosswise of the deposits 

 that were made at different times. It is not 

 a chemical, but purely a mechanical difference. 

 Or, to express it differently the difference is 

 a structural one produced by mechanical 

 causes. 



The origin of cleavage in slate has been the 

 subject of much speculation and investigation, 

 but like many other problems it was solved 

 through the invention and application of the 

 microscope. Thin layers of slate have been 

 made, the same as with limestone and chalk, so 

 thin that the light would readily pass through 

 it and that an examination of the particles 

 could be readily made, showing their arrange- 

 ment under varied conditions. Science is in- 

 debted to the microscope for the solution of 

 very many problems that for ages before had 

 puzzled philosophers. 



