n6 



THE WORLD'S LUMBER ROOM. 



mica, &c., when sufficently pressed as to be reduced to 

 half its original bulk, will be found capable of splitting into 

 almost any number of thin layers at right angles to the 

 direction in which the pressure is applied. It has, in fact, 

 acquired the " slaty cleavage," and the various grains com- 

 posing it have ranged 

 themselves with their flat 

 sides facing the direction 

 from which the pressure 

 came. White wax, and 

 even ice, may be made to 

 acquire this cleavage in the 

 same way by pressure, and 

 it is also developed to a 

 certain extent in biscuit by 

 Fig. 23. SLICE OF SLATE, SEEN 



UNDER THE MICROSCOPE, AND 



HIGHLY MAGNIFIED. 



the mere application of 

 the rolling-pin. 



All mud which has been 



converted into true slate has, therefore, we conclude, been 

 subjected to enormous pressure, and has thence acquired 

 the property of easily splitting into thin plates, which makes 

 it so valuable as a roofing material. 



Fine mud is often rolled into roundish lumps and em- 

 bedded in the coarser materials, and it is to these that 

 are due the greasy whitish-green spots often seen in slates, 

 the grain being too fine to bite the pencil. (Fig. 23.) 



The sediment carried by the Rhine into the German 

 Ocean consists chiefly of silica, alumina, and iron, and 

 is just such as might hereafter form a clay slate, rich in 

 iron. i 



