6 THE WORLD'S LUMBER ROOM. 



glasses are likewise busy with the rocks ; but, besides these 

 very obvious items, the fine gunpowder-like dust, which is 

 sifted out from the other refuse, must contain specimens of 

 Nature's dust in most, if not all, of its varieties. 



Night and day, summer and winter, her great army is 

 ever at work, cutting, carving, grinding, loosening, polishing, 



Fig. i. COSMIC DUST. 



hammering the rocks, and making an impression even on 

 the very hardest of them. One result of all this wear 

 and tear is the soil which almost everywhere covers the 

 earth's crust, sometimes to the depth only of an inch or 

 two, sometimes even less, while sometimes again it attains 

 a thickness of several feet. At the utmost, however, its 

 depth may be measured by feet, and at its greatest thick- 



