26 Pioneer Labourers 



We may have seen, too, how fence-posts are sometimes 

 lifted out of their places, simply by the heaving of the 

 soil beneath them ; the water in the pores of the soil 

 having frozen and swelled, and forced them up. 



But even where there is no rain, and no ice can 

 therefore be formed in their cracks and crevices, the 

 rocks themselves feel changes of temperature ; and 

 where these changes are sudden, severe, and often 

 repeated, no rock is strong enough to stand against 

 them. The rocks of the Sahara and other similar 

 regions are crumbled into sand simply by the intense 

 heat of the day and the sharp frost at night. The one 

 causes them to expand and the other makes them 

 contract; and between the two the outer grains are being 

 constantly loosened and forced out of their placea 



When the ' Glass Road ' was being made in the 

 famous Yellowstone Park (Wyoming, U.S.A.), sqme 

 huge blocks of obsidian, or volcanic glass, were fcamd 

 to come in the way ; and as they were too hard to be 

 either hewn or drilled, and could therefore not be 

 blasted, the engineer in charge had large fires lighted 

 on the top. When the rocks were scorching hot, a 

 sudden deluge of cold water was poured upon them 

 from the neighbouring lake, and by these means they 

 were thoroughly shattered. This is of course a very 

 extreme instance of the effect produced by changes of 

 temperature, and such as would seldom, if ever, occur 

 in nature; but it may serve to show how very real 

 these effects are. 



Of the other ways in which the rocks are broken up, 

 it will not be needful to say much. We must pass 

 over with brief mention the work done by sand, set in 

 motion by wind or water^ which cuts and polishes the 



