11 The Great World's Farm 



fore 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. 



When the Glass Road was being made in the famous 

 Yellowstone Park (Wyoming, U.S.A.), some huge blocks 

 of obsidian, or volcanic glass, were found 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 neighboring 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 very hard- 

 est rocks when driven against them by the former, and 

 when driven by water, has produced the great canons, or 

 narrow gorges some thousands of feet in depth, with 

 which we are familiar in California. 



But a few words must be said about the glaciers, those 

 frozen rivers, which are among the mightiest of nature's 

 grinders. Looking down upon a glacier, and seeing it 

 strewn with the blocks of stone and vast heaps of rubbish 



