PART V--THE EARTH'S CRUST 



CHAPTER XXII 

 HOW ROCK BECOMES SOIL 



279. Questions for Discussion. 1. Sometimes in the coldest winter 

 weather, when a bottle of milk has been left out of doors, it is found 

 that the cover is lifted and the milk stands above the top of the bottle. 

 What lifted the cover? Why? 2. In mountainous regions in winter, 

 even in quiet weather, pieces from the faces of bare rocks are broken 

 off and fall, though nobody has in any way disturbed the rock. What 

 probably causes this to occur? 3. What will take place if a strong 

 metal cylinder filled with water and plugged tightly is placed where the 

 water will freeze? 4. Sometimes the bricks in the most exposed wall 

 of a building crumble much more rapidly than those on less exposed 

 sides. How can you account for this? 5. Why is it that a sharpened 

 stake which in the autumn was driven into the ground is lifted out 

 during winter weather ? 6. Explain how winter or early spring weather 

 lifts clover or alfalfa from the ground. 7. What is a glacier? How do 

 glaciers affect the soil of glaciated regions ? 8. Is the soil of your region 

 of glacial or of sedimentary origin? 9. How do plants and animals 

 contribute toward breaking rocks into smaller pieces? 



280. The surface of the earth. About three fourths of the 

 solid surface of the earth is covered by water. The water 

 has different depths in different places. The floors of oceans 

 and lakes, like the uncovered land surfaces, are undulating 

 in nature. Sometimes whole mountain ranges and the inter- 

 vening valleys are covered by the same body of ocean water. 



Conspicuous irregularities in the uncovered land surfaces 

 are easily noted in mountain regions, and even in the most 

 level plains some irregularities in the surface appear. The 

 nature of this unevenness abrupt or gentle slopes, long or 



279 



