CHAPTER XX 

 RIVERS AND THEIR WORK 



252. Temporary Streams. In places where the surface 

 is bare rock, every rainfall sends sheets of water flowing 

 over the slopes and streams rushing in the ravines. The 

 flow ceases when the rainfall does. Such temporary streams 

 are found on rocky slopes of deforested hillsides and in gul- 

 lies in semi-arid regions. Travelers crossing barren country 

 are sometimes caught by a flood while camping in a seemingly 

 desert hollow. It is important in the study of rivers to 

 know how the loose covering of the earth was made, because 

 that covering holds the rain water for a time and makes 

 possible the beginning of permanent streams. 



253. Weathering. The earliest lands were of rock with 

 no covering of plants and with no animal inhabitants. 

 Animals cannot live without plants, and few plants grow 

 upon bare rock. But the rock began to decay almost as 

 soon as it was formed, and decayed rock is the beginning 

 of soil. 



Many natural forces act together to wear away the sur- 

 face of rocks. The oxygen and carbon dioxide of the air, in 

 combination with water, act chemically upon some minerals 

 to change their composition and cause crumbling and decay. 

 Water enters crevices in a rock; freezing there, it expands 

 about one tenth of its volume and forces the rock apart. 

 The sun's heat also expands the surface layer and loosens 

 it from the under rock, and rain and wind remove the 

 grains which afterward accumulate in hollows. The wearing 

 away of rock in these ways is called weathering. 



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