Viil CONTENTS. 



PAGE 



of dune areas, 32. Migration of dunes, 33. Distribution of 

 dunes, 35. Wind ripples, 37. Abrasion by the wind, 38. 

 Effects of wind en plants, 40. Indirect effects of the wind, 41. 



II. Chemical Work 41 



Precipitation from solution, 41. Oxidation, 42. Carbo- 

 nation, 43. Other chemical changes, 43. Conditions favor- 

 able for chemical changes, 43. 



The Atmosphere as a Conditioning Agency 43 



I. Temperature Effects 44 



II. Evaporation and Precipitation 50 



III. Effects of Electricity 52 



Summary 54 



CHAPTER III. 



THE WORK OF RUNNING WATER. ,, 



Rain and River Erosion ./..... 57 



Subaerial Erosion without Valleys 58 



The Development of Valleys 63 



By the growth of gullies, 63. Limits of growth, 67. The 

 permanent stream, 70. Other modes of valley development, 

 73. Structural valleys, 77. The courses of valleys, 77. The 

 development of tributaries, 78. 



A Cycle of Erosion. Its Stages 80 



General Characteristics of Topography Developed by River 



Erosion 92 



Special Features Resulting from Special Conditions of Erosion. 92 

 Bad-land topography, 93. Special forms of valleys; 

 canyons, 94. 



The Struggle for Existence Among Valleys and Streams 100 



Piracy, 103. 



Rate of Degradation 105 



Material in solution in river water, 107. 



Economic Considerations 108 



Analysis of Erosion. .; 110 



Weathering i 110 



Transportation 115 



