CHAPTER II.

THE ATMOSPHERE AS A GEOLOGICAL AGENT.

+The Atmosphere as a Direct Agency+ 21 I. _Mechanical Work_ 21 Transportation and deposition of dust, 22. Transportation and deposition of sand, 25. Formation of dunes, 26. Shapes of dunes, 26. The topographic map, 30. Topography of dune areas, 32. Migration of dunes, 33. Distribution of dunes, 35. Wind ripples, 37. Abrasion by the wind, 38. Effects of wind on plants, 40. Indirect effects of the wind, 41. II. _Chemical Work_ 41 Precipitation from solution, 41. Oxidation, 42. Carbonation, 43. Other chemical changes, 43. Conditions favorable 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