PART I 



PHYSICAL GEOLOGY 



CHAPTER I 



THE GREAT DIVISIONS OF THE EARTH 



THE great divisions of the earth are the atmosphere or air, 

 the hydrosphere or water portion, and the lithosphere or solid 

 part (Fig. 1). 



The atmosphere. The atmosphere is a mixture of several 

 gases. While nitrogen predominates, the three most impor- 

 tant things in the atmosphere, geologically, are oxygen, carbon 

 dioxide, and water vapor. They combine chemically with many 

 substances of the lithosphere to form new compounds, and 

 are especially important in decomposing surface rocks (p. 103). 

 The condensation of the water vapor leads to the precipitation 

 of rain or snow, and makes possible the work of running water 

 and of ice. The work of the atmosphere in conditioning the 

 rainfall is perhaps its greatest function, geologically. So far 

 as mere volume is concerned, however, these gases are of minor 

 importance. The water vapor, regarded frequently, like dust, 

 as a foreign substance in the air, rather than a constituent of 

 it, varies greatly in amount at different times and places. 

 The carbon dioxide makes about .03 per cent and the oxygen 

 about 21 per cent of the air, or approximately one fifth by 

 volume. The remaining four fifths consists chiefly of nitro- 

 gen, an inactive gas chemically, whose importance geologi- 

 cally is confined largely to its mechanical effects. 



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