CHAPTER II 

 THE FIGURE OF THE EARTH 



The lithosphere and its envelopes. The stony part of the 

 earth is known as the lithosphere, of which only a thin surface 

 shell is known to us from direct observation. The relatively un- 

 known central portion, or " core," is sometimes referred to as the 

 centrosphere. Inclosing the lithosphere is a water envelope, the 

 hydrosphere, which comprises the oceans and inland bodies of 

 water, and has a mass ^Vo that of the lithosphere. If uniformly 

 distributed, the hydrosphere would cover the lithosphere to the 

 depth of about two miles, instead of being collected in basins as it 

 now is. Though apparently not continuous, if we take into account 

 the zone of underground water upon the continents, the hydro- 

 sphere may properly be considered as a continuous film about the 

 lithosphere. It is a fact of much significance that all the ocean 

 basins are connected, so that the levels are adjusted to furnish a 

 common record of deposits over the entire surface that is sea- 

 covered. 



Enveloping the hydrosphere is the gaseous envelope, the atmos- 

 phere, with a mass TTOWUT that of the lithosphere. The atmos- 

 phere is a mixture of the gases oxygen and nitrogen in parts by 

 volume of one of the former to four of the latter, with a relatively 

 small percentage of carbon dioxide. Locally, and at special 

 seasons, the atmosphere may be charged with relatively large 

 percentages of water vapor ; and we shall see that both the carbon 

 dioxide and the vapor contents are of the utmost importance in 

 geological processes and in the influence upon climate. Unlike 

 the water which composes the hydrosphere, the gases of the 

 atmosphere are compressible. Forced down by the weight of 

 superincumbent gas, the layers of the atmosphere at the level of 

 the sea sustain a pressure of about fifteen pounds to the square 

 inch; but this pressure steadily decreases in ascending to higher 

 levels. From direct instrumental observation, the air has now 



8 



