4 PRELIMINARY OUTLINE 



THE GRAND DIVISIONS OF THE EARTH 



The constitution of the earth. The materials of the earth fall 

 into three grand divisions: (i) The atmosphere, (2) the hydrosphere 

 (water sphere), and (3) the lithosphere (rock sphere). 



The atmosphere. Since the atmosphere is a part of the earth, 

 its history falls within the province of geology. It is an intimate 

 mixture of (i) all those substances that do not become liquid or 

 solid under the temperatures and pressures which exist at the earth's 

 surface, together with (2) such transient vapors as the various 

 liquid and solid substances of the earth throw off. The first are 

 the principal gases of the atmosphere, and consist of nitrogen about 

 78 parts, oxygen about 21 parts, carbon dioxide about .03 part, 

 together with small quantities of argon, and several other sub- 

 stances. Chief among the second group is water vapor, which 

 varies greatly in amount from time to time and from place to place. 

 Here, too, belong the gases which issue from volcanoes, and many 

 volatile organic substances. Dust and other matter suspended in 

 the air are regarded as impurities rather than constituents of the 

 atmosphere; but they are important because they affect the tem- 

 perature and light of the air, and the condensation of its moisture. 



The mass of the atmosphere is estimated to be Yiaooooo of the 

 total mass of the earth. It exerts a pressure of about fifteen pounds 

 per square inch at sea-level. Its density decreases upward, but its 

 actual height is not known. There is no direct evidence of its 

 existence above a few hundred miles, but there are theoretical 

 grounds for believing that it reaches much greater heights. 



Geologic activity. The atmosphere is the most mobile and active 

 of the three great subdivisions of the earth. Its direct and indirect 

 effects on water and rocks are so great that it must be regarded as 

 one of the great agents of change in the earth's history. The func- 

 tion of the atmosphere in sustaining life and promoting all that 

 depends on life is obvious. 



The hydrosphere. The water which lies upon the surface of 

 the solid earth is about V4950 part of the earth's mass. Were the 

 solid part of the earth perfectly even, this amount of water would 

 make a universal ocean a little less than two miles deep; but owing 

 to the unevenness of the lithosphere, most of the water is gathered 

 in the great basins which affect its surface. These basins are all 

 connected, so that anything which changes the level of the water 

 in one, changes it in all. 



