PRELIMINARY OUTLINE 7 



and 12,000 feet; under about 53% it sinks to depths of between 

 12,000 and 18,000 feet; and under 4% it ranges from 18,000 feet 

 down to about 30,000. 



Besides the ocean, the hydrosphere includes all the water which 

 constitutes the surface streams and lakes, together with that which 

 permeates the pores and fissures of the outer part of the solid earth. 

 The water of the earth becomes a hydrosphere only when the 

 ground water is considered. All other waters of the earth are 

 small in amount, compared with the great mass of the ocean. 



Geologic activity. Of all geological agents, water is the most 

 obvious and apparently the greatest, though its efficiency is affected 

 by many conditions, especially the relief of the land, and temper- 

 ature. Through the agency of rainfall, surface streams, under- 

 ground waters, and waves, the hydrosphere is constantly modi- 

 fying the surface of the lithosphere, and at the same time carrying 

 sediment from the land and depositing it in the various basins. 

 The hydrosphere thereby becomes the great agency for the degra- 

 dation of the land and the building up of the basin bottoms. It 

 is therefore both destructive and constructive in its action. The 

 beds of sediment which it lays down follow one another in 

 orderly succession, each later one lying above an earlier. In this 

 way, they form a time record. And as relics of the life of each age 

 become more or less embedded in the sediments, they furnish the 

 means of following the history of life from age to age. The historical 

 record of geology is therefore very largely dependent upon the fact 

 that the waters have buried in systematic order, relics of the life 

 of successive ages. 



The special processes of the hydrosphere will be the subject 

 of discussion hereafter. Suffice it here to recognize its great func- 

 tion in the constant degradation of the land, and in the deposition 

 of the derived material in orderly succession in the basins. 



3. The Lithosphere 



The atmosphere and hydrosphere are envelopes or shells, rather 

 than true spheres, though both penetrate the lithosphere to some 

 extent. The lithosphere, on the other hand, is an oblate spheroid 

 with a polar diameter of 7,899.7 miles, and an equatorial diameter 



