THE COMPOSITION OF THE EARTH 



15 



The hydrosphere. The hydrosphere includes all the 

 waters of the earth, the oceans, seas, lakes, streams, and the 

 water underground. 



The oceans occupy nearly three fourths of the earth's sur- 

 face, and contain water sufficient to cover the solid part of the 

 earth nearly two miles deep, were the latter a perfect sphere. 

 The oceans are all connected. If the level of the water in one 

 is changed, all are affected. Streams wear the rocks over and 

 against which they flow, and move loose material to lower 

 levels, much of it to the sea. Together with material 

 worn by waves from the shore, or brought to the sea by other 

 agents, the stream- 

 borne waste of the 

 land is spread out on 

 the floor of the ocean 

 as layers of sediment. 

 The general effect of 

 the work of the hydro- 

 sphere is therefore to 

 wear down the surface 

 of the land, and to 

 build up the bottom of 

 the ocean. The work 

 of the waters beneath 

 the surface of the land 

 is chiefly chemical. 

 Near the surface the 

 general result is to 

 bring about the de- FIG. 

 composition of the 

 rocks ; at greater depths, the general effect is to strengthen 

 them by depositing material in their pores and cracks. 



In the waters of the hydrosphere the same gases which make 

 the air are dissolved, together with many solid substances. 

 Common salt is dissolved in greatest abundance in the ocean, 

 but the lime carbonate (p. 22) and silica (p. 19) in solution 



2. Rock containing several kinds of 

 fossils. (Photograph by Jessup.) 



