234 PHYSICAL GEOLOGY 



more of which forms a monotonously flat plain. As already 

 indicated (p. 62), the absence of the familiar hills, valleys, 

 and many other features of the land is due (1) to the fact 

 that the bottoms of the ocean basins are protected from the 

 attack of wind and weather, of streams and of glaciers, the 

 agents which sculpture land surfaces, and (2) to the effects 

 of the deposition of sediment in the ocean. 



The average depth of the ocean basins is a little less than 

 two and one half miles (about 13,000 feet). This is nearly 

 six times the average elevation (some 2300 feet) of the lands 

 above sea level. 



Offices of the ocean. (1) Nearly all the moisture which 

 is condensed upon the surface of the land as rain or snow, 

 or in less important forms, comes directly or indirectly from 

 the ocean. Together with the atmosphere (p. 86), the 

 ocean therefore makes possible the work of streams, of ground 

 water, and of glaciers. Without the moisture which is 

 evaporated from the ocean and carried by the winds to be 

 precipitated over the land, neither plant nor animal life 

 could flourish. This constitutes perhaps the greatest service 

 which the ocean renders. 



(2) The ocean tends to regulate the distribution of tem- 

 perature over the earth's surface. The temperature of the 

 winds is modified by that of the ocean surfaces across which 

 they blow, and the heat or cold gained is carried over the 

 land for greater or lesser distances. Warm ocean currents 

 from low latitudes carry great quantities of heat poleward. 

 Cold currents from high latitudes carry lower temperatures 

 equatorward. Just as oceanic islands have more uniform 

 climates than great land masses, so in past ages widespread 

 invasions of the lands by the sea resulted in periods of uni- 

 form (oceanic) climate, while great extensions of the land 

 areas coincided with periods of variable (continental) climate. 



(3) In preceding chapters it has been pointed out that 

 the ocean is the ultimate goal of all the waste of the land, 

 which is spread out upon its floor as layers of sediment. 



