16 ELEMENTARY GEOLOGY 
generally in mid-ocean, the depth is over four miles, 
while the average ocean depth is more than two. 
Hence the earth possesses a vast area of salt water, 
and this is not quiet, but has a slow movement in 
the form of ocean currents. (See p. 234.) 
The ocean is also disturbed by two great but very 
low waves which pass around the earth, causing the 
waters to rise and fall twice each day, and on the 
shores producing the phenomenon of tides. The tide 
is mainly caused by the moon, although the sun 
aids in the formation of the tidal wave. 
A more universal movement of the ocean, is one that 
is confined to the mere surface, and is caused by the 
wind which blows over it. Hverywhere, and always, 
the ocean surface is disturbed by the wind wave, some- 
times only a ripple, at other times a billow rising 
twenty or thirty feet into the air, and stirring the 
waters to movement at a depth of two or three 
hundred feet. 
The ocean is of the utmost importance in geology ; 
it is the great dumping ground for the waste of the 
land. ‘The waves beat upon the coast and wear. it 
away, the rivers bear floods of sediment into the sea, 
and the materials thus obtained are spread out over 
the ocean floor. However, the ocean, by covering the 
greater part of the earth’s crust with water, protects it 
from the destructive action of the weather, to which 
