AGENTS AT WORK IN THE OCEAN 221 
most important service performed by this load of dis- 
. solved substances, is the furnishing of materials which 
many animals and plants need for their existence. 
Thousands of tons of carbonate of lime and other sub- 
stances, are being daily extracted from the water by 
animals, and built into their skeletons or shells. This 
process is of no little consequence in the formation of 
rock deposits in the ocean (p. 89). 
A large percentage of this dissolved load is brought 
by streams, but the very presence of sea water in con- 
tact with the rock, furnishes another source. Just as 
the water of the land is able to dissolve and modify 
the minerals of the rocks, so the ocean water works 
constant destruction along the coast line. The rate 
of change caused in this respect by sea water is greater 
than that of rivers, because sea water is more impure, 
and hence a more powerful agent of solution. On 
some rocks, such as limestone, sea water is produ- 
cing marked changes (Fig. 123). One may often see 
its effects upon rocky coasts dashed by the ocean 
spray. 
Wave Action. — Nature of the Wave. In the sea the 
friction of the wind on the surface of the water 
produces waves, whose height in the open ocean is 
often one or two score of feet. Even a slight breeze 
ripples the surface, and the billows which are raised 
by the storm winds, will extend for great distances 
