AQUEOUS AGENCIES. 41 



mountains to sea. In fact, in the phenomena of estuaries 

 and bars, we have already a cooperation of rivers and 

 sea. This brings us very naturally to the next head, viz., 

 Agency of the Ocean. 



. 



SECTION II. THE OCEAN. 



Waves and Tides. 



The ceaseless beating of waves on an exposed shore can 

 not fail to impress the observer as a powerful erosive 

 agent. Tides assist the waves, not only by creating pow- 

 erful currents in all bays, inlets, and estuaries, as already 

 explained, but also by lifting the sea-level and therefore 

 presenting new points of attack. As in the case of rivers 

 the erosive power is greatly increased by the sand, gravel, 

 and pebbles carried by the current, so in the case of waves 

 the sand, gravel, shingle, and rock-fragments torn from 

 the cliffs are taken up again and hurled back with vio- 

 lence, and become the chief agents of further erosion. 

 Although, however, so incessant and violent in action 

 and so conspicuous in effects, yet, being confined wholly 



FIG. 18. View of inclined strata, with faces exposed to waves. 



to the shore-line, the aggregate effect of wave-erosion 

 is far less than that of the universal erosion of rain and 

 rivers. 



