CHEMICAL ACTION 



II 9 



away by the undertow into deeper and quieter waters, leaving 

 the larger quartz fragments to form the pebbles and sand of 

 the beach. 



The action of the waves is limited vertically, ceasing to be effec- 

 tive in quite shallow water, not far below the low-tide mark. In 

 violent storms the waves often accomplish much destruction far 

 above high tide, but the principal work of the waves is confined to 

 a belt extending from a little above high tide to a little below low 

 tide. Below the latter, the wave work is often efficiently supple- 

 mented by tidal currents, which under favourable circumstances 

 acquire great velocity and depth, scouring away loose materials 

 and perhaps even cutting into solid rock. When an island of 

 considerable extent is exposed to the incoming tide, the latter 

 travels around the island in both directions, and if the shape of the 

 mainland is favourable, one of these currents will be much higher 

 than the other, which will produce a " race " between the island 

 and the mainland. Hell Gate, New York, is an example of this; 

 the tide advances through New York Bay and Long Island Sound, 

 being higher at flood, lower at ebb in the sound than in the bay. 

 The consequence is a swift current into the bay at flood tide and 

 into the sound at ebb. By such means as this, the sea cuts away 

 the land to depths much greater than unassisted waves can effec- 

 tively reach. 



The chemical disintegration due to the sea is not well marked in 

 shallow waters, where the mechanical work is so much more effec- 

 tive and striking. In the profound depths of the oceanic basins, 

 where the water is never disturbed and where its motion is 

 extremely slow, chemical activity becomes relatively very impor- 

 tant. Calcareous shells are completely dissolved, and the volcanic 

 debris which covers the sea-bottom over vast areas, is disinte- 

 grated into a characteristic red clay. 



Lakes. — In comparison with the long life of the earth, lakes 

 must be regarded as merely temporary bodies of water, which will 

 sooner or later disappear, either by being drained of their waters 

 or by being filled up with the sediments which are washed into 

 them. The general term lake is employed for any inland body of 



