WATER. 657 



between high and low tides. — Since a wave is a body of water rising 

 above the general surface, and when thus elevated makes its plunge 

 on the shore, it follows that the upper line of wearing action may- 

 be considerably above high-tide level. 



Again, the lower limit of erosion is above low-tide level, for the 

 waves have their least force at low tide, and their greatest during 

 the progressing flood ; and when the waves are in full force, the 

 rocks below are already protected by the waters up to a level above 

 low-tide mark. There is, therefore, a level of greatest wear, which 

 is a little above half tide, and another of no wear, which is just 

 above low tide. 



This feature of wave-action, and the reality of a line of no wear 

 above the level of low tide, are well illustrated by facts on the coasts 

 of Australia and New Zealand. 



In figure 944 (representing in profile a cliff on the coast of New 

 South Wales near Port Jackson), the horizontal strata of the foot 

 of the cliff extend out in a platform a hundred yards beyond the 



Fig. 945. 



Cliff, New South Wale3. " The Old Hat," New Zealand. 



cliff. The tide rises on the platform, and the waves, unable to 

 reach its rocks to tear them up, drive on to batter the lowe#part 

 of the cliff. At the Bay of Islands, New Zealand, the rocks have 

 no horizontal stratification, and, still, there is the same sea-shore 

 platform; and an island in the bay (fig. 945) is called ''The Old 

 Hat." The* sea-shore platform of coral islands has the same origin. 

 The stability of sand-flats in the face of the sea is owing to this cause. 

 In seas of high tides and frequent storms, the platform is narrow 

 or wanting, owing to the tearing action of the heavy waves. 



2. Transportation. 



1. Transportation by currents. — The great oceanic currents are too 

 feeble to transport any material coarser than fine sand, and too 

 remote from coasts to receive detritus of any kind, except sparingly 

 from the very largest of rivers, like the Amazon. Whatever sinks 

 in the main course of the Gulf Stream is carried some distance 

 southward again by the polar current beneath it. 



43 



