664 DYNAMICAL GEOLOGY. 



time immemorial. Many examples inight be cited from the American coast; but none 

 so remarkable have yet been described. 



These effects of the sea on coasts depend on (1) the height of the 

 tides; (2) strength and direction of tidal currents; (3) direction of 

 the prevalent winds and storms ; (4) force of the waves ; (5) nature of 

 the rock of the shores ; (6) outline of the coast. 



Soft sandstones, in horizontal layers, and beds of gravel or earth 

 are easily removed. The harder kinds of granyte, gneiss, quartz rock 

 and trap or basalt, undergo usually but slow wear, while other kinds, 

 looking as firm but really subject to easy decomposition, fall away 

 rapidly before the plunging waters Projecting headlands, which stand 

 out so that the sea can batter them from opposite directions, are es- 

 pecially exposed to degradation, and particularly those on windward 

 coasts. 



3. The wearing action of waves on a coast is mainly confined to a 

 height 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 be- 

 low 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 1096 (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 cliff. 



Fig. 1096. Fig. 1097. 



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



The tide rises on the platform; and the waves, unable to reach its 

 rocks to tear them up, drive on to batter the lower part of the cliff. 

 At the Bay of Islands, New Zealand, the rocks have no horizontal 

 stratification ; and still there is the same seashore platform ; and an 

 island in the bay (Fig. 1097) is called " The Old Hat." The seashore 



