656 DYNAMICAL GEOLOGY. 



mined by the oceanic currents ; but it is not probable that the po 

 sition of the slope has directed the courses of the currents. 



The tidal Jfoio and upper wind-currents may produce results similar 

 to those of fresh-water streams of equal velocity. 



The ebbing tide and the under-currents act on the bottoms of inlets and 

 harbors, and especially their channels, and are an important means 

 of keeping them open to the ocean and of modelling their forms. 



2. Erosion by waves. — The waves bring to bear the violence of a 

 cataract upon whatever is within their reach, — a cataract that girts 

 all the continents and oceanic islands. In stormy seas, they have 

 the force of a Niagara, but with far greater effects ; for Niagara falls 

 into a watery abyss, while in the case of the waves the rocks are 

 made bare anew for each successive plunge. It is not surprising, 

 therefore, that in regions like Cape Horn or the coast of Scotland, 

 where storms are common and the bordering seas deep, the cliffs 

 should undergo constant degradation and be fronted by lofty cas- 

 tellated and needle-shaped rocks. The action of the ordinary 

 breakers is sufficient to wear the rocky shores, reduce stones to 

 gravel and sand, and grind the sands of beaches to a finer powder. 



The cliffs of Norfolk and Suffolk, England, afford an example that has been 

 long under observation, as the country is one of houses and cultivated fields. 

 Lyell states that when the present inn at Shei'ringham was built, in 1805, it was 

 fifty yards from the sea, and it was computed that it would require seventy 

 years for the sea to reach the spot, — the mean loss of land having been calcu- 

 lated, from former experience, to be somewhat less than one yard annually. 

 But it was not considered that the slope of the ground was//ow the sea. Be- 

 tween the years 1824 and 1829, seventeen yards were swept away, bringing the 

 waters to the foot of the garden ; and in 1829 there was depth enough for a 

 frigate (twenty feet) at a spot where a cliff of fifty feet stood forty-eight years 

 before. Farther to the south, the ancient villages of Shipden, Wimpwell, and 

 Eecles have disappeared. This encroachment of the sea has been going on from 

 time immemorial. Many examples might be cited from the American coast, 

 but none as 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. But granite, gneiss, quartz rock, and trap or 

 basalt, undergo usually but slow wear. Projecting headlands, 

 which stand out so that the sea can batter them from opposite 

 directions, are especially exposed to degradation, and particularly 

 those on windward coasts. 



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



