Il6 THE SEA 



Icebergs. — When a glacier enters the sea, it ploughs along the 

 bottom until the buoyant power of the water breaks off great frag- 

 ments of it, which float away as icebergs. These are often of 

 gigantic size, veritable islands of ice, and huge as they appear, 

 only about one-ninth of their bulk is above water. As icebergs 

 are derived from glaciers, they carry away whatever debris the 

 parent glacier had upon or within it. 



2. The Sea 



The destructive work of the sea is accomplished mainly by 

 means of the waves which the wind raises upon its surface. 

 Ocean currents are, as a rule, so far from shore, and flow in such 

 deep water, that their erosive power is comparatively small. The 

 Gulf Stream is said to scour the bottom in the Florida Straits and 

 off the Carolina coast, but this is exceptional. 



Waves act continually upon all coasts, but with very different 

 force at different times and places. According to observations 

 made for the Scotch Lighthouse Board, the average wave press- 

 ure on the coast of Scotland is for the five summer months 

 6n pounds per square foot, and for six winter months 2086 

 pounds. These are average figures and are greatly exceeded in 

 storms, when the force of the breakers often rises to many tons 

 per square foot. 



The effect produced by this great force depends upon the char- 

 acter of the rocks of the coast, its height, and the angle at which 

 it rises out of the water. When the coast is high, steep, and 

 rocky, the waves continually wear away its base, partly by dis- 

 lodging the blocks into which all consolidated rocks are divided, 

 and partly by using as projectiles the blocks which it has dis- 

 lodged, or which have been loosened by the frost. In heavy gales 

 great masses, weighing tons, it may be, are hurled with tremendous 

 violence against the base of the cliffs, cutting them into caverns, 

 which are further excavated by the ordinary surf. Eventually, 

 the cliff is undermined, and the unsupported masses above fall 

 in ruins. 



