36 



AQUEOUS AGENCIES. 



by this cause. The softer parts are worn away into harbors by the 

 waves and scoured out by the tides, while the harder parts reach out 

 like rocky arms far into the sea. Sometimes only small rocky islands, 

 stripped of every vestige of earth, mark the position of the former coast- 

 line. Boston Harbor and the rocky points and islands in its vicinity are 

 good examples. The process is still going on, and its progress may be 

 marked from year to year. 



On the Southern coast examples of a similar process are not want- 

 ing. At Cape May, for instance, the coast is wearing away at a rate 

 of about nine feet per annum. The more exposed portions about 

 Charleston Harbor, such as Sullivan's Island, are said to be wearing 

 away even more rapidly. As a general fact, however, the low, sandy, or 

 muddy shores of the Southern coasts are receiving accessions more 

 rapidly than they are wearing ; while, on the contrary, the New Eng- 

 land coast, as proved by its rocky character, is losing much more than 

 it gains. The shores of Lake Superior (Fig. 29) furnish many beauti- 



Fig. 29.— Lake Superior. 



f ul examples of the action of waves — in this case, of course, unassisted 

 by tides. The general form of the lake along its south shore is deter- 

 mined by the varying hardness of the rock ; the two projecting promon- 

 tories La Pointe (a) and Keweenaw Point (c) being composed of hard, 

 igneous rocks, while the intervening bays b and d are softer sandstone. 

 On the south shore, about e, between La Pointe and Fond du Lac (/), 

 the conditions of rapid erosion are beautif ally seen. The shores are 

 sandstone cliffs, with nearly horizontal strata. These have been eroded 

 beneath by the waves, in some places for hundreds of feet, forming 

 immense overhanging table-rocks, supported by huge sandstone pillars 

 of every conceivable shape. Among these huge pillars, and along these 

 low arches and gloomy corridors, the waves dash with a sound like thun- 

 der. From time to time these overhanging table-rocks, with their load 

 of earth and primeval forests, fall into the lake. 



