34 THE BIOLOGY OF THE SEA-SHORE 



moving with the waves. Large storm waves may break 

 against a cliff without removing the barnacles which are 

 attached to its face, but this merely indicates that the 

 pressure of the liquid mass is so evenly distributed upon 

 all sides of the strong shell, that the excess of pressure on 

 any one side is not sufficiently great nor applied with 

 sufficient suddenness to cause rupture. The same waves 

 will wrench great blocks of rock from their places, and drive 

 air and water into joint-crevices with such force as to loosen 

 large fragments of cliff. According to Lake (19 15), "the 

 waves of the sea continually falling upon the shore 

 gradually wear away the land, whether its margin is a 

 shelving beach or a steep and rocky cliff. If the coast 

 consists of loose material, the waves themselves may wash 

 away the fragments, but if it is formed of firm and solid 

 rock their action is indirect. The pebbles and boulders 

 that they throw against a cliff serve as battering rams, and 

 the face of the cliff is slowly broken up, especially at its base. 

 Most rocks, moreover, are fissured, at least to some extent, 

 and the fissures are filled with air. When a wave rises 

 against a rock or cliff, the air in the fissure is compressed ; 

 when the wave falls away the air expands. By this alternate 

 compression and expansion the fissures are enlarged and 

 masses of rock are broken off. ... If the shore slopes 

 gently into the sea, the action of the waves is almost entirely 

 confined to the part between tide-marks, extending only a 

 little below the true high-water and in heavy storms a little 

 above the true high-water level. Consequently this part 

 of the land is worn away and the material so worn is deposited 

 between tide marks and below low- water mark." The sea 

 will continue to act for some time upon the cliff and the 

 shore will gradually be widened. At length, the beach 

 becomes so broad that the waves no longer reach the cliff ; 

 and unless the beach material is removed by some means, 

 erosion by the sea will practically cease 



Much also depends upon the stratification and jointing 

 of the rock, as to the manner in which erosion will act. 

 Soft rocks erode easily, while the harder they are, the 



