EROSION OF ROCKY COASTS 



117 



Waves are not entirely dependent, as rivers are, upon the hard 

 materials which they dash upon the coast for their efficiency as 

 destructive agents. The force of the mere blow given by a storm 

 breaker is very great, and the hydrostatic pressure which first 

 forces the water into every fine crevice of the rock, and then 

 withdraws it, together with the sudden compression and reexpan- 



FlG. 42. — A rocky shore, coast of Maine. (Photograph by McAllister.) 



sion of the air contained iri these fissures, assists materially in the 

 loosening of the blocks. 



Along coasts which are composed of hard rocks the work of 

 cutting back the land by the sea is comparatively slow, but when 

 the rocks are soft and yielding, and yet rise abruptly from the 

 ocean, the waste is so rapid as to attract every one's attention. 

 The coast of Yorkshire in England is washed away at an average 

 rate of nearly seven feet per annum. The island of Heligoland 

 near the German coast, which has now a circumference of less than 

 three miles, in 1300 a.d. measured forty-five miles around. At 



