EFFECT OF WAVE-ACTION 189 



tinental sliclf borders the shor(; from near New York to 

 near the Rio Grande. Similar areas of recently emerj^red 

 shallow sea bottom occur on almost all the extended sea-coasts 

 of the world, though they are perhaps nowhere else so well 

 exhibited as in the southern seaboard states of this country, 

 where the present coast-line happens to lie near the middle 

 point in the slope of the continental shelf. 



As the form and structure of this continental shelf 

 clearly indicate that the materials have been arranged by 

 wave action, we can readily understand how portions of the 

 detritus may be thrust against the shore by the heavier 

 waves which run from the deep sea toward the coast-line. 

 It is important, however, to perceive in just what manner 

 the wave does this work. We should first note the fact 

 that in the deeper parts of the sea a wave of the first 

 magnitude, though it may have a height of as much as fifty 

 feet from trough to crest, is essentially a superficial move- 

 ment of the waters, in which the particles of the fluid do 

 not go forward in any considerable way, but merely revolve 

 in a kind of orbital movement. The wave motion which 

 we make in shaking a carpet is in all essential respects 

 comparable to that of an ocean surge where the water be- 

 neath its base is a mile or more in depth. Much as the 

 surface of the ocean is heaved and tossed by these waves, 

 the amount of movement imparted to the water is slight. 

 If we could observe what takes place on the sea- floor, 

 five thousand feet below the tempest- swept surface, it would 

 require instruments of exceeding delicacy to indicate the 

 trifling motions which the waves produce. 



As the waves from the deeper seas attain the shallow 

 water next the shore — say, when they come where the sea has 



