THE WORK OF THE OCEAN. 341 



submarine structures are little disturbed at depths of five meters in the 

 Mediterranean and eight meters in the Atlantic/ On the other hand, 

 debris as coarse as gravel, which is transported by rolling on the bottom, 

 is not infrequently carried out to depths of 50 feet, and sometimes even 

 to 150 feet. Fine sediment, like silt, is disturbed at still greater depths, 

 for ripple-marks, which indicate agitation of the water, are said to have 

 been found at depths of 100 fathoms.^ 



AVhen a wave approaches a shehdng shore, its habit is changed. 

 The ATlocity of the undulation is diminished, while the velocity of the 

 advancing particle of water in the crest is increased; the wave-length, 

 measured from trough to trough, is diminished, and the wave-height is 

 increased; the crest becomes acute, vnxh. the front steeper than the 

 back, and these changes culminate in the breaking of the crest, when 

 the undulation proper ceases. Waves of a given height break in about 

 the same depth of water, and the hne along which incoming waves break 

 is the hne of breakers. The line of breakers is in deeper water and farther 

 from shore when the waves are strong than when they are weak. AVaves 

 are repoiled to have broken in 100 fathoms of water, ^ but this must be 

 regarded as very exceptional. The return of the water thrown for- 

 ward in the crests of waATs is accomplished by a current along the bot- 

 tom called the iindertoir. The undertow is sensibly normal to the coast 

 when uninfluenced by obliciue waves, and is efficient in remoA'ing the 

 products of erosion. 



Since the incoming waA'e affects water which is at the same time 

 under the influence of the midertow, it gives to that current a pulsating 

 character, for the wave-motion sometimes supports and sometimes 

 opposes the undertow, and thus endows it A^ith a higher transporting 

 power than belongs to its mean velocity. Xear the breaker-hne, the 

 oscillations communicated by the wave may momentarily overcome 

 and even reverse the movement of the imdertow. Inside the breaker- 

 line, irregular oscillation only is communicated. The broken wave-crest, 

 dashing forward, overcomes the undertow and throws it back, and 

 the water returns as a simple current descending a slope. The power 



^ Delesse. Lithologie des Mers de France, Cited by Geikie,. Text-book of Geology, 

 3d ed., p. 438. 



' Sir G. Airy. Encyclopedia Metropolitana, Art. Waves. Cited by Geikie, loc 

 cit., p. 438. 



' Stevenson. Treatise on Harbors. 



