2i8 SEA AND LAND 



be very few. As it is, there are only a few score of conspicuous 

 deltas in the world, and the limitation in the number is clearly 

 to be explained by the frequent oscillations of the shore- 

 line. 



The mode of action of the solar heat which is brought about 

 by the intervention of waves has a more wide-spread effect on 

 the conditions of harbors, and on the whole a more important 

 one, than that which is induced by the transportation and depo- 

 sition of river-borne detritus. Every coast-line, even if it be 

 only that of an inconsiderable lake, is sure to be affected by 

 wave action. Every cliff shore is a natural factory for the pro- 

 duction of detritus. This material is composed of small discon- 

 nected bits, which are readily moved by the waves and currents. 

 The heavier waves can swing masses each of which weighs 

 many tons; the undulations of less size can move fragments pro- 

 portionate to their energy ; and even the wavelets which, wlien 

 they break, have a height of no more than a foot, can toss bits 

 of rock as laree as marbles several feet to and fro in each oscil- 

 lation. Not only does the dchj'is produced by wave action sway 

 to and fro up and down the beach in the oscillating movement 

 of the water, but it is in all cases liable to be carried along the 

 coast by wave action either directly through their stroke or 

 secondarily by the currents which they induce. If the wave 

 rolls in against the shore at right angles to its trend, the movable 

 debris on the beach merely rolls up and down the slope without 

 changing its position in any important way. If, however, as is 

 oftenest the case, the waves strike at a more or less considerable 

 angle to the shore, the fragments as they move to and fro work 

 along tlie coast in the direction in which the waves are 

 trending. 



Besides the motion imparted to the debr^is of the sea-coast by 



