THE WORK OF THE OCEAN. 339' 



The same is true of landslides where they affect the coast or any part 

 of the sea-bottom. The fall of glacier ends and the capsizing of ice- 

 bergs Ukewise generate strong waves. To the category of exceptional 

 waves also belong those generated by the winds of exceptional storms, 

 such as that w^hich devastated Galveston in 1900.^ 



Summary. — From the point of view of their direct geological results 

 in shallow water, all movements of the sea-water may be grouped into 

 two main classes — (1) waves, with the undertow and the httoral cur- 

 rents they generate, and (2) ocean-currents.^ 



WAVES. 



Wave-motion.^ — The most common waves, and from the present 

 point of view the most important, are those generated by winds. During 

 the passage of a wave, each particle affected by it rises and falls, and 

 moves forward and backward describing an orbit in a vertical plane. 

 If the passing wave is a swell, the orbit of the particle is closed and is, 

 either a circle or an ellipse ; but in the case of a wind-wave the orbit is 

 not closed. In such a wave two things move forward, the undulation 

 and the water. The velocity of the undulation is relatively rapid; that 

 of the water, slow and rhythmic. On the crest of the wind-wave each 

 particle of water moves forward, and in the trough it moves less rapidly 

 backward, and the excess of the forward movement over the backward 

 gives it a slight residual advance. This residual advance is the initiatory 

 element of current. By virtue of it, the upper layer of water is carried 

 forward with reference to the layer below, in the direction toward which 

 the wind blows. The waves of any considerable or long-continued 

 wind, therefore, generate a current tending in the same direction as the 

 wind. 



The agitation of which waves are the superficial manifestation is 

 not restricted to the surface, but is propagated indefinitely downward. 

 Near the surface the amount of motion diminishes rapidly with increas- 



^ National Geographic Magazine, Vol. XI, pp. 377-392. 



2 For causes of ocean-currents, see Croll's Climate and Time: Proc. Roy. Soc, 

 1869-73, and Jour. Roy. Geog. Soc, 1871-77. 



2 In the following pages concerning the waves and their work Gilbert's classic 

 discussion of shore features, in the Fifth Annual Report of the U. S. Geol. Survey, 

 pp. 80-100, is freely drawn on. Another incisive discussion of certain shore phenom- 

 ena is that of Fenneman, Jour, of Geol., Vol. X, pp. 1-32. 



