CHAPTER V. 



MOVEMENTS OF THE HYDROSPHERE AND THEIR GEOLOGICAL 



EFFECTS. 



The movements of the hydrosphere are manifested in the waves, 

 the tides and seiches, the ocean and river currents, and the move- 

 ments of ground waters. The effects of these movements upon the 

 lithosphere are seen in mechanical erosion and the transportation 

 of eroded material, and to a less degree in chemical solution and 

 decomposition of the rock. Diminution or cessation of motion is 

 followed by the deposition of the material in suspension. Deposi- 

 tion of material in solution at a distance from the source of origin 

 is a further result of the movements of water. The effect of the 

 motion upon the biosphere is shown by the increase in the supply 

 of oxygen in the water and the transportation of food. The dis- 

 tribution of organisms is also affected to a considerable degree by 

 the currents. 



Waves. 



These are the undulatory motions of water produced by wind 

 blowing over its surface. In the open sea, the motion of the water 

 is an orbital one ; the particles move in curves, and only the wave 

 form advances. The top of the wave is its crest, the bottom the 

 trough, the lines of both lying at right angles to the direction of the 

 wind. The distance from crest to crest is the waz'c Icngtli, which in 

 stormy weather in the open ocean varies between 60 and 150 meters 

 (200 and 500 feet). Swells may in some cases reach nearly twice 

 this length, however. The rapidity with which wave crests travel 

 is the velocity of the wave, which varies from 10 to 15 meters per 

 second (22 to 33.5 miles per hour, and may be as high as 60 miles 

 per hour). The time taken by a crest to travel a wave length is 

 the period of the wave, which in storm waves varies from 6 to 10 

 seconds or over. The vertical distance between the top of tlie crest 

 and the bottom of the trough is the height of the wave, and its mag- 

 nitude depends on the strength of the wind. In the open sea the 

 average height of the waves varies between 2 m. (6^ feet) and 5 



209 



