674 DYNAMICAL GEOLOGY. 



frith with a velocity of sixteen miles an hour. (Macgowan.) In the 

 eagre of the Amazon, the whole tide passes up the stream in five or 

 six waves, following one another in rapid succession, and each twelve 

 to fifteen feet high. 



The ebbing tide causes the out-flowing current, which is directly 

 the counterpart of the in-flowing current. It is more quiet than the 

 latter in its movement ; but it is often a rapid and powerful current, 

 because more contracted in width than that of the flow, — and espe- 

 cially so in bays in which the waters of a river add to the volume of 

 the ebb. 



The piling of the tide-waters to an unusual height in converging 

 bays, raising them far above their level outside, is another cause of 

 out-flowing currents. The flow is along the bottom, and it often ha* 

 great power. 



In consequence of the tidal movement carrying the waters up coasts, 

 the sea has its flood-grounds, like rivers ; but the floods occur twice a 

 day, with each recurring tide. The tidal movement of the ocean is 

 communicated to the water of rivers and the translation wave travels 

 up stream, the salt waters following. It extends along Hudson River, 

 to Troy, 150 miles, 50 miles up the Connecticut, 400 miles up the 

 Amazon ; and, rising above the level of the wells along the coast and 

 the outlets of the subterranean streams, it acts, siphon-like, to raise 

 their waters, so that such wells also have their tides. In the outflow 

 the rivers regain their normal action and follow the salt waters out to 

 sea, and extend, in the case of large rivers, far out, as a surface layer, 

 over the heavier salt water. 



4. Earthquake Waves. 



In an earthquake, the movement of the earth may be either (1) a 

 simple vibration of a part of the earth's crust; or (2) a vibration with 

 actual elevation or subsidence. If submarine waves are produced, 

 they have a forward impulse, and, in the second case, an actual for- 

 ward movement or amplitude equivalent to the amount of change of 

 level ; in each case, therefore, they are translation-waves. They have 

 great power; and, as there is no narrow limit to the amount of eleva- 

 tion which may attend an earthquake, such a wave may be of enormous 

 height. The velocity of propagation varies as the square root of the 

 depth, the number of miles per hour being 12-2 miles in a depth of 10 

 feet; 38-7 in that of 100 feet; 122-3 in that of 1,000. The wave is 

 made of water taken from either side, and hence along the land the 

 bottom is left bare for some distance out before each rise and plunge. 

 An earthquake at Concepcion, Chili, set in motion a wave that trav- 

 ersed the ocean to the Society and Navigator Islands, 3,000 and 4,000 



