GEOLOGY. 263 



TIDAL PHENOMENA. 



THE following facts and deductions respecting the action of the 

 tides are taken from the report of the lectures of Lieut. Davis before 

 the Smithsonian Institute.* Changes are constantly going on along 

 our coast of the utmost importance to the commerce and navigation of 

 the country. At Sandy Hook, for example, it is well known that 

 where there is now dry land there was in 1836 forty feet of water ; 

 and this in the main ship-channel. "In 1767 there was an open ship- 

 channel from Barnstable Bay to the ocean, and as late as the begin- 

 ning of this century, in heavy storms, the sea occasionally made a 

 breach over the same place ; but the process of construction under the 

 law of tidal action has closed up this opening entirely, and the place 

 is now an important part of Cape Cod. Other curious instances, de- 

 rived from a comparison of the recent surveys with the earliest maps 

 of our coast, have been noticed. Monomoy Point is constantly ex- 

 tending to the south. Considerable numbers of harbours and inlets, 

 particularly along Martha's Vineyard and Long Island, have been 

 gradually closed and converted into ponds. In the course of a few 

 years, the salt water in the ponds thus formed gradually give's place 

 to fresh water. In some cases the bottom of these ponds is deeper 

 than the bottom of the adjoining ocean. This fact is one of interest, 

 since it is found that the inhabited parts of sandy deserts, such as the 

 oases of the Desert of Sahara, present similar depressions, the bottom 

 of the valley being, in some instances, below the present level of 

 the sea. During the changes in the formation of these ponds, they 

 become the home in succession of salt-water, brackish-water, and 

 fresh-water animals, thus affording a beautiful demonstration of the 

 geological formation of basins, such as those of London and Paris, 

 in which the remains of successive races of animals are found in a 

 fossil state. 



Lieut. Davis has deduced from his numerous observations the law 

 of tidal deposits, namely, that all deposits on the external coast are 

 made by the incoming or flood tide, and that the increase of deposit is 

 always in the line of the motion of the tidal current. Thus, if the 

 tide moves to the north along any part of the coast, projecting points, 

 which may serve as nuclei, are found to elongate in a north and south 

 direction. This action is not confined to our coasts, but applies to the 

 explanation of phenomena noticed in France and Holland. Another 

 important deduction is, that the deposits at the mouth of the harbours 

 and estuaries (not rivers), known by the name of bars, are formed 

 from materials deposited by the ocean. The action of the tide is that 

 of constant deposition. Degradation of the coast is the effect of the 

 waves and storms of the ocean. The general action of meteorological 

 causes is to diminish the height of continents and to transport their 

 materials to the sea, while the action of the tide is just the reverse, 

 and tends to keep up and preserve around the coast the materials which 

 have been brought down in geological periods. In this way the belts 



* See Annual of Scientific Discovert/, 1850, p. 242. 



