Shaler.J 466 [March 17, 



These considerations could be used to explain the erosion phe- 

 nomena of many parts of the earth's surface, but in the limits of 

 this note I only desire to present the following propositions, which 

 embody what seem to me to be the essential points in a considera- 

 tion of tidal erosion. 



1. The intensity of the forces of tidal erosion is proportional to 

 the height to which the wave rises. 



2. Given any slight irregularities of a shore, tides tend to deepen 

 the indentations until one of two results is obtained: (A.) that a 

 channel is broken open into another great water basin, or (B) the 

 bay or indentation becomes so narrow that friction arrests the further 

 rising of the wave. 



3. Operating in this way tidal currents tend to form islands by 

 breaking them away from the continents, or other islands ; when the 

 sundering is effected the intensity of the eroding agent is at once 

 lost. 



4. This sort of erosion may, in its results, closely simulate the 

 forms of shore given by glacial action ; in many cases glacial action 

 may have simply developed and given the details to bays that were 

 really of tidal origin. 



5. By familiarizing ourselves with the type of topography formed 

 by tidal action, we may be able to determine the fact that the run of 

 tidal currents has been considerably changed on particular shore 

 lines. For instance, it is quite likely that a close study of the topo- 

 graphy of the north coast of the Mediterranean will show, what seems 

 to me probable on a preliminary inspection, that this coast has been 

 formed during geographical conditions, which allowed strong tidal 

 currents to work upon it. 



6. That in general we must look to this cause as the natural 

 diversifying agent of all shore lines. The elevatory forces merely 

 give the great outlines of the coast. The detail must be worked in 

 by the action of the sea, and this is to a great extent effected by 

 tides. 



7. Tides act most where the waves act least. Were it not for 

 them the action of the sea would be mainly limited to the parts 

 which could get the full surge of the ocean. This latter action tends 

 rather to straighten than to complicate shores. 



The thanks of the Society were voted to Capt. Charles 

 Bendire, U. S. A., for the donation of ten bird-skins to the 

 Museum. 



