1875.] 465 [Shaler. 



A very pure piece of the Tetrahedrite from the vein gave : — 



S . 27.60 



Sb 25.87 



As . . . . . . traces 



Cu 35.85 



Fe 2.66 



Zn 5.15 



Ag 2.30 



99.40 



Note on some Points connected with Tidal Erosion. 

 By N. S. Shaler. 



Owing to the fact that tidal waves heap themselves up in every 

 indentation on the coasts against which they sweep, we have in them 

 a continued diversifying agent, the geological effects of which it is 

 important to consider. Taking, for instance, the most striking exam- 

 ple of tidal wear on this coast, that which is going on in the Bay of 

 Fundy, it is quite easy to see that the erosion there is dependent on 

 the closure of the bay on the north, or, in other words, the preserva- 

 tion of the wedge-like arrangement of the shores. As soon as the 

 eating action has opened a pass into the Gulf of St. Lawrence/ the 

 tides will cease to act with the same vigor, and whenever the current 

 has swept out the opening to considerable width, the extraordinary 

 rise of tide may be almost entirely effaced. 



When we look at the shore lines of the world we find that they are 

 generally much indented and strewn with islands in those regions 

 where tide runs are pretty strong. It happens that in many places, 

 especially in the North Atlantic, the indentations due to the glacial 

 period and to tides are mingled together. In the British Isles we 

 have a remarkable mingling of the two types of erosion, giving us, 

 in many cases, substantially the same results. I am inclined to think 

 that the coast line of Scotland owes little to tides, while southern 

 England probably owes very much to their action. In the formation 

 of the Bristol Channel tidal action has had a very great part, and I 

 am inclined to think that the English Channel is in good part due to 

 this form of erosion. If we could close the Straits of Calais by an 

 isthmus, we would at once cause the tides on both sides to rise very 

 much above their present level, and thereby immensely increase their 

 erosive power. At present the slight value of tidal wear is no fair 

 measure of its value while its work was incomplete. 



PROCEEDINGS B. S. N. H. — VOL.. XVII. 30 JULY, 1875. 



