100 MEMOIR ON BOSTON HARBOR. 



there is found on all alluvial shores, at every point and headland around which the tidal 

 currents turn, a shoal of greater or less extent, joined to the land, and making a continu- 

 ation of it. The precise position of this shoal with regard to the point, that is, whether 

 inside or outside, or in a line with the point, depends upon the strength of the current, the 

 depth of the water, and other circumstances. But, second, if the land in question is an 

 island, there will not only be the eddying action just described, but a case of conflict will 

 occur arising from a division of the stream by the island on the one side, and the meeting 

 of the two branches approaching each other from opposite directions, on the other side. 

 These counteracting forces create a space of still water. The combination of these two 

 separate modes of action increases the amount of the deposit, and alters its form. Under 

 these conditions it will have more of the character of a bank or flat. Boston Harbor, 

 being filled with islands, abounds in examples of this kind of deposit. They are found 

 on two or more sides of the islands, being formed, of course, by the currents of the ebb, 

 as well as of the flood tide. 



Lastly, the detached shoals, which are isolated deposits in deep water, result also from 

 the tidal streams being arrested in their direct course, and being brought into that state 

 of eddies and of rest in which, as has been said before, the suspended matter readily 

 subsides. The place of these shoals may be decided by natural inequalities of the bot- 

 tom, which, interrupting the stream, take up a portion of the matter and cause eddies ; 

 or it may be owing altogether to the meeting of two or more streams from different 

 sources or directions in a central spot, around which they gyrate and fall into repose. 

 The bottom in the lower part of the harbor is rocky, and, as there is a rock near the 

 shoal north of Rainford's Island, we may assume that the position of that shoal has been 

 determined by a nucleus, a natural unevenness in the bottom, as in the case first de- 

 scribed. But the small shoal in the Mystic Channel is the simple result of the conflict 

 of the stream of the channel on the ebb with that returning from Chelsea Creek. This 

 shoal has been bored by Mr. Williams of Charlestown, and found to consist entirely of 

 soft mud ; that is, there is no perceptible nucleus. The Lower Middle probably belongs 

 to the same class, although a single rock is found near the shoal. 



Although pains has been taken to classify the deposits according to their characteris- 

 tic forms, yet it is not to be understood that these precise forms can be distinctly traced 

 in all, or even in many cases, because, as before said, the several modes of action of the 

 tidal currents, or the causes of deposit, are combined in their effects, and produce com- 

 plicated results. For example, the broad South Boston Flats have the general character of 

 a bay deposit ; but this character is modified by other causes. They have accumulated 

 by means of the gradual and long continued deposits made by the currents of the ebb 



