MEMOIR ON BOSTON HARBOR. 101 



returning from the Charles and Mystic Rivers and from Fore Point Channel, and in a less 

 degree by the current of the flood, divided by Castle Island into two branches, which 

 meet above the island and bring in the muddy water of Dorchester and the neighboring 

 flats and the discharges of the Neponset. Upon the surface of the flats generally the 

 Avater is for a large part of the tide in a quiet state, the currents having no appreciable 

 velocity or determinate direction. That part of the flats situated between Castle Island 

 and South Boston Point has been built up by the action of both tides. As the ebb of 

 the upper harbor meets that of Dorchester Bay in this space, so also there is a confluence 

 here of the two branches of the flood, one of which passes to the north, and the other to 

 the south of the island. The result of this double confluence is an excessive increase 

 of the deposit in this place. Between Castle Island and the Point, some threadlike 

 channels or drains are still preserved, but they are of no value to navigation. It is 

 nearly, if not quite, possible to pass on foot from the island to the Point at extreme low 

 water. 



The increased amount of the deposit at the Upper Middle, where it takes a more 

 pointed form, and projects far into the main channel, shows in a prominent manner the 

 double action of the confluent divisions of the flood, and of the greater or less conflict 

 of the concurring streams of the ebb that finally approach each other from different ex- 

 tremities of the bay in the manner previously described as characteristic of the bay de- 

 posit. And this feature of the bay deposit is strengthened by the opposition, as far as 

 it goes, which the ebb current from Dorchester Bay presents to the easy course of that 

 part of the ebb current which tends to drain the South Boston Flats by passing between 

 Castle Island and South Boston Point. 



But the conflict of the stream from South Bay, through the Fore Point Channel, with 

 the ebb passing down from the upper harbor, and winding round on the South Boston 

 Flats, gives rise to that other striking peculiarity of the deposit, its pointed and project- 

 ing shape on the borders of this channel. The water is diverted from its direct course 

 to the bay, running almost at right angles to it, and the channel is constantly getting 

 longer and shallower by means of this accumulation. It will be seen by an inspection 

 of the map, that there is a remarkable correspondence between the outline of the flats 

 and of the shore, the protuberance of the Upper Middle answering to that of the head- 

 land of the heights. On the opposite side of the channel there is a similarity in oudine 

 between the flats and Governor's Island, especially in the spot making off from the south 

 point. 



These flats immediately round Governor's Island have the twofold character of de- 

 posits such as, on alluvial shores, always attach to points and headlands, and of deposits 



VOL. V. NEW SERIES. 15 



