MEMOIR ON BOSTON HARBOR. 99 



From the foregoing recital of tliu proofs of deterioration in the main sliip-cliannel 

 of the harbor, a deterioration slow and gradual, certainly, but which, if suffered to con- 

 tinue, must in course of time impair the commercial advantages of the city, I pass to 

 the treatment of tlie natural causes, by the operation of which, assisted as they unavoid- 

 ably are by artificial constructions, this injury is produced. 



The subaqueous deposits in Boston Harbor may be separated into three classes, the 

 broad Jlais attached to the dry land, from which they have been extended by gradual ac- 

 cumulation, of which the South Boston and Quincy flats arc examples; the shoals and 

 banks connected with the land, which make out in a more or less pointed form, being 

 projected like spits into the channel, and having deep water on both sides of them, of 

 which the spit to the eastward of Castle Island is an example ; and detached shoals 

 wholly disconnected with the land, and surrounded by deep water, of which the shoal in 

 the Mystic Channel (on a small scale), and the Lower Middle, and the shoal to the north 

 of Rainsford's Island, are examples. 



These deposits consist of the silt of the rivers, or of the degraded materials of the 

 neighboring lands, or of both. They are created by the action of the tidal currents, and 

 this action varies under different circumstances, as these distinct forms indicate. The 

 different forms also run into each other and unite under the combined influence of the 

 various modes of action. The broad JIat, or " bay dej)osit," as I have elsewhere named 

 it, is the greatest in extent. It is a skirt of shoal ground, continuous with the beach, 

 and running off some distance under the water, and is formed in those places which re- 

 cede in the manner of a bay from tlie main passages of the harbor. The current car- 

 ried towards the sides and bottom of the bay loses its velocity by degrees as it meets 

 the resistance of the shore; and in bays of every dimension there will be more or less 

 conflict of the tidal streams arising from their approaching each other at last from differ- 

 ent sides of the bay. The quiet condition of the water produced by the first of these 

 causes, and the eddying action following the second, are states in which the water drops 

 its burden witli facility. The external outline of the bay deposit will depend chiefly on 

 the conflict of the tidal streams. 



The spit deposit, running off from the land (whether an island or the main shore), 

 is created by a twofold action of the tidal current. First, when a tidal current freighted 

 with suspended matter presses on a point of land, beyond which it expands into a more 

 open space, it falls into eddies beyond the point, and these eddies again are favorable to 

 an accumulation of the suspended matter. The point or prominence serves as a nucleus 

 to a shoal or bar joined to the land. The stream presses with accumulated force upon 

 the point, eddies around it, and loses its velocity by diffusion. This is so common, that 



