﻿130 GEOLOGICAL ACTION OF 



Where there is a harbour at the bottom of the bay, as at Nantucket, Edgartown, and 

 New York (the bight included between the Jersey shore and Long Island being con- 

 sidered as a bay with regard to the ocean), a part of the bay deposit takes the form of 

 a bar across their entrance. The depth of water on the bar will correspond to the 

 character of the harbour ; so, also, will the depth and breadth and directness of the chan- 

 nels, these last being simply the passages kept open by the great body of the water that 

 advances and retires with every successive flood and ebb. If the capacity of the har- 

 bour is small, as in the case of Nantucket, the bars will in some places rise near the sur- 

 face, and the drains or channels will be contracted and narrow, their direction being 

 controlled by the position of the shoals that accumulate near the mouth of the harbour. 



If, as at Edgartown, there is a double communication with the sea, and a narrow 

 channel that confines the water and gives rapidity to the current, then a greater depth 

 will be preserved over the bar. And finally, if, as is the case at New York, there is a 

 bay of large capacity, a double communication with the sea, and rivers emptying into the 

 harbour, the depth of the channel over the bar must always be considerable. But the 

 width of the entrance, as well as the capacity of the harbour and the action of rivers, 

 will have a controlling influence upon this depth. 



Let us pass for a moment from the harbours and inclosed basins of the North to the 

 peculiar formations of the Southern coast. In Cape Cod, Great Point, Sandy Hook, &c, 

 are exemplified the construction of the external points which shut off from the encroach- 

 ments of the sea the large bays, under the protection of which are formed the smaller 

 hooks and belts ; and these last, as has been seen, may, and finally do, inclose ponds 

 and create harbours. On the southern coast, similar phenomena occur on a large scale, 

 in which the various principles already laid down are brought into play. 



There are low borders of sand, forming for many miles the ocean boundary of the 

 Carolinas and Georgia, inclosing lagoons of considerable space and depth, into which 

 the rivers of the continent are discharged. A glance at the map will suffice to per- 

 ceive the number and peculiarities of these formations. 



There is one peculiarity of these inlets, which is, that they have a communication 

 with the sea that is never destroyed, though its place may be changed. In this respect 

 they differ from those in the Vineyard and Nantucket Sounds, which are eventually 

 turned into ponds. Their characteristic feature, however, is, that they are the receptacles 

 of one or more rivers, and that the capacity of each inlet appears to be proportionate to 

 the number and size of the rivers emptying into it. This fact suggests the explanation 

 of their mode of construction and actual condition. 



In describing the Northern basins, notice was taken of the manner in which the cur- 



