﻿128 GEOLOGICAL ACTION OF 



shoal joined to the point extends four miles from the land in an east-northeast direction. 

 An extraordinary supply of the material is brought to this spot by the tides, the first half 

 of the ebb of the Sound conflicting hereabouts with the last half of the flood from the 

 northern shore, and creating lower down a case of what seamen call " tide and half- 

 tide.'! This shoal is indebted both to the flood and ebb tides for its great extent. 



When similar deposits occur at capes and headlands belonging to an earlier geological 

 period, they may come finally to protect the cliffs against further abrasion. Cape Poge 

 is so protected, but West Chop, having no such defence, is, as has been already men- 

 tioned, constantly wearing away. It sometimes happens that the continued additions to 

 the extremity of a hook unite it at last to the mainland. A sheet of water is then in- 

 closed, making a lake or pond, breached by heavy gales, which may still have an occa- 

 sional communication with the sea. Examples of this pond inclosure are common in all 

 that part of the New England coast subject to alluvial deposit. The water in them will 

 in the course of time become fresh, and as the sand continues to be heaped up on the 

 outside of the separating belt (upon which dunes also will rise), their depth often ex- 

 ceeds that of the neighbouring sea. Mr. Lyell speaks of similar formations on the 

 coast of Norfolk, near Yarmouth. Upon the island of Chappaquiddick, adjoining Mar- 

 tha's Vineyard, the people are now employed in converting one of these inclosures into 

 a herring-pond ; it is but a very few years ago that it was open to the sea, and has since 

 been made fresh by rain and snow. Opposite to Cape Poge there is another pond nearly 

 closed up, and one individual has undertaken during the last year to complete the work 

 of nature by artificial means, for the same purpose.* This explanation of the manner 

 in which a sheet of water may gradually, and without violence, be changed from salt to 

 brackish, and from brackish to fresh, has, as will be shown hereafter, a peculiar interest 

 in fossil geology. 



The hook deposit is one of peculiar importance, on account of its form and of its 

 relation to the bay it shelters. It is, however, of inferior importance in its amount either 

 to the bay deposit or the sound deposit, both which are due to those principles of 

 action of the tidal undulations which are briefly described as follows. The form assumed 



* The lake of Stennis in the Orkney Islands " has been actually converted, within a very recent period, 

 whether by elevation of the land or other cause, from a salt-water loch into a fresh-water and marshy tract." 

 — Murchison, Geology in Russia, &c, p. 302. Is not this result produced in the manner described in the 

 text? Dr. S. Cabot has stated, in a communication to the Natural History Society of Boston, that there is a 

 pond of water six miles in circuit, called Great Pond, now perfectly fresh, on the eastern end of Long Island, 

 which, thirty years ago, within the remembrance of many living people, was a harbour of refuge for small 

 vessels. This is an authentic case of current action. 



