﻿]22 GEOLOGICAL ACTION OF 



The depth of water on the shoal is so small, that the top is subjected to a destructive 

 action of the waves and currents, producing other inequalities of level. There are drains 

 or " slues," as they are called by seamen (probably a corruption of sluices), through the 

 ridges, similar to the channels in the deltas of rivers. These drains result from the agi- 

 tations of the sea, and from the lateral pressure of the tides, which, running parallel to 

 the direction of the ridges, and being higher than their summits, are forced over them, 

 so that at the shoals the tides, both flood and ebb, appear to run at right angles to their 

 real line of direction. 



The drains or slues run in the direction of the ebb tide. This is in accordance with 

 the prevailing law of operation of the tides, which makes the ebb the most influential in 

 preserving and creating channels. 



To return to the materials of the shoals. The particles of sand, even the most minute, 

 are smoothed and rounded by the action of the water ; they appear to be finer as they 

 are farther from the shore, and in their mineralogical character they agree exactly with 

 the sand of Martha's Vineyard, Nantucket, and, it may be added, the coast of Cape 

 Cod. 



Parts of these islands undergo constant changes, sufficiently rapid to be plainly per- 

 ceptible in a few years. I have undoubted testimony, from living and intelligent wit- 

 nesses, that the beach extending along the southeast part of Martha's Vineyard has 

 been washed away for the breadth of one mile in the term of fifty years, leaving the 

 remains of the former beach as a line of detached shoals, between which and the 

 present shore there is now deep water. In 1839-41, the cliff or bank at Siasconsett 

 fell twice, carrying with it the last time about twenty-five feet of the top. One dwell- 

 ing-house, and some small buildings, were precipitated into the sea. Several other 

 houses standing on a line with these were saved by being moved in time. These disas- 

 ters were caused by an unusually high tide, and by a violent gale from the east-north- 

 east. The destructive action of the waves upon prominent headlands is well understood. 

 At a point in the Vineyard Sound called West Chop (see Plate I.), the light-house was 

 removed two years ago farther inland, the water having washed away and undermined 

 that part of the bank on which the old light-house stood. A similar change is taking 

 place at Cape May, and many such instances might be cited from books of authority ; 

 but the general fact is too well known to require additional confirmation. 



From these sources, then, there proceeds a constant supply of loose materials, which 

 are subjected to the action of the water. These materials are undoubtedly carried away 

 by the currents, and as the shoals or other deposits are found in their course, it is a neces- 

 sary conclusion that they are the products of their action. But there should exist some 



