﻿134 GEOLOGICAL ACTION OF 



Tf, on the contrary, the shore is gaining, the beach shows the extent of the construc- 

 tive process. The growth of Nausett Beach, already cited, is an example. 



In some places the changes are irregular, as at Siasconsett, the southeast part of 

 Nantucket. There is now but a narrow strip of beach between the cliff and the sea, 

 where between 1778 and 1792, the beach spread so much that the fishermen dried 

 their fish upon it. The changes both of gain and loss are reported to have been 

 gradual, and were no doubt connected with corresponding changes elsewhere. 



The plastic power of water over so impressible a material gives to all sand beaches a 

 general resemblance. It is impossible, therefore, to distinguish between those that are 

 the remains of former shores, and those that are constantly increasing, without a knowl- 

 edge of local facts and circumstances, such as may be frequently obtained from the 

 neighbouring inhabitants. When hooks are united to the mainland, they become beaches. 

 The same term is given to those necks of loose material which join islands with each 

 other, or with the mainland, and which result from the confluence of two currents ap- 

 proaching from opposite directions. The beach of Nahant is built up in this manner, by 

 the meeting of the two branches of the flood tide, one entering to the north and the 

 other to the south of Nahant Head, at which the stream of the flood is divided. The 

 " Neutral Ground " at Gibraltar has been already mentioned. The effect of this mode 

 of construction is to change the geographical character of the place at which it occurs, by 

 converting two or more islands into one, or an island into a promontory. 



According- to the principles laid down in this memoir, beaches should be found 

 wherever there is a tidal or normal current. And it is so. Even on rocky coasts, 

 where the tides are very rapid, the ocean deposits are made at the bottom of bays and 

 in the indentations of the shores, the tidal currents losing in these situations their 

 velocity, and turning easily into eddies. Phillips's Beach and Chelsea Beach are ex- 

 amples of this kind of bay deposit ; but the rapidity of the currents and the exposure to 

 the sea have prevented the beach formation on the projecting points of rock which inclose 

 these bays, except in very sheltered spots. 



Beaches are constantly undergoing changes, and these changes are likely to be ex- 

 tensive and important in proportion to the abundance of the material. They have 

 always attracted much attention, especially where, as in Holland, the alluvial forma- 

 tions have exercised a real and permanent influence upon the public economy. It is 

 these changes alone that have been treated by the geologists, who, overrating their indi- 

 vidual consequence, have lost sight of the fact that they are merely the present indica- 

 tions of the modes and periods of great operations which have produced much grander 

 results. The destructive agency of the waves upon them has been carefully studied, but 

 the principles of their formation appear to have been wholly overlooked. 



