﻿124 GEOLOGICAL ACTION OF 



upon the quantity of matter transported by the currents, and this again upon the degra- 

 dation of the land. 



The question is often asked, whether the presence of sandy materials in the water is 

 discernible. I do not know how I can answer this question better than by repeating a 

 circumstance related to me by our distinguished fellow-citizen, Mr. T. H. Perkins. 

 About sixty years since he was approaching the coast of New England on his return 

 from the West Indies. By the sudden and unexpected decrease in the depth of water, 

 the vessel was discovered to be in dangerous proximity to the Nantucket South Shoals. 

 The weather was stormy, but the captain found it necessary to carry sail to return again 

 into deep water, and by doing so kept the decks of the vessel washed by the sea. In 

 the morning the decks were covered with the sand which had been lodged there by the 

 waves. The extent to which this part of the ocean is freighted with sand is no doubt 

 always varying, but it may be fairly presumed that a large amount is furnished by every 

 violent storm. 



There is no satisfactory means of ascertaining what changes the present shoals have 

 recently undergone. These are probably not important, though undoubtedly increasing. 

 The causes that have produced them, the shoals themselves, and the general form of the 

 shores, have, according to all authentic information, remained in a great measure the 

 same during the historical period of the country. 



One quality of the shoals which deserves special notice is their firm and compact 

 structure. That this is not to be attributed to the superincumbent pressure of the 

 water only is shown by the softness of the muddy bottom of the ocean, at the depth of 

 a thousand fathoms. The water permeates so freely through the fine matter of which 

 this is composed, that the upper part of the bottom is partially suspended in it, and thus 

 the water, in resisting, shares the pressure from above. The hardness of the shoals, there- 

 fore, must be accounted for partly by the difference of the material, which, being much 

 coarser, is better suited to form a solid structure in the water, and partly, perhaps 

 chiefly, to the beating of the waves, which, hammering with unceasing industry upon 

 their outside, drives the particles of sand closely together, and gives to the whole mass 

 compactness and solidity. 



Section II. — Upon Hooks, Bay-Deposits, Bars, Beaches, fyc. 



Shoals are the result of the constructive action of the tides beneath the surface. 

 They do not appear above it on account of their exposed and disconnected situations, 

 and the constant destructive action of the waves upon their summits. 



