﻿THE CURRENTS OF THE OCEAN. 123 



cause why the suspended matter is found in particular places, and in great quantities : 

 and this cause, it may be readily conceived, must be the natural inequalities in the level 

 of the bottom, which, interrupting the stream, not only take up a portion of its burden, 

 but occasion the eddies that, as I shall have occasion to show hereafter, are especially 

 favorable to deposit. The inequalities need not be very great. Small impediments at the 

 mouths of harbours, or in rivers, serve this purpose of a nucleus. A vessel sunk, in a 

 spot where the water is loaded with sand or mud collects around it a deposit by which 

 it is wholly covered in a short time. This is often exemplified on various parts of our 

 coast. Where the attempt has been made to rescue valuable property from ships 

 that have remained for a long time sunk in such places, a part of the task has gen- 

 erally been to remove the covering under which they were buried. At the late meet- 

 ing of the American Association for the Promotion of Science, Mr. Dickeson re- 

 lated a remarkable incident of this kind, where, at the island of Galveston, in 1839, 

 a vessel from New Orleans was wrecked (at the south end), with a considerable 

 amount of specie. The officers of the custom-house took immediate measures to re- 

 cover the valuable cargo, and in a little time the workmen reported the vessel to be 

 nearly covered with sand. 



How far the eddying of the water produced by the interruption assists in the forma- 

 tion of the shoal will appear more plainly when considering the effect of interruption 

 and diversion of the tidal current by points of land. The results, so far as the eddying 

 action is concerned, are similar in each case, and it is no doubt safe to say that this con- 

 dition is the only one under which a deposit can take place in a strong tide-way. 



In an inclosed space, like Vineyard Sound, shoals are formed both by the ebb and 

 flood tides; their direction is parallel to the course of the tides, the place from which 

 the material is supplied being situated between them. West Chop affords an instance 

 of this. Its wasted cliffs have contributed to build up the " Hedge Fence " and " Squash 

 Meadow " shoals on the flood, and the long rids;e of the " Middle Ground " on the ebb. 

 In bringing forward these particular shoals, however, it is not my purpose to attempt to 

 account for the origin of the materials of all the shoals, but to show, what these cases 

 are especially suited to do, their dependence on the currents. 



Although the interior structure of the shoals is not accessible to examination, yet 

 there is every reason to believe, in arguing by analogy from the beaches, cliffs, and 

 hooks, that the sand composing them is deposited in layers, or strata. This is, in fact, 

 the universal law of aqueous deposit. If the views concerning the formation of the 

 shoals here presented be correct, they must have grown up gradually, and a time can 

 be referred to when they did not exist. The rate of their accumulation would depend 



