﻿THE CURRENTS OF THE OCEAN. 119 



shoals, by which is meant formations composed of loose and light materials, easily sus- 

 pended by the water, and carried about by its motions and agitations. 



Shoals, in the vocabulary of seamen, are, strictly speaking, spots of shallow water, 

 dangerous to vessels, generally isolated in situation, of comparatively small extent, and 

 rising from the surrounding ground in such a manner as to interrupt the channels in or 

 near which they lie. Shoals are most frequently found in great numbers in the same 

 place, and not, like rocks, single ; this feature of their existence is connected with, and 

 dependent upon, the form and material of the neighbouring land. They are also found 

 where there are currents caused by the flux and reflux of the tide, by the regnlarly uni- 

 form changes of the ocean, by occasional and local disturbances, or by the conflict be- 

 tween rivers and the waters into which they empty. 



Examples of shoals the most striking in number, extent, and relative position are 

 to be met with where the action of the tides is equal, permanent, and marked, and 

 where the land around which they are deposited is composed of materials similar to 

 their own. Such is the case on the southeastern coast of New England. 



The navigation here is rendered peculiarly dangerous by numerous shoals, amon^ 

 which the tides ebb and flow with regularity, having a rise and fall of four or five feet ■ 

 and the shores both of the continent and islands consist almost entirely of the same 

 mineralogical substances as the shoals themselves. 



From among these, that group known as the Nantucket South Shoals has been 

 selected as the most characteristic, and as the best suited to illustrate the views which 

 it is the object of this memoir to disclose. Occasional reference, however, will be made 

 to other shoals on the southern coast of Massachusetts. 



It will be readily observed, upon an inspection of the chart of the Nantucket Shoals 

 (Plate I.), that there is a conformity between the shores of Nantucket island and the 

 shoals themselves in their place and direction. On the south side of the island, where 

 the shore runs east and west, the shoals called the " Old Man," " Old South Shoal," and 

 "New South Shoal," lie more nearly in that direction than on the east side. There they 

 follow the north and south direction of the island. There is but little doubt that a com- 

 plete survey of this region will assign the same general conformity of outline to the 

 external soundings. Numerous specimens of the shoals have been collected, and they are 

 found without exception to be identical in composition with the island. Nantucket, as 

 may be easily seen at the sandy bluffs in any part of the island, is composed of quartzose 

 sand deposited in layers; which is the material of the shoals also, with this difference, that 

 the sand of the latter is somewhat finer, as if it had been subjected for a longer time to 

 the action of water. 



