﻿THE CURRENTS OF THE OCEAN. 121 



It may be stated, in genera] terms, that the tidal wave which supplies this part of the 

 coast approaches it in a direction between north and west, but is deflected to the east 

 by the banks that lie between the Gulf Stream and the main, known in the technical 

 language of seamen as "soundings on the coast." The general course of these banks 

 answers to that of the shores, which from New York begin to turn to the eastward. 

 Farther from the island of Nantucket, therefore, the easterly direction of the flood in- 

 clines to the northward, and the westerly direction of the ebb to the southward. Now, 

 the shoals in the vicinity of Nantucket conform so nearly to the courses of the tides as 

 to leave no doubt that they are indebted to them for their form. 



The shoal called the " Old Man," on the south side (see Plate I.), lies nearly east 

 and west, while the " Bass Rip " on the east side has a north and south direction. 



The " Old South Shoal," at a distance of twelve miles, and situated where the 

 flood tide takes a more northerly course, lies about northeast and southwest. 



Without multiplying these instances, it can be safely asserted that the same law of 

 conformity is exemplified in all the principal shoals of this extensive group. This view 

 is further illustrated by noticing the configuration of these shoals, which as a whole may 

 be regarded as a system of curved elevations concentric with the island of Nantucket, 

 and though the members of any one series may be separated by deep water suited to the 

 purposes of navigation, yet the relation below the surface can be frequently traced, either 

 by similarity of material, or by comparatively smaller depths along their line of connec- 

 tion. These separations must be regarded as the channels preserved by the flow and 

 ebb of the tide, through which the great body of the water that passes among the shoals 

 finds its principal vents. 



Between the several series, the intervening space is marked by striking differences of 

 depth and bottom. The shoals themselves are composed, as has been said, of sand ; but 

 in the valleys between there are dead shells, coarse gravel, pebbles, and stones, the latter 

 abounding in animal life, in which the shoals are entirely deficient ; showing that there is 

 not merely a difference of level, but a real distinction in origin and composition. 



Hitherto I have spoken principally of the form and composition of the shoals in their 

 relation to the bottom at large ; but there are peculiarities in their individual structure 

 worthy of special notice on account of their close resemblance to certain forms common 

 in the drift deposits of New England and Northern Europe. One of the most striking 

 of these peculiarities is the ridgy character. The top of the shoal is not an even sur- 

 face, like a plain, but its outline is rounded, and frequently broken into several long and 

 narrow summits, or ridges, crowded closely together, and in general parallel to the main 

 axis of the shoal. 



