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It may be stated, in general 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.