﻿120 GEOLOGICAL ACTION OF 



This correspondence in direction and similarity of material imply the existence of an 

 intimate relation between the deposits above and below the water ; and as it is apparent 

 that the causes which have produced the latter, whatever they may be, are not sudden or 

 accidental, but, on the contrary, continued and regular, it is to be expected that the same 

 or similar causes should now be discovered in operation. 



The most prominent feature of a shoal, which is its rise above the ordinary level of 

 the surrounding ground, proves that the mode of its formation is different from that of 

 the bottom of an inclosed basin of quiet water, where the matter in suspension is de- 

 posited horizontally. This difference has been attributed to the action of the waves of 

 the sea. If this supposition were true, shoals should be more numerous than they really 

 are, and they should be constructed according to the direction of the prevailing winds. 

 Lakes, moreover, are wanting in shoals, which would not be the case if they were pro- 

 duced by waves alone. The effect of the agitation of water by waves over a nearly 

 uniform surface is to distribute materials, not to heap them up; as is shown by the 

 level bottom of a shallow pond. That shoals are not the result of wave-action alone, 

 then, is proved by the universal fact, that their form has no connection with prevailing 

 winds, by their absence in lakes, and by the general distributive, rather than construct- 

 ive, tendency of waves. 



Recurring again to the similarity of materials of the shoals and the adjacent land, to 

 the facility of their transportation, and to the regular and constant movements of the 

 water effected by the tides, it remains to consider the influence of the tidal currents. If 

 this influence operates, it should show itself in various ways ; the shape and direction of 

 the shoals should correspond to the course and action of the tides. The Nantucket South 

 Shoals are in general long and narrow. They lie where the daily ebb and flow of the 

 tides are rapid and, during the greater part of their period, steady in direction ; and the 

 direction of the tides is also that of the shoals. The south shore of the island, from 

 Smith's Point to Siasconsett, runs, as was said before, east and west. At Siasconsett 

 the land turns gradually to the north, so that the shore from this place to Great Point 

 lies nearly north and south. 



The tides that cause the flux and reflux of the waters pressing upon the island follow 

 the outline of its shores. The flood, for example, commencing with a nearly easterly 

 direction on the south side, passes gradually round to the north on the east side, follow- 

 ing the trend of the land. The reverse is the case with the ebb. As the distance from 

 the shore increases on the south and east sides, the direction of the tides is modified by 

 the remoter influences which govern the progress of the tidal wave along this part of 

 the coast. 



