﻿nil: CURRENTS OF THE OCEAN. |,'{7 



they prove beneficial, by lining the inner border of the beach with a range of low hills 

 that guard the land within from the inroads of the sea, or by fencing around a low cavity 

 of water, which, being first converted into a swamp, becomes finally the only fertile 

 ground of the region. Bui the surface of every alluvial soil is a dune formation. Capes, 

 hooks, beaches, &c., owe their growth above water principally to this cause, and where 

 the spot is favorably situated for such mode of accumulation the rate of increase can be 

 satisfactorily estimated. Dunes, in common with deltas, accordingly have been employed 

 as a geological chronometer for determining the periods of the actual alluvial formations. 



In Europe the phei tena of dunes are more extensively developed than in the United 



Stales. At. Cape Cod, however, they exist in every variety and of the greatest size, 

 and have formed with such rapidity, thai the inhabitants of Provincetown, fearing to be 

 buried in their progress, have planted great quantities of the beach-grass (Calamagrostie 

 arenaria) on the northeast end of the cape for protection. This pioneer grass readily 

 takes root, and lays the basis for vegetation. 



One cause why the dune formation is less general in t Ik; United Stales is the 

 absence of any greal level country of sand, like Holland or the Landes of France. The 



chief reason is the hilly and unequal surface of that, pari of the coast in which the 



deposit is made. The sand is frequently deposited al the bottom of high dills, as ai 

 Martha's Vineyard and Nantucket, up which it is blown with such violence, iii strong 



winds, that it is necessary 1o cover the lace to secure it against injury. Upon tin: Surface 



of i In: cliff, which it renders sterile by its constant additions, it is dispersed equally by 



the wind in level beds, following the form of the Stratum on which il lies, and is not 

 heaped up in the dune shape, as on the more level parts of the islands, and where the 

 cliffs are. low. 



At Sankaty cliff, on the southeast pari of Nantucket, there is a deposit of this loose 

 sand, two and a half feet thick, covering (he former soil. Al. the edge of the cliff, 

 Where it has been washed away, it is seen that this loose sand has covered swamps, 



which, having been drained by the destruction of the cliff, are converted into beds of *\iy 



peat. This deposit, of sand must have begun after tin; wood, that once covered the 



iskmd of Nantucket, was cut down. Its rate of increase, therefore, might In: estimated. 

 The wind deposit is distinguished from tin: water deposit by three peculiarities; — it fol- 

 lows the form of the stratum on which il. lies, it. is not: stratified, and it contains no peb- 

 bles. At (-'ape Cod the ripple-mark made by (he wind occurs frequently, and in several 

 spots that are particularly lia hie to its action, the pressure of violent, gales has produced 

 a hardness of surface similar to that, of the shoals, resulting from the action of the 

 water. 



