286 



DUNES OR DOWNS. 



sand are also observed on the islands of Nantucket 

 and Martha's Vineyard, and some of the Elizabeth 

 Islands. 



It is curious to observe, that every obstacle which 

 creates an eddy current in the wind, as a rock, 

 fence, bush, or tree, causes a deposite of sand, 

 which often serves as a nucleus of a hillock, in the 

 same manner as a snowbank is formed, which 

 they resemble also in variety of outline and appear- 

 ance. The sand is often thrown up three or four 

 feet high in a few hours. On the shore of Long 

 Island, these dunes are often from 10 to 40 feet in 

 height, and extend several miles, often drifting over 

 arable land, or filling up ponds of water. In this 

 way the value of farms is often seriously affected. 

 The only practical method of arresting them is by 

 the cultivation of some plants which vegetate only 

 in the most barren sands, as leach-grass^ Hudsonia^ 

 &c. These, by their roots, confine the sand and 

 prevent its drifting. 



A Tabular View of the Rocks of the United States, 

 with their most important Imbedded Minerals. 



( Bog ore (phosphate of iron), carbonic acid, sul- 

 I phuretted hydrogen, carbonate of lime, carbonate of 

 Alluvium. { iron, sulphate of lime, sulphate of magnesia, sul- 

 I phate of alumina and potassa (alum),* peat, marl, 

 \^ manganese, &c. 



( Native gold, native copper, barytes. The various 

 Diluvium. < ores of lead and iron, together with most of those 

 ( found below in the tertiary. 



{ Hydrate of iron (brown hematite, and many other 

 Tertiary. < varieties), manganese, gibbsite, carbonate of lime, 



( iron pyrites, selenite, amber, &c. 

 >^ f ( Native copper, and the other varieties. Ores 



New Red ) of iron, lead, zmc, barytes, strontia, lime, flu- 

 g J Sands to7ie.\ ate of lime, calc spar, tiipoli, bituminous coal, 



I V anthracite, gypsum, salt, mineral waters. 



^ Green sand. { Iron, gypsum, clays, copper, manganese, 

 p [_Cretaceous. ( lime, magnesia, fossil organic remains. 



* In mineral waters. 



