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 
game 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 heach-^rass, Hudsoniay 
&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 
AllumumA iron, sulphate of lime, sulphate of magnesia, sul- 
1 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 
'§ I New Red j of iron, lead, zinc, barytes, strontia, lime, flu- 
3 1 Sandstone. ] ate of lime, calc spar, tripoli, bituminous coal, 
I I, anthracite, gypsum, salt, mineral waters. 
^ I Green sand, i Iron, gypsum, clays, copper, manganese, 
p L Cretaceous. \ lime, magnesia, fossil organic remains. 
