86 
[Assembly 
by the sand and silt carried in by the tide, the deposites from the 
surface waters of the adjacent country, and by sand drifted from 
the beach. Not only marine animals and plants, by their growth 
and decay, add new matter to the gradually shoaling pond or bay, 
but the accumulation of drifted sea-weed, trees, &c. serve to in- 
crease the alluvion at every successive storm. These various 
causes combined, gradually shoal the water with alluvial deposi- 
tions, until marsh grass finally takes root upon the surface. In 
the formation of these marine alluvions, vegetable remains far 
exceed the other materials in volume, so that an imperfect marine 
peat results. The marine peat observed in most localities, was of 
inferior quality: it was light and spongy, containing undecomposed 
vegetable matter. 
Alluvial Sandstones, Sfc, 
Sandstones^ conglomerates and brown iron ore, are continually 
forming, in small quantities, in several localities, by the action of 
mineral springs, and by the decomposition of pyrites. 
The mineral waters producing these effects are chalybeate, and 
on flowing into the low grounds, or on exposure to the air, lose their 
excess of carbonic acid, by means of which the precipitated car- 
bonate of iron had been held in solution. This precipitate, acting 
as a cement, unites the grains of ^and into a ferruginous sandstone. 
It generally occurs in thin plates, from h inch lo 2 inches thick; 
but occasionally in thick masses. At Broad-meadow Point, called 
also Iron Point, 2 or 3 miles east of River-head, in Suffolk county, 
this recent sandstone may be seen, at low water, in thick solid 
masses. It may also be seen in the sand cliffs between Roanoke 
Point and Mattituck Inlet. 
Nodular masses of iron pyrites are not uncommon in the pebble 
beds of Suffolk county, and by their decomposition, form brown 
oxide of iron or haematite, enveloping the adjacent substances 
which serve as nuclei. When nodules of clay, or decomposable 
stones are thus enveloped, geodes of brown haemetite are the re- 
sult. These are abundant at the high cliff on the northeast side 
of Loyd's neck, in Huntington. Where these geodes are numer- 
ous, a kind of ferruginous conglomerate is formed of gravel, peb- 
bles and geodes. At the above locality, the geodes and conglome- 
rate were confined to a stratum of only a few inches in thickness. 
Two geodes were found filled with water. 
