1887, ] NEW YORK ACADEMY OF SCIENCES. 51 
Along the western shore of the Harlem River there is a border 
of salt marsh through its entire length, except where the rocky 
shore at and fora mile above High Bridge projects into the river. 
Here the process of formation is well illustrated. The tide rises 
with its load of sediment, halts for a time, depositing it, and 
retreats. The coarse marsh grass is continually edging up and 
as the deposit rises high enough it seizes and holds it with a 
tough mass of interlacing roots. So far as my observation goes, 
there is no line of demarcation between the meadows and the 
beds of the rivers. ‘That of the Hudson consists of a stratum of 
soft mud known as silt, which rests upon sand or rock. It 
weighs about 100 pounds to the cubic foot, is unstratified, and 
at the top soft, but lower down becomes tougher and more com- 
pact so as to be cut into blocks when excavated. Although its 
deposition would seem to resemble that of clay, it differs from 
clay in several important particulars.’ Gravel and filling placed 
upon it gradually sink through. When mixed with water, it 
readily forms a thin liquid emulsion. Its depth across the river 
at 42d Street varies from 25 to 40 feet. 
As the general level of the land in this vicinity has been shown 
to be slowly sinking* it is probable that this river and meadow 
alluvium is on the increase. Its very presence in the river chan- 
nels, themselves excavated in gneiss, proves that the rivers, or 
rather that the land, was formerly at a higher level. This con- 
clusion will be reinforced further on by the testimony of the 
drift. 
Drift.—By this is understood the mixture of sand, boulders, 
clay, and gravel, which, sometimes locally stratified, sometimes 
indiscriminately mixed, forms the covering of the rocky founda- 
tions of the island. Ags shown on the map, it occurs in three 
chief localities ; first below 30th street ; second filling the Man- 
hattanville depression, and third filling the Inwood depression. 
It also occurs, as stated above, in patches of varying size quite 
generally over the areamapped as gneiss. Its structure has been 
shown by street-cuttings through the hills and by well-borings. 
The structure varies with different hills. In some it is a simple 
pile of earth and boulders without traces of sorting. The boul- 
ders are round and water worn and are much used as paving in 
the less travelled streets. ‘Trap, like the Bergen Hill, predomi- 
nates, but there are representatives of nearly all the formations 
north.* Occasionally the drift affords a bone or tooth. In the 
1'Vide Proc, Am. Assoc. Civ. Eng., Vol. IX., p. 256. Paper on the 
North River Tunnel. 
? Vide Pop. Sci. Monthly, Vol. XIII., p.,652. 
3 A list is given by Mr. Issachar Cozzens in ‘‘ A Geol. Hist. of Man- 
hattan or N. Y. Island.” N. Y. 1848, p. 73. 
