1887. | NEW YORK ACADEMY OF SCIENCES. 61 
tended and thin a stratum could by any possible convulsion be 
doubled up together so as to give these results. The synclinal 
mapped seems beyond dispute; as for dividing up the strata 
each side of it, I do not feel justified in doing it. 
The topography of the upper island has been treated by Prof. 
Dana with great minuteness.’ The level parts or low-lying partsare 
doubtless due, in large measure, as he suggests, to the former pres- 
ence of limestone, which has been eroded away, and that the Har- 
lem River owes its channel to the same cause. But the bend of the 
river to the east at McComb’s Dam requires another explana- 
tion, and as its course corresponds very closely to the direction 
of the glacial scratches, it may have been directed thus by the 
glaciation. The subsequent course of the East River may be 
an ancient outlet of an eastern watershed, under different condi- 
tions, as suggested by Dr. Newberry.* From a local survey, 
I see nothing to contradict or corroborate. Prof. Dana has 
noted evidence tending to show that the Manhattanville depres- 
sion is due to an ‘‘ oblique wrenching and faulting of the rocks,” 
and it is possible that the same movement may have caused the 
breaks at Inwood and Spuyten Duyvil. Prof. Dana ascribes to 
the mountain-making epoch at the close of the Lower Silurian 
the outlining of the island’s important features. This may be 
true, and it likewise may be that the final contortions and fold- 
ings may have a connection with the great trap outburst across 
the river. 
Minerals.—Lists of the minerals found on the island have been 
given in the New York State Report and in Cozzens’ Geology.. 
In the Annais of the Lyceum, Vol. VIII., there is a paper by 
S. C. H. Bailey that treats the subject very fully. It is not my 
intention to repeat anything said in those papers, but simply to 
record afew observations made while studying the geological 
formation.? 
The minerals of the island fall naturally into four classes: 
1. Those indigenous to the gneiss; 2. Those indigenous to the 
granyte; 3. 'Those indigenous to the limestone; 4. Those resulting 
from decomposition. 
1. The gneiss contains masses of quartz, occasionally of some 
size, and of the smoky, ferruginous, and milky varieties. 
Crystals are rarely found. 
The feldspar is of several varieties. Orthoclase occurs, flesh- 
colored, brick-red, and a light, semi-transparent brown. Oligo- 
1 Am, Jour. Sci., XX., p. 440-448. 
2 Pop. Sci. Monthly, XIII., p. 652. 
3In number of species New York Island has been one of the most 
prolific localities in the world. The Transactions will soon contain a 
list complete to the present time. 
