16 ROCK FLOOR OF GREATER NEW YORK. [bull. 270. 
His map has entered upon it also the cross faults located by Dana at 
Manhattanville, an additional parallel fault south of Hell Gate, and a 
third between Randall Island and the Bronx. He distinguishes two 
divisions of the gneiss, the lower or Fordham gneiss of pre-Cambrian 
age, typically represented in Fordham Heights; the upper Hudson 
schist of Silurian age said to be typically represented in Washington 
Heights. Above the lower gneiss formation is a quartzite (Pough- 
quag quartzite) of Cambrian age. which in the area here considered 
is exposed at but two localities, viz. on the eastern shore of the Harlem 
p-oiw near Morris dock, and three-fourths of a mile farther to the east 
in the town of Tremont. Above the quartzite and immediately below 
the Hudson schist is the limestone formation (Stockbridge limestone) 
of Cambro- Silurian age. 'This formation is largely developed at ( Jlare- 
mont, Melrose, and Mott Haven, within the Bronx area; in numerous 
exposures in the vicinity of Inwood and Kingsbridge; and it was for- 
merly to be found also at several localities near the northeast end of 
theBoroughof Manhattan. (PI. II.) Merrill believes the low allu- 
vial flats of Harlem have been eroded in limestone, and draws his 
boundaries to correspond with this view. It is likewise assumed that 
a limestone belt crosses the western end of Randall Island, and that 
two parallel belts cross Wards Island in a north-northeast direction. 
The main river channels about the island he regards as underlain 
throughout by limestone and formed in consequence of this fact. 
Eckel. — In a brief paper published in 1899, Eckel" announces the 
classification of the rocks of southeastern New York, as held by Dr. 
C. R. Van Hise and Dr. F. J. H. Merrill, to be as follows: 
Ordovician: 
1. Manhattan schist — mica schist, containing garnet, fibrolite, kyanite, ami 
staurolite. Hudson River age. 
2. Inwood limestone— crystalline dolomite, containing diopside and tremolite. 
Calciferous-Trenton age. 
Cambrian: 
1. Lowerre quartzite. 
Pre-Cambrian: 
1. Fordham gneiss. Algonkian? 
2. Granites and gneisses. Arvliean'.' 
At a time beginning not later than the Upper Silurian, and possibly continuing at 
intervals to the end of the Paleozoic, these strata were thrown into parallel folds 
having a general northeasterly trend. Transverse folding has resulted in a general 
gentle pitch to the southwest, and overthrown folds are common, i F. .1. II. Merrill ). 
Of the formations mentioned above, the Manhattan schist and Inwood limestone 
form the surface or subsurface strata over the greater pari of Manhattan Island, 
though at one place a small extent of the Fordham gneiss is exposed. 
Gratacap. -In a popular treatise on Greater New York/' Grata- 
cap has brought together the work of many observers and added 
"ivkH. E. C, Intrusivea in the [nwood limestone of Manhattan [sland: Am. Geol., vol. 23, 1899, 
pp. 122-124, pi. ::. 
bGratacap, I- P., Geology of the City of Greater New York, I902,pp.l v. (Privately published.) 
