24 J. D. Dana— Geological Relations of the 
of rare occurrence. Blackish gray beds, owing their dark color 
to the very large proportion of black mica, often alternate with 
whitish or light-colored beds in which muscovite is the most 
abundant mica. Frequently, also, mica schist graduates into 
gneiss in the direction of the be ding as well as transverse to 
The distinction of gneiss from mica schist is, therefore, 
made often with difficulty, and the restriction of the latter term 
to kinds consisting of mica and quartz without feldspar is im- 
practicable. The rocks of New York Island are good exam- 
ples of the ondanasy. rocks of Westchester County, both as to 
kinds and their transitions. 
Sagan qc varieties of the mica schist and gneiss are com- 
A variety of micaceous gneiss in Singsing, contains 
jai elliptical crystallizations of muscovite. A cyanitic mica 
schist is met with on New York Island, as announced by Pro- 
fessor D. S. Martin.* A dark gray fibrolitic enees containing 
some tourmaline has been described by Professor A. A. Julien, 
as occurring at New Rochelle.’ (The saiiafals cyanite and 
fibrolite are alike in composition, they being eo similar 
aluminum silicates, and differing only in crystallizatio 
Hydromica schist, of the slightly crystalline Taeey, resem- 
one ees a glossy roofing slate, occurs in the northwestern 
e County, north and northeast of Peekskill, on the 
a of the Archean of the emesis” It is called talcose 
found in the County. Across the Hudson Riv ver, in Roc Kkland 
County, 1 in the continuation of the same stratum, near Tomp- 
kins’ Cove, the slate is very carbonaceous, as Mather’s report 
states, and ‘much of it is still less me tamorphic in its aspect. 
Quartzyte constitutes a stratum several hundred feet thick in 
pointed out by Mather. But a large part of the rock in that 
region contains more or less feldspar, and often also mica in 
rather indistinct grains; looking either like an underdone mica- 
less granite or eranitoi id gn gneiss. It is usually much jointed aad 
without distinct bedding. The northern part of the mass, at the 
mouth of the river north of Peekskill, is a true siliceous quartz- 
yte, fine-grained, and even in bedding; while on the southern 
side it graduates into the slate. It thus varies greatly in consti- 
tution, but in a way to make it certain, that, although so feld- 
spathic in ortions, the whole of it is one quartzyte forma- 
tion. Further, it is evident, from the facts, that the quartzyte 
and slate are stratigraphically the sam e rock; 0 one changing to 
the other, and taking the same Sitios with reference to an 
associated stratum of limestone. In the Rockland County 
4 Proc. Lyceum Nat. Hist. New ae i, and this Journal, IT, iv, 237. 
5 Amer. Quart. Micr. J., Jan. 18 
