428 J. D. Dana—On Southern Westchester County 
Additional notes on the Rocks.—The crystalline schists of the 
I have us gneiss 
generally garnetiferous, and in some places hornblendic, or 
contain hornblende schist in intercalated beds; and have been 
found to contain cyanite at localities between 4th and 5th 
Avenues and 42d and 5lst Streets.* I have now to report that 
the schist is crowded with minute needles of fibrolite at several 
oints between the same avenues farther to the north (north 
of 115th street), aud also in Westchester County, north of Mott 
aven, near the Mott Avenue bridge over the Hudson River 
railroad track : a fact which adds to the close relations betweer 
the rocks of New York and those of eastern Westchester 
County, at New Rochelle, oe also those of the northern part 
of the county, south of Peeks 
sa ar of the normal type— that i is, Umearey inc only a moder- 
te proportion of mica —is not com A fine- grained, thick- 
heidsied light gray gneiss npaihiag: ‘little black and white 
mica, makes the bluff bounding on the west the limestone area 
between Tremont and Morrisania, and has been quarried 
at several points. By microscopic examination of a thin slice, 
I have found it to be two-thirds granular quartz, so that it is 
in reality quartzytic gneiss or gneissic quartzyte. e other 
ingredients are haga se and microcline, with traces of black 
tourmaline. It is ous, and weathers rather deeply, and 
loses thus its black tits before it does its firmness. mon 
the thicker beds occurs an occasional thin layer of mica schist. 
The preenee of hornblende or hornblendic schist appears to 
have often determined a crowd of subordinate flexures and 
contortions in the beds, and a loss of distinctness in the minor 
layers. I have explained on on the dee ae hornblende 
is relatively a fusible mineral (being of the e 8, on von . 
Kobell’s scale of fusibility), while the feldspar rior whieh ortho- 
clase is the prevailing one) and the mica (black and white) are 
of difficult fusibility (5 to 6, on the same scale); and, in conse- 
uence, beds that become porenene in the metamorphic 
process easily ie and bend. A good example is seen in a 
section on 110th Street, between 9tb and 10th Seba © where 
the micaceous gneiss has high dip (mostly 80° t 90°, but 
varying locally) and in some are contains much e rnblende. 
This street (the 110th) here Hen through the schist vedo 
to its bedding, and has on the north side a vertical face of a 
ix Soa ay was first found on New York Island by cn Pie not Cozzens, as 
by Dr. Torrey in this Journal, vi, 364 (1823); Mr. Cozzens, in his 
«“Geologice! History of Manhattan or New York Island” (843), soap eh io 
neighborhood of the Deaf and Dumb Asylum (which was then si 
49th and 50th Streets and boos and 5th Avenues) as the locality. 
+ This Journal, xx, 24, 200, 206, 
