482 A. A. JULIEN AMPHIBOLE SCHISTS OF MANHATTAN ISLAND 



of symmetr}^ but so small as rarely to show cleavage traces and cracks. 

 A few dull black granules of square or rectangular outlines are prob- 

 ably magnetite, and hematite also occurs in thin but opaque plates of 

 high luster and sometimes rhombic form. 



One mile north of Rye is a similar rock (black hornblendite) in which 

 incipient actinolitic alteration has produced a slight greenish tinge. Red 

 iron garnet is distributed in particles throughout and sometimes gathered 

 in coarse masses, up to 6 centimeters in length, mixed with black horn- 

 blende and greenish white plagioclase. 



Under the microscope hornblende predominates, with the optical 

 characters as described, with maximum extinction angle 18 2 degrees t f\C \ 

 a little feldspar in the interspaces ; some colorless zoisite and reddish 

 epidote, in hexagonal crystals or rodlets, as inclusions in hornblende, 

 and a few minute shining plates of hematite, black and opaque. 



At New Rochelle occurs a finer grained variet}^ of the hornblendite, an 

 epidotic diorite schist, showing bright facets of black hornblende, 2 to 4 

 millimeters long, with a little gra3ash feldspar interspersed. A little 

 quartz and greenish yellow epidote are scattered throughout. Some 

 specimens are streaked on cross-section by parallel seams of yellowish 

 feldspar and epidote, 1 millimeter thick. 



Another variety with less feldspar is a coarse black diorite at Larch- 

 mont manor. 



Under the microscope a thin-section of the quartz-diorite schist of New^ 

 Rochelle is found to consist to about oO per cent of hornblende, mostly 

 in irregular scales, largely fragmental, wnth rounded scolloped indenta- 

 tions ; a few basal sections, with the cleavage at 124 degrees. Absorption 

 scheme ft > C >• a ; t), brownish green ; c, bluish green ; a, pale brownish 

 3^ellow^ Extinction somewhat variable, but 17 degrees in those with 

 traces of eminent cleavage. Plagioclase abundant to about 30 per cent ; 

 clear, colorless, and w^ith the albite twinning. Maximum extinction 

 angles on sections normal to twinning plane seem to indicate a more 

 acid feldspar than usual in these rocks. Occasional grains of orthoclase, 

 marked by absence of polysynthetic striation, cleavage traces approx- 

 imately at right angles and slightly lower interference colors than those 

 of the plagioclase. About 5 per cent of quartz occurs in limpid grains, 

 Avith few inclusions. Hematite common to about 12 per cent in rounded 

 black plates of high luster, some displaying six-sided or rhombic out- 

 lines. These are inclosed in part in the hornblende, and in part among 

 or around the plagioclase grains. A little secondary hornblende or ac- 

 tinolite is found in the quartz as pale greenish to colorless blades. Zoisite 

 in colorless grains, with cleavage traces or rude cracks, rough surfaces, 

 and high relief; elongation parallel toe; parallel extinction; interfer- 



