442 . A. A. .TULIEX AMPJ1IJ30LE SCHISTS OF MANHATTAX ISLAND 



represents only the alteration of that gneiss, with loosening produced by 

 decay. 



In a thin-section of the schist from West One hundred and fifty-fifth 

 street, under the microscope, actinolite predominates in parallel laminse, 

 to about 55 per cent of the volume, as slender fibrous blades, 0.02 to 0.70 

 millimeter in 1 en oth, rarely up to 0.04 millimeter in breadth, with cross- 

 partings ; somewhat dichroic, greenish to colorless, and with maximum 

 extinction 17 degrees. The intervening laminse, up to 0.12 millimeter 

 in thickness, are occupied by clear granules of quartz to about 40 per 

 cent of the rock. A few^ scales of biotite occur, copi)er-colored or 

 orange-yellow ; pleochroism, red to colorless. Zoisite is included in both 

 actinolite and quartz, as colorless rounded granules, wdth high relief. 

 A few small cubes of iron-gray magnetite and films of reddish iron oxide 

 were also distinguished, but no trace of any feldspar. 



Fissile actinolite slate, at One hundredth street and Fifth avenue. 

 An exceedingly fine-grained and thinly lamellated schist of almost green- 

 ish black color, with specific gravity 2.996, which appears to be a strongly 

 sheared form of the more common schist just described. 



Under the microscope, the thin-section reveals an abundance of actin- 

 olite rods and scales and, in the groundmass, quartz predominating, with 

 rare particles of plagioclase feldspar. The dark color of the rock seems 

 to be due to the abundant shreds and scales of leather-brown biotite. A 

 little zoisite is also present. 



Tremolite schist, at West Fifty-ninth street, between Fifth and Sixth 

 avenues. A finely fibrous, grayish white schist, resembling closely in 

 texture and structure the black hornblende schist in the vicinit}'-, but 

 here made up entirely, to the eye, of parallel blades and fibers of tremo- 

 lite, tightly compacted, with many surfaces and division planes stained 

 by reddish and yellowish iron oxide. 



Similar schists, often cream-colored, but with wavy schist planes, were 

 found between West Fiftj^-eighth and Fifty-ninth streets, between Tenth 

 and Eleventh avenues, and elsewhere in that vicinit}" (figure 9), which 

 were made up chiefl}^ of smooth, continuous whitish laminae of aphanitic 

 texture, commonl}^ spotted with tremolite blades of very fine fibration 

 and silky luster, lying confusedly in the foliation planes ; specific gravity 

 of the rock, 2.921. 



Other varieties of tremolite rock or amphibolite are radial granular 

 and with longer fibers, radiated asbestiform, often in part actinolitic, 

 chloritic, talcose, and ophiolitic, first described under " hydrous an- 

 thophyllite" by Thomson, Gale,^ and others. 



*. "Mather: Op. eit., pp. -itll, 582. 



