MACROSCOPIC DESCRIPTION 433 



east, partly covered by small patches of grass. The surface sketched 

 (plate 61) comprises an area of 30 by 110 feet; strike north 31° east, 

 70°> west. Through this stratum, along the foliation planes, run nine 

 parallel sheets (numbered at sides of plate) of quartz diorite or horn- 

 blende schist, passing in places into biotite schist or biotite gneiss. 



These sheets vary in thickness from a few inches up to 7i feet and are 

 separated from 1 to 11 feet. Abundant seams of fine-grained pegmatite 

 or aplite penetrate all the layers of both gneiss and schist, but only along 

 the foliation planes. White quartz also occurs (Q) in scattered lenses, 

 inclosed in or lying along layers of the schist, as well as in crumpled and 

 crushed bunches at many points. The schist layers show more or less 

 disturbance, flexure, and even zigzag folding, with variations in thick- 

 ness and frequent passage at their margins into vaguely defined and 

 crumpled layers of biotitic gneiss. Toward the southwest, crossing One 

 hundred and eighteenth street, two of the sheets, numbers 4 and 5, are 

 continued, the latter splitting again into two. Toward One hundred and 

 seventeenth street three sheets occur, one sending out a small tongue or 

 apophysis, and all die out toward the northwest corner of ( >ne hundred 

 and seventeenth street. In the opposite direction, to the north, several 

 seams of hornblendic and of biotitic gneiss are found, the latter display- 

 ing rather vague outlines and many flexures, mostly within Morningside 

 park, opposite One hundred and twentieth, One hundred and twent3^-first, 

 and One hundred and twenty-second streets. These closely resemble 

 those in the section into which the hornblende schist passes, and appear 

 to mark the former range of the sheets of that rock in this direction. 

 Thus the entire extent of the tract along the strike probabl}^ approxi- 

 mated 1,000 feet in this direction. 



The schist consists mainly of an exceedingly compact, heavy, shining, 

 jet-black rock with slaty lamination, with somewhat gnarled structure 

 in some places, always with the hackly fracture usual in a metamorphic 

 schist. In one of the thicker layers it is divided by a rhomboidal joint- 

 age into diamond- shaped blocks, a structure also observed in the same 

 schist at west end of High bridge, One hundred and seventy-fifth street. 



It also possesses generally a marked fibrous texture, with almost silky 

 or even satiny luster, being a compacted aggregate of thin blades and 

 elongated scales of black hornblende, 1 to 3 millimeters in length, lying 

 parallel to the schist plane and mostly to each other, as well as appar- 

 ently in close contact. Among them a few whitish particles appear, 

 more distinctly on weathered surface (quartz and feldspar), and rarely 

 yellowish particles of pyrite. Along some foliation planes thin seams of 

 aplite penetrate, made up of granules of red orthoclase and gray quartz, 

 and others glisten with films of black and brown biotite. A little 



