OCCLUSION OF IGNEOUS ROCK 397 



meter in thickness, made up of a mixture of white feldspar and 

 gray quartz, amounting to about 25 to 35 per cent, of the 

 volume of the rock. Much variation prevails in the proportions 

 of the two minerals, one or the other predominating in different 

 ledges. A little black to brown biotite, and still less white 

 mica, garnet and pyrite sometimes occur. No trace of epidote 

 or calcite was anywhere detected ; their absence may be corre- 

 lated, in my opinion, with that of the sharp minor foldings and 

 corrugations, only produced by intense pressure.' Occasionally 

 feldspar and quartz fall short and a thin layer of black horn- 

 blende-schist has resulted, with hornblende sometimes in coarse 

 blades, up to 5 centimeters in length. Toward the southern 

 end of the outcrop, where the bed tapers out, the rock becomes 

 fine-grained and more thinly laminated, with hornblende bJades 

 rarely reaching 2 millimeters in length, as in the similar, thinly 

 sheared hornblende schists at other parts of Manhattan Island. 



In a thin section, under the microscope, the minerals are 

 found to be the same as in other dioritic schists of the island. 

 Hornblende, mostly in prismatic or elongated grains, occasion- 

 ally in granules or fibrous scales. Twinning common ; maxi- 

 mum extinction 15° to 16° ; C A (■'- Its rare inclusions consist 

 of rods and spherules of colorless zoisite, plates and twinned 

 crystals of plagioclase, scales of black hematite and rarely 

 brownish red biotite. Plagioclase, abundant in angular parti- 

 cles, showing twinning after the albite law. The low maximum 

 extinction angles, in sections normal to twinning plane, suggest 

 a rather acid feldspar. Apatite and hematite occur as inclusions. 

 Quartz common, in the usual limpid grains. In both feldspar 

 and quartz wavy extinction is very common, sometimes concen- 

 tric. Occasional cloudy colorless grains were referred to 

 orthoclase. 



Joints and Veins. — The mass of schist is intersected by a 

 system of closely contiguous, parallel joints, with general direc- 

 tion N. 5° W. These coincide in direction with well-marked 

 parallel seams of division, often discernible in a hand specimen 

 at intervals of a few millimeters. It is along these planes of 



ijulien, loc. ciL, 446, 493. 



