FETTKE, MANHATTAN SCfflST OF NEW YORK 229 



it consists principally of a dark green hornblende and minor amounts of 

 "<lark brown biotite. Titanite, magnetite and a little pyrite appear as 

 accessory minerals. The hornblende shows marked pleochroism from 

 yellowish brown through greenish brown to deep green. The biotite also 

 exhibits intense pleochroism from light yellowish brown to deep brown. 

 This biotite hornblendite represents the typical composition of the in- 

 trusive mass at the northeast end of the area. 



Along the eastern margin, the contact of the hornblendite with the 

 mica schist may be observed at several places. At one point several 

 apophyses of hornblendite were noticed extending into the schist. Some 

 of these retain the coarse crystalline texture of the main mass, while 

 others are somewhat finer grained. A thin section of this finer grained 

 type under the microscope shows medium granitoid texture and massive 

 structure. It consists almost entirely of hornblende with marked pleo- 

 chroism from light yellowish brown through greenish brown to dark 

 brownish green. It has well developed prismatic cleavage. A little 

 plagioclase and titanite are present as accessory constituents. 



A short distance to the east of this occurrence, several sheets of horn- 

 blende schist similar to those already described occur interstratified with 

 the mica schist. This rock shows a distinct foliated structure. When 

 examined under the microscope, it is seen to consist largely of a deep 

 green hornblende, feldspar and a little quartz. The accessory minerals 

 are magnetite, biotite and apatite. The hornblende shows marked pleo- 

 chroism from light yellowish brown through greenish brown to deep 

 green and has well developed prismatic cleavage. It does not show good 

 cr3^stal boundaries but occurs in irregular grains interlocked with the 

 feldspar and quartz. These grains are usually oriented parallel to the 

 foliation. The feldspar is chiefly plagioclase giving extinction angles 

 up to 26° in sections at right angles to the albite lamellae and is evidently 

 an acid labradorite. Unstriated feldspar also is present. The feldspar 

 grains are irregular in shape and usually elongated parallel to the folia- 

 tion, but they do not show any crystal lographic orientation. It is quite 

 evident that this rock has had a different history from that of the 

 apophyses of hornblendite occurring in the schist. The former was in- 

 truded prior to the period of folding, while the latter either at the close 

 or else after it had ceased entirely. 



At the southwestern end of the area, the hornblendite in places grades 

 into a diorite. A section of the typical hornblendite as developed here 

 was made from a specimen taken from the east side of the railroad at 

 about one-quarter of a mile south of Croton Falls. It is a massive dark 

 green biotite hornblendite. ^ In thin section, it shows a coarse granitoid 



