g30 ANNALS NEW YORK ACADEMY OF SCIENCES 



texture. The principal mineral present is a dark green hornblende as- 

 sociated with minor amounts of dark brown biotite. Small amounts of 

 feldspar, both striated and unstriated, and a very little quartz occur in 

 addition to the above, together with such accessory minerals as magnetite, 

 apatite and titanite. Some of the hornblende is full of little quartz in- 

 clusions. Considerable evidence of strain is present in the section in the 

 form of undulatory extinctions and wedge-shaped twins in the feldspar. 



On the west side of the track in the same cut, the rock has taken on 

 the composition of a diorite. The feldspar and hornblende appear in 

 about equal amounts. Most of the rock is massive, but some of it shows 

 a slightly foliated structure. In thin section, the texture is granitoid. 

 The principal minerals present are a feldspar, a deep green hornblende 

 and a dark brown biotite, together with minor amounts of magnetite, 

 apatite and titanite. The feldspar is mostly plagioclase, giving extinc- 

 tion angles up to 22° 30' in sections at right angles to the albite lamellae, 

 which would indicate andesine. Some orthoclase also is present, as much 

 of the feldspar is unstriated and optically negative. 



A number of inclusions of a schistose rock occur in the diorite at this 

 place. Their contacts with the latter are usually quite sharp. A thin 

 section of one of these, which forms a tabular, nearly vertical mass about 

 four feet wide, shows a marked foliated structure and medium-grained 

 •crystalline texture. The principal minerals present are biotite, horn- 

 blende, feldspar and a little colorless pyroxene. Magnetite, apatite and 

 titanite are accessory constituents. The hornblende is a deep green va- 

 riety showing marked pleochroism from yellowish brown through brown- 

 ish-green to deep green. The biotite shows intense pleochroism from 

 light yellowish brown to deep brown. The feldspar is mostly plagioclase, 

 giving extinction angles up to 20° 30' in sections at right angles to the 

 albite lamellae, and is evidently an andesine. Another section from a 

 somewhat similar inclusion also shows a marked foliated structure. A 

 dark green hornblende is the principal mineral, associated with which 

 are deep brown biotite and minor amounts of apatite, titanite and pyrite. 



In the railroad cut at Croton Falls itself, a dark nearly black foliated 

 rock is exposed. It has a medium coarsely crystalline texture. The thin 

 section reveals a deep green hornblende with well developed prismatic 

 cleavage, deep brown biotite and plagioclase. The accessory minerals are 

 magnetite and titanite. Farther north in the same outcrop, the structure 

 becomes even more foliated. A thin section from a specimen taken near 

 the northern end of the cut shows a marked schistose structure. The 

 minerals making up the rock are deep green hornblende with well de- 

 veloped prismatic cleavage, deep brown biotite, pale green augite and 



