204 ANNALS NEW YORK ACADEMY OF SCIENCES 



A thin section of the light gray gneissic variety from Shaft 18 of the 

 Catskill Aqueduct at West 42nd Street near Fifth Avenue, where it occurs 

 in a belt about two feet wide interbedded with the typical micaceous type, 

 on the other hand shows a medium-grained crystalline texture and only 

 slightly foliated structure (PL XIII, Fig. 2). The principal constituent 

 minerals are quartz, feldspar and biotite. Apatite is present in appre- 

 ciable amounts in minute lath-shaped crystals. The feldspar consists of 

 both orthoclase and plagioclase. The latter has a maximum extinction 

 angle of 22° in sections at right angles to the albite lamellae and is opti- 

 cally positive. It is evidently andesine. Both the quartz and the feldspar 

 occur in allotriomorphic, interlocking grains. The biotite is a dark green- 

 ish brown, intensely pleochroic variety. The chemical composition of this 

 type of schist is given under analysis 2 on page 212. 



Closely related to the gray gneissoid variety just described is a type 

 occasionally found in which the amount of feldspar is very small, the pre- 

 dominant mineral being quartz, so that the rock practically becomes a 

 quartzite. A section of a specimen from West 155th Street and Tenth 

 Avenue shows a medium-grained crystalline texture and slightly foliated 

 structure. The rock is made up largely of quartz with some feldspar and 

 biotite. Magnetite and a little apatite are also present. The biotite oc- 

 curs in small, usually irregular flakes whose basal sections are oriented 

 parallel to the plane of foliation. It shows marked pleochroism from 

 light greenish yellow to deep greenish brown. The quartz and feldspar 

 occur in allotriomorphic, closely interlocking grains. The feldspar con- 

 sists of both orthoclase and plagioclase. The latter shows extinction 

 angles up to 8° in sections at right angles to the albite lamellae and is 

 probably oligoclase. 



Another variety which has a comparatively fine crystalline texture and 

 shows only moderate foliation has the biotite occurring in numerous small 

 flakes showing parallel orientation in a matrix of quartz and feldspar. 

 The rock has a dark color. A specimen collected two and one-half miles 

 north of New Eochelle along the Westchester Eailroad when examined in 

 thin section under the microscope shows the rock to consist mostly of 

 quartz, biotite and feldspar and minor amounts of pyrite, magnetite and 

 upatite. The biotite is a dark reddish brown variety showing intense 

 pleochroism from light yellowish brown to deep reddish brown. The 

 quartz and feldspar occur in allotriomorphic, interlocking grains. Both 

 orthoclase and plagioclase feldspar are present, the latter giving extinc- 

 tion angles running as high as 39° in sections at right angles to the 

 albite lamellae. This would indicate labradorite. 



