210 ANNALS NEW YORK ACADEMY OF SCIENCES 



gneiss making up the hills on the east, while the schist makes up those on 

 the west of the reservoir site, with the limestone occupying the valley be- 

 tween the two. The. formations all dip steeply toward the west. 



A thin section of the Fordham gneiss taken from near the contact with 

 the overlying limestone shows a distinctly gneissoid structure. It is made 

 up largely of feldspar, biotite and quartz, together with a little titanite 

 and an occasional apatite crystal. The feldspar is largely plagioclase, 

 giving extinction angles up to 8° in sections at right angles to the albite 

 lamellae. It is optically positive and is evidently an oligoclase-albite 

 variety. The biotite is an intensely pleochroic, deep brown variety. It 

 occurs in comparatively small crystals. The greater concentration of 

 these in particular bands gives the rock its gneissoid appearance. 



In the limestone, a short distance above the contact with the underly- 

 ing gneiss, an interbedded layer of gneissoid rock several feet thick oc- 

 curs. Under the microscope, it is seen to be made up largely of feldspar, 

 both microcline and orthoclase, diopside, a reddish brown variety of bio- 

 tite, calcite and a little quartz. Minor quantities of titanite were also 

 noticed. 



As the contact of the limestone with the overlying schist is approached, 

 layers of intei'bedded schist begin to appear in the limestone. Some of 

 these are quite garnetiferous. A thin section of a garnetiferous mica 

 schist occurring at this point, when examined under the microscope, was 

 seen to consist largely of biotite, garnet, quartz, sillimanite and a little 

 feldspar, both orthoclase and plagioclase being represented. A few 

 rounded grains of zircon are also present. The garnet, which occasionally 

 contains inclusions of magnetite and biotite, is a light pink variety and 

 reaches a diameter of .5 millimeter. The sillimanite occurs in little 

 needles in the quartz and also as a fibrous aggregate. It is abundant. 

 The biotite is a deep reddish brown variety. The rock contains numerous 

 small stringers of pegmatitic material. For its chemical composition see 

 analysis 4 on page 212. Other varieties of the interbedded schist contain 

 little or no garnet and are made up largely of a deep reddish brown bio-' 

 tite, quartz and feldspar, mainly orthoclase. A few small rounded grains 

 of zircon are usually present. 



The limestone adjoining these layers of interbedded schist is often im-' 

 pure, at times grading into an ophicalcite. A thin section of such an 

 ophicalcite was found to consist essentially of calcite, serpentine and mus- 

 covite. The structure of the serpentine shows that it was derived from a 

 mineral belonging to the olivine group. One piece was found in which a 

 few small cores of the original olivine were still left unaltered. It proved 

 to be optically negative and, therefore, must either be true olivine, with 

 more than 12 per cent iron, or else the variety monticellite (Mg Ca 8104). 



