PEGMATITE, SILEXITE, AND APLITE OF NEW YORK 53 



flow-structure foliation of the latter. Clearly, then, these acidic 

 pegmatites are not only satellites of the gabbro but also formed, 

 probably by magmatic segregation, well before the gabbro magma 

 was consolidated. Another body of pegmatite about six feet wide 

 in this gabbro grades into a mass of practically pure silica (silexite) 

 four feet long. 



A large body of pegmatite exposed for two hundred and fifty feet 

 lies on the east side of the gabbro stock near the top of the hill three 

 and one-half miles south-southeast of Chestertown. This pegmatite 

 is mostly orthoclase and quartz in masses up to a foot or more across, 

 together with considerable muscovite, biotite, and tourmaline in 

 masses up to several inches across. In an old mine pit a moder- 

 ately coarse facies of this pegmatite containing some plagioclase is 

 intricately involved with a highly foliated feldspar-biotite schist 

 phase of the gabbro which grades perfectly through a fine-grained 

 foHated gabbro into normal, medium-grained, non-foliated gabbro. 

 The zone of mixed rocks, which is forty to fifty feet wide, contains 

 many long lenses, strips, and irregular masses of pegmatite arranged 

 roughly parallel to the fluxion-structure foliation of the schistose 

 facies of the gabbro. Evidently this pegmatite, which developed 

 as an early satellitic facies of the gabbro magma, was more or less 

 broken up and involved with the yet molten gabbro. 



Two miles southeast of Chestertown a gabbro stock with highly 

 foliated facies not only contains inclusions of, but also sends dikes 

 into, the granite porphyry country rock. At its north end the 

 fohation of the gabbro perfectly conforms to the rather irregular 

 side of a large mass of pegmatite, which latter is cut by well- 

 foliated, fine-grained gabbro in the form of dikes up to eight feet 

 long, one of these dikes ending in a feldspar-biotite schist. A 

 dike of similar fine-grained gabbro not over six inches wide cuts 

 the granite for thirty feet. In this case it seems certain that the 

 pegmatite must have formed well before the solidification of the 

 parent gabbro, because the pegmatite was intruded by the molten 

 gabbro. 



At the top of Hackensack Mountain near Warrensburg a dike 

 of the gabbro clearly cuts syenite, and the fluxion structure of the 

 gabbro conforms to the borders of a small mass of included peg- 

 matite. 



