38 NEW YORK STATE MUSEUM 



PEGMATITE 



The most interesting thing about the pegmatite is its distribution, 

 because it is very commonly directly associated with the gabbro 

 masses. Many times the pegmatite, in the form of dikes or veins, 

 may be seen cutting the gabbro (see plate 4) and hence the younger 

 age of the pegmatite. This direct association of the very acid 

 pegmatite with basic gabbro and its age intermediate between the 

 basic intrusives — gabbro and diabase — are rather anomalous fea- 

 tures for which the writer can offer no explanation. 



Among other places where pegmatite may be seen cutting the 

 gabbro are: (i) at the top of Hackensack mountain; (2) i^ miles 

 south-southeast of Potter mountain; (3) i^<[ miles a little east of 

 north of The Glen; (4) i mile south-southeast of The Glen; (5) 

 lYs miles northeast of Pottersville ; (6) I7/3 miles southeast of Ches- 

 tertown; (7) 2^ miles southeast of Chestertown; and (8) 2^ 

 miles south-southeast of Chestertown. At most of these places the 

 gabbro is shot through with small pegmatite veins. At the fourth 

 named locality one dike is 50 feet long and 25 feet wide and very 

 rich in big orthoclase and albite feldspar crystals. At the seventh 

 named locality a small pegmatite dike contains fine crystals of biotite, 

 muscovite and black tourmaline. At the sixth and eighth named 

 localities there are pegmatite dikes 50 to 100 feet long with books 

 of muscovite up to 5 or 6 inches across at the eighth locality. 



Large pegmatite dikes are not common away from the gabbro, 

 there being but two examples worthy of mention namely: just east 

 of the old garnet mine south of Daggett pond where there are many 

 poorly formed black tourmaline crystals up to 6 inches long, and one- 

 quarter of a mile above the mouth of Mill creek where there is a 

 dike 200 feet long and 40 feet wide in granitic syenite, 



DIABASE ,/ 



Mode of occurrence and distribution 



In striking contrast with the neighboring gabbro, the diabase 

 invariably occurs in typical dikes which have clearly broken through 

 narrow fissures in the country rock. They vary in length fron: 

 20 or 30 feet to 200 yards, and in width from 55^ inches to 40 feet 



The chief features of occurrence are brought out in the following 

 description of the largest dike of the region which lies at the westeri 

 base of Heath mountain or 3 miles west-northwest of Warrensburg 

 This dike has a maximum width of 40 feet and a length of 20C) 

 yards. It is fine to medium grained toward the interior and ver} 



