§4 NEW YORK STATE MUSEUM 



miles east-northeast of Eddy. About a dozen sheared quartz-feld- 

 spar-tourmaUne dikes, spaced at intervals of a few yards, intersect 

 the banded amphibolite which borders the granite gneiss at this point 

 (see plate 5, upper hgure). They pinch out within 25 to 50 feet 

 from the granite-amphibolite boundar}-, and are not found cutting 

 the nearest outcrops of the adjacent micaceous Grenville schists. 



In occasional dikes the tourmaline is very abundant, and in its 

 cr\'stallization with the quartz often presents the aspect of a peg- 

 matitic or eutectic intergrowth, ver\' similar, with certain modifica- 

 tions, to the analogous and more frequent intergrowth between 

 quartz and orthoclase (see plate 15, lower figure). At the famous 

 black tourmaline-collecting locality on the right bank of Grannis 

 brook about a mile and one-tenth north-northvrest of Pierrepont 

 crossroads, this mineral evidently occurs in a pegmatite dike-cutting 

 limestone ; there may also be f umarole or contact action here, but the 

 relations are not shown so clearly as might be desired. At this place 

 there is also abundant biotite which if belonging to or introduced 

 by the pegmatite is an abnormal associate of the tourmaline. In a 

 nearby ledge of granite-amphibolite injection gneiss, a few rods 

 upstream on the right bank of the brook, a normal tourmaline- 

 bearing pegmatite dike was seen, and in the retaining wall of the 

 adjacent dam a fragment of a pegmatite dike showed a similar 

 association. It seems ver}^ probable, therefore, that this once 

 popular collecting place owes its supply of tourmaline to the con- 

 centration of boron and other mineralizers derived during the difter- 

 entiation of a mass of granite close by, or perhaps not far below it 

 in depth. Lacroix, according to Professor Smyth's citation (1896, 

 page 267), was of the opinion that such was the origin of the tour- 

 maline at this locality, but Sm}-th, in the paper cited, showed that 

 the associated basic gneiss may also have exerted a fumarole action 

 similar to that postulated above for the granite. 



At the southernmost point in the granite boss east of Pyrites is a 

 group of three or four pegmatite dikes cutting the peripheral garnet 

 gneiss, as is usual, nearly at right angles, and prospected for mica. 

 They contain an abundant development of coarse apatite, muscovite 

 and pyroxene, but no tourmaline. The association is unique in the 

 quadrangle, and the apatite and pyroxene commonly show crs'Stal 

 terminations. The apatite and muscovite are common to all members 

 of the group, but the pyroxene is found in quantity only in the east 

 dike, which has been worked most extensively for mica. Quantities 

 of waste material on the dump show the muscovite to have attained 



