6o NEW YORK STATE MUSEUM 



largest is 5 inches wide, visible for 10 or 12 feet, and has a branch 

 bearing off abruptl}^ at right angles. Another is 2 inches wide and 

 traceable for 20 feet in the gabbro. None of these dikes could be 

 traced into the surrounding granite. In certain respects these aplite 

 dikes are quite different from those on Wilson mountain. Most 

 of the dike material is fine grained, particularly so at the mar- 

 gins, but this grades into a medium-grained, narrow, middle por- 

 tion which is very persistent. The dike rock is somewhat weathered 

 and of light-brown color, this latter probably due to stains from 

 iron-bearing minerals in the gabbro. Because these dikes sharply 

 cut the gabbro, which is distinctly later than the granite, and because 

 of their chilled margins, it is believed that they were intruded 

 considerably later than the aplite dikes on Wilson mountain above 

 described. A thin section shows the following mineral percentages : 

 microperthite, 65; oligoclase, i; quartz, 20; diallage, 12; mag- 

 netite, 2 ; and very little zircon. The thin section shows a granitoid 

 texture and no granulation. 



At the eastern end of the Oliver hill gabbro stock a few branch- 

 ing, fine-grained, white, aplite dikes a few inches wide cut the 

 gabbro, the relations being shown in figure 4. Although these dikes 

 are white and do not have distinctly coarse portions, it is probable 

 that the}'' were intruded at about the same time as those on 

 Ledge hill. 



Pegmatite. A number of pegmatite dikes were observed in 

 the granite on top of Ledge hill for one-half of a mile northward 

 from the gabbro stock. None of these is more than 2 feet wide, 

 and they all cut sharply across the foliation of the granite very 

 irregularly. Near the eastern margin of the gabbro stock a very 

 small pegmatite dike cuts across the foliation of the granite, and 

 nearby another cuts the gabbro. 



On the western summit of Wilson mountain near the aplite 

 dikes, a number of very small pegmatite dikes sharply cut the 

 granite. Near the eastern margin of the Grenville area on the 

 western face of Wilson mountain, several small pegmatite dikes 

 cut a mixture of Grenville and granite. 



At the eastern end of the Oliver hill gabbro stock the same ledge 

 which contains the aplite dikes (see figure 4) is also cut by a small 

 pegmatite dike which is without very sharp contacts against the 

 gabbro. \\''hether this is older or younger than the aplite was not 

 determined. 



Pegmatite dikes are scarcer in the Schroon Lake quadrangle 

 than is usual in the Adirondack region. Not one was observed in 



