GEOLOGY OF THE BLUE MOUNTAIN QUADRANGLE 37 



The large stock north of Waterbarrel mountain shows some big 

 ledges of medium to coarse-grained rock with perfect diabasic tex- 

 ture, and feldspar laths up to three-quarters of an inch long (no. 

 8, table 6). Some amphibolite occurs, especially on the north side 

 of the stock, but much of the gabbro is very gneissoid, this being 

 true in places (particularly toward the east) of even the coarsest 

 grained rock, the feldspars having been highly granulated, the dia- 

 basic texture destroyed and occasional garnets developed. This 

 very coarse-grained, gneissoid gabbro so greatly resembles a large 

 mass somewhat doubtfully called gabbro along the West Branch of 

 the Sacandaga river in the Lake Pleasant quadrangle ^ that this 

 latter rock is now quite confidently regarded as a metamorphosed 

 coarse gabbro. Apparently it is a facies of gabbro not often 

 met with. 



The other gabbro masses of the quadrangle are quite typical in 

 every way and require no special description. 



Pegmatite 



Pegmatite dikes or veins were occasionally met wnth in many 

 parts of the quadrangle. One type occurs as narrow masses of 

 moderately coarse grain with long axes essentially parallel to the 

 foliation of, and not very sharply separated from, the inclosing 

 syenite or granite. Dikes of this sort were, no doubt, injected 

 practically contemporaneously with the great intrusives, possibly 

 as a late phase of the intrusions. 



Another type of pegmatite, clearly later in origin, is generally 

 very coarse grained, cuts through rocks of all ages except the dia- 

 base, shows very sharp boundaries and usually cuts across the folia- 

 tion of the country rock. Dikes of this sort are wholly devoid of 

 metamorphism. They range in width from i or 2 inches to lOO 

 feet and in length up to 200 yards or more. A few of the most 

 conspicuous observed examples are as follows : coarse-grained dike 

 10 or 15 feet wide cutting Grenville hornblende gneiss on Cedar 

 river 33^ miles w^est of Indian Lake village; a 15 foot wide dike 

 some 40 or 50 feet long cutting Grenville limestone just southeast 

 of the small gabbro stock i mile north-northeast of Indian Lake 

 village; small but very sharply defined dikes cutting the basic syenite 

 at the summit of Owl's Head mountain ; a dike fully 100 feet wide 

 and 200 yards long at the western border of the gabbro on the 

 largest island in Blue Mountain lake ; small dikes cutting syenite 

 near the western end of Long Lake bridge ; small dikes cutting mixed 



1 N. Y. State Mus. Bui 182, p. 29. 1916. 



