GEOLOGY OF THE BLUE MOUNTAIN QUADRANGLE IQ 



Long Lake bridge. In spite of considerable variations in the char- 

 acter of these rocks, they are distinctly different from any other 

 rocks observed within the quadrangle. In every essential respect 

 they appear to be like the anorthosite-gabbro border facies and small 

 outlying masses of the great anorthosite body within the Long Lake 

 quadrangle as described by Gushing.^ Also they are precisely like 

 the border facies of the anorthosite area along the Hudson river 

 (Newcomb sheet) as seen by the writer during the summer of 1914. 

 Therefore it is with some confidence that the two small masses of 

 anorthosite-gabbro within the Blue Mountain quadrangle are 

 regarded as of the same age as the great anorthosite body, being 

 simply outlying masses or off-shoots of the large body similar to 

 those within the Long Lake quadrangle described by Gushing. 

 Identity of age has of course not been established since the two 

 small areas are separated from the main anorthosite body by an 

 interval of nearly 10 miles. According to Gushing the anorthosiV 

 (and associated anorthosite-gabbro) is distinctly older than the 

 syenite. 



In the area on the eastern shore of the lake the best and most 

 typical exposures are in the immediate vicinity of the southern 

 end of the diabase dike (see map) on the point which there extends 

 into the lake.^ This whole point is practically a solid exposure. 

 \Mthin 30 or 40 feet of the dike the anorthosite-gabbro best shows 

 its variable character. It is mostly light gray to nearly white, 

 medium to very coarse grained and largely devoid of gneissic struc- 

 ture. One patch of the rock is almost pure labradorite feldspar with 

 fresh, bluish gray, rounded cores up to i or 2 inches across clearly 

 showing the twining bands and embedded in a finer grained granular 

 matrix of white plagioclase. These cores clearly represent the 

 portions of the original large labradorite crystals which were 

 uncrushed during the process of metamorphism. Most of the rock, 

 however, is medium to only moderately coarse grained with fewer 

 and smaller uncrushed labradorite cores, some andesine, a consider- 

 able percentage of dark minerals (chiefly hornblende, pyrite and 

 magnetite), and some red garnets. Very locally the dark minerals 

 may reach 30 to 40 per cent, when the rock is dark gray and dis- 

 tinctly gneissoid. The gneissoid structure becomes fainter with 

 diminution of dark minerals, much of the rock not showing it at 

 all. Some of the rock close to the contact with the diabase dike 



IN. Y. State Mus. Bui. 115, p. AlZ-1^- IQO?- 



2 This anorthosite-gabbro, at the water's edge, contains a small inclusion 

 of typical Grenville limestone. 



