58 E. 0. Hovey — Cordierite Gneiss from Connecticut. 



width of more than 175 feet. Their vertical faces are from 

 five to fifteen feet in height. The cordierite is more thickly 

 disseminated through the rock, and is in larger grains in the 

 southern ledges than it is in the northern. The rock is com- 

 posed of biotite, quartz, and cordierite, with some plagioclase. 

 In the hand specimen the cordierite has a deep blue color, 

 and shows its characteristic dichroism plainly. In a thin 

 section it is clear and almost free from alteration, has a deli- 

 cate violet tint which readily escapes detection, and is with- 

 out perceptible dichroism. X o optical characters other than 

 those noted by Posenbusck were observed. Acicular inclu- 

 sions of sillimanite are abundant, but they are arranged with- 

 out apparent accordance with any law. They serve as a con- 

 venient means of distinguishing most of the cordierite from 

 the quartz, which it closely resembles. The axial figure is ex- 

 cellent and is easy to get. 



A few rods west of the line of ledges of cordierite gneiss, 

 and with the same general course, there is a ledge of horn- 

 blende schist and gneiss containing numerous veins of garnet 

 associated with vesuvianite of a dark green color. The vesuvi- 

 anite occurs massive in the veins, showing few crystalline 

 planes and those mostly prismatic. The garnet also is mas- 

 sive, but it shows many crystalline points of clear material ; 

 its color is yellowish red, and its appearance entirely different 

 from that of the deep red individual garnets which are so 

 common in the gneiss. The latter usually show the 2-2 

 planes most prominently, while in the former the I planes 

 modified by the 3-f predominate. The veins vary from half 

 an inch to four inches in width, coincide with the bedding 

 of the gneiss, and are seen for only 100 feet on the east side 

 of a ledge but fifty feet wide. Xo cordierite was found in 

 this ledge. Many garnets of value as gems have been taken 

 from the ledge in the last six years. A quarter of a mile 

 northeast of this point coarse black tourmalines occur in a vein 

 in quartz. 



Percival puts the gneiss of this region into his " Anthophyh 

 lite Formation," which includes the famous Haddam locality, 

 about fifteen miles northeast of the locality here described. 



Magnetite in good octahedral crystals Jin. to 1 in. in diame- 

 ter occurs in the school district of Xut Plains, town of Guil- 

 ford. The locality has long been known to the inhabitants of 

 the district. It is a ledge of gneiss on the north side of the 

 Xorth Madison road, half a mile east of the Xut Plains school 

 house. The ledge is a few rods east of a ruined farm house 

 and near a large brook. 



Yale University. }Iay 28th, 1888. 



