240 The American Qeoloyist, October, 1886 
ose.* In the northeastern part of Westchester Co. it may 
become rather schistose, due to the increased development of 
biotite. 
Dolomitic limestone of later age (probably Trenton-Calcif- 
erous) is found in the larger valleys. 
The ''augend-gneiss extends over an irregularly ovoid area, 
starting from a point about one and a half miles northeast of 
North Castle and extending six miles in a northeastly direc- 
tion to Pound Ridge. Bedford village lies on the northwest- 
ern edge of the area, while the southeastern boundary runs 
approximately parallel to the ('onnecticut state line and about 
one mile distant from it. 
The average width is 2^ miles, thus making an area of 
about 15 sq. miles (Plate IX. Fig. 6). 
Within this area the prevailing strike is N. 30'^-40^' E. and 
the dip 35^-40'-' N. W. 
The ''AtjOEN^-Gxeiss. 
The normal phase of the rock is a line-grained, gray, gneis- 
sic mixture, containing numerous ''augen" of pinkish feld- 
spar, thus giving the rock a distinctly porphyritic aspect. 
These feldspar "augen" usually form a large proportion of the 
rock mass. They vary in size, the average length being one 
inch, although ■3ome are three or four inches long and all show 
twinning after the Carlsbad law. These porphyritic crystals 
are in large part microcline or show "microclinic" structure 
as described later. On some outcrops where weathering has 
taken place the "'augen" appear in bold relief (Plate VIII, 
Fig. 1). 
In most cases the "augen" are elliptical in shape, with 
rounded or rectangular ends, and at times almost showing 
crystallographic boundaries. Again this shape is distinctly 
lenticular, as is well shown by a specimen from L. McDonald's 
quarry, where a portion of the large "auge" has been sheared 
off at the end of the lens (Plate IX, Fig. 5). 
A constant feature is the parallelism which the "augen" 
show with the schistosit^^ of the gneiss, but they are not al- 
ways uniformly distributed through the rock, at times ap- 
pearing in well marked bands of varying dimensions. 
*F. J. H. Merrill. The Metamorphic Strata of Southeastern New 
York. Amer. Jour. Sci-. (3), xxxix, 38.3, 1890. 
