FEATURES OF A BODY OF ANORTHOSITE-GABBRO 
IN NORTHERN NEW YORK* 
WILLIAM J. MILLER 
Smith College, Northampton, Massachusetts 
GENERAL RELATIONS OF THE ANORTHOSITE-GABBRO 
The body of rock considered in this paper varies from true 
anorthosite, through anorthosite-gabbro, to true gabbro. It is 
four and one-third miles long, with a maximum width of one mile. 
It lies a little west of the central part of the Russell quadrangle in 
St. Lawrence County, New York (Fig. 1). 
As far as could be determined, the rock immediately surrounding 
the gabbro is granite which is generally pink in color and carries 
only moderate amounts of dark minerals. The granite varies 
from medium grained to coarse grained, and from scarcely foliated 
to highly foliated. That the granite is younger than the 
anorthosite-gabbro is proved by the presence of both granite and 
granite-pegmatite dikes in the gabbro, and by the more or less 
intimate injection of portions of the borders of the gabbro by the 
granite. The best and most instructive exposures occur within 
the northern one-third of the area. 
About twenty-five years ago Professor C. H. Smyth published. 
a short paper? which is largely a microscopical petrographic de- 
scription of a ledge of gabbro probably lying within the area of 
anorthosite-gabbro. It is the present purpose to rather fully dis- 
cuss the megascopic and microscopic features, field relations, and 
origin of the remarkable lot of rocks which constitute facies of 
the body of anorthosite-gabbro and directly associated rocks 
covering several square miles. 
VARIATIONS IN COMPOSITION 
The whole mass of the rock of the area is called anorthosite- 
gabbro because it shows every possible gradation from true gabbro 
1 Published by permission of the state geologist of New York. 
2C. H. Smyth, Amer. Jour. Sci., 4th series, Vol. I (1896), pp. 273-81. 
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