STRATIGRAPHY AND STRUCTURE CASTLETON AREA VERMONT

elow under "Relations and correlation" it is mapped as the Nassau
formation south of the Castleton River.
The Bird Mountain grit was described by Dale (1893, P. 337-340;
1900, p. 15-23) from the vicinity of Bird Mountain in Ira, Vermont. The
Bird Mountain grit is here considered to be several lenses of coarse
clastics interbedded with rocks of the Nassau formation. Although some
parts of the rock may be called more accurately conglomerate, arkose,
graywacke, and quartzite, on the whole the common angularity of fragments
and the dominant proportion of quartz grains justify the term
grit. The term Bird Mountain grit is used here only in a lithologic sense
to designate a facies of the Nassau formation.
Distribution: The Nassau formation crops out throughout the Taconic
Range south of the Castleton River. Some of the rock that Kaiser (1945)
called Mettawee and Schodack in the Taconic Range north of the Castleton
River may belong to the Nassau formation. The Bird Mountain
grit crops out on the top of Bird Mountain and on the large hill two
miles southwest. Small patches of the grit are exposed near these two
large masses. Other lenses of grit are located in the southern part of
Hampshire Hollow and on the southern slope of Clark Hill in the township
of West Rutland.
Description of the Nassau formation: The Nassau formation is dominantly
a green phyllite containing white and greenish-white quartzite
in beds ranging in thickness from 1 inch to 10 or 15 feet. Grayish-green
and gray phyllite are distinctly subordinate. Lenses and patches of
purple phyllite are common, especially at Ben Slide 1.25 miles north of
the top of Herrick Mountain and interbedded with the coarse clastics of
Bird Mountain and vicinity. The Bird Mountain grit will be discussed
separately below; nevertheless it is obviously an integral part of the
Nassau formation. Small lenses of grit, arkose, and graywacke, similar
in composition to the Bird Mountain, are found at a few places in the
Nassau. A finely banded impure gray quartzite is found at several places
near the trace of the Taconic overthrust south of the Castleton River.
The phyllites of the Nassau have a shimmering luster that is due in
large part to the development of pyrophyllite, the hydrous alu.rninous
silicate that corresponds in crystal structure to the magnesian talc. No
talc was identified. Pyrophyllite is saponaceous to the touch and is
probably the mineral that gives to the "talcose schists" of this region
their peculiar aspect. Pyrite cubes as large as three-eighths inches across
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