STRATIGRAPHY AND STRUCTURE CASTLETON AREA VERMONT

are occasionally found in the green phyllite. In thin section the Nassau
is composed of fibrous chlorite, sericite, and pyrophyllite lying in the
plane of the major cleavage and enclosing irregular elongated pods of
quartz grains. The Nassau formation has two cleavages. The dominant
flow cleavage dips eastward and is cut by a later transverse shear
cleavage. Pyrite and probably magnetite are present as tiny crystals
and as opaque masses that follow both cleavages. Crinkles on the flow
cleavage planes are caused by minute shearing along the transverse
cleavage planes. Such crinkles are by no means confined to the Nassau,
but they are best displayed in it. Although the green color of the Nassau
is traceable to chlorite, the purple phyllites appear to owe their hue to
disseminated hematite (Dale, 1899, p. 191). Dale also reported finding
tiny actinolite crystals with their long axes randomly oriented.
Another feature common to all argillaceous rocks in the area but best
developed in the Nassau formation is the presence of coarse-grained
quartz veins from 1 inch to several feet in width. Most of them are podshaped.
Their origin is clearly not from deep-seated hydrothermal waters
but rather is due to segregation of silica from the surrounding rock by
moving solutions of local genesis. A honeycomb structure of quartz
and an earthy iron oxide is noticeable in some veins. The tabulae separating
successive tiers of cells represent temporary walls of the periodically
widening vein. The original iron mineral was probably siderite. Pyrite
is found rarely in quartz veins.
Description of Bird Mountain grit: The Bird Mountain grit is a fineto
very coarse-grained rock containing dominant quartz, various amounts
of potash and plagioclase feldspar, and occasional fragments of slate
and brown-weathering limestone. Bedding is not readily seen except
where changes in texture occur. Much of the rock is massive and strongly
jointed. Typically the grit weathers dark-gray or green, the surface
being spotted with quartz eyes in a finer matrix. Some of the rock contains
enough fresh angular feldspar grains to be called an arkose. In
other places it is a pure quartzite. The green and brown slate fragments,
heterogeneously jumbled together, attain lengths of 10 cm. in graywacke
zones, and Dale (1900) recorded 3- to 5-inch pieces of limestone at the
summit of Bird Mountain. Grain size in the quartz ordinarily ranges
from silt-sized particles to granules 5 mm. in diameter. One-inch cobbles
of quartz are reported by Dale.
It was Wolff's opinion (Dale, 1893, p. 338) that some of the quartz
41