STRATIGRAPHY AND STRUCTURE CASTLETON AREA VERMONT

flattened in the plane of the regional cleavage. The matrix of the conglomerates
is green slate. Fossil worm-burrows in the boulders are similar
to those from boulders in the conglomerates at the base of the black
Schodack slate. Where the Eddy Hill grit is missing the Mettawee and
Schodack are separated on the basis of color. Consequently conglomerate
lenses lie in both formations. Conglomerates of this sort suggest a shallow
water environment in which bioherms or cliffs were exposed to wave
action.
Petrography: The thin sections of Mettawee studied during this
investigation brought forth no information that is not found in Dale's
exhaustive report (1899, p. 232-265). Transverse sections show a conspicuous
mass extinction because of the parallel orientation of micas
and quartz. Sericite and chlorite shreds, rhombs and irregular masses
of carbonate (dolomite and siderite mostly), quartz grains, multitudinous
rutile needles, and pyrite cubes or masses are the common constituents
of all the slates. The green color is due to the chlorite content, and the
shades of purple and red (rare in the Cambrian slates) are produced by
tiny spots of hematite.
Certain varieties of slate are differentiated in the slate industry on the
basis of shade and permanence of color. Dale could find no explanation
of the distribution of these varieties within the Mettawee other than
chance differences occurring during sedimentation. Using Hillebrand's
chemical analyses Dale was able to show that the slates that fade on
exposure to the atmosphere do so because of the gradual solution of
abundant crystals of iron-bearing dolomite or siderite. Non-fading varieties
have less carbonate. The green siliceous spots in some of the purple
slates probably represent local reduction of iron to the ferrous state by
organic material in the mud. The soluble ferrous iron could be removed
by solutions or could go into pyrite. Excess silica in the green spots may
be explained by infiltration. Cautiously Dale did not say that all green
slate owes its color to reduction of iron compounds. Yet the evident
absence of hematite spots in the green slates throughout the Castleton
area suggests a general lack of oxidizing conditions at the time of their
deposition.
Thickness: The top of the Mettawee is fixed either at the bottom of
the Eddy Hill grit or at the lowest black slate or limestone of the Schodack
formation. The base of the Mettawee rests on the Bomoseen grit,
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