STRATIGRAPHY AND STRUCTURE CASTLETON AREA VERMONT

morphism are here called phyllites. Because Dale regarded the rocks of
the slate belt technically as phyllites, he called the phyllites of the Nassau
formation schists. The gradational series shale-slate-phyllite-schist can
be divided only by arbitrary decision. It seems reasonable here to draw
the line between commercial roofing slate and phyllite at the zone where
recrystallization and deformation has produced a warped, coarse-grained
rock that is worthless as slate. The typical shimmering luster of phyllite
is not present to any great degree in roofing slate, and phyllite usually
lacks the lumpy crystalloblastic surface that is found in a schist. The
phyllite-schist boundary has here been placed approximately at the
zone where recrystallization has been complete enough to remove the
"dirty" appearance of the rock, where the fine-grained ferruginous and
carbonaceous matter has aggregated into larger crystals. Obviously
these terms are used arbitrarily.
As judged by the mineral suites in the aluminous rocks, most of the
Castleton area is in the low-grade metamorphic zone and is characterized
by such minerals as chlorite, sericite, and pyrophyllite. Some actinolite
needles were reported in the Nassau by previous investigators. Metamorphism
increases in intensity from the slate belt eastward until in the
schists of the Mendon series in the Cox Mountain area chlorite plates
can be seen in thin section to be partially altered to highly pleochroic
fresh biotite. The northeastern corner of the quadrangle should therefore
be placed in the middle-grade zone.
An unquestionable correlation between deformation of the rocks and
intensity of metamorphism is found in this area. The connection between
the formation of shear cleavage and recrystallization has been mentioned.
In the marbles "association of abnormally coarse texture with high local
intensity of folding is illustrated by the Pittsford Valley, West Rutland,
and Danby deposits" (Bain, 1934, p. 131). Bain saw that the increase in
grain size of the marbles from northern to southern Vermont is accompanied
by more intense fiowage and overturning of the rocks. It is not
likely that stress is the only agent, although it may be the ultimate cause,
of the metamorphism here. Temperature must be raised, and probably
solutions are necessary to redistribute materials and heat. The metamorphism
is therefore dynamothermal.
The heat may be due in part to burial and in part to friction, but
probably an additional source is required. Bain (1934, 1938, p. 18) appealed
to "both tectonic and magmatic metamorphism" in the alteration
of the carbonate rocks. The change in color in the Lower Cambrian
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