STRATIGRAPHY AND STRUCTURE CASTLETON AREA VERMONT

dolomites of \ermont from red at the north to white and gray to the
south is accomplished by "deoxidation, dehydration, and high-temperature
carbonatization of the ferric compounds." The marbles are silicated,
and the mineral assemblage indicates "high-temperature hydrothermal
action." Evidently Bain regarded the magmatic solutions as more or
less syntectonic. No igneous rocks adequate to accomplish the metamorphism
of this region are exposed within dozens of miles of the Castleton
area. It may be safer to speak of a regional rise of rock temperature
during metamorphism than to infer the presence of magma. The great
abundance of hydrous minerals in the argillaceous rocks belies a general
dehydration during metamorphism.
An instance of apparent de-dolomitization adjacent to the large
Mississippian (?) dike in Furnace Brook at Pittsford Mills was observed
during field work. The Dunham dolomite within a few feet of the dike
is a white marble that effervesces strongly in acid. Nowhere else in this
area and in very few other places in Vermont is the Dunham calcareous.
In general, however, the dikes have had no metamorphic effect on their
surroundings.
No evidence of retrograde metamorphism was observed in this area,
although Bain has interpreted some of the Mendon strata at Cox
Mountain as phyllonite. In the Taconic quadrangle (Prindle and Knopf,
1932, p. 298-301) phyllonites are described.
The time of thermal metamorphism was the time of shear cleavage
formation, which is judged to have been late in the Taconic disturbance.
Metamorphism must be related to the rise of the Green Mountain axis
and probably to the formation of similar domes in eastern Vermont. It is
not unlikely that the ultimate elevation of these great domal features
climaxed the orogenic episode and was accompanied by a regional rise
of temperature in the rocks.
HISTORICAL GEOLOGY
Before the beginning of the Cambrian period there existed in Vermont
and probably western New Hampshire a northeast-trending geosyncline
that received coarse sediments from nearby highlands. There may have
been islands within the basin in addition to the lands on the western and
eastern borders, and the eroded sediments from these lands were probably
of different texture and composition in various parts of the basin. This
depositional trough continued south and north for great distances as the
embryonic Appalachian geosyncline. Sometime near the beginning of the
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