STRATIGRAPHY AND STRUCTURE CASTLETON AREA VERMONT

grains came from veins and some from granite or gneiss. The opalescent
blue quartz found in the grit is identical to that found in some rocks
of the Mendon series near Cox Mountain and in the widespread Pinnacle
formation of the Green Mountains. The cause of the color is not known,
but the ultimate source of Vermont blue quartz is undoubtedly the
ancient gneisses that crop out beneath the oldest strafified rocks in the
state. Some quartz appears under the microscope to be eroded fragments
of previously indurated quartzite. The source of the slate and limestone
fragments cannot be discussed until it is certain where the Taconic
rocks were laid down.
The cement in this rock is chiefly quartz and chlorite. Minor detrital
and secondary minerals include: pyrite, sericite, chlorite, magnetite,
sphene, tourmaline, and calcite. Secondary siderite and quartz coat
some joint faces.
Relations and correlation: The argument that follows seeks to prove
that most of the rock that Dale (1899) called the Berkshire schist in the
southern half of the Castleton quadrangle is to be correlated with the
Nassau formation of eastern New York (Plate I). The name Berkshire
schist is applied to this rock until the discussion is concluded.
As heretofore mentioned, Dale's position throughout his life was that
the Berkshire schist of the Taconic Range is Ordovician. He held that it
lies, probably unconformably, above both the Cambro-Ordovician slates
of the slate belt at the west and the Cambro-Ordovician carbonates of
the Vermont Valley at the east. He believed that sudden changes of
facies were the rule during the deposition of the sediments of the entire
Taconic region. Under this view the marbles and dolomites of the Valley
sequence must change to the slates of the Taconic sequence over a
lateral distance of a few miles. Dale concluded that the Berkshire schist
must be later Ordovician, because it appears to overlie beds of Middle
Ordovician age on both sides of the Taconic Range.
The consistency of Dale's hypothesis cannot be questioned, but it
contains one assumption that is now believed to be unjustified: namely,
that the Berkshire overlies the rocks of the slate belt. The Berkshire
obviously overlies the Valley sequence along the Taconic Mountain
Front; but, as will be shown below, it appears to be impossible to prove
that the Berkshire overlies the rocks of the slate belt.
Inasmuch as no fossils other than worm trails have been found
throughout the Berkshire, age determination must be made on the basis
42