Mineral Industries and Geology of Certain Areas - Vermont Agency ...

80 REPORT OF THE VERMONT STATE GEOLOGIST.
slate 4 feet wide and 10 feet deep. It is a picturesque and popular
resort.
Illustration, Plate XVII, shows the cascade, the eroded channel,
and the rustic bridge above them.
In the northeast corner of Thetford, about one mile south of
Ely station and 50 rods west of the Passumpsic Railroad, there
is an old quarry of extremely fissile, soft, argillaceous slate, with
apparent strike north io° east, cleavage dip 85 ° east 10° south,
strata (lip 45° north. It affords a most striking illustration of
the nonconformity of planes of bedding and fissility. Crossing
the entire exposed surface of ioo feet by 25 remarkably
distinct lines of original bedding are seen. The cleavage planes
are induced by subsequent pressure at right angles to the lines
of fissility.
About one-third of a mile to the northeast of Thetford Hill,
at the old galena quarry, the cleavage is 40° east 30° south
with no evidence of characteristic planes of bedding, unless the
dip of the mineral vein, 21 ° to the southeast, affords us that evidence.
It carries galena and sphalerite in considerable abundance.
This Thetford argillite is a narrow band varying from onefourth
of a mile in width, in Fairlee, to i mile in Norwich. It
was not known to extend continuously north of Ely station until
the author succeeded in the summer of '96 in tracing it through
Fairlee and into Bradford, where the hydromica schist takes
its place.
The strike varies from north 10° east in Fairlee to north 50 °
east in Hartford. South of North Hartland it is north and
south, and in Norwich north 45° east. The dip changes from
6o° west in Hartford to 85 ° east in Fairlee.
With the general dip of the eastern flank to the west, or
vertical, and a low dip of the planes of bedding just north and
south of where the slate has been entirely eroded in Bradford,
is sufficient evidence that it lies unconformably upoii the Huronian
hydromica schist. The lapse of time between the deposition
of the Huronian schists and the Lower Trenton slates was
Ll
I
PLATE XVTT.
Glen Falls, Fairlee.