The Geological structure of this parish is precisely similar to that
of Callington; but, as it is better developed, it will admit of a
little more detail.
The northern part consists of the granite of Hingston Down, which is
crystalline, and it is extensively quarried for economical purposes.
The quarries near the summit of Kitt Hill afford excellent
illustrations of the internal structure of the granitic mass. On this
Down, beds of fine-grained granite project here and there above the
surface, resembling that which occurs as Elvan courses in the adjacent
slate. The latter rock consists of a basis of granular felspar,
spotted and spangled with a shining mineral like mica. This slate, as
well as the granite, have been long explored for tin and copper. Shorl
and mica generally abound in the lodes; and the former mineral is
often so intimately combined with the quartz, as to form a
dark-coloured compact shorl rock. Southward the slate becomes more
blue, and is fissile into extended slabs. Near the village of Calstock
it is soft and lamellar, having evidently graduated into the
calcareous series. At Cotehele, near the landing place, a beautiful
oved-coloured calcareous schist is quarried, which is said to prove a
good material for lining kilns and ovens: it has a shining talcose
appearance, resembling that of Trenalt, near Pallephant, in Alternon.
Nearly opposite to the Rectory, built by Bishop Blackbourn, and
pleasantly situated on the river, may be seen the debouch of a canal
from Tavistock, with an inclined plane, descending into the Tamar.
This canal, in one part of its line, passes through a hill at the
depth of seventy fathoms.