The northern part of this parish rests on the porphyritic series, but
no where reaches the granite hills: its rocks are very felspathic and
metalliferous. The southern part comes into the calcareous series, and
its rocks are like those of Creed, Cuby, and Cornelly; the most
interesting of these rocks may be seen on the hill to the westward of
Grampound. They appear to afford a most decided example of greywacke,
exhibiting large grains, and even nodules of quartz on their fractured
surfaces; this however can only be seen on surfaces that have been for
some time exposed to the weather; for no such appearance can be
observed when the fractures are recent and perfect. These rocks are
probably masses of compact felspar, in the bases of which silica so
predominated at the time of their formation, as to have given rise to
large concretions of quartz.
* * * * *
Since Doctor Boase made the geological examination of Cornwall, an
abridgment of which he has had the kindness and the liberality to
communicate for this work, the road leading westward from Grampound,
has been turned to the north, and thus obviated a steep and even
dangerous hill, and in consequence the sections referred to can no
longer be seen on the highway.