St. Enedellyan has been long celebrated for its mines of antimony.
This metal occurs in the state of a sulphuret, associated with iron
pyrites, in quartz veins, which run east and west, traversing a blue
lamellar slate, very similar to that which frequently abounds in the
calcareous series, and to which, indeed, this slate belongs. Some
curious varieties of rocks in this series are found in St. Enedellyan,
one of which, in particular, abounds between the church and Port
Isaac, which was noticed by Sir Humphrey Davy in a decomposing state,
under the name of mandelstone. In this state it is a dull earthy
argillaceous rock, of an ochreous colour, full of small, roundish
cavities. In its perfect state this rock is a greenish-grey, glossy,
compact felspar, containing granules of flesh-coloured calcareous
spar, and minute prisms of hornblende. During decomposition, the
calcareous spar is dissolved, and washed away by the rainwater, which
produces the honeycomb appearance; and the ferruginous stain is
derived from the iron contained in the hornblende. This rock has been
described by Mr. Prideaux as occurring in Devon. It is very rare in
Cornwall.