This parish is situated on rocks of the calcareous series, which are
for the most part like those of Laneast; but where Trewen joins
Alternun, bounded only by the small river Inney, some strata occur
which deserve more particular notice. It has already been stated, that
on the opposite side of this stream, a potstone or ollareous
serpentine occurs; on the Trewen side a talcose schist first appears,
which is followed by a calcareous schist, with its surface talcose and
glossy, resembling the slate at Cotele on the Tamar. This slate
gradually passes into a compact limestone, which is light-coloured and
talcose, especially in such parts as come in contact with masses of
hornstone diffused through the mass. This limestone is quarried and
burnt on the spot, but after the selection of such portions as abound
with calcareous spar.