This parish is situated on rocks belonging to the calcareous series.
On the eastern boundary, however, near the church, the slate is nearly
connected with the porphyritic series; here at Treburget is a lead
mine in a blue pyrituous slate; the lodes run north-east and
south-west, varying from two feet to five feet in thickness. The
matrix of the ores consists of angular pieces of slate like fragments
cemented by quartz, in which galena, blend, iron pyrites, and spathous
iron occur.
Proceeding northward, this lamillar blue slate is succeeded by a
shining talcose slate; and at Rediver Mills on the road to Port Isaac,
a hard compact rock is quarried for the roads, which contains veins of
magnesian minerals; this rock occurs on the side of a steep
round-backed hill; and those circumstances, combined with the talcose
slate, render it probable that magnesian rocks may exist in the
neighbourhood. These rocks are succeeded by slate traversed by veins
of antimony, similar to what occurs in Endellion.