The western and south-western parts of this extensive parish are
composed of granite and of hornblend rocks, which surround the former,
and recline upon them. This portion is in every respect similar to the
parish of Advent. At Trewint a very fine compact felspar occurs,
imbedded in green stone; and proceeding eastward on the Launceston
road, hornblend slate prevails, and affords a very fertile soil.
Descending the hill to Pellaphant, masses of compact rock protrude
from beneath the soil. They are composed of compact felspar,
aggregated with lamellar hornblend, forming a kind of sienite. But the
most interesting object in this parish is to be found a little to the
north of this last place, on the side of the hill sloping down to the
river Inny. It is a magnesian rock, and it is quarried for
building-stones, yielding large blocks, which are sufficiently soft at
the time of their being raised, to allow of their being cut through by
a common handsaw. A considerable quantity of this stone has been used
in building a very large house, denominated a cottage by the Duke of
Bedford, at Ensleigh, on the eastern bank of the Tamma. The stone
appears, however, to have one bad quality: the surface, after a slight
disintegration from exposure to the atmosphere, becomes covered over
with ocherous spots, owing to the presence of a scaly mineral
resembling diallage, which is disseminated through the rock. This
magnesian rock has all the characters of pitstones, the lapis ollaris
of the ancients; and it is a curious circumstance, that on opening
some old workings near this place, several antique vessels were found
resembling pipkins and shallow pans made out of this material.
The immediate connection of this bed of ollareous serpentine with the
adjacent rocks, is not disclosed; but it is succeeded by talc ore,
slate, and limestone, on the other side of the river.