This extensive parish includes nearly the whole of that granitic patch
known by the names of Tregonning and Godolphin Hills; and it also
comprises the greater part of the country lying between those hills
and those of the opposite range of granite in Wendron and Crowan,
called the Forest. Its mines, quarries, and sea cliffs afford most
interesting geological sections.
The granite of Godolphin Hill is of the common kind, containing in
several places an intermixture of shorl, and it is traversed by
numerous thick veins of quartz, which sometimes pass into compact
shorl rock. The granite of Tregonning Hill is of two kinds; one
fine-grained like free-stone, which is extensively quarried on the
western side of the hill, and used for ornamental building, under the
name of Breage stone; the other, abounding in talc, and in a state of
considerable decomposition, affording, like the similar granites of
St. Austell and St. Stephen’s, the china clay, which is here worked
for economical purposes, but not to any great extent.
The western part of the celebrated mine Whele Vor is situated in
Breage; and, as the workings approach the granite, they exhibit a
highly interesting arrangement of rocks, the granite and slate
alternating in the same manner as they have been observed to do at
Delcoath in Cambourne. The composition of these rocks, and the nature
of their connection, are very evidently seen in the heaps of fragments
piled round the shafts; but they are better and more clearly
illustrated in the cliffs near Trewaras Head.
It would occupy two much space to enter into details on this important
subject. It may, however, be noticed that both the granite and the
slate gradually pass into each other; and that they appear to differ
very little in their mineral composition. These facts seem to explain,
in a satisfactory manner, the nature of granite veins. For, if both
rocks have a similar composition, and have been produced at the same
time, the form, position, contents, and other circumstances of these
veins, are no longer perplexing.