The whole of this parish, with the exception of a small patch of slate
at Rosemodris, rests on granite. Judging from what occurs in the
eastern part of Cornwall, one might be led to expect that the land of
St. Burian must be sterile. In some elevated spots it undoubtedly is
so, but in general the parish is well cultivated and highly
productive.
This difference in the granitic soils of east and of west Cornwall,
may be, in part, explained by the gradual diminution of height towards
the west, accompanied by a corresponding improvement of the climate;
but in this part of the county more of the debris, especially of
diluvial clay, is retained on the surface, that of the more elevated
eastern ridges having been in great measure swept away.
This circumstance must not, however, be omitted. The granite of Burian
exhibits more varieties than have been yet found in the eastern
district. The slate in the cliffs at Rosemodris is a felspar rock, and
its contact with the granite is distinctly seen; where it may be
observed at the eastern extremity traversed by numerous granite veins;
and the granite near this junction abounds in shorl.