Note 64 — EXCURSION III.

This rock is a binary compound of Quartz and Schorl, without any, or scarcely any, admixture of the other constituents of Granite; and yet when we consider its various relations, it must be regarded as rather a variety of the latter than a distinct rock. The locality now mentioned and that singular group of rocks between Truro and Bodmin, known by the name of Roach Rock, are, as far as we know, the only places in Cornwall where this modification of granite is found in mass. In the form of veins its occurrence is not unusual, especially at the junction of granite and slate, where it would often seem to exist as an intermediate rock.