The pegmatites of Trolvis Quarry, Carnmenellis, are small lenticular masses which were developed before the joints of the associated granite. Both mineralogically simple and complex types occur: the latter are drusy. Whilst the earlier minerals lining the druses may have crystallized in a closed system, the later minerals were clearly deposited from a moving phase. The disposition of the later minerals was largely determined by earlier crystals acting as baffles to the mineral-depositing agents. The pegmatites appear to occupy spaces formed by the solution of the granite by a pegmatitic fraction of the magma.