In the summer of 1890, in the company of Mr. L. W. Fulcher, I again visited Mynydd Mawr, on the west of the Snowdon area, in the hope of obtaining specimens of Riebeckite larger than those on which Mr. Harker and Professor Bonney based their observations in 1888, when they independently pointed out the occurrence of this mineral for the first time in the British Isles. Previous traverses of the dome-shaped mass had shown how uniform in structure and how minutely crystalline the rock of Mynydd Mawr was over all its surface; we accordingly examined the columnar cliffs on the north and west, paying especial attention to the great western hollow, where denudation allows one to stand almost in the centre of the intrusive neck. But even here we were somewhat disappoinied, and our specimens are only slightly coarser in grain than those studied with such striking results by Professor Bonney.