In 1872 I published in the Geological Magazine (Vol. IX. p. 1), a brief account of the Arran Pitchstones, in which I figured and described the singularly beautiful groups of minute green crystals which are so characteristic of the rocks on the eastern shore of the island. I have since cut many additional thin slices, and have long been aware that my paper contains an error, which I will take the present opportunity to rectify. The groups of crystallites to which I refer are described on page 2 and illustrated, Plate I. Fig. 1, and contain the “long slender prisms of a green pyroxenic mineral” there mentioned. In the last paragraph, p. 9, these green prismsare again mentioned, and are said to be augite, instead of hornblende, as stated by Zirkel in his description of the same rock. The grounds for this conclusion were twofold, the absence of dichroism, and the presence of undoubted crystals of augite. After pointing out that the pale green crystals exhibit no trace of dichroism, I added, “Unless, therefore, it can be shown that some hornblende is not in the least dichroic. the mineral in question must, on this ground alone, be regarded as augite.” As regards the latter mineral, there can be no doubt whatever of its occurrence, for in addition to the crystal described in the paragraph just quoted, I may now add, that it is far from uncommon, perfectly characteristic examples having been observed by me in specimens from several different localities in the island.