In the Geological Magazine, Vol. VIII. 1871, p. 247, I gave a short account of the composition and structure of a Phonolite which forms the mass of the Wolf Rock lying between the Land's End and the Scilly Islands. The account there given has been noticed by Prof. Zirkel in a work recently published, in which he refers to my description of some of the crystals stated to be nepheline, and suggests that they may be nosean. The passage to which, he refers is as follows:—“The grey dust filling some of the crystals is frequently collected together so as to form a dark or even black mass in the centre, the edges of which are sharply defined, and correspond exactly with those of the crystal. Hexagonal crystals, for example, exhibit a border filled with a fine grey dust, and a central portion occupied by a well-defined black hexagon, or there is sometimes a black band running parallel with, and at some distance from the sides.” To this I may now add, that some of the crystals and irregular grains are traversed by a number of very fine straight lines of a bluish-black colour, and that with a high magnifying power these lines are resolved into rows of extremely minute dark granules similar to those forming the dust. This remarkable structure, combined with that previously given, is so thoroughly characteristic of nosean that there can be no doubt of its presence in the rock; the mineralogical composition of which is thus found to be in complete accordance with that of the large series examined by Zirkel.