The micas presented by the granites of the Co. Dublin have been analysed by Haughton, and determined by him to be margarodites and lepidomel~lnes (see Proc. Roy. Irish Ac. Vol. VI. 1855, pp. 176-79, and Quart. Journ. Geol. Soc. Vol. XV. 1859, p. 129). In this latter paper he says:--" In my first paper on the Granites of Leinster (Quart. Journ. Geol. Soc. Vol. XII. 1856 p. 171), I have mentioned the black mica which is found accompanying the white margarodite of the Leinster granite in small flakes, and noticed the curious fact that these flakes are sometimes physically embedded in the plates of white mica without injuring their fissility or lustre, hut always eifecting a reduction of about 20° in the angle between the optic axes of the latter."