Between the lapilli and the minute pumice-dust that constitute the matrix of this tuff much calcite may be detected. Though this mineral may have been partly derived from the decay of the felspar in the lava-fragments, I believe that it is mainly to be attributed to the intermingling of fine calcareous ooze with the ash accumulated on the sea-floor. A more remarkable association of the same kind will be described in later pages from King's County in Ireland. That abundant calcareous organisms peopled the sea in which the Manx Carboniferous volcanoes were active is shown by the contemporaneously deposited limestones. The tuffs themselves are occasionally fossiliferous. Species of _Spirifer_, _Productus_ and other brachiopods, together with broken stems of encrinites, may be found in them, and doubtless the diffused calcite, though now crystalline, as in the limestones, and showing no organic structure, owes its presence to the detritus of once living organisms. The stones imbedded in the tuff consist almost exclusively of slightly different varieties of the same pale, always vesicular rock, and sometimes pass into a coarse slag. They vary up to six feet or more in length. In many cases, they appear to have been derived from the disruption of already solidified lava, for their vesicles are not elongated or arranged with reference to the form of the block, but have been broken across and appear in section on the outer surface. In other instances, however, the cavities are large and irregular in the centre of the block, while on the outside they are smaller and are drawn out round the rudely spherical shape of the mass, as in true volcanic bombs. The limestone fragments enclosed in the tuff include pieces of the dark carbonaceous and of the pale encrinal varieties. In no case did I observe any sensible alteration of these fragments. They seem to have been derived from material disrupted and ejected during the opening of successive vents, and not to have been exposed for any considerable time to the metamorphic influence of volcanic heat and vapours.