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.
Narrow though the strip of volcanic material is along the south coast of the Isle of Man, it has fortunately preserved for us some of the vents from which the tuffs were ejected. A group of these vents, three or four in number, may be traced along the shore in a general W.N.W. and E.S.E. line from Scarlet Point for rather more than a mile. Their margins are in some places exceedingly well defined. The most striking example of this feature occurs in the most westerly vent, where a neck of remarkably coarse volcanic agglomerate rises vertically through well-bedded, westerly-dipping tuff ([Fig. 187]). In other portions of their boundaries no sharp line can be drawn between the material filling the vent and that of the surrounding tuffs. Hence it is difficult to define precisely the form and size of the vents. I am inclined to believe from this indefiniteness of outline, and from the remarkable structure of the dykes, to which I shall afterwards refer, that the presently visible parts of these necks must lie close to the mouths of the original vents, if indeed they do not actually contain parts of the craters and of their surrounding walls.
The materials that have filled up the eruptive vents consist chiefly of agglomerate, but partly also of intrusive portions of vesicular lava. The agglomerate is composed of similar materials to the tuffs. Its matrix shows the same extraordinarily abundant fine greenish-grey basic pumiceous lapilli, with the same kind of plentiful loose felspar-crystals. The large blocks of lava, too, resemble in composition and structure those of the bedded tuffs, but greatly exceed them in size and abundance.
Besides the fragments of vesicular lava, there occur also occasional blocks of limestone. Some of these are several yards in length. Messrs. Strahan and Lamplugh have mapped a large mass of limestone at the Scarlet vent, which, so far as can be observed, lies in the agglomerate—a large cake of white limestone with pebbles of quartz, which has probably been broken off from some underlying bed and carried up in the chimney of the volcano.
As a rule the agglomerate is a tumultuous, unstratified mass. But in many places it shows lines of bedding and, as already stated, passes outward into ordinary bedded tuff, the number and size of the ejected blocks rapidly diminishing. Where this transition occurs we seem to see a remnant of the base of the actual volcanic cone. Thus, in the most westerly vent already cited, while the wall of the vent has been laid bare on the side next the sea, so that the agglomerate on the beach descends vertically through the surrounding bedded tuffs, on the western side the cliffs have preserved a portion of the material that accumulated outside the orifice ([Fig. 187]). In this section we observe that the coarse agglomerate which fills up the main part of the vent has been left with a hummocky, uneven surface, and that a subsequent and perhaps feebler eruption of finer material has covered over these inequalities, and has extended to the left above the fine tuffs through which the agglomerate has been drilled.
Fig. 187.—Section of part of a volcanic neck on shore to the south-east of Poyll Vaaish Bay, Isle of Man.
