The volcanic materials consist mainly of bedded tuffs, but include also several necks of agglomerate and a number of dykes and sills. So far as I have observed, they comprise no true lava-streams.[57] These Manx tuffs present many of the familiar features of those belonging to the puy-eruptions of Central Scotland, but with some peculiarities worthy of attention. They are on the whole distinctly bedded, and as their inclination is generally in a westerly direction, an ascending order can be traced in them from the eastern end of the section to the highest parts of the group associated with the Poyll Vaaish limestones. Their colour is the usual dull yellowish-green, varying slightly in tint with changes in the texture of the materials, the palest bands consisting of the finest dust or volcanic mud. Great differences in the size of their fragmentary constituents may be observed in successive beds, coarse and fine bands rapidly alternating, with no admixture of non-volcanic sediment, though occasional layers of fine ash or mudstone, showing distinct current-bedding, may be noticed. [Footnote 57: The occurrence of intercalated lavas has been described in this series, but, as I shall show in the sequel, they are probably intrusive masses.] Pauses in the succession of eruptions are marked by the intercalation of seams of limestone or groups of limestone, shale and black impure chert. Such interstratifications are sometimes curiously local and interrupted. They may be observed to die out rapidly, thereby allowing the tuff above and below them to unite into one continuous mass. They seem to have been accumulated in hollows of the tuff during somewhat prolonged intervals of volcanic quiescence, and to have been suddenly brought to an end by a renewal of the eruptions. There are some four or five such intercalated groups of calcareous strata in the thick series of tuffs, and we may regard them as marking the chief pauses in the continuity or energy of the volcanic explosions.