(_a_) _Volcanic Agglomerates._--In the tumultuous unstratified masses of fragmentary materials which fill eruptive vents in and around the plateaux, the stones, which vary in size up to blocks several feet in diameter, consist for the most part of basalts, often highly slaggy and scoriaceous. They include also fragments of different acid eruptive rocks (generally felsitic or rhyolitic in texture), with pieces of the non-volcanic rocks through which the volcanic pipes have been drilled. The paste is granular, dirty-green or brown in colour, and seems generally to consist chiefly of comminuted basalt. As in the Carboniferous and Permian necks, the Tertiary agglomerates contain abundant detritus of a basic minutely cellular pumice. (_b_) _Volcanic Conglomerates and Breccias in beds intercalated between the flows of Basalt._--These are of at least three kinds. (_a_) Basalt-conglomerates, composed mainly of rounded and subangular blocks of basalt (or allied basic lava), sometimes a yard or more in diameter, not unfrequently in the form of pieces of rough slag or even of true bombs, imbedded in a granular matrix of comminuted basalt-debris. In some cases, the stones form by far the most abundant constituents of the rock, which then resembles some of the coarse agglomerates just described. Perhaps the most remarkable accumulations of this kind are those intercalated among the basalts in the islands of Canna and Sanday, of which a detailed account will be given in Chapter xxxviii. These conglomerates, besides their volcanic materials, contain rounded blocks of Torridon sandstone and other rocks, which must have been carried from the east by some tolerably powerful river that flowed across the basalt-plains during the volcanic period. Again, on the east side of Mull, the slaggy basalts of Beinn Chreagach Mhor are occasionally separated by volcanic conglomerates. As a rule, however, such intercalations are seldom more than a few feet or yards in thickness. Their coarseness and repetition on successive horizons indicate that they probably accumulated in the near neighbourhood of one or more small vents, from which discharges of fragmentary materials took place at the beginning or at the close of an outflow of lava, and that the stones were sometimes swept away from the cones and rolled about by streams before being buried under the succeeding lava-sheets. More commonly the dirty-green or dark-brown granular matrix exceeds in bulk the stones embedded in it. It has obviously been derived mainly from the trituration of already cooled basalt--masses, and probably also from explosions of the still molten rock in the vents. A striking illustration of this type of rock may be seen on the south side of Portree Harbour, where a mass of dark-green basalt-conglomerate, with a coaly layer above it, lies near the base of the bedded basalts, and attains at one part of its course a thickness of about 200 feet. This