In consequence of the circumstance that the eruption of lavas of intermediate composition usually precedes that of other varieties, we usually find the central and older portions of great volcanoes to be formed of andesites, trachytes, or phonolites, while the outer and newer portions of the mass are made up of acid or basic lavas. This is strikingly exemplified in the great volcanoes of the Auvergne and the Western Isles of Scotland, in all of which we find that great mountain masses have, in the first instance, been built up by extrusions of lava of the intermediate types, and that through this central core fissures have been opened conveying basic lavas to the surface. From these fissures great numbers of basaltic lava-streams have issued, greatly increasing the height and bulk of the volcanic cones and deluging the country all around. The lavas of intermediate composition--the andesites, trachytes, and phonolites--possess, as we have already seen, but very imperfect liquidity as they flow from the volcanic vents. Hence we find them either accumulating in great dome-shaped masses above the vent or forming lava-streams which are of great bulk and thickness, but do not flow far from the orifices whence they issue. The more fusible basaltic lavas, on the other hand, spread out evenly on issuing from a vent, and sometimes flow to the distance of many miles from it. This difference in the behaviour of the intermediate and basic lavas is admirably illustrated in the volcanic districts of the Auvergne and the Western Isles of Scotland. In other cases, like Vesuvius, we find that great volcanic cones of trachytic tuff have been built up, and that these masses of fragmentary trachytic materials have been surrounded and enclosed by the ejection, at a later date, of great outbursts of basaltic lavas. In still other cases, of which Rocca Monfina in Southern Italy constitutes an excellent example, we find that a great crater-ring of trachytic tuffs has been formed in the first instance, and in the midst of this a cone, composed of more basic materials, has been thrown up. [Sidenote: EXCEPTIONS TO THE GENERAL LAW.]