We perceive on studying the ground-plan of these volcanoes that great masses of granite, syenite, and diorite--the crystalline representatives of the first-extruded lavas--are penetrated by intrusions of gabbro--the granitic form of the later-ejected lavas. These features are admirably illustrated by the ruined volcano now constituting the Island of Mull, one of the Inner Hebrides, a plan of which is given in fig. 57, and a section in fig. 58. This volcano probably had a diameter at its base of nearly thirty miles, and a height of from 10,000 to 12,000 feet, but is now reduced to a group of hills few of which exceed 3,000 feet in height. From these great intrusive masses of highly crystalline rocks there proceed in every direction great spurs or dykes, which are evidently the radiating fissures formed during the outwelling of igneous materials from below, injected by these fluid substances. The rock forming these dykes is often less perfectly crystalline than that which constitutes the centre of the mass, and we may indeed detect among the materials of these dykes examples of every variety of structure, from the perfectly crystalline granite to the more or less glassy substance of lavas. Besides the vertical or oblique dykes we also find horizontal sheets, which, passing from these central masses, have penetrated between the surrounding strata, often, as we have seen, to enormous distances. For the sake of simplicity, we have spoken of these ground-plans, or basal wrecks of volcanoes, as constituting a flat plain; as a matter of fact, however, the unequal hardness of the materials composing volcanic mountains causes them to assume, under the influence of denuding agencies, a very rugged and uneven surface. The hard crystalline materials filling the central vent stand up as great mountain groups; each large dyke, by the removal of the surrounding softer materials, is left as a huge wall-like mass, while the remnants of lava-streams are seen constituting a number of isolated plateaux.