The great Island of Skye is the basal wreck of another volcano which was also in eruption during Tertiary times; probably, many millions of years ago. This immense volcano had originally a diameter at its base of about thirty miles, and a height of 12,000 to 15,000 feet, and must have been comparable to Etna or Teneriffe in its dimensions. At the present time, there is nothing left of this vast pile but the highly crystalline granites and gabbros filling up the great fissures through which the eruption of igneous materials took place. These, worn by denudation into rounded dome-like masses and wild rugged peaks, constitute the Red Mountains and Coolin Hills of Skye, which rise to the height of more than 3,000 feet above the sea-level. From these great, central masses of crystalline rocks, innumerable radiating dykes may be found rising through the surrounding rock-masses, with isolated patches of the scoriæ and lapilli ejected from the volcano, which have here and there escaped removal by denudation. Along what were the outskirts of this great mountain-mass are found flat-topped hills, built up of lava-streams, only small portions of which have escaped removal by denudation. [Sidenote: RESERVOIRS BENEATH VOLCANOES.] But this wearing away of the structure of a volcanic cone by the denuding forces may proceed even one stage farther, and we may then have revealed for our inspection and study the mass of originally fluid materials, from which one or more volcanoes have been fed, cooled and consolidated in their original reservoir. There are many examples of masses of granitic or highly crystalline rocks, having precisely the same composition as the different varieties of lavas, which are found lying in the midst of the sedimentary rocks, and sending off into these rocks veins and dykes of the same composition with themselves. No one who has carefully studied the appearances presented by volcanic mountains in different stages of dissection, by the action of denuding forces, can avoid recognising these great granitic masses as the cooled reservoirs from which volcanoes have in all probability been supplied during earlier periods of the earth's history.