In this region we see the once boiling and seething masses of rock which fed the old volcanoes, now no longer endowed with life-like power by the force of steam, but lying in deathlike cold and stiffness, with their beautiful crystals of mica and felspar sparkling in the sun. The volcanic fires have died out; but the traces of their work are unmistakable, among which we must not forget to reckon the beautiful minerals made by the action of heated water upon the surrounding rocks. The beautiful cairngorm stones are still sometimes found on the mountain from which they take their name, and in all volcanic regions minerals are plentiful. The well-known hill called Arthur's Seat, close to Edinburgh, marks the site of an old volcano. The "neck," or central opening, may be seen at the top of the hill, but choked up with volcanic rocks and débris. The crater has long since disappeared, but Salisbury Craigs and St. Leonard's Craigs are formed of a great sheet of basalt that intruded itself among the stratified rocks that had been formed there, and so belong really to a great intrusive dyke. In the Castle Rock we see the same basalt again. During a much later age, known as the Miocene Period (see chap. x., p. 324), enormous outpourings of lava took place in Western Europe, covering hundreds of square miles. Of these the most important is that which occupies a large part of the northeast of Ireland, and extends in patches through the Inner Hebrides and the Faröe Islands into Iceland. These eruptive rocks, unlike those above referred to, must have poured out at the surface, and have taken the form of successive sheets, such as we now see in the terraced plateaux of Skye, Eigg, Canna, Muck, Mull, and Morven. These, then, are patches of what once formed a great plain of basalt. During later times this volcanic platform has been so greatly cut up by the agents of denudation that it has been reduced to mere scattered fragments; thousands of feet of basalt have been worn away from it; deep and wide valleys have been carved out of it; and in many cases it has been almost entirely stripped off from the wide areas it once covered. Where, as in the Isle of Eigg, the lava has been piled up in successive sheets, with some layers of volcanic ash between, the latter has been worn away rather faster than the hard layers of basalt, and each lava-flow is clearly marked by a terrace. These volcanic eruptions have thus had a great influence in moulding the scenery of this region. In Ireland the old basalts are well seen at the Giant's Causeway, and on the Scottish coast we see them again at the well-known Fingal's cave at Staffa. This island, like the others, is just a patch of the old lava-streams.