Its curious six-sided columns illustrate a fact with regard to the subsequent cooling of lava-flows. Some internal forces, analogous to that which regulates the shapes of crystals, have caused it to crack along three sets of lines, so placed with regard to each other as to produce six-sided columns. In Ireland the basalts attain a thickness of nine hundred feet; in Mull they are about three thousand feet thick. It has been clearly proved that Mull is the site of one of the old volcanoes of this period, but very few others have as yet been detected. Perhaps the eruptions took place mainly from large fissures, instead of from volcanic cones, for it is known that the ground below the lava-sheets has been rent by earthquakes into innumerable fissures, into which the basalt was injected from below. In this way a vast number of "dykes" were formed. These have been traced by hundreds eastwards from this region across Scotland, and even the north of England. In this case the molten rock was struggling to get through the overlying rocks and escape at the surface; but apparently it did not succeed in so doing, for we do not find lava-flows to the east and south. These basalt dykes are found as far south as Yorkshire, and can be traced over an area of one hundred thousand square miles. It is thus evident that in the Miocene Period a great and extensive mass of molten basalt was underlying a large part of the British Isles, and probably the weight of the thick rocks overlying it was sufficient to prevent its escape to the surface. If it had succeeded in so escaping and overflowing, how different the scenery of much of Scotland and Northern England might have been! [Illustration: COLUMNAR BASALT AT CLAMSHELL CAVE, STAFFA. FROM A PHOTOGRAPH BY J. VALENTINE.] CHAPTER IX. MOUNTAIN ARCHITECTURE. The splendour falls on castle walls And snowy summits old in story; The long light shakes across the lakes, And the wild cataract leaps in glory. Blow, bugle, blow, set the wild echoes flying; Blow, bugle; answer, echoes, dying, dying, dying. TENNYSON. The dying splendours of the sun slowly sinking and entering the "gates of the West" may well serve as a fitting emblem of the mountains in their beautiful old age, awaiting in silent and calm dignity the time when they also must be brought low, and sink in the waters of the ocean, as the sun appears daily to do. Yes, they too have their day. They too had their rising, when mighty forces brought them up out of their watery bed. Many of them have passed their hey-day of youth, and their midday; while others, far advanced in old age, are nearing the end of their course. But as the sun rises once more over eastern seas to begin another day, so will the substance of the mountains be again heaved up after a long, long rest under the sea, and here and there will rise up from the plains to form the lofty mountain-ranges of a distant future.