The Story of the Hills: A Book About Mountains for General Readers.

by H. N. Hutchinson

There can be no doubt that the present area of the Grampian Hills was once the site of a considerable number of volcanoes, only at a much higher level than their present surface, elevated though that is to the region of the clouds; but in this case subsequent denudation has been so enormous that the old mountain surface has been planed away until all we can now see is a series of separate patches of granite, that were once in a fused and highly heated state far below the surface, and formed part of the subterranean reservoirs from which the volcanoes derived their great supplies of lava and steam. It is indeed difficult to imagine the enormous amount of denudation which has taken place in the Highlands of Scotland, and to realise that the magnificent range of the Cairngorms, for instance, has been for ages worn down until now they are but a remnant of what they once were.

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.

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!