[Illustration: Fig. 1. THE RANGES OF THE GREAT BASIN, WESTERN STATES OF NORTH AMERICA, SHOWING A SERIES OF GREAT FRACTURES AND TILTED MASSES OF ROCK.] [Illustration: Fig. 2. SECTION THROUGH SNOWDON.] Thus Snowdon, Cader Idris, the Arans, Arenig Mountain, and others, are very largely made up of these ancient volcanic materials. The writer has picked up specimens of fossil shell-fish near the summit of Snowdon from a bed of fine volcanic ash that forms the summit. Fig. 2 represents a section through Snowdon, from which it will be seen that we have first a few sedimentary strata, _S_, then a great lava-flow, _L_; and that volcanic ashes accumulated on the top of this, of which _A A_ are patches still left. _B_ is an intrusive dyke of a basaltic rock that forced its way through afterwards. Again, in the Lake District there is a well-known volcanic series of stratified rocks of the same age, consisting mostly of lavas and ashes, the total thickness of which is about twelve thousand feet (known as the "Green Slates and Porphyries"), so that a large part of some of the mountains there have also been built up by volcanic action; but no traces of the old volcanoes remain. Going farther north we find abundant proof that volcanic action on a prodigious scale took place in Scotland during the very ancient period of the Old Red Sandstone, with which the name of Hugh Miller will always be associated. In Central Scotland we see lava-flows and strata formed of volcanic ash, with a thickness of more than six thousand feet, fragments of which, having escaped the destructive agents of denudation, now form important chains of hills, such as the Pentland, Ochil, and Sidlaw ranges. Nor was the volcanic action confined to this region. In the district of the Cheviot Hills similar volcanic rocks are to be seen. But here again the old volcanoes have long since been swept away, leaving us only portions of their outpourings buried in the hills. 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.