This long level line of the Highland mountain-tops may be seen very well from the lower country outside; for example, from the isles of Skye and Eigg, where one may see the panorama between the heights of Applecross and the Point of Ardnamurchan showing very clearly the traces of the old table-land. How are we to explain this curious fact, so opposed to our first impressions of a mountain region? It is quite clear that the old plateau thus marked out cannot be caused by the arrangement or position of the rocks of which the Highlands are composed. If these rocks were found to be lying pretty evenly in flat layers, or strata, undisturbed by great earth-movements, we could readily understand that they would form a plateau. But the reverse is the case: the rocks are everywhere thrown into folds, and frequently greatly displaced by "faults;" yet these important geological features have little or no connection with the external aspect of the country. It is therefore useless to look to internal structure for an explanation. We must look outside, and consider what has been for ages and ages taking place here. As already pointed out, an enormous amount of solid rock has been removed from this region--thousands and thousands of feet. It was long ago planed down by the action of water, so that a table-land once existed of which the tops of the present mountains are isolated fragments. No other conclusion is possible. To the geologist every hill and valley throughout the whole length and breadth of the Highlands bears striking testimony to this enormous erosion. The explanation we are seeking may therefore be summed up in one word, "denudation." The valleys that now intersect the table-land have been carved out of it. If we could in imagination put back again onto the present surface what has been removed, we should have a mental picture of the Highlands as a wide, undulating table-land; and this rolling plain would suggest the bottom of the sea. The long flat surfaces of the Highland ridges, cut across the edges of inclined or even upright strata, are the fragments of a former base-line of erosion; that is, they represent the general submarine level to which the Highlands were reduced after exposure to the action of "rain and rivers," and finally of the sea. As the sea gradually spread over it, it planed down everything that had not been previously worn away, and so reduced the whole surface to one general level like the sea-bed of the present day. But it is not necessary to suppose that the whole region was under water at the same time, and it is probable that there were separate inland seas or lakes. In these the rocks of the Old Red Sandstone were formed; and they in their turn have suffered so much denudation that only patches and long strips of them are left on the borders of the Highlands.