[Footnote 13: The actual depth of the Mid-Lothian coal-field, to the base of the carboniferous limestone, is rather more than 3000 feet. It is, perhaps, rather under than over the truth to allow 3000 feet for the total thickness of beds from the limestone to the conglomerate of Liberton, though, owing to the curved and contorted position of the strata from Edinburgh to Stirlingshire, it is impossible to obtain a measurement of their real thickness. I have attributed the isolation of the Falkirk and Mid-Lothian coal-fields to the effect of faults and general depressions of their areas. This was assuredly the case in the latter coal-field, and probably in the former also. The trap which occurs between them, though in great abundance, has certainly not acted as an elevating agent. It occurs in beds among the strata, and, judging from the number of associated tufas, appears to have been to a considerable extent erupted while the lower carboniferous series was forming. Mr. M'Laren, in his excellent work, p. 100, states his opinion that the traps may have materially contributed to push up the coal strata. A careful and extended examination of the district has convinced me that this view is incorrect.] But, perhaps, the most striking instances of denudation in the British Islands are the three famous Ross-shire hills--Suil Veinn, Coul Mor, and Coul Bheig. They are formed of piles of sandstone beds like tiers of regular masonry, and reach a height of 3000 feet over the sea. The sandstone of which they are composed must once have formed a bed or set of beds fully 2000 feet thick, that covered the whole district for many miles around. Yet of this extensive deposit there now exist only a few isolated fragments. I have watched the sunshine and shadow of an autumn sky resting alternately on these strange pyramidal hills, as they towered in their giant proportions like the last remnants of a mighty rampart that had stood the brunt of a long siege, and, breached at last in many places, had been all but levelled to the ground. How long-continued and how potent must that agency have been which could cut down and disperse the massive barrier that flanked the western coast of Ross-shire to a height of 2000 feet!