the one extremity to the other--from Benlomond to the Maidenpaps of Caithness--of a continuous tract of Old Red Sandstone; though, ere the land finally emerged, the ocean currents of ages had swept it away, all except in the lower and last-raised borders, and in the detached localities, where it still remains, as in the pyramidal hills of western Ross-shire, to show the amazing depth to which it had once overlaid the inferior rocks. The Old Red Sandstone of Morvheim, in Caithness, overlooks all the primary hills of the district, from an elevation of three thousand five hundred feet. [Footnote E: Suil Veinn, Coul Beg, and Coul More.] The depth of the system, on both the eastern and western coasts of Scotland, is amazingly great--how great, I shall not venture to say. There are no calculations more doubtful than those of the geologist. The hill just instanced (Morvheim) is apparently composed from top to bottom of what in Scotland forms the lowest member of the system--a coarse conglomerate; and yet I have nowhere observed this inferior member, when I succeeded in finding a section of it directly vertical, more than a hundred yards in thickness--less than one tenth the height of the hill. It would be well nigh as unsafe to infer that the three thousand five hundred feet of altitude formed the real thickness of the conglomerate, as to infer that the thickness of the lead which covers the dome of St. Paul's is equal to the height of the dome. It is always perilous to estimate the depth of a deposit by the height of a hill that seems externally composed of it, unless, indeed, like the pyramidal hills of Ross-shire, it be unequivocally a hill dug out by denudation, as the sculptor digs his eminences out of the mass. In most of our hills, the upheaving agency has been actively at work, and the space within is occupied by an immense nucleus of inferior rock, around which the upper formation is wrapped like a caul, just as the vegetable mould or the diluvium wraps up this superior covering in turn. One of our best known Scottish mountains--the gigantic Ben Nevis--furnishes an admirable illustration of this latter construction of hill. It is composed of three zones or rings of rock, the one rising over and out of the other, like the cases of an opera-glass drawn out. The lower zone is composed of gneiss and mica-slate, the middle zone of granite, the terminating zone of porphyry. The elevating power appears to have acted in the centre, as in the well-known case of Jorullo, in the neighborhood of the city of Mexico, where a level tract four square miles in extent rose, about the middle of the last century, into a high dome of more than double the height of Arthur's Seat.[F] In the formation of our Scottish mountain, the gneiss and mica-slate of the district seem to have been upheaved, during the first period of Plutonic action in the locality, into a rounded hill of moderate altitude, but of huge base. The upheaving power