These details may appear tediously minute; but let the reader mark how very much they involve. The occurrence of recent shells largely diffused throughout the boulder-clays of Caithness, at all heights and distances from the sea at which the clay itself occurs, and not only connected with the iceberg phenomena by the closest juxtaposition, but also testifying distinctly to its agency by the extremely comminuted state in which we find them, tell us, not only according to old John Busby, "that the ocean covered the inland country at some former period of time," but that it covered it to a great height at a time geologically recent, when our seas were inhabited by exactly the same mollusca as inhabit them now, and so far as yet appears, by none others. I have not yet detected the boulder-clay at more than from six to eight hundred feet over the level of the sea; but the travelled boulders I have often found at more than a thousand feet over it; and Dr. John Fleming, the correctness of whose observations few men acquainted with the character of his researches or of his mind will be disposed to challenge, has informed me that he has detected the dressed and polished surfaces at least four hundred feet higher. There occurs a greenstone boulder, of from twelve to fourteen tons weight, says Mr. M'Laren, in his "Geology of Fife and the Lothians," on the south side of Black Hill (one of the Pentland range), at about fourteen hundred feet over the sea. Now fourteen or fifteen hundred feet, taken as the extreme height of the dressings, though they are said to occur greatly higher, would serve to submerge in the iceberg ocean almost the whole agricultural region of Scotland. The common hazel (_Corylus avellana_) ceases to grow in the latitude of the Grampians, at from one thousand two hundred to one thousand five hundred feet over the sea level; the common bracken (_Pteris aquilina_) at about the same height; and corn is never successfully cultivated at a greater altitude. Where the hazel and bracken cease to grow, it is in vain to attempt growing corn.[16] In the period of the boulder-clay, then, when the existing shells of our coasts lived in those inland sounds and friths of the country that now exist as broad plains or fertile valleys, the sub-aerial superficies of Scotland was restricted to what are now its barren and mossy regions, and formed, instead of one continuous land, merely three detached groups of islands,--the small Cheviot and Hartfell group,--the greatly larger Grampian and Ben Nevis group,--and a group intermediate in size, extending from Mealfourvonny, on the northern shores of Loch Ness, to the Maiden Paps of Caithness.