The upper subsoils of Scotland bear frequent mark of the elevatory period which succeeded this period of depression. The boulder-clay has its numerous intercalated arenaceous and gravelly beds, which belong evidently to its own era; but the numerous surface-beds of stratified sand and gravel by which in so many localities it is overlaid belong evidently to a later time. When, after possibly a long protracted period, the land again began to rise, or the sea to fall, the superior portions of the boulder-clay must have been exposed to the action of the tides and waves; and the same process of separation of parts must have taken place on a large scale, which one occasionally sees taking place in the present time on a comparatively small one, in ravines of the same clay swept by a streamlet. After every shower, the stream comes down red and turbid with the finer and more argillaceous portions of the deposit; minute accumulations of sand are swept to the gorge of the ravine, or cast down in ripple-marked patches in its deeper pools; beds of pebbles and gravel are heaped up in every inflection of its banks; and boulders are laid bare along its sides. Now, a separation, by a sort of washing process of an analogous character, must have taken place in the materials of the more exposed portions of the boulder-clay, during the gradual emergence of the land; and hence, apparently, those extensive beds of sand and gravel which in so many parts of the kingdom exist, in relation to the clay, as a superior or upper subsoil; hence, too, occasional beds of a purer clay than that beneath, divested of a considerable portion of its arenaceous components, and of almost all its pebbles and boulders. This _washed_ clay,--a re-formation of the boulder deposit, cast down, mostly in insulated beds in quiet localities, where the absence of currents suffered the purer particles held in suspension by the water to settle,--forms, in Scotland at least, with, of course, the exception of the ancient fire-clays of the Coal Measures, the true brick and tile clays of the agriculturist and architect.
It is to these superior beds that all the recent shells yet found above the existing sea-level in Scotland, from the Dornoch Frith and beyond it, to beyond the Frith of Forth, seem to belong. Their period is much less remote than that of the shells of the boulder-clay, and they rarely occur in the same comminuted condition. They existed, it would appear, not during the chill twilight period, when the land was in a state of subsidence, but during the after period of cheerful dawn, when hill-top after hill-top was emerging from the deep, and the close of each passing century witnessed a broader area of dry land in what is now Scotland, than the close of the century which had gone before. Scandinavia is similarly rising at the present day, and presents with every succeeding age a more extended breadth of surface. Many of the boulder-stones seem to have been cast down where they now lie, during this latter time. When they occur, as in many instances, high on bare hill-tops, from five to fifteen hundred feet over the sea-level, with neither gravel nor boulder-clay beside them, we of course cannot fix their period. They may have been dropped by ice-floes or shore-ice, where we now find them, at the commencement of the period of elevation, after the clay had been formed; or they may have been deposited by more ponderous icebergs during its formation, when the land was yet sinking, though during the subsequent rise the clay may have been washed from around them to lower levels. The boulders, however, which we find scattered over the plains and less elevated hill-sides, with beds of the washed gravel or sand interposed between them and the clay, must have been cast down where they lie, during the elevatory ages. For, had they been washed out of the clay, they would have lain, not _over_ the greatly lighter sands and gravels, but _under_ them. Would that they could write their own histories! The autobiography of a single boulder, with notes on the various floras which had sprung up around it, and the various classes of birds, beasts, and insects by which it had been visited, would be worth nine-tenths of all the autobiographies ever published, and a moiety of the remainder to boot.