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 h