nt 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, extend