But besides the wooden islands there are others, still extant in
several of our Scottish lakes, which appear to be composed entirely
of stones and earth irregularly heaped together. In the absence of
any historical knowledge as to their age there is no _prima facie_
reason why some of these should not be contemporary with the former,
as it cannot be assumed that the crannog-builders made wood a _sine
quâ non_ in the structure of islands. There were, no doubt, certain
stagnant marshes and small lochs in which a wooden foundation was
essential for the construction of an island, owing to the softness and
yielding nature of the mud; but, on the other hand, there were others
with compact rocky or gravelly beds, in which any solid materials, as
stones, earth, turf, etc., would be equally applicable. The outlets of
the larger lakes, more especially such as were formed in glacial and
rock-cut basins, were more adapted for the latter, and as far as my
observations have enabled me to form an opinion, these are the very
situations in which the lake stone-dwellings abound. Some of them are
mere shapeless cairns, without any indications of having been formerly
inhabited, while on others some remains of stone buildings are to
be seen. As to wooden huts or houses, had such structures ever been
erected on them, it is not likely that they could, for any length of
time, have resisted the decaying tendencies of atmospheric agencies,
so that all traces of them would have disappeared long ago.
The social or military exigencies that led people to construct
artificial islands would also lead them to take advantage of such
natural ones as would be found most suitable, and we may reasonably
infer that it is in the absence of the latter that the former would
be resorted to. The great and primary object of the island-builder
was the protection afforded by the surrounding lake or morass, the
securing of which has continued to be the ruling principle in the
erection of defensive works down to the Middle Ages, long after the
wooden islands ceased to be constructed. The transition from the
crannog to the massive mediæval castle, with its moat and drawbridge,
is but a stage in the progressive march of civilisation.