170 COMPARATIVE ARCHEOLOGY. 
of flagstone set on edge, characteristic specimens of which 
have been described at Uley (Gloucester), Stoney Littleton 
(Somerset), Park Cwn (Gower Peninsula), Achnacree (Argyll), 
Maeshowe (Orkney), New Grange (Ireland), etc., ete. But 
between dolmens, cairns, tumuli, barrows, etc., there is some- 
times no clear distinction—so much do they overlap in con- 
structive details. 
VitriFieD Forts.—Among the forts, camps, huts, and 
other inhabited sites, I must restrict my remarks to vitrified 
forts, so called because their surrounding stone walls are 
partly cemented by a vitreous slag, caused by the external 
application of heat, which liquified the fragments of trap-rock 
in their structure—thus forming an excellent substitute for 
mortar. Hill-forts, with or without vitrifaction, were con- 
structed for defensive purposes, the wall following the contour 
of the summit, which it enclosed. The chief problem at issue 
is, therefore, to account for the raison d’étre of the vitri- 
faction, which, to a greater or less extent, is, or rather was, 
to be seen on the surrounding walls of some fifty stone-built 
forts scattered throughout the northern and south-western 
districts of Scotland. I have satisfied myself, from a practical 
examination of the more important examples in Scotland, 
that the vitrification was effected by the external application 
of fire after the wall had been constructed. The wall was 
composed of small stones, such as could be gathered around 
the site, and the sole object of firing it was to consolidate the 
loose stones into a uniform mass. All trap-rocks are readily 
fused under a moderate heat without a flux, so that, with the 
addition of an alkali such as could be supplied by wood ashes 
or dried seaweeds, most of the stones could be converted into 
the pudding-stone appearance of the walls of vitrified forts. 
It is also noteworthy that vitrified walls are scarcely half as 
thick as those great stone walls with well-built double facings, 
such as may be seen in the forts of Burghead, Forgandenny, 
and Abernethy; so that, without some cementing element, 
the small stones could not be of permanent value as a defen- 
sive barrier, as they would soon fall asunder. 
Outside the Scottish area the distribution of vitrified forts 
is somewhat remarkable. Four are stated, on the authority 
