is an accurately cut tenon which dovetails into two mortices cut one at
each end of the lower surface of the horizontal block. Each upright of
the outer circle had a double tenon, and the lintels, besides being
morticed to take these tenons, were also dovetailed each into its two
neighbours.

Within the horseshoe and close up to it stand the famous blue-stones,
now twelve in number, but originally perhaps more. These stones are not
so high as the trilithons, the tallest reaching only 7-1/2 feet. They
are nearly all of porphyritic diabase. It has often been asserted that
these blue-stones must have been brought to Stonehenge from a distance,
as they do not occur anywhere in the district. Some have suggested that
they came from Wales or Cornwall, or even by sea from Ireland. Now, the
recent excavations have shown that the blue-stones were brought to
Stonehenge in a rough state, and that all the trimming was done on the
spot where they were erected. It seems unlikely that if they had been
brought from a distance the rough trimming should not have been done on
the spot where they were found, in order to decrease their weight for
transport. It is therefore possible that the stones were erratic blocks
found near Stonehenge.

Within the horseshoe, and near its apex, lies the famous "Altar Stone"
(A), a block measuring about 16 feet by 4. Between the horseshoe and the
outer circle another circle of diabase stones is sometimes said to have
existed, but very little of it now remains.

The whole building is surrounded by a rampart of earth several feet
high, forming a circle about 300 feet in diameter. An avenue still 1200
feet in length, bordered by two walls of earth, leads up to the rampart
from the north-east. On the axis of this avenue and nearly at its