ess boulders had to
be relied on. The orthostatic slabs were often deeply sunk into the
ground where this consisted of earth or soft rock; of the latter case
there are good examples at Stonehenge, where the rock is a soft chalk.
When the ground had an uneven surface of hard rock, the slabs were set
upright on it and small stones wedged in beneath them to make them stand
firm. Occasionally, as at Mnaidra and Hagiar Kim, a course of horizontal
blocks set at the foot of the uprights served to keep them more securely
in position. With the upright block technique went hand in hand the
roofing of narrow spaces by means of horizontal slabs laid across the
top of the uprights.

The second principle of megalithic architecture was the use of more or
less coursed masonry set without mortar, each block lying on its side
and not on its edge. It is quite possible that this principle is less
ancient in origin than that of the orthostatic slab, for it usually
occurs in structures of a more advanced type. Thus in simple and
primitive types of building such as the dolmen it is most rare to find
dry masonry, but in the advanced corridor-tombs of Ireland, the Giants'
Graves and _nuraghi_ of Sardinia, and in the 'temples' of Malta this
technique is largely used, often in combination with the upright slab
system. Indeed, this combination is quite typical of the best megalithic
work: a series of uprights is first set in position, and over this are
laid several horizontal courses of rather smaller stones. We must note
that the dry masonry which we are describing is still strictly
megalithic, as the blocks used are never small and often of enormous
size.

Buildings in which this system is used are occasionally roofed with
slabs, but more often corbelling is employed. At a certain height each
succeeding course in the wall begins to project inwards over the last,
so that the walls, as it were, lean together and finally meet to form a
false barrel-vault or a false dome, according as the structure is
rectangular or round. Occasionally, when the building was wide, it was
impossible to corbel the walls sufficiently to make them meet. In this
case they were corbelled as far as possible and the open space still
left was covered with long flat slabs.

It has often been commented on as a matter of wonder that a people
living in the stone age, or at the best possessing a few simple tools of
metal, should have been able to move and place in position suc