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 such enormous blocks of stone. With modern cranes and traction engines all would be simple, but it might have been thought that in the stone age such building would be impossible. Thus, for instance, in the 'temple' of Hagiar Kim in Malta, there is one block of stone which measures 21 feet by 9, and must weigh many tons. In reality there is little that is marvellous in the moving and setting up of these blocks, for the tools needed are ready to the hand of every savage; but there is something to wonder at and to admire in the patience displayed and in the organization necessary to carry out such vast pieces of labour. Great, indeed, must have been the power of the cult which could combine the force of hundreds and even thousands of individuals for long periods of time in the construction of the great megalithic temples. Perhaps slave labour played a part in the work, but in any case it is clear that we are in the presence of strongly organized governments backed by a powerful religion which required the building of temples for the gods and vast tombs for the dead. Let us consider for a moment what was the procedure in building a simple megalithic monument. It was fourfold, for it involved the finding and possibly the quarrying of the stones, the moving of them to the desired spot, the erection of the uprights in their places, and the placing of the cover-slab or slabs on top of them. With regard to the first step it is probable that in most cases the place chosen for a tomb or cemetery was one in which numbers of great stones lay on the surface ready to hand. By this means labour was greatly economized. On the other hand, there are certainly cases where the stones were brought long distances in order to be used. Thus, in Charente in France there is at La Perotte a block weighing nearly 40 tons which must have travelled over 18 miles. We have no evidence as to whether stones were ever actually quarried. If they were, the means used must have been the stone axe, fire, and water. It was not usual in the older and simpler dolmens to dress the stones in any way, though in the later and more complicated structures well-worked blocks were often used.