It is also convenient here to note, though it anticipates our story, the more elaborate example of this principle of roofing which is shown in the so-called =Treasury of Atreus= at =Mycenæ= in Greece. In this instance, moreover, there is a farther approximation toward the arch, since the projections of the stones have been cut so as to present a continuous line. And these contour lines are slightly concave and meet at the top in a point, for which reason this class of tomb is known as _bee-hive_. Another form of this method of angular roofing is seen in an =Arch at Delos=, which is part of a system of masonry that is known as _Cyclopean_, after the name of the one-eyed giant whom Ulysses and his followers encountered in Sicily, during their return from Troy. For the masonry is composed of large blocks of unshaped stone, the interstices of which are filled in with smaller stones. Here, too, the actual arch is composed of a repetition of huge, upright monoliths, supporting a series of single blocks, set up one against the other at an angle. While, however, these primitive forms of roof construction prefigure the later development of the true arch, the student is warned in advance that they represent rather a feeling of the need of some such method of construction than any approach to a solution of the problem. For the latter, as we shall find later, consisted in discovering how to counteract the thrust of the arch; its tendency, that is, to press outward and collapse; whereas in the primitive construction this danger was evaded by embedding the roof in a mass of masonry or earth that made lateral strains impossible. The system, in fact, was more like that employed in shoring up the excavations in modern tunnelling and mining. Meanwhile, this rude method of spanning an opening with more than one piece of stone was the primitive germ of the later development of arch, vault, and dome construction, just as the placing of a single horizontal stone on two upright ones is the prototype of columns and entablature. Thus the instinct of man, in earliest times, reached out toward the two fundamental principles of architectural construction. The most interesting examples of primitive structure are the so-called _Cromlechs_, of which that of =Stonehenge=, in England, is the best preserved. The unit of this and like remains is the “post and beam” formation, composed of a block of stone, supported on two uprights. In the case of Stonehenge this formation was repeated so as to form a continuous circle one hundred feet in diameter. Within this was a concentric circle, composed of smaller slabs, which enclosed a series of five separate post and beam structures on a horse-shoe plan. The latter is repeated by another series of slabs and in the centre stands the flat altar stone. Seventeen stones of the outer circle, varying from sixteen to eighteen feet in height, are still standing and in part connected by their beam slabs.