The built-up tumulus of masonry took a form very different, in its definite lines, from the rounded slopes of the mound. The squared forms of brick or cut stone infallibly give to the edifice upon which they are employed one of those more or less rigid forms which are defined by geometry. When they leave the hands of the builder they are either cubes or parallelopipeds, pyramids or prisms, cylinders or cones. They present the general appearance, they possess the essential properties, of one of those forms. We may say that architecture was born on the day when man began to use the unyielding materials by which definite geometrical forms can alone be given. As soon as this early development was reached he set to work to combine those elementary forms in different proportions and to add to their effect by elegance and richness of decoration, and so in the end to form national architectures. When the first pyramid was built upon the borders of the desert man was on the threshold of the movement to which we have referred. The form adopted for the royal tomb was one of the most simple which could be chosen for a building. The most simple of all would have been the _tetrahedron_, or pyramid built upon a triangular base. But not a single pyramid of that kind has been discovered in Egypt. The whole of the pyramids, large or small, are built upon a right-angled base, and in most instances upon one with sides practically equal.[185] Mystic reasons for this shape have been given. It has been said that each face was dedicated to one of the four powers of Amen, which corresponded to the cardinal points of heaven.[186] We are not yet sufficiently well acquainted with the genesis of the Egyptian religion to be able to decide how far into the past the four powers of Amen may be traced: but it is quite possible that they were derived from the four faces of the strictly oriented pyramids. Were we inclined to enter into this discussion we should rather, perhaps, attribute the shape of the pyramid to the prevailing Egyptian desire to turn one face of their tombs towards the west, the abode of the dead, and another to the east, whence the hoped-for resurrection was to come. The three-sided pyramid would not have lent itself to such an arrangement. [185] The base of the great pyramid at Sakkarah is a rectangle, measuring 390 feet from north to south, and 347 from east to west. The three great pyramids at Gizeh like most of these structures, are built upon a base which is practically square. [186] MARIETTE, _Itinéraire de la Haute-Égypte_, p. 96.