also at Copan, the stairway is furnished with ornamental balustrades, those at Chichen being carved in the form of two monstrous snakes of which the heads are extended on the ground at the foot of the pyramid. Practically all pyramid-mounds served as the support for buildings, though a certain number, without stairways, have been found which are simply burial-mounds. An exception occurs at Tikal (according to Tozzer), and perhaps in other places, where high pyramids with stairways appear to have had no crowning structures, and were possibly used as sites for offerings made in the open air. The material of which such structures are built is earth and rubble, and they have usually been faced with stone dressed with more or less accuracy, any imperfections being concealed with stucco. In some cases, notably at Copan, excavation has revealed the presence of a cement layer at some depth beneath the surface. This is probably an indication that the pyramid at some period has been enlarged, and it may be said that similar evidence of the practice of adding to existing structures is found elsewhere in the Maya area. 'The buildings which crown the foundation-mounds vary in type from simple, single-chambered edifices to elaborate complexes such as are found at Palenque and Menché, but as a matter of fact the construction is essentially the same throughout. The form of the typical Maya building was to a great extent conditioned by the fact that the primitive architect was ignorant of the principle of the true arch. It is possible that some buildings may have been furnished with flat roofs by means of wooden beams, but if so the beams have decayed and the buildings have fallen in; such structures as have survived were built as follows (Figs. 74 and 77; pp. 323 and 329): The walls, built very thick, were carried vertically up to the desired height, and then the mason commenced to build inwards at a very wide angle, allowing successive courses to overlap, until those on opposite
