In some of the buildings, however, the angles of the quadrangle are rounded, and in one instance—that of the Peñasca Blanco—the structure is elliptical. From the nature of the plan of any of these buildings it is evident that many of the apartments on the ground floor were dark, and were probably used for granaries and store-rooms. There are no doors whatever in the outer walls, and no windows except in the upper stories. Windows and doors opening into the courts are, on the contrary, numerous in all the stories but the first. The doors are quite small, in many cases not exceeding two and a half feet square. The lintels of the doors and windows are in most cases stone slabs, but in some instances are small round timbers tied together with withes. A remarkable feature of the construction is the presence of the Yucatan arch formed of overlapping stones, illustrations of which may be seen in our next chapter. Dr. Hammond, a companion of Lieutenant Simpson, has minutely described a room of very perfect finish.[473] Each edifice was provided with the sacred estufa, and some of the houses had as many as seven, circular in form, excavated several feet deep in the earth and enclosed with circular walls. One in the Pueblo Bonito was of remarkable size, having been sixty feet in diameter, extending twelve feet below the surface and rising two or three stories high. Lieutenant Simpson found in close proximity to one of the ruins an excavation in the cliff which had been enclosed with a front wall of well-laid stone and mortar, thus associating one of the simplest of the cave-dwellings to which we shall refer presently, with one of the most extensive and perfect of the Pueblo buildings; a fact of no little value in identifying the architects of both as one and the same.[474] This introduces us to another class of ruins, which, with a couple of exceptions, were not discovered prior to the summer of 1874. We refer to the cliff-dwellings, the most remarkable habitations ever occupied by man. The descriptions of them seem more suitable to form parts of the most romantic works of fiction than of sober and scientific memoirs from the pens of government explorers. One hundred miles westward from the ruins of the Chaco lies the Chelly Valley or Cañon. The Chelly is one of the tributaries of the Rio San Juan from the south, having its source in the Navajo country. The Chelly Cañon is described as from one hundred and fifty to nine hundred feet wide, with perpendicular sides between three hundred and five hundred feet high. Simpson in 1849 found several caves built up in front with stone and mortar in a side cañon. About four miles from its foot or mouth he observed on a shelf fifty feet high, accessible only by ladders, a stone ruin, the plan of which resembles that of the Chaco Valley pueblos, except that it was constructed on a considerably smaller scale. Three miles further up the cañon a double ruin of an extraordinary nature was discovered. At the