Much of the masonry in the canyon is of the type described, but examples differ widely in degree of finish and in material selected. Some of the walls appear very rough and even crude, so much so that they almost appear to be the first efforts of a people at an unknown art, but a closer inspection shows that even the rudest walls were erected with a knowledge of the principles which were followed in the best ones, and that the difference resulted only from the care or lack of care employed. The rudest walls are much superior to the masonry of the Navaho cists which are found in conjunction with them and which are constructed on a different method. Although walls were often built on sloping rock, and the builders had experience and at times disastrous experience to guide them, the necessity for a fiat and solid foundation was never appreciated. Walls were sometimes built on loose debris; even refuse which had been covered and formed an artificial soil was considered sufficient. There are many instances in the canyon where lack of foresight or lack of knowledge in this respect has brought about the destruction of walls. Walls resting on foreign material occur throughout the region; they are not confined to anyone class of ruins or to any part of the canyon, but are found as much or more in the most recent as in the most ancient examples. Mummy Cave ruin and Casa Blanca are good examples. In the latter the small room on the left of the upper group (plate XLVII) is especially interesting. The side walls appear to rest on a deposit of refuse nearly 2 feet thick, which in turn rests on the sloping rock. The front wall is supported by a buttress as shown; without this support it would certainly have been pushed out. The buttress appears to have been built at the same time as the front wall, although its use in this way is not aboriginal. The whole arrangement is such as would result if this room, originally represented by a low front wall perhaps, were constructed when the site became inadequate and consequently at a late period in its occupancy. The character of the refuse and debris upon which some of the walls rest is worth notice. It is well known that sheep were introduced into this country by the Spaniards, and the presence in the ruins of sheep dung, or of a material which closely resembles it, is important. Much of this is due to subsequent Navaho occupancy, and many ruins are used today by these Indians as sheepfolds. It is said, moreover, that at the time of the Navaho war, when the soldiers bayoneted all the sheep they could find, large flocks were driven up into some cliff ruins that are almost inaccessible, and kept there for a time in security. But many instances are found where the walls rest directly upon layers of compacted dung. An example is shown in plate LII, and others are mentioned in the text under the descriptions of various ruins. [Illustration: Plate LVIII Chinked Walls in Canyon De Chelly]