As artifacts were not found in or near the buildings on the Hill Canyon cliffs, and as the ruins show no evidence of former habitation, it is evident that they were not dwellings. Their use and the kinship of the people who built them can be judged only by what is left of their walls and the character of their masonry. As has been pointed out, the most prominent of these ruins are circular rooms or towers, arranged in clusters, for an interpretation of which we may look to similar architectural forms found elsewhere in the Southwest. Their commanding position suggests that these towers were constructed for lookouts and for defense, but the questions might very pertinently be asked, Why should either of these uses necessitate three or four almost identical buildings grouped together, when one would be sufficient? Why are some of them in places where there is no broad outlook? The massive character of the walls suggests a fortification, but why if defense were the only explanation of their use would not one large building be preferable to many, especially as it would be more easily constructed. It might be urged that they were granaries; but if so, why were they placed in such a conspicuous situation? In searching for an explanation for the construction of these buildings, an examination was made of aboriginal towers in the valley of the San Juan and its tributaries, especially the Yellow Jacket Canyon and those tributaries entering it on the northern side. In the Mesa Verde National Park the author has also discovered several towers which are in a comparatively good state of preservation. Some of these are situated on high cliffs, others stand in valleys hidden by dense forests of cedar. Towers are, roughly speaking, scattered sporadically in numbers over a wide extent of country, bounded on the east by Dolores River and on the south by the Mancos River and the San Juan. They extend as far west as Montezuma Creek, following it up north as far as exploration has gone and occurring as far south as Zuñi. Rarely, if ever, however, do we find towers in the dry, sandy, wastes south of the San Juan, and they are unrepresented in the great ruins of the Chaco Canyon. Although there seemed to be certain minor differences in the construction of towers found at different places in this area of distribution, all are identical in essential features.