The nature of the cliffs in which the ruins of the Navaho Monument are situated favored the construction of cliff-dwellings rather than of open pueblos in this region. These cliffs are full of caverns, large and small, presenting much the same condition as the cliffs of the red sandstone elsewhere in the Southwest, as the Mesa Verde, Canyon de Chelly, the Red Rocks south of Flagstaff, and other sections where caverns abound. Fragments of fallen rocks present good plane surfaces for walls of masonry, and there is abundant clay for plastering. Trees suitable for rafters and beams are not wanting. In short, all conditions are favorable for stone and adobe houses in the cliffs. The neighboring Sethlagini mesa is of different geological formation; in it are no caverns, the mesa top is broad, and ruins thereon are necessarily open pueblos. The effect of difference in geological structure is nowhere more evident than in these adjacent formations. [Illustration: BUREAU OF AMERICAN ETHNOLOGY BULLETIN 50 PLATE 18 _a._ LARGE BLACK-AND-WHITE VASE Cat. No. 257774. Height, 17 inches. _b._ LARGE VASE WITH HANDLE Cat. No. 257787. Height, 8½ inches. POTTERY FROM NAVAHO NATIONAL MONUMENT] If environment has had so marked an influence on the character of building, we can readily see how it has affected arts and crafts. We can hardly imagine a people living any length of time in this region without being mentally influenced by the precipitous cliffs that rise on all sides. The summits of these heights are eroded into fantastic shapes resembling animals or grotesque human forms. The constant presence of these marvelous forms, of awe-inspiring size and weird appearance, exerted a profound influence on the supernatural ideas of the inhabitants. Here were born many conceptions of earth gods and the like, survivals of which still remain among the Hopi. As a rule the cliff-houses are not situated in sight of the main stream, but are hidden away in secluded side canyons, approached by narrow entrances, their sites having been determined no doubt by the position of the springs with their constant water supply. Almost every side canyon, even in a dry season, has its spring of water which, trickling out of the rocks, follows the canyon bed until it is finally drunk up by the thirsty sands. Often water seeps out of a soft stratum of rock in the cave itself, where it was gathered in artificial reservoirs that in ancient times furnished an adequate supply for the inhabitants. One feature of these side canyons is that they enlarge into basins surrounded on all sides by lofty cliffs. Many of these basins are so hidden that they can be discovered only by following dry stream-beds from their junction with the creeks. How many of these basins are still undiscovered no one can yet tell. In these basins now covered with bushes the aboriginal farms were probably situated.