The adaptation, of this architecture to the peculiar environment indicates that it has long been practiced under the same conditions that now prevail. Nearly all of the ancient pueblos were built of the sandstone found in natural quarries at the bases of hundreds of cliffs throughout these table-lands. This stone readily breaks into small pieces of regular form, suitable for use in the simple masonry of the pueblos without receiving any artificial treatment. The walls themselves give an exaggerated idea of finish, owing to the care and neatness with which the component stones are placed. Some of the illustrations in the last chapter, from photographs, show clearly that the material of the walls was much ruder than the appearance of the finished masonry would suggest, and that this finish depended on the careful selection and arrangement of the fragments. This is even more noticeable in the Chaco ruins, in which the walls were wrought to a high degrees of surface finish. The core of the wall was laid up with the larger and more irregular stones, and was afterwards brought to a smooth face by carefully filling in and chinking the joints with smaller stones and fragments, sometimes not more than a quarter of an inch thick; this method is still roughly followed by both Tusayan and Cibolan builders. Although many details of construction and arrangement display remarkable adaptation to the physical character of the country, yet the influence of such environment would not alone suffice to produce this architectural type. In order to develop the results found, another element was necessary. This element was the necessity for defense. The pueblo population was probably subjected to the more or less continuous influence of this defensive motive throughout the period of their occupation of this territory. A strong independent race of people, who had to fear no invasion by stronger foes, would necessarily have been influenced more by the physical environment and would have progressed further in the art of building, but the motive for building rectangular rooms--the initial point of departure in the development of pueblo architecture--would not have been brought into action. The crowding of many habitations upon a small cliff ledge or other restricted site, resulting in the rectangular form of rooms, was most likely due to the conditions imposed by this necessity for defense. [Illustration: Plate CXI. Zuñi eagle-cage.]