An hypothesis as to the order in which sites of the various classes were occupied can not be based on the present condition of the ruins. It is more than likely that the older ruins served as quarries of building material for succeeding structures erected near them, and probably some of the cliff ruins themselves served in this way for the erection of others, for there are many sites from which the building stone has been almost entirely removed; yet there is no doubt that these sites were formerly occupied. The Navaho also have contributed to the destruction. Notwithstanding their horror of contact with the remains of the dead, quite a number of buildings have been erected by these Indians with material derived from adjacent ruins. It is evident that the gathering of this material would be a much lighter task than to quarry and prepare it, no matter how roughly the latter might be done. In a study of some ruins in the valley of the Rio Verde, made a few years ago, a suggestion was made of the order in which ruins of various kinds succeeded one another--a sort of chronologic sequence, of which the beginning in time could not be determined. Studies of the ruins and inhabited villages of the old province of Tusayan (Moki) and Cibola (Zuni), and a cursory examination of ruins on Gila river, show that they all fall easily into the same general order, which is somewhat as follows: 1. The earliest form of pueblo house is doubtful. As a rule, in most localities the earliest forms are already well advanced. As it is now known that the ancient pueblo region was not inhabited by a vast number of people, but by a comparatively small number of little bands, each in constant though slow movement, this condition is what we would expect to find. It is probable that the earliest settlements consisted of single houses or small clusters located in valleys convenient to areas of cultivable land and on streams or near water. 2. The next step gives us villages, generally of small size, located on the foothills of mesas and overlooking large areas of good land which were doubtless under cultivation. This class comprises more examples perhaps than any other, and many of them come well within the historic period, such as six of the seven villages of Tusayan at the time of the Spanish conquest in 1540, all of the Cibolan villages of the same date, and some of the Rio Grande pueblos of that time. 3. In some localities, though not in all, the small villages were at a later period moved to higher and more inaccessible sites. This change has taken place in Tusayan within the historic period, and in fact was not wholly completed even fifty years ago. The pueblo of Acoma was in this stage at the time of the conquest, and has remained so to the present day. As a rule each of the small villages preserved its independence, but in some cases they combined together to occupy together a high defensive site. Such combination is, however, unusual.