erosion or equal antiquity. Surface erosion does not become a prominent factor until after the walls have fallen, and one wall may easily last for a century or two centuries longer than another similarly situated. The surface erosion of a standing wall of grout, such as these under discussion, is very slight; photographs of the Casa Grande ruin, extending over a period of sixteen years, and made from practically the same point of view, show that the skyline or silhouette remained essentially unchanged during that period, every little knob and projection remaining the same. It is through sapping or undermining at the ground surface that walls are destroyed. An inspection of the illustrations accompanying this paper will show what is meant by sapping: the external walls are cut away at the ground surface to a depth varying from a few inches to nearly 2 feet. After a rain the ground, and that portion of the walls at present below its surface, retains moisture much longer than the part of the walls which stands clear; the moisture rises by capillary attraction a foot or two above the ground surface, rendering the walls at this level softer than elsewhere, and as this portion is more exposed to the flying sand which the wind sweeps over the ground it is here that erosion attains its maximum. The wall is gradually cut away at and just above the ground surface until finally the base becomes too small to support it and it falls en masse. Then and not till then surface erosion becomes an important factor and the profile of the mass becomes finally rounded. But it will be readily seen that a slight difference of texture, or thickness, or exposure, or some trifling difference too minute for observation, might easily add many decades to the apparent age of a mound. The walls once fallen, however, the rounding or smoothing of the mounds would probably proceed at an equal rate throughout the group, and study of the profile gives a fairly good estimate as to the comparative age of the mounds. On this basis the most ancient mounds are those specified above, while the most recent are those in the immediate vicinity of the Casa Grande ruin. This estimate accords well with the limited historical data and with the Pima traditions, which recount that the Casa Grande ruin was the last inhabited village in this vicinity.

Fig. 328.—Map of large mound.

Probably intermediate in time between the Casa Grande ruin and the rounded mounds described above should be placed the large structure occupying the northern-central part of the map. This mound is deserving of more than a passing notice. It consists of two mounds, each four or five times the size of the Casa Grande ruin, resting on a flat-topped pedestal or terrace about 5 feet above the general level. The summits of these mounds, which are nearly flat, are some 13 feet above this level. The sides of the mounds slope very sharply, and have suffered somewhat from erosion, being cut by deep gullies, as shown in figure 328, which is an enlargement from the map. It has been stated that these structures were mounds, pure and simple, used for sacrifice or worship, resembling somewhat the well-known pyramid of Cholula; but there is no doubt that they are the remains of house-structures, for a careful examination of the surface on the slopes, reveals the ends of regular walls. The height is not exceptional, the mound on the east being less than 3 feet lower, while the one on the southeast lacks less than 4 feet of its height. The characteristic feature, however, and one difficult to explain, except on the hypothesis stated, is the sharp slope of the sides. It will be noticed that the raised base or terrace on which the mounds are located is not perfectly flat, but on the contrary has a raised rim. This rim seems quite inconsistent with the theory which has been advanced that the terrace was built up solidly as a terrace or base, as in that case it would seem natural that the slope from the base of the mounds to the edge of the terrace would be continuous.

There is an abundance of room between the crest of the rim and the base of the terrace for a row of single rooms, inclosing a court within which the main structures stood, or such a court may have been covered, wholly or partly with clusters of rooms, single storied outside, but rising in the center, in two main clusters, three or more stories high. Such an agglomeration of rooms might under certain conditions produce the result seen here, although a circumscribing heavy wall, occupying the position of the crest of the rim and inclosing two main clusters each rising three or more stories, might also produce this result. The difficulty with the latter hypothesis is, however, that under it we should expect to find a greater depression between the base of the mounds and the edge of the terrace. The most reasonable hypothesis, therefore, is that the space between the base of the mounds and the edge of the terrace was occupied by rooms of one story. This would also help to explain the steepness of the slopes of the mounds themselves. The walls of the structures they represent, being protected by the adjacent low walls of the one-story rooms, would not suffer appreciably by undermining at the ground level, and if the central room or rooms of each cluster were higher than the surrounding rooms, as is the case in the Casa Grande ruin, the exterior walls, being usually heavier than the inner walls, would be the last to succumb, the clusters would be filled up by the disintegration of the inner walls, and not until the spaces between the low one-story walls surrounding the central cluster were nearly filled up would the pronounced disintegration of the outer walls of the structures commence. At that period the walls were probably covered and protected by debris dropping from above, and possibly the profile of the mounds was already established, being only slightly modified by surface erosion since.

PLATE LII

GROUND PLAN OF CASA GRANDE RUIN

About the center of the eastern side of the terrace, and also on the western side, the water which falls on the surface of the structure is discharged through rather pronounced depressions at these points. These depressions are not the work of running water, though doubtless emphasized by that agency, but represent low or open spaces in the original structure, probably passageways or gateways. Furthermore, before or inside each gateway there is a slightly depressed area, just where we would expect to find it under our hypothesis, and showing that the process of filling in is not yet completed. If the structure were to remain undisturbed for some decades longer these spaces would doubtless be filled up from material washed from the mounds, giving eventually a continuous slope from the base of the mounds to the edge of the terrace.

On the eastern margin of the map and in the southeastern corner two small and sharply defined mounds, differing in character from any others of the group, are represented. That shown on the eastern margin rises about 6 feet and the other about 10 feet above the surrounding level, and both stand out alone, no other remains occurring within a hundred yards in any direction. These mounds seem a thing apart from the other remains in the group; and it is probable that they represent the latest period in the occupancy of this site, or possibly a period subsequent to