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.

[Illustration:  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.